The only able to test the rechargeable batteries!

Alkaline batteries have much voltage drop that with the discharge, as rechargeable batteries have an almost constant voltage before a rapid fall (and here it's too late!). Thus conventional voltage testers evaluate without asking the battery. They walk correctly alkaline battery but can not say what is a rechargeable battery nicd, nimh, lion or lipo.
This one though! Why? Simply because it stresses the battery / cell for 2 seconds before testing the tension. It then displays an evaluation of the remaining percentage (0-10-20 ... 100%) and the battery voltage.
As a AAA battery nimh worn displaying at 1.32V rest will be perceived as "good" for conventional testers then the tester Ansmann found in the 0.98V after 2 seconds and effectively discharge the battery is recharged urgently!