1. Passive balance

　　 Passive equalization generally discharges the battery with a higher voltage through resistance discharge, and releases the power in the form of heat, so as to obtain 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. If the voltage during charging exceeds this value, which is commonly known as "overcharge", the lithium battery may burn or explode. therefore,

　　The advantages of passive balancing are low cost and simple circuit design. The disadvantage is that balancing is based on the lowest battery residual 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 an overcharge protection function to prevent the battery from overcharging.

As shown in the figure above, during the charging process, the No. 2 battery is first charged to the protection voltage value, which triggers the protection mechanism of the lithium battery protection board to stop the charging of the battery system, which directly causes the No. 1 and No. 3 batteries to fail to be fully charged. The full charge capacity of the entire system is limited by the No. 2 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. After the equalization starts, the lithium battery protection board will discharge the No. 2 battery, delaying the time for it to reach the protection voltage value, so that the charging time of the No. 1 and No. 3 batteries will be correspondingly extended, thereby increasing the power of the entire battery system. 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 above, in addition to overcharging will have a serious impact on the battery, overdischarging will 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 No. 2 battery reaches the discharge protection value, the protection mechanism of the lithium battery protection board is triggered to stop the system discharge, which directly causes the battery remaining capacity of the No. 1 and No. 3 batteries to be not fully used, and the system will be improved after the equalization starts. Over-discharge.

　　The advantages of passive balancing are low cost and simple circuit design. The disadvantage is that balancing is based on the lowest battery residual 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.

　　 2. Active balance

　　 Active balance is the balance of power transfer, with high efficiency and low loss. Different manufacturers have different methods, and the equalizing current also ranges from 1 to 10 A. At present, many active equalization technologies appearing on the market are immature, leading to battery over-discharge and accelerating battery degradation. Most of the active equilibrium in the market adopts the principle of voltage transformation, relying on the expensive chips of chip manufacturers. In addition to the equalization chip, this method also requires expensive transformers and other peripheral components, which is larger in size and higher in cost.

　　 Passive equalization is suitable for small-capacity, low-string lithium battery pack applications, and active equalization is suitable for high-volume, high-capacity power lithium battery pack applications. For BMS, in addition to the balance function is very important, the balance strategy behind it is even more important.

As shown in the figure above, every 6 strings of batteries are a group, and the total power of the 6 strings of batteries is transferred to a battery with a small capacity. Inductive active balancing is based on physical conversion, integrating power switches and micro-inductors, and adopts a two-way balancing method to balance the battery through charge transfer between adjacent or adjacent batteries, and it can be used regardless of whether the battery is discharged, charged or static. Perform equalization, and the equalization efficiency is as high as 92%.

The working principle of discharging and charging, as shown in the figure above, the No. 2 battery transfers the power to the No. 1 and No. 3 batteries. Efficient charge transfer keeps the voltage of the three batteries in an equilibrium state during charging, so that all batteries can be fully charged. The lithium battery protection board can also balance the battery when discharging. The No. 1 and No. 3 batteries transfer the power to the No. 2 battery, and the voltages of the 3 batteries are always discharged in a balanced state, so that all the batteries can be used up.

Battery protection board active balance circuit:

The topology diagram of the active equalization charging system of the battery protection board of the lithium iron phosphate power battery pack used in the figure below. Active balancing of the battery protection board is a balancing method relative to passive balancing. In terms of its definition, it refers to the method of using active components to balance the capacity of the battery pack.

The system is mainly composed of a single battery protection board management circuit and a vehicle controller. The single battery management circuit is composed of a single battery detection unit and a single battery equalization charging unit. The detection unit uses an adaptive Kalman filter algorithm to calculate the single battery sOC, the equalizing charging unit adopts a flyback converter to realize the supplementary charge from the battery pack to the battery with a lower SOC, so as to realize the consistency of the SOC of each single battery within the controllable range.

The communication mode between each single battery management circuit adopts CAN bus communication, and the communication between the single battery management circuit and the vehicle controller is realized through a CAN-USB bus adapter. The vehicle controller is connected to the CAN bus of the single battery management circuit through the USB-CAN bus adapter, and realizes the sending and receiving of the information of the single battery management circuit. By adopting the distributed structure of CAN bus, the connection of the system is reduced, which is convenient for system installation and debugging.

　　The battery management system is composed of n cell management circuits. As shown in the figure, it is the connection line diagram of the cell management circuit and the cell when n=4.

The single battery protection board management circuit adopts the CAN bus for communication and connection. The CAN bus is composed of 4 signal wires. GND_CAN is the ground wire of the external power supply interface, CANL is the CAN bus L signal, CANH is the CAN bus H signal, +5VCAN External power supply interface for CAN module +5V power supply. odd stands for odd modules, and even stands for even modules. The single battery management circuit respectively leads out two wires to connect to the single battery, so that the single battery management circuit can be disconnected from the single battery at any time.