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WHY BATTERY BMS ARE IMPORTANT IN LiFePO4 BATTERIES

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Himax-home-page-design-news-2

Overview

Lithium iron phosphate batteries pack a lot of power and value into a small package. The chemistry of these batteries is a big part of their superior performance. But all reputable commercial lithium-ion batteries also include another important element along with the battery cells themselves: a carefully designed electronic battery management system (BMS). A well-designed battery management system protects, and monitors a lithium-ion battery to optimize performance, maximize lifetime, and ensure safe operation over a wide range of conditions.

At RELiON, all our lithium iron phosphate batteries include an internal or external BMS. Let’s have a look at how a RELiON BMS protects and optimizes the operation of a lithium iron phosphate battery.

1. Over and Under Voltage

Lithium iron phosphate cells operate safely over a range of voltages, typically from 2.0V to 4.2V. Some lithium chemistries result in cells that are highly sensitive to overvoltage, but LiFePO4 cells are more tolerant. Still, significant overvoltage for a prolonged period during charging can cause plating of metallic lithium on the battery’s anode which permanently degrades performance. Also, the cathode material may oxidize, become less stable, and produce carbon dioxide which may lead to a buildup of pressure in the cell. All RELiON battery management systems limit each cell and the battery itself to a maximum voltage. The BMS in the LiFePO4 battery, for example, protects each cell in the battery and limits the voltage in the battery to 15.6V.

Undervoltage during battery discharge is also a concern since discharging a LiFePO4 cell below approximately 2.0V may result in a breakdown of the electrode materials. Lithium batteries have a recommended minimum operational voltage. In the Himax 12.8V 100Ah, for example, the minimum recommended voltage is 11V. The BMS acts as a failsafe to disconnect the battery from the circuit if any cell drops below 2.0V.

2. Overcurrent and Short Circuit Protection

Every battery has a maximum specified current for safe operation. If a load is applied to the battery which draws a higher current, it can result in overheating the battery. While it’s important to use the battery in a way to keep the current draw below the maximum specification, the BMS again acts as a backstop against overcurrent conditions and disconnects the battery from operation.

Again, using the RB100 as an example, the maximum continuous discharge current is specified at 100A, the peak discharge current is 200A, and the BMS disconnects the battery from the circuit if the load draws about 280A.

A short circuit of the battery is the most serious form of overcurrent condition. It most commonly happens when the electrodes are accidentally connected with a piece of metal. The BMS must quickly detect a short circuit condition before the sudden and massive current draw overheats the battery and causes catastrophic damage. In the RB100, the battery shuts down within 200-600 microseconds of an external short circuit, then resumes normal operation if the short circuit condition is removed.

3. Over Temperature

Unlike lead-acid or lithium cobalt oxide batteries, lithium iron phosphate batteries operate efficiently and safely at temperatures up to 60oC or more. But at higher operating and storage temperatures, as with all batteries, the electrode materials will begin to degrade. The BMS of a lithium battery uses embedded thermistors to actively monitor the temperature during operation, and it will disconnect the battery from the circuit at a specified temperature. In the example of the RELiON RB100, the BMS disconnects the battery at 80oC (176oF) and reconnects the battery at 50oC (122oF).

4. Cell Imbalance

Lithium-ion batteries have a major difference from lead-acid batteries when it comes to balancing the voltage in each individual cell during charging. Because of small differences in manufacturing or operating conditions, each cell in a battery charges at a slightly different rate. In a lead-acid battery, if one cell charges faster and reaches its full voltage, the typical low end of charge current, along with the excess charge-return, will ensure the other cells get fully charged. In a sense, the cells in a lead-acid battery are self-equalizing during charge.

This is not the case with lithium-ion batteries. When a lithium-ion cell is fully charged, its voltage begins to rise further which may lead to electrode damage. If the charge of the entire battery is stopped when only one cell is fully charged, the remaining cells do not reach full charge and the battery will operate below peak capacity. A well-designed BMS will ensure each cell safely and fully charges before the entire charging process is complete.

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How to Increase the Solar Battery Lifespan?

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Himax-home-page-design-news-1

The solar battery lifespan is a very essential factor that needs to be put into consideration by the manufacturers to ensure their batteries are reliable, durable and facilitate the production of energy. The design alone should enable them to resist could and heat cycles.

Therefore, various manufacturers need to have extensive knowledge regarding the solar batteries by ensuring proper steps are followed in order to increase their lifespan. The type of battery should also not be left out.
What You Should Understand?
Solar batteries have many factors and technical details that need to be taken into consideration when sizing up the backup required for a system. Battery system sizing also allows for a long life of service.

What Factor Could Affect the Lifespan of a Battery?
There are different types of batteries, where some are more durable compared to others despite having the same source of power. There are 3 main factors which may affect the durability of solar batteries. Some of them include cyclic life, their temperature, and depth of discharge.
The Cyclic Life
The lifespan of solar battery can easily be determined through its cyclic life or the number of use cycles it has. For example, a lead-acid battery which is flooded is expected to provide 300 to 700 cycles. A GEL cell battery is capable of providing 500 to 5000 cycles. Lithium batteries are capable of offering 2000 cycles.

Depth of Discharge (DoD)
The depth of discharge refers to the extent to which a solar battery can be used relatively to its total capacity. Batteries go down as they are discharged or charged. This, therefore, lowers their ability to store more energy. A battery that comes alongside a nominal capacity of 100 kWh at 60 % DoD will have a remaining charge of 40 kWh

Temperature
A battery attains higher chemical activity when kept under high temperatures. This makes the solar batteries less efficient in colder climates. However, the cyclic life of a battery decreases with the increase in temperature.

How to Increase the Battery Lifespan?
Despite to design of the solar battery, it may not provide longer services if not properly maintained. The following are steps involved in extending its lifespan.
Regulate the Number of Batteries
Try to lower the number of batteries used at the bank. Use of several batteries may increase resistance and connection that is likely to result into unequal charging. Therefore, regulate the number of batteries used in your bank up to 4 or maybe less.

Enhance Equalization on Solar Batteries
Equalization of battery refers to the overcharging process of your solar batteries at a regulated manner. Unequal charging results to plate’s sulphation. Overcharging gets rid of this through gassing. There are those solar batteries that are built with a solar charge controller to suppress overcharging.
Ensure Solar Batteries do not go Uncharged for a Long Time
Solar batteries are likely to be damaged if they sit for a long time in storage. You need to ensure your source of charging is always turned on to enable the battery charge continuously to facilitate a continuous solar light.

Make use of the Appropriate Solar Batteries
Batteries sized appropriately for the application will ensure a long lifespan. Lithium batteries are starting to build up steam since they have a long lifespan and are safer and conducive for the environment. However, GEL cell batteries are still the battery of choice because of their proven life, typically five to seven years in the field when sized properly. GEL cell batteries are still a fraction of the cost of Lithium battery technology, but they are starting to become more and more cost-effective as technologies improve and their share of the market increases. Make sure the kind of battery you use has a voltage rating of 12.8V or 25.6V to make sure it lasts longer.