Inconsistencies of EV battery cells
The battery pack consists of cells, but the SOC, internal resistance, self discharge current, and capacity of each cell are not consistent. Equilibrium cannot completely solve these four differences. Equilibrium can only compensate for SOC differences and also solve the problem of inconsistent self discharge. But for internal resistance and capacity, balance is powerless.
So how is the inconsistency of the battery caused?
There are mainly two aspects: one is the inconsistency caused by battery production and processing, and the other is the inconsistency caused by the battery usage environment. The reason for inconsistent production comes from factors such as processing technology and materials. I said this is relatively simple, but the actual things inside are very complex; The inconsistency of the environment is easy to understand. Because the position of each battery in the PACK is different, the environment will also be different. For example, the temperature may vary slightly, and long-term accumulation may cause inconsistent batteries.
As mentioned earlier, equalization is used to eliminate SOC differences in batteries. In an ideal state, it always maintains the SOC of each battery constant, enabling all batteries to synchronously reach the upper and lower voltage limits for charging and discharging, thereby increasing the usable capacity of the battery pack. Under balance, all batteries maintain the same SOC for charging and discharging.
Therefore, equalization is a very important function of current batteries. There are two options for achieving balance function, active balance and passive balance. Passive balance is discharge with resistance, while active balance allows charge to flow between batteries. Passive equilibrium is widely used in reality, while active equilibrium is relatively rare.