1.Passive equalization

Passive equalization generally discharges the higher voltage lithium-ion battery by means of resistive discharge, releasing power in the form of heat to buy more charging time for other batteries. This way the power of the whole system is limited by the battery with the least capacity. During charging, lithium-ion batteries generally have a charge limit protection voltage value, when a string of batteries reaches this voltage value, the lithium-ion battery protection board will cut off the charging circuit and stop charging. If the charging voltage exceeds this value, which is commonly known as overcharge, the lithium-ion battery may burn or explode. Therefore, lithium-ion battery protection panels are generally equipped with overcharge protection to prevent the battery from being overcharged.

The advantage of passive equalization is the low cost and simple circuit design; and the disadvantage is that the minimum battery residual is used as the benchmark for equalization, so it is impossible to increase the capacity of the battery with less residual, and 100% of the equalized power is wasted in the form of heat.

2. Active equalization

Active equalization is equalization by power transfer with high efficiency and low loss. The method varies from manufacturer to manufacturer and the equalization current varies from 1 to 10?A. Many of the active equalization technologies currently available on the market are immature, leading to over-discharge and accelerated battery decay. Most of the active equalization on the market use the principle of variable voltage, relying on the expensive chips of chip manufacturers. And this way, in addition to the equalization chip, but also expensive transformers and other peripheral parts, larger and more costly.

The benefits of active equalization are obvious: high efficiency, energy is transferred, the loss is only the transformer coil loss, accounting for a small percentage; equalization current can be designed to reach a few amps or even 10A level, the equalization effect is fast. Despite these benefits, active equalization also brings new problems. First, the structure is complex, especially the transformer method. How to design the switching matrix for dozens or even hundreds of strings of batteries, and how to control the driver, are all headaches. Now the price of BMS with active equalization function will be much higher than that of passive equalization, which also limits the promotion of active equalization BMS more or less.

Passive equalization is suitable for small-capacity, low-series lithium-ion battery pack applications, while active equalization is suitable for high-series, high-capacity power lithium-ion battery pack applications. For BMS, besides the equalization function is very important, the equalization strategy behind is more important.

Lithium-ion battery protection board equalization principle

Commonly used equalization charging techniques include constant shunt resistor equalization charging, on-off shunt resistor equalization charging, average cell voltage equalization charging, switched capacitor equalization charging, buck converter equalization charging, inductor equalization charging, etc. When charging lithium-ion batteries in series in groups, each battery should be guaranteed to be charged equally, otherwise the performance and life of the whole battery will be affected during use.