What is cell balancing in a BMS and why is it important

Cell balancing refers to the process of equalizing the charge across all cells in an electric vehicle (EV) battery pack, ensuring each cell charges and discharges at the same rate. The process is beneficial in a battery management system (BMS) to enhance the availability of a battery pack with multiple cells and increase each cell’s longevity and safety.

Cell balancing is not limited to EV battery packs; it applies to any electrical system, such as renewable energy, where a battery pack with several individual connected batteries is used. Figure 1 illustrates the two methods by which cell balancing is achieved in the context of EV. One way is called passive cell balancing, while the other is called active cell balancing.

Figure 1. An illustration of how passive and active cell balancing perform. (Image: Elsevier)

Passive cell balancing occurs when a cell’s voltage exceeds a certain threshold, and the BMS activates a resistor to dissipate the excess energy. This process continues until the cell’s voltage matches the other cells in the pack. The advantage of this method is that it’s cost-effective and straightforward.

However, any electrical engineer can quickly deduce that it leads to energy waste and reduces overall efficiency. Therefore, this method is useful when the degree of imbalances in the cell (SOC or voltage) is minimal. It’s ideal for low-power applications within EVs.

Active cell balancing uses various methods, such as capacitors, inductors, or transformers, to redistribute energy among cells. Energy is actively moved from cells with higher voltage to those with lower voltage, maintaining balance without wasting energy as heat. As no resistors are used to dissipate the heat, this method is more efficient than passive cell balancing. Active cell balancing is complex and costly to implement and requires more components, which also makes the system bulky.

Figure 2 shows a list of different passive and active cell balancing methods. The number of ways passive cell balancing can be implemented is much lower than its counterpart.

Figure 2. Different cell balancing topologies based on passive and active cell balancing. (Image: Energies, MDPI)

Why is cell balancing important in EVs?

Before discussing the benefits of cell balancing in EVs, a closer look at Figure 3 makes it obvious what happens when cell balancing is not implemented. There are two cases to observe: one is during battery discharging, and the other is during battery charging.

Figure 3. An illustration showing what happens when cell balancing is ignored. (Image: World Electric Vehicle Journal, MDPI)

When an unbalanced battery pack discharges, at one point in time, one of the cells in the battery pack with the least SOC will dry up to 0% first. However, the other cells have a residual SOC still left to discharge. In this case, the battery pack as a unit is still considered to be left with some charge for discharging, and the cells continue to discharge despite one of them already being at 0%. This is practically impossible, and therefore, it may lead to severe physical damage to the battery pack due to a single cell.

In the other case, when the unbalanced battery pack is charging, one of the cells with the highest SOC will reach 100% first. The rest of the cells are still charging as the battery pack is still not fully charged. Therefore, it pressures the cell with the 100% SOC, leading to fire and safety concerns.

Now that the root cause of not implementing cell balancing in an EV battery pack is clear, let’s look at its benefits.

Maximizing battery capacity: cell balancing ensures that all cells in the battery pack are charged and discharged uniformly. Without balancing, some cells may become overcharged while others remain undercharged. This imbalance can prevent the battery pack from reaching its full capacity, as the overall performance is limited by the weakest cell.
Improved battery life: imbalanced cells can lead to overcharging or over-discharging of individual cells, which accelerates the aging process and reduces the battery’s overall lifespan. Overcharged cells can suffer from thermal runaway, while undercharged cells can experience capacity loss. Cell balancing mitigates these risks by ensuring that no cell is subjected to extreme conditions, extending the battery pack’s life.
Increased charging efficiency: cell balancing allows for more efficient charging by ensuring all cells simultaneously reach their full charge. This reduces the time required for charging and ensures that the energy is distributed evenly across all cells. Efficient charging saves time and reduces energy losses, contributing to the overall efficiency of the battery pack.
Improved performance consistency: balanced cells ensure the battery pack delivers consistent power and performance. Imbalances can cause fluctuations in power output, leading to inefficiencies and damage to the connected systems. The BMS ensures a stable and reliable power supply by keeping the cells balanced.

Summary

EV driving range anxiety has been a pain point for EV drivers and users. The main reason is attributed to batteries, which deplete fast when driving. Therefore, many consider increasing the number of batteries and using high-energy density batteries such as lithium-ion. However, an often overlooked aspect is cell balancing, which can give notable results and help reduce range anxiety.

Active cell balancing is preferred over passive cell balancing in EVs because the magnitude of power loss with passive cell balancing is bothersome and unacceptable. Active cell balancing is complex and makes circuits costly and bulky, but the advantage of increased efficiency outweighs the disadvantages.