I believe everyone has been introduced to the concept of cell balance, mainly because the current consistency of cells is not good enough, and it needs to be improved through balance. Just like in the world, we cannot find two identical leaves, and we cannot find two identical cells. Balance ultimately aims to address the shortcomings of battery cells and is a means of compensation. Below is a brief introduction to passive and active balancing of lithium batteries:
01 Passive Equilibrium
Passive balancing generally discharges batteries with higher voltage through resistance discharge, releasing electricity in the form of heat to gain more charging time for other batteries. In this way, the power of the entire system is limited by the battery with the least capacity. During the charging process, lithium batteries generally have a charging upper limit protection voltage value. When a certain string of batteries reaches this voltage value, the lithium battery protection board will cut off the charging circuit and stop charging. Therefore, the advantages of passive equilibrium are low cost and simple circuit design; The disadvantage is that the balance is based on the lowest remaining battery capacity, which cannot increase the capacity of batteries with low residual capacity, and 100% of the balanced power is wasted in the form of heat. Lithium battery protection boards generally have overcharge protection function to prevent overcharging of the battery.
As shown in Figure 2, during the charging process, the second battery is first charged to the protection voltage value, triggering the protection mechanism of the lithium battery protection board and stopping the charging of the battery system, which directly leads to the inability to fully charge the first and third batteries. The full charge of the entire system is limited by the second battery, which is the system loss. In order to increase the power of the battery system, the lithium battery protection board will balance the battery during charging. As shown in Figure 3, after balanced start-up, the lithium battery protection board will discharge the second battery, delaying the time for it to reach the protection voltage value. This will also extend the charging time of the first and third batteries, thereby increasing the overall power of the battery system. However, 100% of the discharged power of the No. 2 battery is converted into heat release, resulting in significant waste (the heat dissipation of the No. 2 battery is a loss of the system and also a waste of power).
As shown in Figure 4, in addition to the serious impact of overcharging on the battery, overdischarging can also cause serious damage to the battery. Similarly, the lithium battery protection board has an over discharge protection function. When discharging, when the voltage of the second battery reaches the discharge protection value, the protection mechanism of the lithium battery protection board is triggered, stopping the system discharge, directly causing the remaining battery capacity of the first and third batteries to be unable to be fully used. After balanced start-up, the system over discharge will be improved.
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02 Active Equilibrium
Active balancing is achieved through the transfer of electricity, which is highly efficient and results in minimal losses. Different manufacturers have different methods, and the balanced current varies from 1 to 10A.
Passive balancing is suitable for lithium battery pack applications with small capacity and low string count, while active balancing is suitable for power type lithium battery pack applications with high string count and large capacity. For BMS, in addition to the balancing function, the underlying balancing strategy is even more important.
As shown in Figure 5, every 6 strings of batteries form a group, and the total power of the 6 strings of batteries is transferred to the battery with the smaller capacity. Inductive active balancing is based on physical conversion, integrating power switches and micro inductors. It uses a bidirectional balancing method to balance batteries through charge transfer between adjacent or neighboring batteries, and can achieve balancing regardless of whether the battery is in discharge, charge, or static state. The balancing efficiency is as high as 92%.
The discharge and charging working principle is shown in Figures 6 and 7, where Battery 2 transfers power to Batteries 1 and 3. Efficient charge transfer ensures that the voltage of the three batteries remains balanced during charging, allowing all batteries to be fully charged. The lithium battery protection board can also balance the battery during discharge. The first and third batteries transfer their power to the second battery, and the voltage of the three batteries is constantly discharged in a balanced state, so that all battery power can be used up.