Skip the battery tester and use this quick and easy way to test if your batteries are dead: just drop them. This video, by electrical engineer Lee Hite, explains why dead batteries bounce.
On YouTube, he says:
This test works equally well for AA, AAA, C, D and 9 Volt alkaline batteries.
It is our understanding that the following chemical reactions occur and helps to explain our observations.
A non-rechargeable alkaline battery begins life using zinc powder mixed into a gel containing a potassium hydroxide electrolyte separated from a paste of manganese dioxide powder mixed with carbon powder using a porous membrane. To minimize hydrogen outgassing an extra measure of manganese dioxide is added. As the battery discharges manganese dioxide powder changes to manganese oxide causing the powdered granules to bond both chemically and physically. This packed-sand consistency reduces the antibounce effect exhibited by the gel mixture when the battery was fully charged.
Got that? No matter, the takeaway is you can easily tell if a battery is still good or not. All you need is a hard surface.