Abstract: The apparatus and method of the present invention measure the remaining capacity of a battery connected to a fluctuating load. The apparatus senses dispersive terminal voltages and dispersive discharge currents from the battery, and samples the sensed terminal voltages and discharge currents. On the basis of the sampled terminal voltages and discharge currents, the apparatus calculates a watt-hour value of the battery for a predetermined time interval, and checks whether the calculated watt-hour value reaches a predetermined value. If the calculated watt-hour value reaches the predetermined value, the apparatus calculates an approximate linear function indicative of a trend of the sampled terminal voltages and discharge currents, and calculates a voltage corresponding to a reference discharge current in accordance with the calculated approximate linear function as a remaining capacity voltage.

Abstract: A sensor contactlessly driven by a single power supply is disclosed. A current flows in a resistor R.sub.L through the secondary winding of a coil in the direction corresponding to the direction of magnetic fluxes generated in the coil in accordance with discharge or charge mode of a main battery. The input terminal of a voltage follower is connected to the voltage-dividing point of a series circuit including resistors R.sub.1 and R.sub.2 and a reference voltage control circuit. The reference voltage control circuit changes the input voltage of the voltage follower in accordance with the direction of the current flowing in the resistor R.sub.L. It follows therefore that the measurement range is switched in accordance with discharge or charge mode of the main battery, with the result that the overall measurement range can be widened in spite of the fact that only a single power supply +Vcc is used.