Battery Discharge Measurement Circuit

The Battery Discharge Measurement Circuit is an analog system designed to track how long a battery can sustain a voltage above a specific cutoff point. At the heart of the circuit is the MAX921 comparator, which continuously monitors the voltage level of the battery under test. A potentiometer is configured to supply a precise, adjustable reference voltage to one of the comparator's inputs, serving as the user-defined threshold limit. The battery being tested feeds the other input of the comparator. As long as the battery voltage remains above the preset threshold, the comparator's output keeps the VN0300L MOSFET (IRF1) in a conducting state. While the MOSFET is turned on, it completes the power path for an ordinary analog clock, which runs on its own separate 1.5V AA-size battery. When the battery under test gradually discharges and dips below the comparator's reference threshold, the comparator output flips state. This transition immediately turns off the MOSFET, breaking the circuit path and stopping the analog clock. Because the clock stops the exact moment the cutoff voltage is reached, the elapsed time displayed on the clock face directly corresponds to the discharge duration of the battery. The entire discrete analog build is assembled on a single-side PCB and housed inside a compact plastic enclosure for safe benchtop testing.