Not all battery energy can or should be used on discharge; some reserve is almost always left behind on purpose after the equipment cuts off. There are several reasons for this.
Most mobile phones, laptops and other portable devices turn off when the lithium-ion battery reaches 3.00V/cell on discharge. At this point the battery has about 5 percent capacity left. Manufacturers choose this voltage threshold to preserve some energy for housekeeping, as well as to reduce battery stress and allow for some self-discharge if the battery is not immediately recharged. This grace period in empty state can last several months until self-discharge lowers the voltage of Li-ion to about 2.50V/cell, at which point the protection circuit opens and most packs become unserviceable with a regular charger.
Power tools and medical devices drawing high current tend to push the battery voltage to an early cut-off prematurely. This is especially apparent at cold temperatures and in cells with high internal resistance. These batteries may still have ample capacity left after the cutoff; discharging them with a battery analyzer at a moderate load will often give a residual capacity of 30 percent. Figure 1 illustrates the cut-off voltage graphically.
Figure 1: Illustration of equipment with high cut-off voltage.
Portable devices do not utilize all available battery power and leave some energy behind.
To prevent triggering premature cutoff at a high load or cold temperature, some device manufacturers may lower the end-of-discharge voltage. Li-ion in a power tool may discharge the battery to 2.70V/cell instead of 3.00V/cell; Li-phosphate may go to 2.45V/cell instead of 2.70V/cell, lead acid to 1.40V/cell instead of the customary 1.75V/cell, and NiCd/NiMH to 0.90V/cell instead of 1.00V/cell.
Industrial applications aim to attain maximum service life rather than optimize runtime, as it is done with consumer products. This also applies to the electric powertrain; batteries in a hybrid cars and electric vehicle electric vehicles are seldom fully discharged or charged; most operate between 30 and 80 percent state-of-charge when new. This is the most effective working bandwidth; it also delivers the longest service life. A deep discharge to empty followed a full charge would cause undue stress for the Li-ion. Similarly, satellitesuse only the mid-band of a battery called the “sweet zone.” Figure 2 illustrates the “sweet zone” of a battery.
Figure 2: Sweet zone of a Lithium-ion battery to extend life.
Operating Li-ion in the “sweet zone” prolongs battery life because a partial cycle is less stressful than a full cycle. As the capacity fades with use, the battery management system (BMS) may engage the full working range of the battery.
Elevated internal resistance makes alkaline and other primary batteries unsuitable for high load applications. The resistance rises further as the cell depletes. This causes an early cutoff with the device drawing some current, and much energy is left behind. Primary batteries have high capacities and perform well when new, but they soon lose power like a deflating balloon.
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Become familiar with battery fade and how the ready light can deceive the user.
Folks have been using rechargeable batteries for over 100 years but this marvelous power source is still poorly understood. The battery is a silent worker that delivers energy until it quits of exhaustion and old age. It is more prone to failure than most other parts in a system. Much is expected but little is given in return. With a shorter life span than the host device, battery replacement becomes an issue, and the “when” and “what if” are not well defined by the device manufacturer. Some batteries are replaced too soon but most stay too long.
A portable system works well when the batteries are new but confidence drops after the first packs need replacing due to capacity fade. In time, the battery fleet becomes a jumble of good and bad batteries, and that’s when the headache begins. Battery management mandates that all batteries in a fleet are kept at an acceptable capacity level. Packs that fall below a given threshold must be replaced to keep system integrity. Battery failure occurs most often on a heavy traffic day or in an emergency when more than normal service is demanded.
Batteries exhibit human-like qualities and need good nutrition. Care begins by operating at room temperate and discharging them at a moderate current. There is some truth as to why batteries cared for by an individual user outperform those in a fleet; studies can back this up.
Charging is generally well understood, but the “ready” light is misconstrued. Ready does not mean “able.” There is no link to battery performance, nor does the green light promise full runtime. All batteries charge fully, even if weak; “ready” simply means that the battery is full.
The capacity a battery can hold diminishes with age and the charge time shortens with nickel-based batteries and in part also with lead acid, but not necessarily with Li-ion. Lower charge transfer capability that inhibits the flow of free electrons prolongs the charge time with aged Li-ion. (See BU-409a: Why do Old Li-ion Batteries Take Long to Charge?)
A short charging time propels faded batteries to the top, disguised as combat ready. System collapse is imminent when workers scramble for freshly charged batteries in an emergency; those that are lit-up may be deadwood. (Note that the charge time of a partially charged battery is also shorter.) Figure 1 shows the “ready” light that is known to lie.
Figure 1: The “ready” light lies. The READY light indicates that the battery is fully charged. This does not mean “able” as there is no link between “ready” and battery performance.
The amount of energy a battery can hold is measured in capacity. Capacity is the leading health indicator that determines runtime and predicts end of battery life when low. A new battery is rated at 100 percent, but few packs in service deliver the full amount: a workable capacity bandwidth is 80–100 percent. As a simple guideline, a battery on a two-way radio having a capacity of 100 percent would typically provide a runtime of 10 hours, 80 percent is 8 hours and 70 percent, 7 hours.
The service life of a battery is specified in number of cycles. Lithium- and nickel-based batteries deliver between 300 and 500 full discharge/charge cycles before the capacity drops below 80 percent.
Cycling is not the only cause of capacity loss; keeping a battery at elevated temperatures also induces stress. A fully charged Li-ion kept at 40°C (104°F) loses about 35 percent of its capacity in a year without being used. ( See BU:808: How to Prolong Lithium-based Batteries ). Ultra-fast chargers and harsh discharging is also harmful. This cuts battery life to half, and hobbyists can attest to this.
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Discover what a battery needs to get going and maintain a long life.
In many ways, a battery behaves like a human being. It senses the kindness given and delivers on the care given. It is as if the battery has feelings and returns on the benevolence bestowed. But there are exceptions, as any parent raising a family will know; and the generosity conferred may not always deliver the anticipated returns.
To become a good custodian, you must understand the basic needs of a battery, a subject that is not taught in school. This section teaches what to do when the battery is new, how to feed it the right “food” and what to do when putting the pack aside for a while. Chapter 7 also looks into restrictions when traveling with batteries by air and how to dispose of them when their useful life has passed.
Just as a person’s life expectancy cannot be predicted at birth, neither can we date stamp a battery. Some packs live to a great old age while others die young. Incorrect charging, harsh discharge loads and exposure to heat are the battery’s worst enemies. Although there are ways to protect a battery, the ideal situation is not always attainable. This chapter discusses how to get the most from our batteries.
Priming a New Battery
Not all rechargeable batteries deliver the rated capacity when new, and they require formatting. While this applies to most battery systems, manufacturers of lithium-ion batteries disagree. They say that Li-ion is ready at birth and does not need priming. Although this may be true, users have reported some capacity gains by cycling after a long storage.
“What’s the difference between formatting and priming?” people ask. Both address capacities that are not optimized and can be improved with cycling. Formatting completes the fabrication process that occurs naturally during use when the battery is being cycled. A typical example is lead- and nickel-based batteries that improve with usage until fully formatted. Priming, on the other hand, is a conditioning cycle that is applied as a service to improve battery performance during usage or after prolonged storage. Priming relates mainly to nickel-based batteries.
Lead Acid
Formatting a lead acid battery occurs by applying a charge, followed by a discharge and recharge. This is done at the factory and is completed in the field as part of regular use. Experts advise not to strain a new battery by giving it heavy duty discharges at first but gradually working it in with moderate discharges, like an athlete trains for weight lifting or long-distance running. This, however, may not be possible with a starter battery in a vehicle and other uses. Lead acid typically reaches the full capacity potential after 50 to 100 cycles. Figure 1 illustrates the lifespan of lead acid.
Figure 1: Lifespan of Lead Acid
A new lead acid battery may not by fully formatted and only attains full performance after 50 or more cycles. Formatting occurs during use; deliberate cycling is not recommended as this would wear down the battery unnecessarily.
Deep-cycle batteries are at about 85 percent when new and will increase to 100 percent, or close to full capacity, when fully formatted. There are some outliers that are as low as 65 percent when tested with a battery analyzer. The question is asked, “Will these low-performers recover and stand up to their stronger brothers when formatted?” A seasoned battery expert said that “these batteries will improve somewhat but they are the first to fail.”
The function of a starter battery lies in delivering high load currents to crank the engine, and this attribute is present from the beginning without the need to format and prime. To the surprise of many motorists, the capacity of a starter battery can fade to 30 percent and still crank the engine; however, a further drop may get the driver stranded one morning. See also BU-904: How to Measure Capacity)
Nickel-based
Manufacturers advise to trickle charge a nickel-based battery for 16–24 hours when new and after a long storage. This allows the cells to adjust to each other and to bring them to an equal charge level. A slow charge also helps to redistribute the electrolyte to eliminate dry spots on the separator that might have developed by gravitation.
