That’s because of the way these scales work. They use a material that deforms under stress and when it deforms the resistance changes. By putting current through this material and measuring the voltage drop, it can be mapped to how much stress the material is under and thus how much weight is on the scale.

This is a pretty roundabout way and has a lot of caveats, but it is very cheap. So cheap scales always work this way. That’s why they aren’t super accurate and have deviations depending on things like temperature. Another big downside is any permanent deformation ruins the calibration, giving incorrect results. That’s why you never put more weight on kitchen scales than it says, it will break them.

The issue you are running into is the way it measures. It applies a very specific voltage and current in order to get the result. The lookup table it uses is only valid within a narrow range. When the battery voltage goes outside that range, it can no longer perform the measurement. Even though there’s plenty of juice for things like the little processing chip and the LCD display. They don’t need a lot of power and can do with low voltages. But it can no longer weigh anything so it just errors out with a low battery warning.

Yes I hate them as well, they always get in the way and putting the cap back on is super annoying.

However, since these bottles were introduced data has shown they work. Bottle caps were one of the most found items in trash picked up from the streets. The number of bottle caps has gone way down since these were introduced.

So I’ve accepted them. Can’t argue with data. I’ve never returned a bottle without a cap in my life. I’ve never thrown away a cap separate from the bottle. But turns out the world is full of psychopaths who throw the cap in the street. Probably the same kind of person who throws their trash by the side of the road from their car. Fuck those people.