Since most batteries don't come with built in meters to tell you when they are out of power it can be hard to tell when household batteries can be used or should be replaced without testing them first. Without a battery tester or multimeter, the only way to do this is by popping the battery or batteries into an electronic device and trying to turn it on. This is a time consuming process, though, and battery testers let you test single batteries quickly and accurately.
Battery testers work by testing the current that comes from a battery. When something conductive is touched to both the positive and negative contacts on the battery, the current is released. Many adventurous individuals test 9 volt batteries, using this principle, by sticking the contacts to their tongue, and anyone who has tried this knows that a charged 9 volt will offer a small jolt. A battery tester does the same thing, without the pain, by using a conductive medium to measure the charge (amperage) that comes from the battery. Each tester also has a gauge which reflects the level of the charge.
The most rudimentary battery testers, those found on battery packages, use a conductive ink that responds to minute temperature changes. If a battery has a charge, the ink heats up as the current passes through it. If the temperature is higher, the battery has more current. The gauge changes color based upon the temperature. Handheld battery testers and multimeters use resistors to measure the current, and, depending upon the tester, other components to move the gauge, which is usually a pin. As the resistor measures more amperage, the pin moves further on the gauge.
Battery testers only measure current. On many batteries, such as lithiums, the results of a reading will be inaccurate. This is because the current does not wear down evenly, as it does in an alkaline battery, but takes a significant drop at a certain charge levels. These batteries will read a fully charged even if they only have half a charge remaining. Testers also will not tell you if a rechargeable battery can be charged again. To test this, you'll need to use a battery conditioner or a battery tester that is specially designed for rechargeable batteries.