The Basics of a Battery Management System
The a battery pack’s Battery Management System (BMS) is a circuit board that is wired in between the battery cells and the battery pack leads. The BMS serves multiple functions. It will disconnect the cells from the leads if one of the following conditions is detected 1) over-temperature, 2) excessively high voltage, or 3) excessively low voltage. The BMS also prevents excessively high charging or discharging currents.
Finally, the BMS acts as a distributor, that actively controls how much current is allowed to enter each cell, or group of cells in parallel. This is called cell balancing. The primary purpose of cell balancing is to maximize the lifetime of the pack by making sure that small variations in cell capacity are not amplified over time. For example, if one cell has a slightly lower capacity, it will complete its charging cycle before neighboring cells. When this happens, the BMS can switch off the charging current to that cell, while continuing to charge the neighboring cells to full capacity. In this way, the pack maintains full capacity over its lifetime.