LiFePO4 Battery Storage of SOC


LiFePO4 Battery Storage of SOC
LiFePO4 Battery Pack Storage
Lithium-ion batteries have a life standard aging life, which means that even if you don't use the battery, the battery capacity and performance will decrease at a rate.
The phenomenon is that lithium ions are transferred from the positive electrode to the negative electrode if battery is charging. If all of ions run out from positive to negative, the structure of positive electrode will is changed unstably, and if this status lasts for too long, the positive electrode may be damaged and the battery performance will be decreased and it will be irreversible.

In normally, 90% lithium ions are transferred from positive to negative if the battery is fully charged and 10% lithium ions is remaining in the positive electrode which is responsible for keeping for a stable positive electrode structure and SEI film. When LiFePO4 battery is charged up to 3.8V, all the ions are embedded in the negative electrode, at this time, the positive electrode structure will be very unstable, for a long time, the battery performance will be decrease.
Therefore, in order to maintain battery life and performance stability, it is necessary to ensure that there are enough lithium ions on the positive and negative electrodes. And we know that if it is 50% charging, the amount of ions on both sides of the positive and negative are on balance, and they will be in a relative stable station. So we suppose 50% station of charge storage seem to be more reasonable.

Battery Test after Storage
To verify our 50% station of charge storage is better than 100% station of charge storage. We arranged self-discharge test and capacity recovering rate test after 28 days storage at ambient temperature 25℃. We take 20pcs IFR18650EC-1.5Ah batteries in the same lot of production:
1. 10pcs 100% charged and 10pcs 50% charged and record the Voltage, Impedance before storage
2. Store all of them for 28days in the storage condition.
3. Record Impedance and Retention rate (with 0.2C discharge current).
4. Recharge all of them for fully charge (with 0.5C charge current).
5. Discharge all of them to 2.0V (with 0.2C discharge current) and record capacity recovering rate.

Test Result
100% charging station and stored for 28days 
       Voltage(V)  Impedance(mΩ)  Voltage(V)  Impedance(mΩ)   Retention Rate%    Recovering Rate%
Max     3.341         23            3.339          22.8             97.72%                 98.56%
Min     3.339         22.1          3.338          22.5             97.55%                 98.49%
Average 3.344         21.8          3.336          22.1             97.69%                 98.50%

50% charging station and stored for 28days 
       Voltage(V)   Impedance(mΩ) Voltage(V) Impedance(mΩ)    Retention Rate%    Recovering Rate %
Max     3.310          22           3.309        22.3               98.4%                   99.43%
Min     3.309          23.6         3.308        22.9               98.25%                  99.20%
Average 3.311          23.5         3.311        22.8               98.31%                  99.35%

50% station of charge storage is better than 100% station of charge storage.

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