1. Passive Equilibrium
Passive balancing typically uses resistive discharge to discharge the higher-voltage lithium-ion batteries, releasing energy as heat to buy more charging time for the other batteries. In this case, the overall system's capacity is limited to the battery with the smallest capacity. During charging, lithium-ion batteries generally have a charging upper limit protection voltage. When a string of batteries reaches this voltage, the lithium-ion battery protection board cuts off the charging circuit and stops charging. If the charging voltage exceeds this value, commonly known as overcharging, the lithium-ion battery may burn or explode. Therefore, lithium-ion battery protection boards generally have overcharge protection functions to prevent overcharging.
The advantages of passive balancing are low cost and simple circuit design; however, the disadvantages are that it balances based on the lowest remaining battery capacity, making it impossible to add capacity to batteries with low remaining capacity, and 100% of the balanced charge is wasted as heat.
2. Active Equilibrium
Active balancing uses power transfer to equalize batteries, resulting in high efficiency and minimal loss. Different manufacturers use different methods, and the balancing current varies from 1 to 10 µA. Currently, many active balancing technologies on the market are immature, leading to frequent cases of over-discharge and accelerated battery degradation. Most active balancing solutions on the market employ a transformer principle, relying on expensive chips from chip manufacturers. Furthermore, this method requires not only the balancing chip but also expensive transformers and other peripheral components, resulting in a larger size and higher cost.
The benefits of active balancing are obvious: high efficiency, energy transfer, and losses are limited to transformer coil losses, which account for a small proportion; the balancing current can be designed to be large, reaching several amps or even 10A, and balancing is effective quickly. Despite these advantages, active balancing also brings new problems. Firstly, it involves structural complexity, especially with transformer-based methods. Designing the switch matrix for dozens or even hundreds of battery cells and controlling the drive are both daunting challenges. Currently, BMSs with active balancing capabilities are significantly more expensive than those with passive balancing, which somewhat limits the widespread adoption of active balancing BMSs.
Passive balancing is suitable for small-capacity, low-cell-count lithium-ion battery packs, while active balancing is suitable for high-cell-count, large-capacity power lithium-ion battery packs. For a battery management system (BMS), in addition to the balancing function being very important, the underlying balancing strategy is even more crucial.
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 battery voltage equalization charging, switched capacitor equalization charging, buck converter equalization charging, and inductor equalization charging. When charging a series of lithium-ion batteries, it is essential to ensure that each cell is charged evenly; otherwise, the performance and lifespan of the entire battery pack will be affected during use.