Nickel-based batteries are not always fully formatted when leaving the factory. Applying several charge/discharge cycles through normal use or with a battery analyzer completes the formatting process. The number of cycles required to attain full capacity differs between cell manufacturers. Quality cells perform to specification after 5–7 cycles, while lower-cost alternatives may need 50 or more cycles to reach acceptable capacity levels.
Lack of formatting causes a problem when the user expects a new battery to work at full capacity out of the box. Organizations using batteries for mission-critical applications should verify the performance through a discharge/charge cycle as part of quality control. The “prime” program of automated battery analyzers (Cadex) applies as many cycles as needed to attain full capacity.
Cycling also restores lost capacity when a nickel-based battery has been stored for a few months. Storage time, state-of-charge and temperature under which the battery is stored govern the ease of recovery. The longer the storage and the warmer the temperature, the more cycles will be required to regain full capacity. Battery analyzers help in the priming functions and assure that the desired capacity has been achieved.
Lithium-ion
Some battery users insist that a passivation layer develops on the cathode of a lithium-ion cell after storage. Also known as interfacial protective film (IPF), this layer is said to restrict ion flow, cause an increase in internal resistance and in the worst case, lead to lithium plating. Charging, and more effectively cycling, is known to dissolve the layer and some battery users claim to have gained extra runtime after the second or third cycle on a smartphone, albeit by a small amount.
Scientists do not fully understand the nature of this layer, and the few published resources on this subject only speculate that performance restoration with cycling is connected to the removal of the passivation layer. Some scientists outright deny the existence of the IPF, saying that the idea is highly speculative and inconsistent with existing studies. Whatever the outcome on the passivation of Li-ion may be, there is no parallel to the “memory” effect with NiCd batteries that require periodic cycling to prevent capacity loss. The symptoms may appear similar but the mechanics are different. Nor can the effect be compared to sulfation of lead acid batteries.
A well-known layer that builds up on the anode is the solid electrolyte solid electrolyte interface (SEI). SEI is an electrical insulation but has sufficient ionic conductivity to allow the battery to function normally. While the SEI layer lowers the capacity, it also protects the battery. Without SEI, Li-ion might not get the longevity that it has. (See BU-307: How does Electrolyte Work?)
The SEI layer develops as part of a formation process and manufacturers take great care to do this right, as a batched job can cause permanent capacity loss and a rise in internal resistance. The process includes several cycles, float charges at elevated temperatures and rest periods that can take many weeks to complete. This formation period also provides quality control and assists in cell matching, as well as observing self-discharge by measuring the cell voltage after a rest. High self-discharge hints to impurity as part of a potential manufacturing defect.
Electrolyte oxidation (EO) also occurs on the cathode. This causes a permanent capacity loss and increases the internal resistance. No remedy exists to remove the layer once formed but electrolyte additives lessen the impact. Keeping Li-ion at a voltage above 4.10V/cell while at an elevated temperature promotes electrolyte oxidation. Field observation shows that the combination of heat and high voltage can stress Li-ion more than harsh cycling.
Lithium-ion is a very clean system that does not need additional priming once it leaves the factory, nor does it require the level of maintenance that nickel-based batteries do. Additional formatting makes little difference because the maximum capacity is available right from the beginning, (the exception may be a small capacity gain after a long storage). A full discharge does not improve the capacity once the battery has faded — a low capacity signals the end of life. A discharge/charge may calibrate a “smart” battery but this does little to improve the chemical battery. (See BU-601: Inner Working of a Smart Battery.) Instructions recommending charging a new Li-ion for 8 hours are written off as “old school,” a left-over from the old nickel battery days.
Non-rechargeable Lithium
Primary lithium batteries, such as lithium-thionyl chloride (LTC), benefit from passivation in storage. Passivation is a thin layer that forms as part of a reaction between the electrolyte, the lithium anode and the carbon-based cathode. (Note that the anode of a primary lithium battery is lithium and the cathode is graphite, the reverse of Li-ion.)
Without this layer, most lithium batteries could not function because the lithium would cause a rapid self-discharge and degrade the battery quickly. Battery scientists even say that the battery would explode without the formation of lithium chloride layers and that the passivation layer is responsible for the battery’s existence and the ability to store for 10 years.
Temperature and state-of-charge promote the buildup of the passivation layer. A fully charged LTC is harder to depassivate after long storage than one that was kept at a low charge. While LTC should be stored at cool temperatures, depassivation works better when warm as the increased thermal conductivity and mobility of the ions helps in the process.
CAUTION
Do not apply physical tension or excessive heat to the battery. Explosions due to careless handling have caused serious injuries to workers.
The passivation layer causes a voltage delay when first applying a load to the battery, and Figure 2 illustrates the drop and recovery with batteries affected by different passivation levels. Battery A demonstrates a minimal voltage drop while Battery C needs time to recover.
Figure 2: Voltage behavior when applying a load to a passivated battery.
Battery A has mild passivation, B takes longer to restore, and C is affected the most. Courtesy EE Times
LTC in devices drawing very low current, such as a sensor for a road toll or metering, may develop a passivation layer that can lead to malfunction, and heat promotes such growth. This can often be solved by adding a large capacitor in parallel with the battery. The battery that has developed a high internal resistance is still capable of charging the capacitor to deliver the occasional high pulses; the standby time in between is devoted to recharging the capacitor.
To assist in sulfation prevention during storage, some lithium batteries are shipped with a 36kΩ resistor to serve as a parasitic load. The steady low discharge current prevents the layer from growing too thick, but this will reduce the storage life. After 2-year storage with the 36kΩ resistor, the batteries are said to still have 90 percent capacity. Another remedy is attaching a device that applies periodic discharge pulses during storage.
Not all primary lithium batteries recover when installed in a device and when a load is applied. The current may be too low to reverse the passivation. It is also possible that the equipment rejects a passivated battery as being low state-of-charge or defective. Many of these batteries can be prepared with a battery analyzer (Cadex) by applying a controlled load. The analyzer then verifies proper function before engaging the battery in the field.
The required discharge current for depassivation is a C-rate of 1C to 3C (1 to 3 times of the rated capacity). The cell voltage must recover to 3.2V when applying the load; the service time is typically 20 seconds. The process can be repeated but it should take no longer than 5 minutes. With a load of 1C, the voltage of a correctly functioning cell should stay above 3.0V. A drop to below 2.7V means end-of-life. (See BU-106: Primary Batteries)
These lithium-metal batteries have high lithium content and must follow more stringent shipping requirements than Li-ion of the same Ah. (See BU-704a: Shipping Lithium-based Batteries by air) Because of the high specific energy, special care must be taken in handling these cells.
CAUTION
When charging an SLA with over-voltage, current limiting must be applied to protect the battery. Always set the current limit to the lowest practical setting and observe the battery voltage and temperature during charge.
In case of rupture, leaking electrolyte or any other cause of exposure to the electrolyte, flush with water immediately. If eye exposure occurs, flush with water for 15 minutes and consult a physician immediately.
Wear approved gloves when touching the electrolyte, lead and cadmium. On exposure to the skin, flush with water immediately.
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Know how to maintain a battery fleet and eliminate the risk of unexpected downtime.
A battery performs well when new but the capacity soon begins to fade with use and time. To assure reliable service during the life span of the battery, design engineers oversize the pack to include some spare capacity. This is similar to carrying extra fuel in an airplane to enable a waiting pattern or attempt a second landing approach when so required.
New batteries operate (should operate) at a capacity of 100 percent; replacement occurs when the packs fade to about 80 percent. All batteries must include a secure level of spare capacity to cover worst-case scenarios.
In addition to normal capacity fade, cold temperature lowers the capacity, especially Li-ion. The capacity loss of a Li-ion Energy Cell is about 17 percent at 0°C (32°F), 34 percent at –10°C (14°F) and 47 percent at –20°C (–4°F). Power Cells perform better at cold temperature with lower cold-related capacity losses than Energy Cells.
Lack of spare capacity is a common cause of system failures. This commonly happens during heavier than normal traffic or in an emergency. During routine operations, marginal batteries can hide comfortably among their peers, but they will fail when put to the test. A battery maintenance program as part of quality control assures that all batteries in the fleet are within the required performance range.
