Between the individual cells of a battery it can come to unacceptably large differences of the cell voltage under maintainance charge. In this case an equalizing or calibration charge is necessary to compensate the differences and to bring all cells back to a consistent voltage level. Other reasons for an equalisation charge may be a deep discharge, sulphation of the battery due to insufficient charging or temperature differences between the cells over a longer period of time. Deviations in electrolyte density or voltage values are also characteristics of a necessary equalizing charge. Such charging is carried out in accordance with the manufacturer's specifications and can be carried out using existing charging technology or with the an external charger.