How to maximize the capacity efficiency of the battery pack?

　　This must also be done when the battery pack is discharged, otherwise, when the battery with the smallest capacity reaches the turn off voltage, the entire battery pack will stop discharging, while other batteries still have residual capacity, thus reducing the total capacity. In the long run, the battery with the smallest capacity will decline faster than other batteries, and the capacity loss will be accelerated after multiple charge/discharge cycles.

　　By matching the voltage of the battery in series, more current will be drawn from the high-capacity battery. When discharging, it is required to consume some additional voltage through equalization. When all batteries reach 0SOC in the end, the total electric energy obtained from the battery pack will still increase compared with that before equalization.

　　Generally, the quality control of cylindrical lithium-ion battery is good, and the difference of battery capacity is not more than ± 3%. The input capacity is basically accurate, and the difference does not exceed several mAs (milliamperes per second). Therefore, the absolute value of battery capacity is basically accurate, and the difference of SOC is within a few percentage points.

　　The chemical properties of lithium ion materials themselves do not cause the battery imbalance problem, nor do they have a reversible self discharge mechanism. However, lithium ion batteries must go through a process to make their performance stable, and will produce irreversible losses. Most of these losses occur before the battery leaves the factory. A small amount of such irreversible loss will also occur when discharging at high temperature and placing at room temperature for too long. The maximum proportion of such loss is within 10%. All batteries stored or used at the same time have the same probability of such loss, so this situation will not cause the problem of battery imbalance.

　　When irreversible loss occurs, there is also a small amount of reversible loss, and there is a certain proportion between them. Most of the reversible losses occurred in the factory have been recharged before the battery capacity classification, so this kind of loss is very small, and each battery is the same, which will not cause the battery imbalance problem. Another reason for battery imbalance is that batteries are left idle for too long before assembly. If several batches of batteries with different production time are put into the same battery pack, this imbalance will be aggravated. In this case, batteries with different reversible losses classified by capacity but not yet assembled will accumulate greater differences over time. Even after the battery pack is assembled, if the battery in the pack changes greatly (especially when the battery pack is placed for too long), the imbalance of the battery in the battery pack will increase over time.

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