Figure 1 illustrates the breakdown of a battery that includes capacity fade and spare capacity. Adding 20 percent for fade and 20 percent for spare as a safety net leaves only 60 percent for the actual capacity. Such a generous allowance may not be practical in all cases.
Figure 1: Calculating spare battery capacity.
Spare capacity should be calculated for a worst-case scenario. The allowable capacity range is 80-100%; a spare capacity of 20 percent is recommended for critical use. Allow more capacity reserve when operating at cold temperature.
To verify sufficient spare capacity in a battery fleet, identify batteries that are close to retirement and spot-check their capacities after a busy day with a battery analyzer. The Cadex analyzer provides this function on the “Prime” program in that it applies a discharge before charge. The first reading on the display reflects the spare capacity and the second represents the full capacity after a charge.
If packs with fringe capacity levels come back from a full-day shift with less than 10 percent of spare capacity, raise the pass/fail target capacity from 80 to 85 percent to gain five extra points. If, on the other hand, these old-timers come back with 30 percent before charging, keep them longer by lowering the target capacity to, say, 70 percent. Knowing the energy needs for each application during a typical shift increases battery transparency. This improves reliability and creates a sweet spot between risk management and economics.
While most batteries are replaced when the capacity fades to 80 percent, scanners in some warehouses can be kept longer because they may not require all available capacity during an 8-hour shift. If this is the case, the target capacity can safely be set to 70 percent while maintaining ample spare capacity. A starter battery in a vehicle still cranks the motor with a capacity of 40 percent. The discharge is short and the battery recharges right away. Allowing the capacity to drop much further might prevent the battery from turning the engine on a cold morning, stranding the driver.
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Discover familiar battery formats, some of which going back to the late 1800s.
Early batteries of the 1700s and 1800s developed in Europe were mostly encased in glass jars. As batteries grew in size, jars shifted to sealed wooden containers and composite materials. In the 1890s, battery manufacturing spread from Europe to the United States and in 1896 the National Carbon Company successfully produced a standard cell for widespread consumer use. It was the zinc-carbon Columbia Dry Cell Battery producing 1.5 volts and measuring 6 inches in length.
With the move to portability, sealed cylindrical cells emerged that led to standards sizes. The International Electrochemical Commission (IEC), a non-governmental standards organization founded in 1906, developed standards for most rechargeable batteries. In around 1917, the National Institute of Standards and Technology formalized the alphabet nomenclature that is still used today. Table 1 summarizes these historic and current battery sizes.
Size
Dimensions
History
F cell
33 x 91 mm
Introduced in 1896 for lanterns; later used for radios; only available in nickel-cadmium today.
E cell
N/A
Introduced ca. 1905 to power box lanterns and hobby applications. Discontinued ca. 1980.
D cell
34.2 x 61.5mm
Introduced in 1898 for flashlights and radios; still current.
C cell
25.5 x 50mm
Introduced ca. 1900 to attain smaller form factor.
Sub-C
22.2 x 42.9mm
16.1mL
Cordless tool battery. Other sizes are ½, 4/5 and 5/4 sub-C lengths. Mostly NiCd.
B cell
20.1 x 56.8mm
Introduced in 1900 for portable lighting, including bicycle lights in Europe; discontinued in in North America in 2001.
A cell
17 x 50mm
Available in NiCd, NiMH and primary lithium; also in 2/3 and 4/5 sizes. Popular in older laptops and hobby applications.
AA cell
14.5 x 50mm
Introduced in 1907 as penlight battery for pocket lights and spy tool in WWI; added to ANSI standard in 1947.
AAA cell
10.5 x 44.5mm
Developed in 1954 to reduce size for Kodak and Polaroid cameras. Added to ANSI standard in 1959.
AAAA cell
8.3 x 42.5mm
Offshoot of 9V, since 1990s; used for laser pointers, LED penlights, computer styli, headphone amplifiers.
4.5V battery
67 x 62
x 22mm
Three cells form a flat pack; short terminal strip is positive, long strip is negative; common in Europe, Russia.
9V battery
48.5 x 26.5
x 17.5mm
Introduced in 1956 for transistor radios; contains six prismatic or AAAA cells. Added to ANSI standard in 1959.
18650
18 x 65mm
16.5mL
Developed in the mid-1990s for lithium-ion; commonly used in laptops, e-bikes, including Tesla EV cars.
26650
26 x 65mm
34.5mL
Larger Li-ion. Some measure 26x70mm sold as 26700. Common chemistry is LiFeO4 for UPS, hobby, automotive.
14500
14x 50mm
Li-ion, similar size to AA. (Observe voltage incompatibility: NiCd/NiMH = 1.2V, alkaline = 1.5V, Li-ion = 3.6V)
21700*
21 x 70mm
New (2016), used for the Tesla Model 3 and other applications, made by Panasonic, Samsung, Molicel, etc.
32650
32 x 65mm
Primarily in LiFePO4 (Lithium Iron Phosphate)
Table 1: Common old and new battery norms. * The 21700 cell is also known as 2170. IEC norm calls for the second zero at the end to denote cylindrical format.
Standardization included primary cells, mostly in zinc-carbon; alkaline emerged only in the early 1960s. With the growing popularity of the sealed nickel-cadmium in the 1950s and 1960s, new sizes appeared, many of which were derived from the “A” and “C” sizes. Beginning in the 1990s, makers of Li-ion departed from conventional sizes and invented their own standards.
A successful standard is the 18650 cylindrical cell. Developed in the early 1990s for lithium-ion, these cells are used in laptops, electric bicycles and even electric vehicles (Tesla). The first two digits of 18650 designate the diameter in millimeters; the next three digits are the length in tenths of millimeters. The 18650 cell is 18mm in diameter and 65.0mm in length.
Other sizes are identified with a similar numbering scheme. For example, a prismatic cell carries the number 564656P. It is 5.6mm thick, 46mm wide and 56mm long. P stands for prismatic. Because of the large variety of chemistries and their diversity within, battery cells do not show the chemistry.
Few popular new standards have immerged since the 18650 appeared in ca. 1991. Several battery manufacturers started experimenting using slightly larger diameters with sizes of 20x70mm, 21x70mm and 22x70mm. Panasonic and Tesla decided on the 21×70, so has Samsung, and other manufacturers followed. The “2170” is only slightly larger than the 18650 it but has 35% more energy (by volume). This new cell is used in the Tesla Model 3 while Samsung is looking at new applications in laptops, power tools, e-bikes and more. It is said that the best diameters in terms of manufacturability is between 18mm and 26mm and the 2170 sits in between. (The 2170 is also known as the 21700.) The 26650 introduced earlier never became a best-seller.
The 32650 is primarily available in LiFePO4 (Lithium Iron Phosphate) with a nominal voltage of 3.2V/cell and a typical capacity of 5,000mAh. The dimensions are 32x65mm; true sizes may be slightly larger to allow for insulation and labels.
On the prismatic and pouch cell front, new cells are being developed for the electric vehicle (EV) and energy storage systems (ESS). Some of these formats may one day also become readily available similar to the 18650, made in high energy and high power versions, sourced by several manufacturers and sold at a competitive prices. Prismatic and pouch cells currently carry a higher price tag per Wh than the 18650.
The EV and ESS markets advance with two distinct philosophies: The use of a large number of small cells produced by an automated process as low cost, as done by Tesla, versus larger cells in the prismatic and pouch formats at a higher price per Wh for now, as done by other EV manufacturers. We have not seen clear winners of either format; time will tell.
Looking at the batteries in mobile phones and laptops, one sees a departure from established standards. This is due in part to the manufacturers’ inability to agree on a standard, meaning that most consumer devices come with custom-made cells or battery packs. Compact design and market demand are swaying manufacturers to go their own way. High volume with planned obsolescence allows the production of unique sizes in consumer products.
In the early days, a battery was perceived “big” by nature, and this is reflected in the sizing convention. While the “F” nomenclature may have been seen as mid-sized in the late 1800s, our forefathers did not anticipate that a battery resembling a credit card could power computers, phones and cameras. Running out of letters towards the smaller sizes led to the awkward numbering of AA, AAA and AAAA.
Since the introduction of the 9V battery in 1956, no new formats have emerged. Meanwhile portable devices lowered the operating voltages to between 3V and 5V. Switching six cells (6S) in series to attain 9V is expensive to manufacture, and a 3.6V alternative would serve better. This imaginary new pack would have a coding system to prevent charging primaries and select the correct charge algorithm for secondary chemistries.
