A empirically based mathematical model of a lead-acid battery for use in the Texas A&M University's Electrically Peaking Hybrid (ELPH) computer simulation is presented. The battery model is intended to overcome intuitive difficulties with currently available models by employing direct relationships between state-of-charge, voltage, and power demand. The model input is the power demand or load. Model outputs include voltage, an instantaneous battery efficiency coefficient and a state-of-charge indicator. A time and current dependent voltage hysteresis is employed to ensure correct voltage tracking inherent with the highly transient nature of a hybrid electric drivetrain.