Charge Retention of Li-Polymer (LiPO) Battery Packs

1. What is Charge Retention?

Charge retention refers to a battery’s ability to retain its stored electrical energy over time when not in use. This is inversely related to the self-discharge rate, which measures the rate at which a battery loses capacity on its own, without any load.

For LiPO batteries, charge retention is one of their strengths compared to older technologies like NiCd or NiMH.

2. Typical Self-Discharge Rates for LiPO Batteries

Condition	Monthly Self-Discharge Rate	Explanation
New, premium quality cells	1% – 3%	High manufacturing quality, low internal resistance
Typical LiPO cells	3% – 5%	Industry average
Aged or damaged cells	5% – 10%+	Increased impedance, side reactions inside cell
Stored in hot conditions	Up to 15%+	Accelerated chemical reactions

 

Note: LiPO batteries have lower self-discharge than NiMH (≈30%/month) or lead-acid (≈5–10%/month).

3. Technical Factors Influencing Charge Retention

A. Electrochemical Stability

LiPO batteries use lithium-ion intercalation chemistry.

• They typically use a lithium cobalt oxide (LCO) or nickel manganese cobalt (NMC) cathode with a carbon-based anode.

• The tight molecular structure of these materials leads to low ionic leakage, reducing self-discharge.

B. Temperature

Heat accelerates electrolyte decomposition and SEI (solid electrolyte interphase) layer growth.

 

Cold slows down ion mobility, reducing discharge rate — but too cold can cause lithium plating when recharging.

 

Temperature	Expected Self-Discharge Impact
0–15°C	Minimal, ~1–2%/month
20–25°C	Normal, ~2–4%/month
30–40°C	Elevated, ~4–6%/month
>45°C	Risk of degradation, >10%/month

C. State of Charge (SoC) During Storage

 

Storing LiPO batteries at:

• 100% SoCaccelerates electrolyte oxidation and gas formation
• 0% SoCcan cause over-discharge and permanent damage
• Ideal: 40–60% SoCfor long-term storage (also called “storage mode”)

D. Manufacturing Quality

• Well-balanced cells, high-quality separator materials, and uniform coatings lead to lower parasitic losses.
• Example: Grade A 18650 LiPOpouch cells can show <2% self-discharge per month under optimal storage.

 

4. Real-World Charge Retention Over Time

Let’s assume a 5000mAh 3S 11.1V LiPO pack is stored at 50% SoC (~11.4V) at room temperature:

Time Stored	Estimated Retained Charge	Assumed Conditions
1 month	~4750–4900 mAh	2–5% loss
3 months	~4500–4750 mAh
6 months	~4100–4500 mAh
12 months	~3700–4200 mAh	20–30% loss in worst case

After 6 months or more, periodic checks and balance charging are recommended.

 

5.Risks of Poor Storage (Charge Retention Failure)

 

Problem	Cause	Consequence
Over-discharge	Stored at 0% SoC	Permanent capacity loss, safety risk
Cell swelling	Stored at high SoC + heat	Gas formation in electrolyte
Voltage imbalance	Poor cell matching + time	Reduced performance and cycle life
Thermal runaway	Overcharging or long-term full charge in heat	Fire/explosion risk

 

6.How to Maximize Charge Retention

Parameter	Recommendation
Storage SOC	40–60%
Storage Temp	15–25°C
Humidity	<60% RH (dry storage is best)
Charger use	Use “Storage Mode” if available
Inspection	Check voltage every 1–2 months

Example Storage Voltage

For 3S (11.1V) battery: store at 11.1V–11.4V
For 4S (14.8V) battery: store at 14.8V–15.2V

 

7.Charge Retention vs. Other Battery Types

Battery Type	Monthly Self-Discharge Rate	Charge Retention Advantage
LiPO	2–5%	🔹 Excellent
LiFePO₄	1–3%	🔹 Excellent
NiMH	20–30%	🔻 Poor
Lead-Acid	5–10%	⚠️ Moderate
Alkaline	~0.3% (primary)	🔹 Excellent (non-rechargeable)

8. Internal Mechanisms of Self-Discharge in LiPO

 

• SEI Layer Instability:Breakdown or growth of the protective layer on the anode.
• Electrolyte Decomposition:Accelerated by temperature and SoC.
• Parasitic Currents:Micro-leakage through separator or from cell defects.
• Impedance Increase:As battery ages, internal resistance increases → higher leakage current.

9. Industrial Standards/Testing for Charge Retention

Standards for charge retention testing:

• IEC 61960– Secondary lithium cells and batteries
• UL 1642– Safety for lithium batteries
• UN 38.3– Transportation testing
• JEITAor GB/T 18287-2013 – Often used in Chinese battery manufacturing

Testing typically involves storing a fully charged cell at room temperature for 28 days, followed by capacity test to evaluate % charge retained.

Conclusion

LiPO batteries have excellent charge retention (~2–5% per month under good conditions).

Long-term storage without proper SoC or at high temps can result in serious damage and reduced lifespan.

Proper storage, routine maintenance, and storage at optimal voltage and temperature are critical for extending both charge retention and overall battery life.