Starter batteries for vehicles also follow battery norms that are based on the North American BCI, the European DIN and the Japanese JIS standards. These batteries are similar in footprint to allow swapping. Deep-cycle and stationary batteries follow no standardized norms and the replacement packs must be sourced from the original maker. The attempt to standardize electric vehicle batteries may not work and might follow the failed attempt to standardize laptop batteries in the 1990s.
Future Cell Formats
Standardization for Li-ion cell formats is diverse, especially for the electric vehicle. Research teams, including Fraunhofer,* examine and evaluate various formats and the most promising cell types until 2025 will be the pouch and the 21700 cylindrical formats. Looking further, experts predict the large-size prismatic Li-ion cell to domineer in the EV battery market. Meanwhile, Samsung and others bet on the prismatic cell, LG gravitates towards the pouch format and Panasonic is most comfortable with the 18650 and 21700 cylindrical cells.
Large battery systems for ESS, UPS, marine vessels and traction use mostly large format pouch cells stacked with light pressure to prolong longevity and prevent delamination. Thermal management is often done by plates drawing the heat between layers to the outside and liquid cooling.
Safety Concerns with Rechargeable Cells
Off-the-shelf cells have primarily been non-rechargeable and for public use. Typical applications are spares for flashlights, portable entertainment devices and remote controls. Accidental shorting with keys or coins in a jean pocket only causes an alkaline cell to heat up and not catch fire. The voltage collapses on an electrical short because of high internal resistance; removing the short stops the reaction. (See BU-304c: Battery Safety in Public.)
Rechargeable cells are normally encapsulated in a for-purpose pack. The exception is the 18650 available as a spare cell for vaping. Looking like a large AA cell, these Li-ion cells can inflict acute injury, even death, if mishandled. If shorted, an unprotected Li-ion cell will vent with flame. Once the jet-like explosion is in progress, removing the short no longer stops the reaction and the cell burns out. Li-ion’s ability to deliver high power is a characteristic that must be respected. (See also BU-304c: Battery Safety in Public.)
The 18650 cell can be made safe with built-in safety circuits described in BU-304b: Making Lithium-ion Safe. With protection, excessive current shuts the cell down, either momentarily by a heat element or permanently by an electric fuse. But the fused 18650 has the disadvantage of shutting down when high current is needed on purpose, such as vaping. Spare cells for vaping are normally unprotected.
Another cause of fire is low quality no-brand cells. Li-ion batteries are safe if made by a reputable manufacturer. Many aftermarket cells do not have the same rigorous safety checks as brand name products have. (See BU-810: What Everyone Should Know About Aftermarket Batteries.) Cells can also be damaged by stress related to heat, shock, vibration and incorrect charging or loading.
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Discover which charger is best for your application
A good battery charger provides the base for batteries that are durable and perform well. In a price-sensitive market, chargers often receive low priority and get the “after-thought” status. Battery and charger must go together like a horse and carriage. Prudent planning gives the power source top priority by placing it at the beginning of the project rather than after the hardware is completed, as is a common practice. Engineers are often unaware of the complexity involving the power source, especially when charging under adverse conditions.
Figure 1: Battery and charger must go together like horse and carriage.
One does not deliver without the other.
Chargers are commonly identified by their charging speed. Consumer products come with a low-cost personal charger that performs well when used as directed. The industrial charger is often made by a third party and includes special features, such as charging at adverse temperatures. Although batteries operate below freezing, not all chemistries can be charged when cold and most Li-ions fall into this category. Lead- and nickel-based batteries accept charge when cold but at a lower rate. (See BU-410: Charging at High and Low Temperature)
Some Li-ion chargers (Cadex) include a wake-up feature, or “boost,” to allow recharging if a Li-ion battery has fallen asleep due to over-discharge. A sleep condition can occur when storing the battery in a discharged state in which self-discharge brings the voltage to the cut-off point. A regular charger treats such a battery as unserviceable and the pack is often discarded. Boost applies a small charge current to raise the voltage to between 2.2V/cell and 2.9V/cell to activate the protection circuit, at which point a normal charge commences. Caution is required if a Li-ion has dwelled below 1.5V/cell for a week or longer. Dendrites may have developed that could compromise safety. (See BU-802b: What does Elevated Self-discharge Do? in which Figures 5 examines the elevated self-discharge after a Li-ion cell had been exposed to deep discharge. See also BU-808a: How to Awaken Sleeping Li-ion.)
Lead- and lithium-based chargers operate on constant current constant voltage (CCCV). The charge current is constant and the voltage is capped when it reaches a set limit. Reaching the voltage limit, the battery saturates; the current drops until the battery can no longer accept further charge and the fast charge terminates. Each battery has its own low-current threshold.
Nickel-based batteries charge with constant current and the voltage is allowed to rise freely. This can be compared to lifting a weight with a rubber band where the hand advances higher than the load. Full charge detection occurs when observing a slight voltage drop after a steady rise. To safeguard against anomalies, such as shorted or mismatched cells, the charger should include a plateau timer to assure a safe charge termination if no voltage delta is detected. Temperature sensing should also be added that measures temperature rise over time. Such a method is known as delta temperature over delta time, or dT/dt, and works well with rapid and fast charge.
A temperature rise is normal with nickel-based batteries, especially when reaching the 70 percent charge level. A decrease in charge efficiency causes this, and the charge current should be lowered to limit stress. When “ready,” the charger switches to trickle charge and the battery must cool down. If the temperature stays above ambient, then the charger is not performing correctly and the battery should be removed because the trickle charge could be too high.
NiCd and NiMH should not be left in the charger unattended for weeks and months. Until required, store the batteries in a cool place and apply a charge before use.
Lithium-based batteries should always stay cool on charge. Discontinue the use of a battery or charger if the temperature rises more than 10ºC (18ºF) above ambient under a normal charge. Li ion cannot absorb over-charge and does not receive trickle charge when full. It is not necessary to remove Li-ion from the charger; however, if not used for a week or more, it is best to place the pack in a cool place and recharge before use.
Types of Chargers
The most basic charger was the overnight charger, also known as a slow charger. This goes back to the old nickel-cadmium days where a simple charger applied a fixed charge of about 0.1C (one-tenth of the rated capacity) as long as the battery was connected. Slow chargers have no full-charge detection; the charge stays engaged and a full charge of an empty battery takes 14–16 hours. When fully charged, the slow charger keeps NiCd lukewarm to the touch. Because of its reduced ability to absorb over-charge, NiMH should not be charged on a slow charger. Low-cost consumer chargers charging AAA, AA and C cells often deploy this charge method, so do some children’s toys. Remove the batteries when warm.
The rapid charger falls between the slow and fast charger and is used in consumer products. The charge time of an empty pack is 3–6 hours. When full, the charger switches to “ready.” Most rapid chargers include temperature sensing to safely charge a faulty battery.
The fast charger offers several advantages and the obvious one is shorter charge times. This demands tighter communication between the charger and battery. At a charge rate of 1C, (see BU-402:What is C-rate?) which a fast charger typically uses, an empty NiCd and NiMH charges in a little more than an hour. As the battery approaches full charge, some nickel-based chargers reduce the current to adjust to the lower charge acceptance. The fully charged battery switches the charger to trickle charge, also known as maintenance charge. Most of today’s nickel-based chargers have a reduced trickle charge to also accommodate NiMH.
Li-ion has minimal losses during charge and the coulombic efficiency is better than 99 percent. At 1C, the battery charges to 70 percent state-of-charge (SoC) in less than an hour; the extra time is devoted to the saturation charge. Li-ion does not require the saturation charge as lead acid does; in fact it is better not to fully charge Li-ion — the batteries will last longer but the runtime will be a little less. Of all chargers, Li-ion is the simplest. No trickery applies that promises to improve battery performance as is often claimed by makers of chargers for lead- and nickel-based batteries. Only the rudimentary CCCV method works.
Lead acid cannot be fast charged and the term “fast-charge” is a misnomer. Most lead acid chargers charge the battery in 14–16 hours; anything slower is a compromise. Lead acid can be charged to 70 percent in about 8 hours; the all-important saturation charge takes up the remaining time. A partial charge is fine provided the lead acid occasionally receives a fully saturated charge to prevent sulfation.
The standby current on a charger should be low to save energy. Energy Star assigns five stars to mobile phone chargers and other small chargers drawing 30mW or less on standby. Four stars go to chargers with 30–150mW, three stars to 150–250mW and two stars to 250–350mW. The average consumption is 300mW and these units get one star. Energy Star aims to reduce current consumption of personal chargers that are mostly left plugged in when not in use. There are over one billion such chargers connected to the gird globally at any given time.
