A battery management system (BMS) monitors and reports on an operational state of a battery. In some embodiments the BMS can be part of a battery assembly. In at least one embodiment the BMS can monitor battery voltage, charge and discharge current, temperature and (via a measurement utilizing magnetism) chemical change in a battery. In some embodiments, a permanent magnet is placed within the battery and a magnetoresistance sensor on the battery management system circuit board is employed to measure the affected magnetic field from the chemical change within the battery as it moves from a discharged state to a charged state and back to a discharged state. This measurement provides for an accurate statement of the battery's present day state of charge and state of health.