Cell balancing in a Battery Management System (BMS) is the process of equalizing the voltages among individual battery cells within a battery pack. It is vital in applications that require the connection of many battery cells either in series or parallel configuration.

Need for Cell Balancing

• Equalizing cell voltages and state of charge (SOC): The inherent differences and inconsistencies among individual cells within a battery pack create the need for battery balancing. These disparities can arise from manufacturing variations, aging, temperature effects, or differing load conditions. Cell balancing ensures that all cells are maintained at the same SOC. 
• Preventing overcharging or over-discharging of individual cells: Charging must cease when any cell reaches its maximum voltage and discharging must halt when any cell depletes its charge, even if other cells still retain charge. Cell balancing addresses this issue by equalizing the voltages, ensuring that all cells are charged and discharged evenly.
• Ensuring safety and preventing thermal runaway: Imbalanced cells can trigger thermal runaway and result in catastrophic failures. Therefore, cell balancing is essential to keep the battery within its safe operating range and prevent these incidents.
• Maintaining pack capacity at the average of all cells: Active cell balancing techniques ensure that the overall battery capacity reflects the average capacity of all cells, rather than being restricted by the weakest cell.

Types of Cell Balancing: There are two types of cell balancing techniques to equalize the charge among individual cells of the battery pack.

• Passive Cell Balancing: Passive cell balancing uses passive elements such as resistors or capacitors to release excess energy in cells with higher SoC. The balancing helps in bringing the cells with higher charge levels in line with other cells having lower SoC.
• The energy from higher energy cells is drained using a resistor connected in parallel to each cell and the drained energy is lost in the form of heat. Alternatively, shunt capacitors are used to temporarily store the energy from higher energy cells on a temporary basis. This energy is later dissipated through a resistor.
• Active Cell Balancing: Active cell balancing uses active elements to redistribute the energy of higher energy cells to lower energy cells, thereby balancing the energy of all the cells in the battery pack. It uses capacitors, inductors or DC-DC converters to distribute the charge among the cells. This technique results in a battery pack SoC that is equal to the average of all individual cell SoC. Active cell balancing provides increased efficiency though it comes with complex architecture and additional electronic cost.

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