The simplest way is measuring the specific density or gravity of the liquid container in the accumulator (electrolyte). The density expresses the weight of the electrolyte in comparison with the same amount of water and it is measured with a densimeter o hydrometer.

The most common densimeter is used for the automotive industry and it indicates the charge in percentage. It has the inconvenient that it is calibrated for the electrolyte used in non-stationary starting accumulators, so it will always display less of the real (50% for a fully loaded stationary accumulator). Typical density and voltage per vessel in a lead-acid accumulator, the higher is the specific gravity of the electrolyte, the higher will be the state of the load. The voltage of each vessel and therefore of the battery is also higher.

The measuring of the density during the unload process will give a good indication of the state of the loading. During the loading process, the density will delay the measuring of the state of the load because the complete mixture of the electrolyte is not produced until the beginning of the gasification, near the end of the loading period. In any case, this must not be considered as an absolute measure of the battery’s capacity and must be combined with other techniques.