In addition to ensure that the battery pack not overcharged and overdischarged, the battery management system BMS must have a good balancing function to keep the consistency of the battery pack.
Almost all the BMS on the market now have balancing function, mainly divided into two kinds: active balancing function, and passive balancing function.
Active balancing is also called lossless balancing.
Active balancing is equalization by means of energy transfer, transferring a single cell with high energy to the one with low energy, thus equalizing the voltage of the whole group, with almost no energy loss involved in the process of transfer.
*Start-up conditions:
Whether the battery is charging, discharging, or resting, balancing starts as soon as the differential voltage is greater than the set value. Therefore, as long as there is a differential voltage, the active balancing should work 24 hours a day until the differential voltage is less than the set range.
*Balanced current:
As the active equalization is the way of energy transfer, no heat, all equalization current can be larger without affecting heat dissipation, generally active equalization current can be 1~2A more common.
Since active equalization is not limited by charging time, the equalization time is longer and the equalization current is larger, so it is more suitable for large capacity battery pack.
Passive balancing is also called lossy balancing.
Passive balancing is generally discharges the higher voltage cells by resistor discharge, releasing power in the form of heat to balancing the whole group voltage and obtain more charging time for other batteries.
*Start-up conditions:
Passive balancing is generally started when the battery is close to full voltage discharge equalization. So the start equalization time of passive equalization is relatively short, from the battery is close to full to full cutoff, depending on the charger is generally a few hours equalization time.
*Balanced current:
Since passive balancing is a resistive energy consumption equalization, the energy consumed is released in the form of heat, so the BMS will heat up during the equalization process, resulting in the equalization current cannot be set too high, otherwise it will cause the battery pack temperature to be too high. The general passive balancing current ranges from 35mA to 200mA, the higher the equalization current, the more serious of the heat.
If the equalization current is small, then the efficiency of the power balancing in large-capacity battery packs with large power differences is very low, and it takes a long time to reach the balance, which is a kind of itchy feeling in the application. Passive equalization circuit is simple, low cost, and suitable for low capacity battery pack.
