How does capacitive energy transfer work in active cell balancing?

Capacitive energy transfer in active cell balancing works by using capacitors to temporarily store energy from stronger cells (higher voltage) and then transfer that energy to weaker cells (lower voltage), achieving a more balanced state of charge across all cells in the battery pack. This method improves energy efficiency compared to passive balancing, which dissipates excess energy as heat. In a typical capacitive balancing system, a capacitor (or multiple capacitors) is selectively connected to different cells in the battery pack through a switching network, controlled by a Battery Management System (BMS). The process involves two main phases: charging the capacitor and discharging the capacitor. During the charging phase, the capacitor is connected to a stronger cell with a higher voltage. The capacitor charges up to a voltage close to that of the stronger cell, storing a certain amount of energy. The amount of energy stored depends on the capacitance value and the voltage difference between the capacitor and the cell. Once the capacitor is sufficiently charged, the switching network disconnects it from the stronger cell. In the discharging phase, the capacitor is connected to a weaker cell with a lower voltage. The capacitor then discharges its stored energy into the weaker cell, increasing its voltage and state of charge. The switching network ensures that the energy flows from the capacitor to the weaker cell until the voltage difference between them is minimized or the capacitor is fully discharged. The charging and discharging phases are repeated cyclically, transferring energy from the stronger cells to the weaker cells until all cells are balanced. The BMS monitors the voltage of each cell and controls the switching network to ensure that the energy is transferred efficiently and safely. There are different configurations for capacitive balancing, such as switched capacitor circuits and flying capacitor circuits. In a switched capacitor circuit, multiple capacitors are used to transfer energy between adjacent cells. In a flying capacitor circuit, a single capacitor is used to transfer energy between any two cells in the pack. Capacitive balancing offers several advantages, including relatively simple implementation and low cost compared to other active balancing methods. However, it also has some limitations, such as lower energy transfer efficiency and slower balancing speed compared to inductive balancing. The balancing speed depends on the capacitance value, the switching frequency, and the voltage difference between the cells. Overall, capacitive energy transfer provides an effective and energy-efficient way to balance cells in a battery pack, improving the pack's performance, lifespan, and safety.