Simple Guidelines when Buying a Charger
Charging a battery is most effective when its state-of-charge (SoC) is low. Charge acceptance decreases when the battery reaches a SoC of 70% and higher. A fully charged battery can no longer convert electric energy into chemical energy and charge must be lowered to trickle or terminated.
Filling a battery beyond full state-of-charge turns excess energy into heat and gas. With Li-ion, this can result in a deposit of unwanted materials. Prolonged over-charge causes permanent damage.
Use the correct charger for the intended battery chemistry. Most chargers serve one chemistry only. Make sure that the battery voltage agrees with the charger. Do not charge if different.
The Ah rating of a battery can be marginally different than specified. Charging a larger battery will take a bit longer than a smaller pack and vice versa. Do not charge if the Ah rating deviates too much (more than 25 percent).
A high-wattage charger shortens the charge time but there are limitations as to how fast a battery can be charged. Ultra-fast charging causes stress.
A lead acid charger should switch to float charge when fully saturated; a nickel-based charger must switch to trickle charge when full. Li-ion cannot absorb overcharge and receives no trickle charge. Trickle charge and float charges compensate for the losses incurred by self-discharge.
Chargers should have a temperature override to end charge on a faulty battery.
Observe charge temperature. Lead acid batteries should stay lukewarm to the touch; nickel-based batteries will get warm towards the end of charge but must cool down on “ready.” Li-ion should not rise more than 10ºC (18ºF) above ambient when reaching full charge.
Check battery temperature when using a low-cost charger. Remove battery when warm.
Charge at room temperature. Charge acceptance drops when cold. Li-ion cannot be charged below freezing.
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Learn about the differences within the lead acid family and find out what the cons and pros are.
Invented by the French physician Gaston Planté in 1859, lead acid was the first rechargeable battery for commercial use. Despite its advanced age, the lead chemistry continues to be in wide use today. There are good reasons for its popularity; lead acid is dependable and inexpensive on a cost-per-watt base. There are few other batteries that deliver bulk power as cheaply as lead acid, and this makes the battery cost-effective for automobiles, golf cars, forklifts, marine and uninterruptible power supplies (UPS).
The grid structure of the lead acid battery is made from a lead alloy. Pure lead is too soft and would not support itself, so small quantities of other metals are added to get the mechanical strength and improve electrical properties. The most common additives are antimony, calcium, tin and selenium. These batteries are often known as “lead-antimony” and “leadcalcium.”
Adding antimony and tin improves deep cycling but this increases water consumption and escalates the need to equalize. Calcium reduces self-discharge, but the positive lead-calcium plate has the side effect of growing due to grid oxidation when being over-charged. Modern lead acid batteries also make use of doping agents such as selenium, cadmium, tin and arsenic to lower the antimony and calcium content.
Lead acid is heavy and is less durable than nickel- and lithium-based systems when deep cycled. A full discharge causes strain and each discharge/charge cycle permanently robs the battery of a small amount of capacity. This loss is small while the battery is in good operating condition, but the fading increases once the performance drops to half the nominal capacity. This wear-down characteristic applies to all batteries in various degrees.
Depending on the depth of discharge, lead acid for deep-cycle applications provides 200 to 300 discharge/charge cycles. The primary reasons for its relatively short cycle life are grid corrosion on the positive electrode, depletion of the active material and expansion of the positive plates. This aging phenomenon is accelerated at elevated operating temperatures and when drawing high discharge currents. (See BU-804:How to Prolong Lead Acid Batteries)
Charging a lead acid battery is simple, but the correct voltage limits must be observed. Choosing a low voltage limit shelters the battery, but this produces poor performance and causes a buildup of sulfation on the negative plate. A high voltage limit improves performance but forms grid corrosion on the positive plate. While sulfation can be reversed if serviced in time, corrosion is permanent. (See BU-403: Charging Lead Acid.)
Lead acid does not lend itself to fast charging and with most types, a full charge takes 14–16 hours. The battery must always be stored at full state-of-charge. Low charge causes sulfation, a condition that robs the battery of performance. Adding carbon on the negative electrode reduces this problem but this lowers the specific energy. ( See BU-202: New Lead Acid Systems. )
Lead acid has a moderate life span, but it is not subject to memory as nickel-based systems are, and the charge retention is best among rechargeable batteries. While NiCd loses approximately 40 percent of their stored energy in three months, lead acid self-discharges the same amount in one year. The lead acid battery works well at cold temperatures and is superior to lithium-ion when operating in subzero conditions. According to RWTH, Aachen, Germany (2018), the cost of the flooded lead acid is about $150 per kWh, one of the lowest in batteries.
Sealed Lead Acid
The first sealed, or maintenance-free, lead acid emerged in the mid-1970s. Engineers argued that the term “sealed lead acid” was a misnomer because no lead acid battery can be totally sealed. To control venting during stressful charge and rapid discharge, valves have been added that release gases if pressure builds up. Rather than submerging the plates in a liquid, the electrolyte is impregnated into a moistened separator, a design that resembles nickel- and lithium-based systems. This enables operating the battery in any physical orientation without leakage.
The sealed battery contains less electrolyte than the flooded type, hence the term “acid-starved.” Perhaps the most significant advantage of sealed lead acid is the ability to combine oxygen and hydrogen to create water and prevent dry out during cycling. The recombination occurs at a moderate pressure of 0.14 bar (2psi). The valve serves as a safety vent if the gas buildup rises. Repeated venting should be avoided as this will lead to an eventual dry-out. According to RWTH, Aachen, Germany (2018), the cost of VRLA is about $260 per kWh.
Several types of sealed lead acid have emerged and the most common are gel, also known as valve-regulated lead acid (VRLA), and absorbent glass mat (AGM). The gel cell contains a silica type gel that suspends the electrolyte in a paste. Smaller packs with capacities of up to 30Ah are often called SLA (sealed lead acid). Packaged in a plastic container, these batteries are used for small UPS, emergency lighting and wheelchairs. Because of low price, dependable service and low maintenance, the SLA remains the preferred choice for healthcare in hospitals and retirement homes. The larger VRLA is used as power backup for cellular repeater towers, Internet hubs, banks, hospitals, airports and more.
The AGM suspends the electrolyte in a specially designed glass mat. This offers several advantages to lead acid systems, including faster charging and instant high load currents on demand. AGM works best as a mid-range battery with capacities of 30 to 100Ah and is less suited for large systems, such as UPS. Typical uses are starter batteries for motorcycles, start-stop function for micro-hybrid cars, as well as marine and RV that need some cycling.
With cycling and age, the capacity of AGM fades gradually; gel, on the other hand, has a dome shaped performance curve and stays in the high performance range longer but then drops suddenly towards the end of life. AGM is more expensive than flooded, but is cheaper than gel. (Gel would be too expensive for start/stop use in cars.)
Unlike the flooded, the sealed lead acid battery is designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge. Excess charging causes gassing, venting and subsequent water depletion and dry-out. Consequently, gel, and in part also AGM, cannot be charged to their full potential and the charge voltage limit must be set lower than that of a flooded. This also applies to the float charge on full charge. In respect to charging, the gel and AGM are no direct replacements for the flooded type. If no designated charger is available for AGM with lower voltage settings, disconnect the charger after 24 hours of charge. This prevents gassing due to a float voltage that is set too high. ( See BU-403: Charging Lead Acid )
The optimum operating temperature for a VRLA battery is 25°C (77°F); every 8°C (15°F) rise above this temperature threshold cuts battery life in half. ( See BU-806a: How Heat and Loading affect Battery Life ) Lead acid batteries are rated at a 5-hour (0.2C) and 20-hour (0.05C) discharge rate. The battery performs best when discharged slowly; the capacity readings are substantially higher at a slower discharge than at the 1C-rate. Lead acid can, however, deliver high pulse currents of several C if done for only a few seconds. This makes the lead acid well suited as a starter battery, also known as starter-light-ignition (SLI). The high lead content and the sulfuric acid make lead acid environmentally unfriendly.
Lead acid batteries are commonly classified into three usages: Automotive (starter or SLI), motive power (traction or deep cycle) and stationary (UPS).
Starter Batteries
The starter battery is designed to crank an engine with a momentary high-power load lasting a second or so. For its size, the battery is able to deliver high current but it cannot be deep-cycled. Starter batteries are rated with Ah or RS (reserve capacity) to indicate energy storage capability, as well as CCA (cold cranking amps) to signify the current a battery can deliver at cold temperature. SAE J537 specifies 30 seconds of discharge at –18°C (0°F) at the rated CCA ampere without the battery voltage dropping below 7.2 volts. RC reflects the runtime in minutes at a steady discharge of 25. (SAE stands for Society of Automotive Engineers.) See also BU-902a: How to Measure CCA.
