I've got these & they do exactly what they are supposed to.
from the website;
2 QUALITY BATTERY TESTERS FOR JUST £2.99 - Just the job for quickly checking the condition/state of your batteries. Every House should have one (or even two perhaps!). Quickly shows you the remaining power on the easy to read scale. Suitable for testing the very popular AA, AAA, C, D and also 9 volt (block) sizes as well as smaller button cells. Small matchbox size widget to save you hassle and uncertainty - easily stored in a camera bag pocket etc.
Measures 5x5x2.3cm and weighs in at just 36 grams.