Abstract: An apparatus and methods for determining dynamic state-of-charge of a battery that is subject to periodic charging and discharging within a system. Correction factors &eegr; are determined for various operational conditions of current, temperature and state of charge which relate relative discharge and charge efficiency with a reference set of conditions. According to conditions, the correction factor, either calculated in real-time or retrieved as a stored value, is then applied periodically to the charge and discharge currents of the battery to adjust an estimated state-of-charge value (SOC) for the battery within the given system.