Starter batteries have a very low internal resistance that is achieved by adding extra plates for maximum surface area (Figure 1). The plates are thin and the lead is applied in a sponge-like form that has the appearance of fine foam, expanding the surface area further. Plate thickness, which is important for a deep-cycle battery is less important because the discharge is short and the battery is recharged while driving; the emphasis is on power rather than capacity.
Figure 1: Starter battery
The starter battery has many thin plates in parallel to achieve low resistance with high surface area. The starter battery does not allow deep cycling. Courtesy of Cadex
Deep-cycle Battery
The deep-cycle battery is built to provide continuous power for wheelchairs, golf cars, forklifts and more. This battery is built for maximum capacity and a reasonably high cycle count. This is achieved by making the lead plates thick (Figure 2). Although the battery is designed for cycling, full discharges still induce stress and the cycle count relates to the depth-of-discharge (DoD). Deep-cycle batteries are marked in Ah or minutes of runtime. The capacity is typically rated as a 5-hour and 20-hour discharge.
Figure 2: Deep-cycle battery
The deep-cycle battery has thick plates for improved cycling abilities. The deep-cycle battery generally allows about 300 cycles. Courtesy of Cadex
A starter battery cannot be swapped with a deep-cycle battery or vice versa. While an inventive senior may be tempted to install a starter battery instead of the more expensive deep-cycle on his wheelchair to save money, the starter battery would not last because the thin sponge-like plates would quickly dissolve with repeated deep cycling.
There are combination starter/deep-cycle batteries available for trucks, buses, public safety and military vehicles, but these units are big and heavy. As a simple guideline, the heavier the battery is, the more lead it contains, and the longer it will last. Table 3 compares the typical life of starter and deep-cycle batteries when deep cycled.
Depth of discharge
Starter battery
Deep-cycle battery
100%
50%
30%
12–15 cycles
100–120 cycles
130–150 cycles
150–200 cycles
400–500 cycles
1,000 and more cycles
Table 3: Cycle performance of starter and deep-cycle batteries. A discharge of 100% refers to a full discharge; 50% is half and 30% is a moderate discharge with 70% remaining.
Lead Acid or Li-ion in your Car?
Ever since Cadillac introduced the starter motor in 1912, lead acid batteries served well as battery of choice. Thomas Edison tried to replace lead acid with nickel-iron (NiFe), but lead acid prevailed because of its rugged and forgiving nature, as well as low cost. Now the lead acid serving as starter battery in vehicles is being challenged by Li-ion.
Figure 4 illustrates the characteristics of lead acid and Li-ion. Both chemistries perform similarly in cold cranking. Lead acid is slightly better in W/kg, but Li-ion delivers large improvements in cycle life, better specific energy in Wh/kg and good dynamic charge acceptance. Where Li-ion falls short is high cost per kWh, complex recycling and less stellar safety record than lead acid.
Figure 4: Comparison of lead acid and Li-ion as starter battery.
Lead acid maintains a strong lead in starter battery. Credit goes to good cold temperature performance, low cost, good safety record and ease of recycling. Source: Johnson Control
Lead is toxic and environmentalists would like to replace the lead acid battery with an alternative chemistry. Europe succeeded in keeping NiCd out of consumer products, and similar efforts are being made with the starter battery. The choices are NiMH and Li-ion, but the price is too high and low temperature performance is poor. With a 99 percent recycling rate, the lead acid battery poses little environmental hazard and will likely continue to be the battery of choice.
Table 4 lists advantages and limitations of common lead acid batteries in use today. The table does not include the new lead acid chemistries. (See also BU-202: New Lead Acid Systems.)
Advantages
Inexpensive and simple to manufacture; low cost per watt-hour
Low self-discharge; lowest among rechargeable batteries
High specific power, capable of high discharge currents
Must be stored in charged condition to prevent sulfation
Limited cycle life; repeated deep-cycling reduces battery life
Flooded version requires watering
Transportation restrictions on the flooded type
Not environmentally friendly
Table 4: Advantages and limitations of lead acid batteries. Dry systems have advantages over flooded but are less rugged.
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Explore SoC measurements and why they are not accurate.
Voltage Method
Measuring state-of-charge by voltage is simple, but it can be inaccurate because cell materials and temperature affect the voltage. The most blatant error of the voltage-based SoC occurs when disturbing a battery with a charge or discharge. The resulting agitation distorts the voltage and it no longer represents a correct SoC reference. To get accurate readings, the battery needs to rest in the open circuit state for at least four hours; battery manufacturers recommend 24 hours for lead acid. This makes the voltage-based SoC method impractical for a battery in active duty.
Each battery chemistry delivers its own unique discharge signature. While voltage-based SoC works reasonably well for a lead acid battery that has rested, the flat discharge curve of nickel- and lithium-based batteries renders the voltage method impracticable.
The discharge voltage curves of Li-manganese, Li-phosphate and NMC are very flat, and 80 percent of the stored energy remains in the flat voltage profile. While this characteristic is desirable as an energy source, it presents a challenge for voltage-based fuel gauging as it only indicates full charge and low charge; the important middle section cannot be estimated accurately. Figure 1 reveals the flat voltage profile of Li-phosphate (LiFePO) batteries.
Figure 1: Discharge voltage of lithium iron phosphate.
Li-phosphate has a very flat discharge profile, making voltage estimations for SoC estimation difficult.
Lead acid comes with different plate compositions that must be considered when measuring SoC by voltage. Calcium, an additive that makes the battery maintenance-free, raises the voltage by 5–8 percent. In addition, heat raises the voltage while cold causes a decrease. Surface charge further fools SoC estimations by showing an elevated voltage immediately after charge; a brief discharge before measurement counteracts the error. Finally, AGM batteries produce a slightly higher voltage than the flooded equivalent.
When measuring SoC by open circuit voltage (OCV), the battery voltage must be “floating” with no load attached. This is not the case with modern vehicles. Parasitic loads for housekeeping functions puts the battery into a quasi-closed circuit voltage (CCV) condition.
In spite of inaccuracies, most SoC measurements rely in part or completely on voltage because of simplicity. Voltage-based SoC is popular in wheelchairs, scooters and golf cars. Some innovative BMS (battery management systems) use the rest periods to adjust the SoC readings as part of a “learn” function. Figure 2 illustrates the voltage band of a 12V lead acid monoblock from fully discharged to full charged.
Figure 2: Voltage band of a 12V lead acid monoblock from fully discharged to fully charged.Source: Power-Sonic
Hydrometer
The hydrometer offers an alternative to measuring SoC of flooded lead acid batteries. Here is how it works: When the lead acid battery accepts charge, the sulfuric acid gets heavier, causing the specific gravity (SG) to increase. As the SoC decreases through discharge, the sulfuric acid removes itself from the electrolyte and binds to the plate, forming lead sulfate. The density of the electrolyte becomes lighter and more water-like, and the specific gravity gets lower. Table 2 provides the BCI readings of starter batteries.
Approximate
state-of-charge
Average
specific gravity
Open circuit voltage
2V
6V
8V
12V
100%
1.265
2.10
6.32
8.43
12.65
75%
1.225
2.08
6.22
8.30
12.45
50%
1.190
2.04
6.12
8.16
12.24
25%
1.155
2.01
6.03
8.04
12.06
0%
1.120
1.98
5.95
7.72
11.89
Table 2: BCI standard for SoC estimation of a starter battery with antimony.
Readings are taken at 26°C (78°F) after a 24h rest.
While BCI (Battery Council International) specifies the specific gravity of a fully charged starter battery at 1.265, battery manufacturers may go for 1.280 and higher. Increasing the specific gravity will move the SoC readings upwards on the look-up table. A higher SG will improve battery performance but shorten battery life because of increased corrosion activity.
Besides charge level and acid density, a low fluid level will also change the SG. When water evaporates, the SG reading rises because of higher concentration. The battery can also be overfilled, which lowers the number. When adding water, allow time for mixing before taking the SG measurement.
Specific gravity varies with battery applications. Deep-cycle batteries use a dense electrolyte with an SG of up to 1.330 to get maximum specific energy; aviation batteries have an SG of about 1.285; traction batteries for forklifts are typically at 1.280; starter batteries come in at 1.265; and stationary batteries have a low specific gravity of 1.225. This reduces corrosion and prolongs life but it decreases the specific energy, or capacity.
Nothing in the battery world is absolute. The specific gravity of fully charged deep-cycle batteries of the same model can range from 1.270 to 1.305; fully discharged, these batteries may vary between 1.097 and 1.201. Temperature is another variable that alters the specific gravity reading. The colder the temperature drops, the higher (more dense) the SG value becomes. Table 3 illustrates the SG gravity of a deep-cycle battery at various temperatures.
