A state of charge control system and method uses a region of voltage linearity and a long-term average voltage to estimate an average battery state of charge for storage batteries and to eliminate errors due to hysteresis, drift and sensor noncompliance. An average state of charge Sn is calculated using a current integral method and an average state of charge Sv based on an average observed voltage Vn for a time period exceeding a predefined minimum time but before a predefined maximum time has elapsed is also calculated. If the absolute value of the difference between the Sv and Sn are not are within an acceptable error band, then an offset value is calculated to correct the Sn by subtracting the Sv from the Sn, and thus, to maintain a more accurate Sn. Additionally, a closed current integral method is used to reduce errors due to hysteresis.