Test batteries without a voltmeter

Nam Phan, Pasadena, CA -- EDN, 11/9/2000

The circuit in Figure 1 is an easy approach to testing batteries without exiting the voltmeter. The battery holders in sizes AAA, AA, C, and D make this tester so much faster than a voltmeter. You just put the battery into the holder and look at the circuit meter instead of getting the voltmeter out of the case, plugging in the probe, and turning on the meter. Holding the tips of the probes to the tips of the battery is clumsy.

The heart of this circuit comprises op amps that the circuit configures as comparators. When the voltages at the plus (noninverting) inputs are higher than the voltages at the minus (inverting) inputs, the op-amp outputs are equal to V_CC. When the plus inputs are lower than the minus inputs, the outputs are equal to V_DD. Every plus input connects to a potentiometer that controls the voltages going into the plus-input pin. The minus inputs all connect to battery holders.

You adjust the potentiometers in increments of 0.05V starting at 1.2V and ending at 1.55V. You can change this adjustment to increments of 0.2 or 0.3V, depending on how accurate you want the tester to be.

The output of each op amp connects to a 20-pin LED bar, which you place vertically to look like a meter. The circuit uses only eight of the LEDs. If the battery voltage is higher than 1.4V, the bottom five LEDs will light up because the minus input is greater than the plus input on the bottom five op amps. The top three LEDs do not light up because 1.4 is not higher than 1.45, 1.5, or 1.55.