if its truly a DC generator and regulator.....the vibrating points of the voltage regulation coil will be producing spikes read by the digital meter......So,if the battery stays charged ,and doesnt boil over,then its best to leave well alone...If its a AC system,then lights may work,but the battery will not charge,and generally boil dry ...........Also ,no system will give meaningful readings if the battery is sulfated/flat/dry.,although the lights will usually work.