PURPOSE: to enable the monitoring of the capacity of a battery simply by connection to a terminal with the indication of a virtual discharge curve on an indicator such as meter as obtained by a method wherein open voltage of an intermittently discharged battery is charged into a capacitor for memory at the stable peak part when it is at rest and discharged through an internal impedance to effect an impedance conversion.

CONSTITUTION: A capacitor 5 connected in parallel to a circuit 4 through a reverse current preventing element 6 is charged with a voltage equivalent to the stable peak part of the open voltage of a battery 1 obtained from the circuit 4 by subtracting a fixed voltage from the open voltage. This charge of the capacitor won't be discharged to the circuit 4. Current proportional to the voltage of the charge of the capacitor 5 flows with an amplifier 8 to operate an ammeter 9, which gives a reading proportional to the voltage of the capacitor 5. The charge of the caoacitor 5 is discharged through an impedance in the amplifier 8 and a resultant voltage loss is corrected by recharge with the open voltage when the discharge of the battery 1 is at rest subsequently. Thus, the charge of the capacitor always provides a voltage close to the open voltage of the battery 1 thereby allowing the detection of the capacity or the electrolyte specific gravity of the batterty 1.