1. Use a multimeter
Set a digital multimeter to DC volts. Touch the red probe to the positive end and the black probe to the negative end. A fresh coin cell reads about 3 volts and a fresh AA about 1.5 volts.
2. When to replace it
Replace a coin cell that reads below about 2.8 volts, or an AA below about 1.2 volts. These are the same limits as in the guide on shelf life.
3. A good reading can still fail
A battery can read well with nothing connected, then drop sharply once a device draws current. A weak cell may still read 1.4 volts and then fail in a torch.
4. Check the contacts too
Before you blame the battery, check it’s the right way round and that the contacts are clean. Corroded contacts can stop a fresh battery from working.
5. Know the limits
A cheap tester gives a rough answer. For a battery that runs something important, such as a smoke alarm or a car key, replacing it on a regular schedule is safer than relying on a test.