Active and Passive Battery Balancing Mechanisms: Working Principles and Comparative Analysis

The rapid development of new energy vehicles is inseparable from an efficient and reliable battery management system (BMS). As the core component of electric vehicles, the performance of the battery pack directly affects the operating efficiency and safety of the vehicle. As one of the important functions of BMS, battery balancing is designed to ensure that the state of charge (SOC) and voltage of each single cell in the battery pack are maintained at similar levels to improve the overall performance of the battery pack, extend its service life and ensure safety. This article will explore in detail the working principles of active and passive battery balancing mechanisms and their advantages and disadvantages.

Passive cell balancing mechanism

Passive battery balancing is a relatively simple and low-cost balancing method that mainly dissipates excess energy through external resistors or discharge devices. When the voltage difference between battery cells exceeds a certain threshold, the passive balancing system will be activated and the excess energy will be converted into heat energy through resistors or loads, thereby adjusting the voltage level of the battery cells to a similar state.

How it works

Passive balancing is usually achieved using switches and bleeder resistors in parallel with each cell. During the charging cycle, when the voltage of a cell is higher than the others, the BMS activates the switch to release the excess energy as heat through the bleeder resistor. This process continues until the voltage of all cells is close to the same. Passive balancing relies mainly on resistor discharge to release the charge in the higher voltage cell as heat, thus creating more charging time for the other cells.

Pros and Cons

advantage:

Low cost: The design and implementation of the passive balancing system are relatively simple and low cost.

Simple circuit design: no complex control system required, easy to implement and maintain.

Correction of self-discharge mismatch: It can correct the self-discharge current mismatch between different battery cells over a long period of time.

shortcoming:

Energy waste: The presence of discharge resistance causes energy to be released in the form of heat, which is completely wasted.

Heat generation: The heat released during the balancing process increases the thermal burden on the system and may affect battery life.

System power is limited: The power of the entire system is limited by the battery with the smallest capacity, resulting in power loss.

Active cell balancing mechanism

Active cell balancing is a more complex balancing technology that achieves balancing by controlling the charge and discharge between battery cells. This requires an intelligent battery management system (BMS) that can monitor and control the charge and discharge process of each battery cell to ensure that the SOC and voltage between them remain within the appropriate range.

How it works

The active balancing mechanism achieves balancing by transferring power. During the charge and discharge cycle, the BMS monitors the SOC and voltage of each battery cell, and when the difference exceeds the set threshold, the balancing strategy is initiated. This usually involves redistributing power from battery cells with more remaining power (strong batteries) to battery cells with less remaining power (weak batteries), thereby ensuring that the energy in the battery pack is fully utilized.

There are many ways to implement active balancing, including but not limited to:

Capacitive balancing: Use capacitors as intermediate media for energy transfer, and transfer power from strong batteries to weak batteries through switch control.

Transformer balancing: uses the transformer principle to transfer power from high-voltage batteries to low-voltage batteries.

Battery-to-battery direct balancing: Direct charging and discharging between battery cells is achieved through intelligent BMS control.

Pros and Cons

advantage:

High energy utilization: Active balancing does not waste energy, but redistributes energy to other battery cells in the battery pack.

Extended system run time: Active balancing can extend the run time of the battery pack by optimizing energy distribution.

Improve charging efficiency: Active balancing can shorten charging time and improve charging efficiency.

shortcoming:

Complex system: requires more complex control systems and hardware support, which is difficult to design and implement.

Higher cost: Due to the need for intelligent BMS and additional balancing hardware, the cost is relatively high.

Balancing time: Although active balancing is more efficient, the balancing process may take some time, especially when the battery pack capacity is large.

Comparative Analysis

Application scenarios: Passive balancing is more suitable for small-capacity, low-string lithium battery pack applications, while active balancing is more suitable for high-string, large-capacity power lithium battery pack applications.

Energy efficiency: Active balancing is significantly better than passive balancing in terms of energy utilization, which can reduce energy waste and improve the overall performance of the battery pack.

System complexity and cost: Active balancing requires a more complex control system and hardware support, so it is more expensive and more difficult to design and implement. Passive balancing is relatively simple and less expensive.

Balancing effect: Active balancing can more accurately control the SOC and voltage differences between battery cells, achieving a more efficient balancing effect. Passive balancing, on the other hand, may limit the balancing effect due to energy waste and heat generation.

in conclusion

As an important part of the battery management system of new energy vehicles, battery balancing is of great significance for improving the performance of battery packs, extending service life and ensuring safety. Active and passive battery balancing mechanisms have their own advantages and disadvantages and should be selected according to specific application scenarios and needs. With the continuous development of new energy vehicle technology, more efficient and intelligent battery balancing technology may appear in the future, providing strong support for the development of new energy vehicles.