kmagn13
A battery Testers work by using a chemical reaction to show about how much "Juice" it has left.

pathfinder
The pressure buttons on some alkaline batteries does not give you a true picture of how good the battery is. A good battery tester tests the battery by testing it under a load,by adding resistance to it. This tell you more accurately the condition of the battery under working conditions.

nachotrack
They use thermocromic and conductive ink. The first one changes colour with temperature and conductive ink can conduct electricity. Check this site

kirchwey
The conducting ink forms a resistor which does load the battery. The thermochromic strip lies along the resistor and responds to the heat it generates. The resistor is tapered, so its resistance per unit length increases as it gets narrower, and so does the temperature due to current flow. The point along the strip where the temperature reaches the color-change value depends on the amount of current flowing in the resistor. Thus the length of the changed color section indicates battery freshness. This is a very approximate test. A better tester loads the battery similarly, but measures the battery voltage under such loading.