Electrolyte temperature
Gravity at full charge
Table 3: Relationship of specific gravity and temperature of deep-cycle battery.
Colder temperatures provide higher specific gravity readings.
40°C
104°F
1.266
30°C
86°F
1.273
20°C
68°F
1.280
10°C
50°F
1.287
0°C
32°F
1.294
Inaccuracies in SG readings can also occur if the battery has stratified, meaning the concentration is light on top and heavy on the bottom. (See BU-804c: Water Loss, Acid Stratification and Surface Charge.). High acid concentration artificially raises the open circuit voltage, which can fool SoC estimations through false SG and voltage indication. The electrolyte needs to stabilize after charge and discharge before taking the SG reading.
Coulomb Counting
Laptops, medical equipment and other professional portable devices use coulomb counting to estimate SoC by measuring the in-and-out-flowing current. Ampere-second (As) is used for both charge and discharge. The name “coulomb” was given in honor of Charles-Augustin de Coulomb (1736–1806) who is best known for developing Coulomb’s law. (See BU-601: How does a Smart Battery Work?)
While this is an elegant solution to a challenging issue, losses reduce the total energy delivered, and what’s available at the end is always less than what had been put in. In spite of this, coulomb counting works well, especially with Li-ion that offer high coulombinc efficiency and low self-discharge. Improvements have been made by also taking aging and temperature-based self-discharge into consideration but periodic calibration is still recommended to bring the “digital battery” in harmony with the “chemical battery.” (See BU-603: How to Calibrate a “Smart” Battery)
To overcome calibration, modern fuel gauges use a “learn” function that estimates how much energy the battery delivered on the previous discharge. Some systems also observe the charge time because a faded battery charges more quickly than a good one.
Makers of advanced BMS claim high accuracies but real life often shows otherwise. Much of the make-believe is hidden behind a fancy readout. Smartphones may show a 100 percent charge when the battery is only 90 percent charged. Design engineers say that the SoC readings on new EV batteries can be off by 15 percent. There are reported cases where EV drivers ran out of charge with a 25 percent SoC reading still on the fuel gauge.
Impedance Spectroscopy
Battery state-of-charge can also be estimated with impedance spectroscopy using the Spectro™ complex modeling method. This allows taking SoC readings with a steady parasitic load of 30A. Voltage polarization and surface charge do not affect the reading as SoC is measured independently of voltage. This opens applications in automotive manufacturing where some batteries are discharged longer than others during testing and debugging and need charging before transit. Measuring SoC by impedance spectroscopy can also be used for load leveling systems where a battery is continuously under charge and discharge.
Measuring SoC independently of voltage also supports dock arrivals and showrooms. Opening the car door applies a parasitic load of about 20A that agitates the battery and falsifies voltage-based SoC measurement. The Spectro™ method helps to identify a low-charge battery from one with a genuine defect.
SoC measurement by impedance spectroscopy is restricted to a new battery with a known good capacity; capacity must be nailed down and have a non-varying value. While SoC readings are possible with a steady load, the battery cannot be on charge during the test.
Figure 4 demonstrates the test results of impedance spectroscopy after a parasitic load of 50A is removed from the battery. As expected, the open terminal voltage rises as part of recovery but the Spectro™ readings remains stable. Steady SoC results are also observed after removing charge during when the voltage normalizes as part of polarization.
Figure 4: Relationship of voltage and measurements taken by impedance spectroscopy after removing a load.
Battery is recovering after removing a load. Spectro SoC readings remain stable as the voltage rises.
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Learn about storage temperatures and state-of-charge conditions.
The recommended storage temperature for most batteries is 15°C (59°F); the extreme allowable temperature is –40°C to 50°C (–40°C to 122°F) for most chemistries.
You can store a sealed lead acid battery for up to 2 years. Since all batteries gradually self-discharge over time, it is important to check the voltage and/or specific gravity, and then apply a charge when the battery falls to 70 percent state-of-charge, which reflects 2.07V/cell open circuit or 12.42V for a 12V pack. (The specific gravity at 70 percent charge is roughly 1.218.) Lead acid batteries may have different readings, and it is best to check the manufacturer’s instruction manual. Some battery manufacturer may further let a lead acid to drop to 60 percent before recharge. See BU-903: How to Measure State-of-charge.)
Low charge induces sulfation, an oxidation layer on the negative plate that inhibits current flow. Topping charge and/or cycling may restore some of the capacity losses in the early stages of sulfation. (See BU-804b: Sulfation and How to Prevent it.)
Sulfation may prevent charging small sealed lead acid cells, such as the Cyclone by Hawker, after prolonged storage. These batteries can often be reactivated by applying an elevated voltage. At first, the cell voltage under charge may go up to 5V and draw very little current. Within 2 hours or so, the charging current converts the large sulfate crystals into active material, the cell resistance drops and the charge voltage gradually normalizes. At between 2.10V and 2.40V the cell is able to accept a normal charge. To prevent damage, set the current limit to a very low level. Do not attempt to perform this service if the power supply does not have current limiting. (See BU-405: Charging with a Power Supply.)
Recommended storage is around 40 percent state-of-charge (SoC). This minimizes age-related capacity loss while keeping the battery operational and allowing for some self-discharge. Nickel-based batteries can be stored in a fully discharged state with no apparent side effect.
Measuring SoC by voltage is difficult on nickel-based batteries. A flat discharge curve, agitation after charge and discharge and temperature affects the voltage. The good news is that the charge level for storage is not critical for this chemistry, so simply apply some charge if the battery is empty and store it in a cool and dry place. With some charge, priming should be quicker than if stored in a totally discharged state.
Nickel-metal-hydride can be stored for 3–5 years. The capacity drop that occurs during storage is partially reversible with priming. Nickel-cadmium stores well. The US Air Force was able to deploy NiCd batteries that had been in storage for 5 years with good recovered capacities after priming. It is believed that priming becomes necessary if the voltage drops below 1V/cell. Primary alkaline and lithium batteries can be stored for up to 10 years with only moderate capacity loss.
There is virtually no self-discharge below about 4.0V at 20C (68F); storing at 3.7V yields amazing longevity for most Li-ion systems. Finding the exact 40–50 percent SoC level to store Li-ion is not that important. At 40 percent charge, most Li-ion has an OCV of 3.82V/cell at room temperature. To get the correct reading after a charge or discharge, rest the battery for 90 minutes before taking the reading. If this is not practical, overshoot the discharge voltage by 50mV or go 50mV higher on charge. This means discharging to 3.77V/cell or charging to 3.87V/cell at a C-rate of 1C or less. The rubber band effect will settle the voltage at roughly 3.82V. Figure 1 shows the typical discharge voltage of a Li-ion battery.
Figure 1: Discharge voltage as a function of state-of-charge. Battery SoC is reflected in OCV. Lithium manganese oxide reads 3.82V at 40% SoC (25°C), and about 3.70V at 30% (shipping requirement). Temperature and previous charge and discharge activities affect the reading. Allow the battery to rest for 90 minutes before taking the reading.
Li-ion cannot dip below 2V/cell for any length of time. Copper shunts form inside the cells that can lead to elevated self-discharge or a partial electrical short. (See BU-802b: Elevated Self-discharge.) If recharged, the cells might become unstable, causing excessive heat or showing other anomalies. Li-ion batteries that have been under stress may function normally but are more sensitive to mechanical abuse. Liability for incorrect handling should go to the user and not the battery manufacturer.
Alkaline and other primary batteries are easy to store. For best results, keep the cells at cool room temperature and at a relative humidity of about 50 percent. Do not freeze alkaline cells, or any battery, as this may change the molecular structure. Some lithium-based primary batteries need special care that is described in BU-106a: Choices of Primary Batteries.
Capacity Loss during Storage
Storage induces two forms of losses: Self-discharge that can be refilled with charging before use, and non-recoverable losses that permanently lower the capacity. Table 2 illustrates the remaining capacities of lithium- and nickel-based batteries after one year of storage at various temperatures. Li-ion has higher losses if stored fully charged rather than at a SoC of 40 percent. (See BU-808: How to Prolong Lithium-based Batteries to study capacity loss in Li-ion.)
Temperature
Lead acid
at full charge
Nickel-based
at any charge
Lithium-ion (Li-cobalt)
40% charge
100% charge
0°C
25°C
40°C
60°C
97%
90%
62%
38%
(after 6 months)
99%
97%
95%
70%
98%
96%
85%
75%
94%
80%
65%
60%
(after 3 months)
Table 2: Estimated recoverable capacity when storing a battery for one year. Elevated temperature hastens permanent capacity loss. Depending on battery type, lithium-ion is also sensitive to charge levels.
