A fluid consuming battery is provided with a fluid regulating system for regulating fluid entry into the battery. Process variables ) from the adjacent cyclically determined process variables, s_) are determined as being event-related as a function of the time ) of the event within the cycle. A spectral transmission T of light transmitted by the light-absorbing medium changes from T0. The method of virtual reacquisition renders unnecessary the physical reacquisition of the analog data in case the digital data obtained by the acquisition device are of insufficient quality.