Voltage Monitoring- Over-voltage
One of the most straightforward aspects of a BMS system is monitoring the lithium-ion cell voltage, which must be kept within an upper limit (typically around 4.2 volts) and a lower limit (typically 2.0 volts) to prevent permanent damage to the cell.
During charging, if the voltage increases beyond the recommended upper cell voltage, typically 4.2 Volts, an excessive amount of current can flow through the battery cell. The excess current promotes the deposition of metallic lithium onto the surface of the graphite anode. This reduces the number of free lithium ions available for reaction and so can cause an irreversible reduction of cell capacity. The plating of the lithium can also form spikey, needle-like crystals, called dendrites, that can grow large enough to reach the cathode and short out the cell, potentially causing a fire. Excessive current also causes increased heating of the cell, accompanied by an increase in the temperature of the pack.