Super AAA

It seems I found an supercharged AA battery today.

While for moment this seemed very interesting, after few seconds of consideration I came into a conclusion that my multimeter is more or less at fault here.

This specific meter one is one I have at home. It retails (in brick-and-mortar store) for something like 20€ (or at least it was, some 10 years ago), and today you could get similar one for some 5-10€ or so on ebay. While it claims to be CAT III rated, I wouldn't let let it anywhere close of high voltages. In other words, it's a PoS that is barely sufficient for checking if a battery is full or empty. And apparently it can't do even that that well.

But I don't blame the meter for all that. I knew what I was buying when I got that one.

Now, if you take a closed look at the picture above, you may see a small "battery" symbol on the screen of the meter, meaning that its battery is almost empty. Unlike newer models, this one has toggle-button for power. You always must remember to turn it off after you're done with it.

What isn't immediately obvious is that the same battery symbol also means that the readings aren't to be trusted anymore, as we can see here. I haven't really bothered to check how these things work (in circuit level), but I guess that there is a reference voltage somewhere - generated from battery voltage by some kind of accurate reference, be it of shunt or whatever type - which isn't correct anymore due to low battery voltage and this is causing readings to be higher than they should be.

As proven by quick battery change. With fresh 9v battery the reading is much closer to what I'd expect it to be. Old battery read about 4,9v when I took it out so it clearly was at end of its life.

So, in short, if your multimeter reading seems suspicious, take another meter and cross-check. And never, ever, take these cheap ones anywhere close to mains voltages, for any reason. The protections they contain are simply insufficient and they will kill you for that.