What is a battery management system (BMS)? It is a system that uses circuitry to monitor battery life and catastrophic conditions. The BMS circuitry can also extend the on time and lifetime of a battery. A complete lithium-ion battery system consists of a battery, BMS circuitry, a load and a source, usually a charger. It is important to know the load and the source voltages and currents when designing the system. The BMS circuitry consists of protection/monitors, balancing circuitry and system management. The protection/monitors include cutoff FETs or drivers to disconnect the battery from the load or the charger, current sense circuitry to monitor battery usage and over current conditions, external temperature sensors to monitor cell temperature, voltage cell monitors to determine the state of charge of each cell, and regulators and references to power up peripheral circuitry. The balancing circuitry consists of the cell balancing FET or cell balancing drivers for larger current bearing battery packs. The cell balancing circuitry extends the life of a battery pack and the on time of the battery per charge. The system management block consists of an external MCU or internal state machine that maintains the battery in an optimal state for the given application. It makes decisions based on circuit measurements, has analog and digital comparators to acknowledge catastrophic alert conditions such as cell voltage being too large or the load current being too large, and memory for recording diagnostic data (such as what happened before the battery went bad).