12V27A battery.The key reason why BMS cannot solve the consistency problem

The fundamental reason why BMS cannot solve the consistency problem of the battery pack is caused by the balancing control strategy and functional defects it provides. Its balancing control strategy cannot solve the problem of equal-rate charging and discharging. Even if the active balancing charging function is configured, it cannot solve the problem fundamentally. To solve the problem, it only has a superficial or psychological effect, and does not help to solve the consistency problem.

Detailed analysis is as follows:

First, even if the balanced charging function works normally, the best effect can only ensure that small-capacity batteries do not overcharge. During the charging and discharging process of the battery pack, there is a concept and actual usage that is often ignored, that is, the charge and discharge rate. For the same series battery pack, due to the existence of consistency problems, the charge and discharge rates of batteries with different capacities are different. The difference is that the smaller the capacity, the greater the charge and discharge rate, and the larger the capacity, the smaller the charge and discharge rate.

Research and application data show that the impact of charge and discharge rate on battery attenuation is regular, that is, the greater the charge and discharge rate, the faster the battery decay rate. It can be seen that if a small-capacity battery is always under a relatively high charge and discharge rate, the attenuation speed is always the fastest. Under high-rate charge and discharge conditions, a small-capacity battery cannot avoid its capacity loss regardless of whether it has a balanced charging function or not. The fact of continued rapid decay and decline.

Another very serious negative effect caused by high-rate charging and discharging is the accelerated increase in attenuation battery temperature, which can easily cause "thermal runaway" failure, resulting in spontaneous combustion and explosion of the battery pack during vehicle charging and driving. On the contrary, long-term continuous use of balanced charging will make the capacity of small-capacity batteries less balanced and the capacity difference will become larger and larger. This is the fundamental reason why the charging balancing time of electric vehicles is getting longer and longer.

Second, equal-rate charging and discharging is the fundamental strategy to solve the rapid decay of small-capacity batteries. Equal-rate charging and discharging means that in a series battery pack, batteries with different capacities achieve the same rate of charging and discharging with the intervention of a transfer battery equalizer. Constant rate charging and discharging is the key to ensuring the cycle life of the battery pack. At the same time, constant rate charging and discharging actively reduces the actual charging and discharging current of small-capacity batteries, thus effectively reducing the temperature rise of the attenuated battery and preventing thermal runaway failure.

The reason why new electric vehicles have long cruising range and large charging capacity is mainly because the consistency of the batteries is relatively good, and the charge and discharge rates of each battery are basically the same. As the battery consistency problem appears and worsens, the difference in charge and discharge rates also increases. The gradual increase further aggravates the attenuation of the battery pack and reduces the available capacity. Therefore, the cruising range gradually shrinks and the charging capacity becomes less and less.

It can be seen that unless the problem of battery consistency is completely solved, people's criticism of the rapid decline of the cruising range of electric vehicles will not be eliminated. So, why do domestic electric vehicle manufacturers only use passive balancing without considering the energy-saving and efficient transfer active battery balancing technology? This is because:

The first is the cost factor. Passive balancing technology is mature, very low cost, and easy to implement; while the transfer battery balancing technology, a typical representative of active balancing, is efficient and energy-saving, but it is technically complex, difficult to implement, large in size, and high in cost; the second is No transfer cell balancing technology has been developed or suitable, especially a high-efficiency transfer real-time cell balancing technology.

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