Recently, more and more friends are asking questions about our BMS. In this blog, we will explain the difference between active balancing and passive balancing.
Believe everyone has come across the concept of cell balancing. The main reason is that the current cell consistency is not good enough and needs to be improved through balancing. Just like you can't find two identical leaves in the world, you can't find them either. to two identical cells. Therefore, in the final analysis, balancing is to solve the shortcomings of the battery core and is a means of compensation. The following is a brief introduction to the lithium battery passive balancing and active balancing solutions and the role of the lithium battery balancer:
Passive balancing:
Passive balancing generally discharges batteries with higher voltages through resistor discharge, releasing power 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 smallest capacity. During the charging process, lithium batteries generally have a charging upper limit protection voltage value. When a certain battery string reaches this voltage value, the lithium battery protection board will cut off the charging circuit and stop charging. If the voltage during charging exceeds this value, which is commonly known as "overcharging", the lithium battery may burn or explode. Therefore, the advantages of passive balancing are low cost and simple circuit design; but the disadvantage is that the balance is based on the lowest remaining battery capacity, which cannot increase the capacity of batteries with low remaining capacity, and 100% of the balanced power is wasted in the form of heat. Lithium battery protection boards generally have overcharge protection functions to prevent the battery from overcharging.
As shown in the figure, during the charging process, battery No. 2 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. This directly causes batteries No. 1 and No. 3 to be unable to be fully charged. The full charge capacity of the entire system is limited by the AA battery, which is a system loss. To increase the capacity of the battery system, the lithium battery protection board balances the battery during charging. As shown in Figure 3, after the equalization is started, the lithium battery protection board will discharge the No. 2 battery, delaying the time for it to reach the protection voltage value. In this way, the charging time of the No. 1 and No. 3 batteries will be correspondingly extended, thereby improving the entire battery system. of power. However, 100% of the discharged power of the No. 2 battery is converted into heat release, causing a lot of waste (the heat dissipation of the No. 2 battery is a loss of the system and a waste of power).
As shown in the figure, in addition to overcharging having a serious impact on the battery, overdischarging can also cause serious damage to the battery. Similarly, the lithium battery protection board has over-discharge protection function. During discharge, when the voltage of battery No. 2 reaches the discharge protection value, the protection mechanism of the lithium battery protection board is triggered and the system is stopped from discharging. This directly results in the remaining battery capacity of batteries No. 1 and No. 3 not being fully used. After balanced startup, the system will be improved. Over release.
The advantages of passive balancing 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 remaining capacity, and 100% of the balanced power is wasted in the form of heat.
active balancing ：
Active balancing uses power transfer to balance, which has high efficiency and small loss. Different manufacturers have different methods, and the balancing current also varies from 1 to 10 A. Many active balancing technologies currently on the market are immature, resulting in battery over-discharge and accelerated battery degradation. Most active equalizers on the market use the transformer principle and rely on expensive chips from chip manufacturers. In addition to the balancing chip, this method also requires expensive transformers and other peripheral components, which are larger in size.
Passive balancing is suitable for lithium battery pack applications with small capacity and low string count, while active balancing is suitable for power lithium battery pack applications with high string count and large capacity. For BMS, in addition to the important balancing function, the balancing strategy behind it is even more important.
As shown in the figure, every 6 strings of batteries are a group, and the total power of the 6 strings of batteries is transferred to the battery with a smaller capacity. Inductive active balancing is based on physical conversion and integrates a power switch and a micro-inductor. It adopts a two-way balancing method to balance the battery through charge transfer between similar or adjacent batteries, and can be used regardless of whether the battery is discharging, charging or resting. For equalization, the equalization efficiency is as high as 92%.
Battery life decreases as variability increases. Therefore, when differences in batteries are found, processing should be carried out to reduce the impact of inconsistencies on the batteries and avoid forming a vicious cycle.
ring. Cell inconsistency cannot be completely eliminated, but it can be minimized through cell balancing techniques.
Thank you for your patience in reading, see you in the next blog.
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