The main problem in the application of lithium batteries is the application of battery packs. In order to solve the "battery consistency problem", battery balancing technology is commonly used in the industry. At present, the industry divides the mainstream battery balancing technology into three types: passive balancing (energy shunt method), active balancing (dynamic balancing), and internal balancing (natural balancing).

The passive equalization method is to make the battery voltage in the battery pack tend to be consistent through the discharge equalization method. In the traditional energy-consuming BMS system, passive balance is the main method, and the single battery is connected in parallel to shunt energy-consuming resistors. The balance can only be done during the charging process. The excess energy is consumed on the consumption resistors, and the efficiency is zero. At the same time, the equalization current is very small, usually less than 100mA, and its effect on large-capacity batteries is negligible, and the SOC estimation accuracy is also very low.

The passive equalization method is characterized by its simple principle and easy implementation. When the equalization current is small, the device cost is relatively low. However, there are two major problems: one is that the resistor can consume discharge, waste energy, and generate heat; second, because the discharge resistor cannot be selected too small, at the end of the charge, according to the battery characteristics, the voltage of the small-capacity battery is often the highest. In static equilibrium , It is precisely the power of the small-capacity battery that is discharged, which increases the mutual difference between the batteries.

The active balancing method is to actively balance the individual capacity generated by the battery during use and the voltage difference caused by the self-discharge rate. Its main function is to actively balance the differences between battery cells within the battery pack during charging, discharging or placing the battery pack, so as to eliminate the various problems caused by the battery itself and during use. Inconsistency.

The main characteristics of the active equalization method are the use of a DC/CD bidirectional active equalization circuit, which has high equalization efficiency; equalization is done during charging, discharging and static processes; the equalization current is large and the equalization speed is faster. But there are also two major problems: one is the complexity of the technology, high cost, and difficulty in implementation; the other is the frequent switching of the equalization circuit, which causes great damage to the battery and affects the life of the battery.

At present, neither passive equalization technology nor active equalization technology can solve the problem well. The reason is that the currently commonly used charge and discharge control process uses a fixed voltage as the termination condition for charge and discharge, while the effective voltage range of the battery during operation is constantly changing with conditions such as temperature, charge and discharge current, and cycle period. Therefore, using a fixed voltage to control charge and discharge can easily cause overcharge or overdischarge of the battery. The change of temperature will cause the aging of the internal materials of the lithium battery, and make the battery degrade prematurely. Although researchers have been exploring and proposing some solutions, and modeling and simulating the battery, the algorithm is too complicated to run on the single-chip computer of the battery management system. Due to the lack of universality, complex identification and modeling of batteries of specific brands and models is required, and continuous improvements are made on the charging equipment of the battery pack to obtain a more stable charging power source, but the effect is still limited.

The internal equalization method uses the BMS to adjust the charging current and control the charging voltage topology algorithm in the process of charging the battery in series, so that the charge of each single battery in the battery pack is basically the same.

The characteristics of internal equalization technology are simple algorithm; no energy loss; no additional charging and discharging process, no impact on battery life; no additional hardware equipment. But if the battery charge varies greatly, it will take a long time to balance.

In recent years, BMS has done a lot of practical verification in more than 80 power battery factories and electric vehicle manufacturers of different models at home and abroad by adopting the method of internal equilibrium, and has achieved good application effects and has been widely praised by the industry.

Through the comparison of the three equalization modes and a large number of practical cases, the BMS "internal equalization technology" follows the "battery inflection point" theory, which not only saves complicated hardware costs that violate the inherent laws of batteries, but also reduces the cost of lithium batteries. The operating cost of group applications is widely recognized by the industry.