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How does active balancing differ from passive?

April 24, 2025

How does active balancing differ from passive?

Active balancing and passive balancing are two common cell balancing methods in Battery Management Systems (BMS), differing mainly in the following aspects:

1. Balancing Principle

 

  • Passive Balancing: The principle is to discharge cells with higher capacity through parallel resistors or similar components, dissipating excess energy in the form of heat. This brings the SOC (State of Charge) of all cells closer together. However, it wastes energy during the balancing process and can lead to energy loss. 

  • Active Balancing: The principle involves transferring energy from cells with higher SOC to those with lower SOC or to a storage device. It achieves balancing by redistributing energy within the battery pack, maximizing energy utilization. For example, in a lithium-ion battery system, active balancing can use a DC-DC converter to transfer the excess energy of high-SOC cells to low-SOC cells.

2. Energy Utilization Efficiency

  • Passive Balancing: Energy is wasted in the form of heat during the balancing process, leading to relatively low energy utilization efficiency. It is more suitable for scenarios where balancing requirements are not high and balancing time is not critical. 

  • Active Balancing: Energy is transferred and redistributed rather than directly dissipated. This improves energy utilization efficiency, reduces energy loss, and enhances the overall performance of the battery system, making it suitable for high-power and fast-charging/discharging scenarios.

3. Circuit Complexity

  • Passive Balancing: The circuit design is relatively simple and low-cost. It typically only requires resistors and switches to achieve balancing. However, it has limited balancing capability and may not achieve high balancing precision. 

  • Active Balancing: The circuit is more complex and requires additional components such as converters, transformers, and inductors. The control strategy is also more sophisticated, with higher hardware costs. It offers strong balancing capability and high balancing precision but requires higher technical expertise to implement.

4. Balancing Speed

 

  • Passive Balancing: Balancing speed is relatively slow. It usually takes a longer time to achieve a balanced state, which may affect system efficiency.

  • Active Balancing: Balancing speed is faster. It can quickly adjust the SOC of cells to achieve a balanced state, improving system efficiency.

5. Heat Dissipation Characteristics

 

  • Passive Balancing: During the balancing process, the resistor consumes energy and generates heat, which may increase the temperature of the battery pack and affect battery performance and lifespan. Additional heat dissipation measures may be required.

  • Active Balancing: Energy is transferred and redistributed rather than directly converted into heat. The heat generated is relatively low, reducing its impact on battery pack temperature and potentially improving battery lifespan.

6. Impact on Battery Lifespan

 

  • Passive Balancing: The process of discharging cells with higher SOC may accelerate battery aging, especially if the balancing frequency is high. This could negatively affect battery lifespan.

  • Active Balancing: By effectively transferring and redistributing energy, it reduces the likelihood of overcharge and over-discharge of individual cells, helping to extend battery lifespan.

7. Typical Applications

 

  • Passive Balancing: Often used in small battery systems with low balancing requirements, such as consumer electronics and low-power battery packs.

  • Active Balancing: More commonly used in high-power battery systems like electric vehicles and energy storage systems.

In summary, active balancing and passive balancing each have advantages and disadvantages. The choice between them depends on specific application scenarios and requirements. In practice, a combination of active and passive balancing can be employed to leverage their complementary strengths, achieving better balancing effects.