Charge monitor for electric battery

TLDR: In this paper, an electric battery is continuously tested for its state of charge by a bridge circuit in parallel with a voltage stabilizer such as a Zener diode which forms part of a voltage divider connected across the battery terminals, the other part of that divider being a resistance circuit.

Abstract: An electric battery, such as that of an automotive vehicle, is continuously tested for its state of charge by a bridge circuit in parallel with a voltage stabilizer such as a Zener diode which forms part of a voltage divider connected across the battery terminals, the other part of that divider being a resistance circuit. The bridge circuit has an output diagonal with one corner connected to the battery terminal which is tied to the resistance circuit so that a reduction of battery voltage unbalances the bridge in one sense, a compensatory change in the opposite sense being brought about by a sensor responsive to the load current drawn from the battery. The current sensor may be a small resistor inserted between two adjoining bridge arms. An integrating operational amplifier connected across the output diagonal of the bridge controls a pulse generator which steps an associated pulse counter whenever the amplifier output passes a threshold indicating a predetermined degree of depletion of the battery; the counter, after measuring a cumulative time interval during which this depletion condition is present, triggers a switch which generates an alarm signal, e.g. by eliminating certain options in the operation of the vehicle. The operational amplifier and/or pulse generator also work into visual charge indicators. The resistance circuit of the voltage divider may include a transistor designed to compensate changes in battery current due to the de-energization of a switching relay. A feedback connection between the counter and the pulse generator causes continuing stepping of the counter after the switchover to maintain the alarm signal until the counter resets itself after an extended period, such resetting also occurring upon reclosure of the load circuit after a prolonged interruption.