What Is The Difference Between Active And Passive Battery Balancing?

Lithium batteries are the power source for new energy vehicles. However, due to the significant differences in parameters such as voltage and capacity of individual cells, inconsistencies can easily occur during use. This leads to unstable battery performance and seriously affects the lifespan of electric vehicles.

Balancing technology is an effective means to address the inconsistency problem of power batteries. At present, there are mainly two types of technologies: active balancing and passive balancing. This article focuses on active and passive balancing technologies, analyzing and comparing them from aspects such as principles, circuit structure, control strategies, and provides application design of both balancing technologies in the power batteries of electric vehicles.

Why Battery Balancing?

Through battery balancing, each cell in the battery pack can be effectively monitored and maintain a healthy state of charge (SoC). This not only increases the number of battery cycle operations but also provides additional protection to prevent damage to battery cells due to overcharging or deep discharging.

Active Balancing and Passive Balancing

Passive balancing consumes excess charge through discharge resistors, ensuring that all battery cells have roughly equivalent SoC. However, it does not extend system operating time. Typically, any balancing that uses resistance to dissipate energy is referred to as passive balancing. Active balancing is a more complex balancing technique. As the charge within the battery cells is redistributed during the charging and discharging cycles, the total available charge in the battery pack also increases, thus extending the system’s operating time. Compared to passive balancing, active balancing can shorten charging time and reduce heat generated during balancing. Generally, any balancing achieved through energy transfer is referred to as active balancing.

Active battery balance discharge

In the below image of a typical battery pack at full capacity, full capacity refers to a charge level of 90%, as maintaining the battery at (or close to) 100% capacity for an extended period can quickly reduce its lifespan. Total discharge means discharging to 30% to prevent the battery from entering a deep discharge state.

Over time, some batteries may perform worse than others, leading to the discharge characteristics of the battery pack as shown in the figure below.

Even if some battery cells still retain a large capacity, the weak battery cells limit the system’s operating time. A 5% mismatch in battery capacity will result in 5% of the energy not being utilized. For high-capacity batteries, this means a significant amount of energy is wasted, which is particularly crucial for remote systems and systems that are difficult to maintain. Unused energy also leads to an increase in the number of battery charging and discharging cycles, reducing the battery’s lifespan and resulting in higher costs due to frequent battery replacements.

Through active balancing, the charge is redistributed from strong battery cells to weak ones, allowing the energy in the battery pack to be fully depleted.

Active Battery Equalization Charging

If the battery pack is charged without equalization, weak battery cells will reach full capacity before strong battery cells. Once again, these weak cells become a limiting factor, constraining the total energy that can be accommodated in the system.

Active equalization redistributes charge during charging, enabling the battery pack to reach full capacity. We will not delve into the effects of equalization time proportion and the impact of equalization current on time here.

Advantages and Disadvantages of Active and Passive Equalization

Both active and passive battery equalization can effectively promote the health of the battery system by monitoring and matching the charge status of each battery cell. Unlike passive battery equalization, which only consumes excess charge during charging, active battery equalization can redistribute charge during both charging and discharging. Therefore, active battery equalization can extend system operating time and improve charging efficiency. However, the solutions required for active battery equalization are often more complex and larger in size, while passive battery equalization is more cost-effective.

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