Application of battery power systems increases in the marine and offshore industry. Most applications target to reduce the energy consumptions when the battery power system is wholly or partially used. The hardware and software of the battery power system design for underwater application are described. The testing of the battery power system is successfully performed in a water tank at low temperature of 4 °C that mimics subsea operating condition where most remotely-operated vehicle (ROV) operates. The average variation of state-of-charge for the 12-cell is approximately 5 percent after active cell balancing. The battery management system is capable of estimating the state-of-charge and using the data to perform the active cell balancing on each cell with different imbalanced state-of-charge values of at least 30 percent.