Determining the capacity and discharge reserve time of a valve regulated lead acid battery is a highly non-trivial task. This is due to the dependence of the capacity and discharge reserve time on the discharge operating conditions as well as the battery condition. Operating conditions include discharge rate, ambient temperature and initial state of charge (SOC), while battery conditions include battery state of health and battery type. A simple approach is presented for estimating the SOC and discharge reserve time throughout a discharge. In addition, an estimation of capacity can be made at the completion of a shallow discharge. The approach employs a unified discharge characteristic that is shown to be robust to variations in operating conditions as well as battery condition. The approach provides a good degree of accuracy without the cost of complexity. Results are presented that demonstrate the estimation of the SOC to be well within 10% throughout the discharge and the reserve time (and capacity) to be within 10% from the early stages of the discharge. (C) 2003 Elsevier Ltd. All rights reserved.