Guanay II is an autonomous underwater vehicle (AUV) designed to perform measurements in a water column. In this paper the aspects of the vehicle’s power system are presented with particular focus on the power elements and the state of charge of the batteries. The system performs both measurement and monitoring tasks and also controls the state of charge (SoC) of the batteries. It allows simultaneous charging of all batteries from outside the vehicle and has a wireless connection/disconnection mode. Guanay II uses a NiCd battery and for this reason the current integration as a SoC methodology has been selected. Moreover, it has been validated that it is possible to obtain instant consumption from the SoC circuit. Finally, laboratory and vehicle navigation tests have been performed to validate the correct operation of the systems and the reliability of the measured data