Passive equalization and active equalization of lithium ion battery protection board
1. Passive equalization
Passive equalization generally discharges lithium-ion batteries with high voltage by means of resistance discharge to release electricity in the form of heat, so as to strive for more charging time for other batteries. In this way, the power of the whole system is limited by the battery with the least capacity. During charging, the lithium-ion battery generally has a charging upper 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 voltage during charging exceeds this value, which is commonly known as overcharge, the lithium-ion battery may burn or explode. Therefore, lithium ion battery protection panels generally have overcharge protection function to prevent battery overcharge.
The advantages of passive equalization are low cost and simple circuit design; The disadvantage is that the balance is based on the minimum battery residue, the capacity of the battery with less residue cannot be added, and 100% of the balanced power is wasted in the form of heat.
2. Active equalization
Active equalization is based on power transfer, which has high efficiency and small loss. The methods of different manufacturers are different, and the equalization current is also from 1 to 10? A varies. At present, many active equalization technologies in the market are immature, resulting in battery over discharge and accelerating battery attenuation from time to time. Active equalization in the market mostly adopts the principle of voltage transformation, relying on the expensive chips of chip manufacturers. In addition to the equalization chip, this method also needs expensive transformer and other peripheral parts, which has large volume and high cost.
The benefits of active equalization are obvious: high efficiency, energy is transferred, and the loss is only the loss of transformer coil, accounting for a small proportion; The equalizing current can be designed to be large, up to several amperes or even 10a, and the equalizing effect is fast. Despite these benefits, active equilibrium also brings new problems. The first is the complex structure, especially the transformer method. How to design the switch matrix of dozens or even hundreds of batteries and how to control the drive are all headache problems. Now the price of BMS with active equalization function will be much higher than that of passive equalization, which limits the promotion of active equalization BMS.
Passive equalization is suitable for the application of lithium-ion battery pack with small capacity and low string number, and active equalization is suitable for the application of power lithium-ion battery pack with high string number and large capacity. For BMS, in addition to the balance function, the balance strategy behind it is more important.
Equalization principle of lithium ion battery protection board
Common equalization charging technologies include constant shunt resistance equalization charging, on-off shunt resistance equalization charging, average battery voltage equalization charging, switched capacitor equalization charging, step-down converter equalization charging, inductance equalization charging, etc. When groups of lithium-ion batteries are charged in series, ensure that each battery is charged evenly, otherwise the performance and service life of the whole group of batteries will be affected during use.
During the charging process of lithium-ion battery, each lithium-ion battery is equipped with an equalization circuit. During charging, the voltage of each battery is controlled through the equalization circuit of lithium-ion battery protection board to keep each string of batteries in the same state and ensure the performance and service life of lithium-ion battery.