Despite the invention of new chemical sources of electricity, acid batteries, due to their price, are still widely used. They are also used on conventional submarines as the primary source of power. Due to their properties, lead-acid batteries require a lot of attention during their operation, including charging according to fairly rigorous rules. Because of their special operating conditions on submarines, it is not possible to maintain optimal conditions for such batteries. The battery’s integrated management system allows for remote and online monitoring. However, it gives only general information about how much energy remains in the battery for the use in operating conditions. The submarine’s operational capabilities, such as range and operation time, depend to a great extent on battery capacity, therefore, the information of its condition and status is very important. The proposed method of evaluating the capacity of lead-acid batteries allows them to be processed during their operation, without the need of a time-consuming and expensive capacity test. The authors proposed an original algorithm for evaluating the battery capacity, based on measurement data from a typical monitoring system, assembled with a battery on a ship. The measurements needed to implement the proposed algorithm are already implemented in the existing battery monitoring system – no additional equipment is required. Based on the results of the proposed method, it will be possible to easily determine how much energy is available for the submarine, what is crucial in planning combat missions at the sea.