This article presents an original method to accurately predict the end of discharge of rechargeable batteries inserted in portable electronic equipments. The proposed method is based on two neural networks organized in a master-slave relation. A prediction accuracy of 3% (18 minutes) is reached. A further improvement of the system is introduced by adapting on-line another neural network to the actual battery currently in use. This adaptive method reduces the average error to 10 minutes. Results are promising and implementation, carried out in a portable multimeter prototype, only requires a small amount of the computing power already available inside most portable equipments.