There is a metal strip inside that gets narrower on one side and as it gets narrower the resistance increases and once you press both sides it gets in contact with the battery terminals and current flows through. The lower the remaining battery capacity, the less the wide part of the strip heats up. Over the top is a heat activated colour strip thingy that shows how much of the strip is heated and thus approximately how much juice is left in the battery.
If it didn't use power, it could remain on all the time.
There is pretty much no way around this. All voltmeters use power (although digital ones use their own source and draw very little from the measured voltage) but this one consumes more than most.
I think they could make one with some LCD or electrochromic display (both use a negligible amount of power) but it might be too expensive to include on each battery. Also, the voltage to state-of-charge relationship is not a simple one: discharged batteries’ voltage will rise back when not in use but the internal resistance had gone up so they no longer allow discharge at a practical current. The little heater test accounts for this, an LCD or ECD does not.
It actually did work reasonably well I always thought, but only if you did it just right. It was more a gimmick than a feature IMO - rarely did we keep batteries long enough to wonder if they were dead, and we usually determined it by putting them in a device.
I think it’s because many people misunderstood how to use it: you’re not supposed to jam your thumb on the sharp corner, rather just gently push on the flat end.
Huh, I haven't seen these things for many many years. And thought about them a few months ago and actually just bought a battery tester / charger. I don't think they exist in Germany?
Honestly, I've never heard of theses things until now.
I'd wager at least 50% of the 66 people who upvoted /u/amio haven't heard of it until now, either. They just upvote things because other people upvote them.
Were a thing in Italy I think in the 90s. Don't live in eu anymore but haven't seen one since.
They did work fine, it was just incredibly painful, and took a very long time to get a reading. Gee I might have just made some progress with my therapy.