Lithium-ion batteries will discharge naturally by self-discharge.
Lithium-ion batteries have smaller self-discharge than other rechargeable batteries, but it is not zero.
If self-discharge is large, the voltage drop will increase when the battery is left unused.
For lithium-ion batteries with 18650 size,when the environment temperature rises 10°C, self-discharge doubles and the higher the environment temperature, the larger self-discharge will be.
For lithium-ion batteries, the self-discharge amount varies even for each cell.
It is rare that even the lithium-ion battery whose self-discharge has become large due to mixing of minute metal scraps inside the electrode is recognized.
If it is left for a long time after charging, the voltage drops so the size of self-discharge can be seen, but when charge and discharge are repeated, the decrease in capacity due to self-discharge is extremely small and cannot be recognized.
Self-discharge becomes a problem in the case of a battery pack being connected in series and raising the voltage.
If cells A and B with capacity of 1000mAh are connected in series and the self-discharge current A is 50μA and B is 250μA, the amount of charge was the same at a time of manufacturing, but after 3 months, since there is a difference in self-discharge amount between A and B, B’s charging amount will be smaller than that of A by 200μA × 24H × 30 days × 3 months = 432mAh.
When discharging in such a state, B reaches 3V faster than A.
Over discharge protection of the protection circuit may operate and stop discharging.
When the battery pack is charged, cell A first reaches 4.35V and the overcharge protection function of the protection circuit operates and automatically stops charging.
As a result, the lithium-ion battery pack has capacity of only about 568mAh instead of 1000mAh.
Even after that, the difference in charge amount will increase with time.