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Battery Testers are available in two versions:
Active testers apply a small load to a battery while testing to test a battery "in use". This is more accurate, but requires more advanced electronics, as well as a power source, making the tester more expensive.
Passive testers simply take a voltage measurement of any battery inserted. They cost a lot less, and can still be used for a reasonably accurate reading if you test immediately after removing the battery from use.