- Electronic battery testers usually have one or two leads. Testers with a single lead usually have metallic studs for specific types of batteries. Either the positive or negative pole is pressed on the stud while the lead is pressed to the other pole. Electronic battery testers with two leads have a red lead and a black lead. The red lead is pressed to the positive pole of the battery and the black lead is pressed to the negative.
- In battery testers with studs and a single lead, a capacitor calibrated to each type of battery supported by the tester creates a simulated load. Electronic circuitry in the tester calculates how long the battery would last under the simulated load. Battery testers with two leads often have a rotary switch to select the type of battery being tested. The switch selects the appropriate capacitor and load for the particular type of battery being tested.
- When the battery's current is tested under an appropriate load, the battery tester calculates the percentage of time under the specified load that the battery would last. That number is compared to an ideal maximum amount of time the type of battery should last, and the result is shown as a percentage of charge remaining in the battery. Some battery testers show this as a percentage, others show the remaining charge using green for near full capacity, yellow for partially discharged, and red for nearly fully discharged.
- Different batteries discharge at different rates. Different batteries are expected to be operated under differing loads. A good modern electronic battery testers compensate for differences in discharge times and patterns. For example, a AA lithium battery will discharge in a different way than a AA Nickel-Cadmium (NiCd) battery. Modern electronic battery testers either automatically sense the type of battery or have separate settings for the various battery technologies available in a particular standard battery size.
