Why do lithium batteries need battery balancing?

In today's context of increasing awareness of environmental protection, lithium batteries, as an efficient and reliable energy storage device, are gradually becoming the mainstream choice. However, lithium batteries may have problems such as capacity mismatch and excessive voltage difference during long-term use, which requires the application of battery balancing technology. This article will explore why lithium batteries need cell balancing, and explain its importance and how to achieve it.

Lithium batteries pass through battery balancing, and each unit in the battery pack can be effectively monitored and maintained in a healthy state of charge (State of Charge, SoC). This not only increases the number of battery cycles, but also provides additional protection against damage to the battery cells due to overcharging/deep discharging.

Active and passive equalization

Passive equalization consumes excess charge through bleeder resistors, so that all battery cells have approximately the same SoC, but it does not prolong the running time of the system. Usually, the equalization that uses resistors to dissipate energy is called passive equalization.

Active balancing is a more sophisticated balancing technique that increases system runtime by increasing the total charge available in the pack as charge is redistributed within the cells during charge and discharge cycles. Compared with passive equalization, active equalization can shorten the charging time and reduce the heat generated during equalization. Usually, the equalization achieved through capacity transfer is called active equalization.

Active Cell Balanced Discharge

As shown in the figure below, it is a typical battery pack at full capacity. Full capacity means that the charge capacity reaches 90%, because keeping the battery at (or close to) 100% capacity for a long time will reduce its service life soon. And full discharge refers to discharge to 30%, which prevents the battery from entering a deep discharge state.
Over time, the characteristics of some batteries become worse than others. Even if some battery cells still have a lot of capacity left, weak battery cells limit the system run time, 5% of the battery capacity mismatch will cause 5% of the energy to be ineffective. For large-capacity batteries, it means that a lot of energy is wasted. This situation is particularly critical for remote systems and systems that are not easy to maintain. Unused energy also leads to increased battery charge and discharge cycles, reduced battery life, and higher costs due to frequent battery replacement.

With active balancing, charge is redistributed from strong cells to weak cells, completely draining the battery pack of energy.

Active Cell Balanced Charging

If the pack is charged without equalization, the weak cells will reach full capacity before the strong cells, again becoming the limiting factor; at this point, they limit the total energy that can be held in the system.

Active equalization can make the battery pack reach full capacity by redistributing the charge during charging. The proportion of equalization time and the impact of equalization current on time will not be discussed in depth here.

Advantages and disadvantages of active and passive equalization

Active battery balancing and passive battery balancing can effectively promote the health of the battery system by monitoring and matching the state of charge of each battery cell. Unlike passive battery balancing, which only consumes excess charge during charging, active battery balancing can be used during charging and discharging. charge redistribution during this period. Therefore, active cell balancing can prolong system runtime and also improve charging efficiency. Active cell balancing requires solutions that are often more complex and larger in size, while passive cell balancing is more cost-effective.

The application of battery balancing technology in the field of lithium batteries has become an important means to improve the efficiency of energy storage systems and prolong battery life. By monitoring and adjusting the status of each single battery in the battery pack, the battery equalization system can make the overall performance of the lithium battery pack more stable and reliable. With the advancement of science and technology, we can expect the emergence of more advanced battery equalization technology to provide better support for the development of lithium battery energy storage systems.