The handy pocket-sized battery tester operates without a battery or any external power source. This means it is always ready for use, without the need to worry about power connections or fully charged batteries. It reliably and quickly tests alkaline batteries, NiMH, and NiCd rechargeable batteries in sizes Micro AAA, Mignon AA, Baby C, Mono D, 9V block E, and button cells. The condition is displayed on a scale.