Batteries are often exposed to unfavorable temperatures, and leaving a mobile phone or camera on the dashboard of a car or in the hot sun are such examples. Laptops get warm when in use and this increases the battery temperature. Sitting at full charge while plugged into the mains shortens battery life. Elevated temperature also stresses lead- and nickel-based batteries. (See BU-808: How to Prolong Lithium-based Batteries.)
Nickel-metal-hydride can be stored for 3–5 years. The capacity drop that occurs during storage is partially reversible with priming. Nickel-cadmium stores well. The US Air Force was able to deploy NiCd batteries that had been in storage for 5 years with good recovered capacities after priming. It is believed that priming becomes necessary if the voltage drops below 1V/cell. Primary alkaline and lithium batteries can be stored for up to 10 years with only moderate capacity loss.
You can store a sealed lead acid battery for up to 2 years. Since all batteries gradually self-discharge over time, it is important to check the voltage and/or specific gravity, and then apply a charge when the battery falls to 70 percent state-of-charge, which reflects 2.07V/cell open circuit or 12.42V for a 12V pack. (The specific gravity at 70 percent charge is roughly 1.218.) Lead acid batteries may have different readings, and it is best to check the manufacturer’s instruction manual. Some battery manufacturer may further let a lead acid to drop to 60 percent before recharge. Low charge induces sulfation, an oxidation layer on the negative plate that inhibits current flow. Topping charge and/or cycling may restore some of the capacity losses in the early stages of sulfation. (See BU-804b: Sulfation and How to Prevent it.)
Sulfation may prevent charging small sealed lead acid cells, such as the Cyclone by Hawker, after prolonged storage. These batteries can often be reactivated by applying an elevated voltage. At first, the cell voltage under charge may go up to 5V and draw very little current. Within 2 hours or so, the charging current converts the large sulfate crystals into active material, the cell resistance drops and the charge voltage gradually normalizes. At between 2.10V and 2.40V the cell is able to accept a normal charge. To prevent damage, set the current limit to a very low level. Do not attempt to perform this service if the power supply does not have current limiting. (See BU-405: Charging with a Power Supply.)
Alkaline batteries are easy to store. For best results, keep the cells at cool room temperature and at a relative humidity of about 50 percent. Do not freeze alkaline cells, or any battery, as this may change the molecular structure.
AirShip
Li-ion batteries not only live longer when stored partially charged; they are also less volatile in shipment should an anomaly occur. The International Air Transport Association (IATA) and FAA mandate that all removable Li-ion packs be shipped at 30% state-of-charge. (More on BU-704a: Shipping Lithium-based Batteries by Air.) SoC can be estimated by measuring the open circuit voltage of a rested battery. (See also BU-903: How to Measure State-of-charge.)
Relating SoC to voltage can be inaccurate as the voltage curve of Li-ion between 20% to 100% charge is flat, as Figure 1 demonstrates. Temperature also plays a role, so do the active materials used in a cell. Aviation authorities seem less concerned about the exact 30% SoC but the importance of shipping Li-ion below 50% SoC. Larger misgivings are wrong labeling by passing Li-ion as a benign nickel-based chemistry.
To bring Li-ion to 30% SoC, discharge the battery in a device featuring a fuel gauge and terminate the discharge at 30% charge. The Embedded Battery Management System (BMS) does a reasonably good job giving SoC information but the measurements are seldom accurate. A full discharge to “Low Batt” is acceptable as long as the battery receives a charge at destination. Keeping Li-ion in a discharged state for a few months could slip the pack to sleep mode. (See BU-808a: How to Awaken a Sleeping Li-ion.)
Modern chargers feature the “AirShip” program that prepares a Li-ion pack for air shipment by discharging or charging the battery to 30% SoC on command. Typical methods are a full discharge with subsequent recharge to 30% using coulomb counting or advanced Kalman filters. Li-ion batteries built into devices have less stringent SoC requirements than removable packs.
Simple Guidelines for Storing Batteries
Primary batteries store well. Alkaline and primary lithium batteries can be stored for 10 years with moderate loss capacity.
When storing, remove the battery from the equipment and place in a dry and cool place.
Avoid freezing. Batteries freeze more easily if kept in discharged state.
Charge lead acid before storing and monitor the voltage or specific gravity frequently; apply a charge if below 2.07V/cell or if SG is below 1.225 (most starter batteries).
Nickel-based batteries can be stored for 3–5years, even at zero voltage; prime before use.
Lithium-ion must be stored in a charged state, ideally at 40 percent. This prevents the battery from dropping below 2.50V/cell, triggering sleep mode.
Discard Li-ion if kept below 2.00/V/cell for more than a week. Also discard if the voltage does not recover normally after storage. (See BU-802b: What does Elevated Self-discharge do?)
CAUTION
When charging an SLA with over-voltage, current limiting must be applied to protect the battery. Always set the current limit to the lowest practical setting and observe the battery voltage and temperature during charge.In case of rupture, leaking electrolyte or any other cause of exposure to the electrolyte, flush with water immediately. If eye exposure occurs, flush with water for 15 minutes and consult a physician immediately.
Wear approved gloves when touching electrolyte, lead and cadmium. On exposure to skin, flush with water immediately.
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Learn the unique advantages of lead acid batteries
The early gelled lead acid battery developed in the 1950s by Sonnenschein (Germany) became popular in the 1970s. Mixing sulfuric acid with a silica-gelling agent converts liquid electrolyte into a semi-stiff paste to make the gel maintenance free. The AGM that arrived in the early 1980s offers similar performance to gel but each system offers slightly different characteristics to fill unique market needs. Gel batteries are commonly used in UPS, big and small, while AGM has carved out a market with starter and deep-cycle applications. Gel and AGM batteries are part of the valve-regulated lead acid (VRLA) family to make the traditional flooded lead acid maintenance free.
Energy storage systems (ESS) deployed for frequency regulation and energy buffering use lithium-ion batteries. Unlike lead acid, Li-ion can be rapid charged when excess energy is available. While UPS normally dwells at full-charge and is only discharged occasionally, Li-ion in an ESS can operate at mid-state-of-charge of 40 to 60 percent without inducing sulfation. UPS for standby applications continue to be served by lead acid batteries because of economical cost, ruggedness and superior safety, Li-ion is making inroads into applications that need cycling by delivering the best price per cycle.
A gel battery generally lasts longer than AGM; improved heat transfer to the outside is one reason. (The gel separator moves heat whereas the absorbent glass mat of the AGM acts as insulator.) A further advantage of gel is the dome shaped performance curve that allows the battery to stay in the high performance range during most of its service life before dropping rapidly towards the end of life; AGM, in comparison, fades gradually.
Gel is known for good performance at high ambient temperatures, is less prone to sulfation than other systems, but it needs the correct charge and float voltages. In comparison, AGM is superior at low temperatures with better current delivery because of low internal resistance. The cycle count on gel is said to be larger than AGM and the secret lies in holding more acid due to its design. Because of higher internals resistance, gel batteries are not used for high current applications.
One of the secrets of building a good gel battery lies in the valve construction. Small and economical gel batteries use a valve consisting of EPDM-rubber (EPDM stands for ethylene propylene diene monomer). High quality large gel batteries for use in high and low temperatures use a more elaborate valve design to improve moisture retention.
In terms of suitability and cost, the flooded lead acid is most durable when used in standby operation, but it is also the most expensive and requires maintenance by replenishing water. Gel is cheaper than flooded and is the preferred battery for the UPS installations in communications. AGM comes at a lower cost and is also superior in load capabilities to gel. Both systems have a promising future and will continue to serve for standby applications that require limited deep cycling. Table 1 illustrates the advantages and disadvantages of the gel battery over other lead acid systems.
Advantages
Maintenance free; can be mounted sideways; low self-discharge
Long lasting due to its ability to transfer heat to the outside
Performance stays high until the end of life, then drops rapidly
Produces water by combining oxygen and hydrogen
Safe operation and forgiving if abused; less dry-out than AGM
High cycle count, tolerance to abuse and heat
Large variety of battery sizes available
Limitations
Higher manufacturing cost than AGM
Sensitive to overcharging (gel is more tolerant than AGM)
Moderate specific energy and load current
Subject to release gases. Ventilation needed
Must be stored in charged condition (less critical than flooded)
Table 1: Advantages and limitations of the gel battery.
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