The paper presents a new mathematical approach to the behaviour of lead-acid storage batteries in Stand Alone Photovoltaic Power Systems (SAPV). The approach takes into consideration different aspects affecting the battery discharge and charge behaviour, such as discharge and charge rate, battery capacity and current and power efficiency, among others. The modelling has been matched against experimental results taken out from an operational SAPV. The matching index of the modelling is of excellence, 99.9% on average. (C) 2003 Elsevier Science Ltd. All rights reserved.