A battery tester is a device used to measure the voltage and overall health of batteries. It helps determine whether a battery is still functional or needs to be replaced. Battery testers can be used for various types of batteries, including alkaline, lithium-ion, and nickel-cadmium. Using a battery tester is straightforward. The user connects the tester's probes to the battery terminals, and the device displays the voltage level. Some testers also provide additional information, such as the battery's remaining capacity or charge status, making it easier to assess the battery's performance and longevity.