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 Equalisation 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 Equalisation

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.

FAQs

1. What exactly is the difference between active and passive battery balancing?

Passive balancing is a method that dissipates excess energy from high-voltage cells as heat through resistors, while active balancing is a more advanced technology that transfers energy from stronger cells to weaker ones to maximize the pack’s total capacity.

2. Why do I need battery balancing for my lithium pack?

Battery balancing ensures each cell maintains a healthy State of Charge (SoC), which prevents individual cells from overcharging or deep discharging; according to technical data, effective balancing can significantly increase the number of battery cycle operations and protect the overall system health.

3. How does passive balancing actually work?

The process is simple: passive balancing uses discharge resistors to “burn off” the extra charge of the highest cells until their SoC matches the rest of the pack, ensuring all cells reach a uniform voltage level during the charging process.

4. What is the working principle of active battery balancing?

Active balancing works by redistributing charge through energy transfer components like inductors or capacitors; it moves energy from high-capacity cells to low-capacity ones during both the charging and discharging cycles.

5. How much energy is wasted if my battery cells are mismatched?

A mismatch in battery capacity can lead to significant inefficiency: even a 5% mismatch in cell capacity will result in 5% of the total energy in the pack being unutilized, which is particularly critical for remote systems and long-range electric vehicles.

6. Can active balancing really extend my EV’s driving range?

Yes, it can. Compared to passive methods, active balancing increases the total available charge in the battery pack by utilizing energy that would otherwise be “trapped” in weak cells, effectively extending the system’s operating time and range.

7. Is active balancing faster than passive balancing for charging?

Statistically, active balancing can shorten overall charging time because it actively pulls energy into lagging cells rather than just waiting for high cells to bleed off energy through heat, making the equalization process much more efficient.

8. Which is more cost-effective: active or passive balancing?

If your primary concern is upfront cost, passive balancing is the winner because it uses simpler circuit structures; however, active balancing offers better long-term ROI by reducing heat generation and slowing down battery degradation.

9. What are the main disadvantages of passive equalization?

The biggest drawback is energy waste; since passive balancing dissipates energy as heat, it does not extend system operating time and can lead to thermal management challenges in high-density battery packs.

10. When should I choose an active balancing BMS over a passive one?

You should opt for active balancing when dealing with high-capacity packs (like those in EVs or heavy equipment) or in remote systems where maintenance is difficult, as the 3x to 5x higher efficiency in energy redistribution justifies the complex hardware.

11. Does balancing happen only during charging?

While passive balancing typically only occurs during the final stages of charging, active battery equalization is capable of redistributing charge during both the charging and discharging phases, providing 24/7 cell protection.

12. How does active balancing reduce heat in a battery pack?

Unlike passive systems that generate heat through resistors, active balancing uses energy transfer; by moving energy instead of burning it, the thermal load on the pack is significantly reduced, which further preserves the lithium chemistry.

13. What happens if I don’t use any balancing technology?

Without balancing, the weakest cell in the pack dictates the performance of the entire system; once the weak cell hits the “empty” threshold, the BMS will shut down the whole pack, leaving a large portion of energy in other cells completely unused.

14. Can I use active balancing for heavy-duty forklifts or mining machines?

Absolutely. For high-voltage systems (300V–600V) used in heavy equipment, active balancing is the preferred choice because it handles the large capacity mismatches common in heavy-duty cycles more effectively than passive resistors.

15. What are the three steps to implement a balancing strategy in a custom battery pack?

To implement a strategy: first, monitor the individual cell voltages via a BMS; second, determine the SoC mismatch threshold (e.g., 50mV); and third, activate the balancing circuit—either by engaging resistors (passive) or triggering energy transfer (active)—until uniformity is restored.

16. Is active balancing better for the environment?

Compared to traditional passive methods, active balancing is more eco-friendly because it minimizes energy loss and extends the useful life of the battery pack, thereby reducing the frequency of battery replacements and recycling needs.

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