it's like 50% to 60% percent.
The tester basically does what it is supposed to do and that is to show the battery value. The question is whether it's not too little for about five kilos. This is more of an indicative measurement of the batteries in terms of whether the battery is no longer to be discarded.
Regarding accuracy - I'm not so sure about the exact values. I just did a quick measurement at home - no laboratory conditions - measured with EMOS and Voltcraft multimeters.
AA batteries - Varta - EMOS multimeter 1,35V, Voltcraft multimeter 1,35V, this tester 1,33V
AA batteries-Panasonic-Mult. EMOS 1.35, Multim. Voltcraft 1,35, this tester MS-229 1,30V
AAA batteries - Varta - EMOS multimeter 1,48, Voltcraft multimeter 1,49, this tester MS229 1,45V
We are on tenths in total, hundredths is worse, for five hundred I might expect more accuracy (resp. ok both multimeters can be bad). Otherwise, other minus points have already been written here by other customers. The fact is that it also switches off when measuring, and that's a bitch. Like, when you measure 10 batteries and it goes out every third time, it's a pain in the ass. Also that after each switching off you have to choose the measuring range again (whether it is 1.2V or 1.5V and others). Resp. every time you use on/off. If you have it turned on it will only darken the display, but the voltage value will remain. When you put OFF/ ON you set it again. Furthermore, it bothers me that there is no CZ manual in the package. The display has quite small letters, you can see it, but it could be backlit.
For people who don't have a battery tester at home, why not (but consider the price). It's good for a "yep, I can throw this one away" orientation measurement. Those who have a multimeter at home will be fine. Resp. in general, you can use a cheaper multimeter, which will show more accurate values and can cost about 250 or 300Kč (you will only "fight" with cables and it will not be a pocket pack)