What is the purpose of cell balancing in a battery pack?

The purpose of cell balancing in a battery pack is to equalize the State of Charge (SOC) among all individual cells in the pack, ensuring that each cell operates within its safe and optimal operating range. Battery packs consist of multiple cells connected in series and parallel to achieve the desired voltage and capacity. Due to manufacturing variations, temperature gradients, and aging effects, individual cells in a pack can have slightly different capacities, internal resistances, and self-discharge rates. These differences can lead to imbalances in SOC, where some cells become overcharged while others become undercharged during charging and discharging cycles. If left unaddressed, these imbalances can cause several problems. Overcharged cells can experience accelerated degradation and may even undergo thermal runaway, while undercharged cells can experience accelerated degradation due to deep discharge. Cell balancing redistributes charge among the cells, preventing overcharging and over-discharging, thereby maximizing the pack's overall capacity and lifespan. By ensuring that all cells are utilized effectively, cell balancing also improves the pack's energy efficiency. There are two main types of cell balancing techniques: passive balancing and active balancing. Passive balancing dissipates excess energy from the stronger cells through resistors, while active balancing transfers charge from the stronger cells to the weaker cells. Active balancing is more efficient but also more complex and costly. Effective cell balancing is essential for maximizing the performance, lifespan, and safety of battery packs in energy storage systems.