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The discharge reserve time of a valve regulated lead acid (VRLA) battery is dependent on both discharge operating conditions as well as battery condition. Operating conditions include discharge type and rate, ambient temperature and initial state of charge while battery conditions include battery state of health and battery type. For this reason determining the discharge reserve time can be a very complex problem. This paper presents a simple approach for estimating VRLA battery state of charge (SOC) and thus discharge reserve time during discharge over a wide range of operating and battery conditions. A discharge characteristic, referred to as the unified characteristic, is employed that is shown to be robust to variations in operating conditions as well as battery condition. Furthermore, the resulting accurate estimations of SOC (within 10%) and reserve time (within 10% from the early stages of the discharge) do not come at the cost of complexity.