Abstract: A method is provided for determining a state-of-charge of a battery for a vehicle. The vehicle is in a charging state when the engine is operating and a non-charging state when the engine is not operating. A first battery voltage is measured at a first predetermined time period after battery charging is discontinued in the non-charging state. A first temperature of the battery is measured that coincides with the first battery voltage. A second battery voltage is measured at a second predetermined time. The second predetermined time is greater than the first predetermined time. A second temperature of the battery is measured that coincides with the second battery voltage. An average temperature is calculated based on the first temperature measurement and the second temperature measurement. A fixed time constant is determined based on the average temperature. An open circuit voltage is estimated as a function of the first voltage measurement, the second voltage measurement, and the fixed time constant.