How to Test a Battery Under Load

There can be a difference when measuring a battery voltage when the battery is under load and not under load. When a battery is under load, it is connected to the circuit in which it is intended to be used and the circuit or device is turned on. For example, a flashlight battery is under load when it is installed in a flashlight and the flashlight is turned on. For devices such as smoke detectors or cell phones, which don't draw much current from the battery, the battery voltage can typically be accurately measured when the battery is not under load. For larger batteries in which the current draw can be higher, such as car batteries, the battery voltage can drop dramatically when it is under load.

Things You'll Need

• Voltmeter
• Voltmeter probes
• Jumper wires

Instructions

1. Connect the voltage probes to the voltmeter and turn it on. Most voltmeters have automatic range finders, so you don't have to select a range. If your voltmeter doesn't have an automatic range finder, select a range in which the listed voltage of your battery falls inside.

2. Connect the ends of the voltmeter probes to the terminals of your battery. If the batteries are difficult to access while in the device, such as in a flashlight, connect one jumper wire to each end of the battery and run the jumper wires outside the device, where you can connect the probes to the ends of the wires. If the batteries are in a battery pack, you can connect the probes to the wires leading from the battery pack.

3. Turn on the voltmeter and note the voltage shown on the voltmeter display. Batteries supply direct current (DC) voltages, so the number displayed will be a measurement in units of DC volts.

4. Turn on the device while monitoring the voltmeter display. For a flashlight, simply turn it on. If it's a smoke alarm, press the "Test" button, which will sound the alarm and draw more current from the battery. If the battery is weak, the voltage may drop. If you're testing a car battery, turn the ignition switch on to draw current from the battery.

5. Record the voltage when your device is under load. If it matches the voltage when the device is not under load, the battery will likely be good. If the voltage drops, the battery could be defective.

Tips & Warnings

• Store batteries in dry locations to help preserve their capacity to provide a DC voltage.

• Old batteries, especially automobile batteries, can create toxic chemicals around the battery terminals. Use gloves when handling the batteries and wash your hands afterward.

Resources

• Smoke Alarms