I hate this scale, it says low battery and shuts off after just 3 months of sitting in the drawer. It infuriates me that there’s still a lot of energy in the battery, I can use that in remote controls with no issues
If there’s enough battery to say “low battery”, then there’s enough battery to show the measurements!
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.
Mighty informative, thank you
Look up piezolectric materials if you wanna know more
The GP is not describing a piezoelectric scale. And you won’t be able to find any piezoelectric scale that is anything similar to “cheap”.
Well which principle do the cheap ones operate on?
It’s actually a plastic ribbon with a metallic foil in a zigzag pattern on top of it. It’s extremely cheap and does a pretty good job. It usually sits on top of a metal bar that can deflect a tiny little bit.
You can read all about it here: https://en.wikipedia.org/wiki/Strain_gauge
The main effect is that the resistance depends on the geometry. So as that changes, the resistance changes as well.
Ah okay.
All of which is fixed by a voltage regulator
Except the cheap part. But likely not by much.
You can order 3000 3.3V low drop out (LDO) voltage regulators on LCSC for $25.50. That’s less than a penny each.
Yeah, but it’s more than 0 pennies each.
Doesn’t help when you need 3.3V and the batteries are now down to 2.5V they are not putting a boost converter in there.
Right. If your design requires 3.3V minimum then putting in a 3.3V battery and no boost converter is just dumb (or extremely user-hostile).
Batteries have one advantage over over supplies: extremely low noise. Even an good LDO will bump up the noise floor, and a cheap lcsc part will do so too. Plus you’s want a reasonably low dropout and quiescent current, which also increases price. Maybe 10ct in volume is reasonable for such a part - and yes, that will absolutely eat the margin
That’s definitely true. But I would definitely pay more for a scale with ultra long battery life.
I made the mistake of buying an off brand digital calliper and now like an idiot I find myself removing the battery when it’s not in use just to avoid damn thing running flat in one month thanks to its atrocious standby current which enables the display to turn on instantly when I move the slide (rendering the on/off entirely moot).
Next time I’ll just bite the bullet and buy a Mitutoyo.
Same story here. You described it perfectly. Mitutoyo comes next.
Lol I must have one similar. Burned through a battery in storage and when I replaced it, I noticed that the display is still active even when off - such a weird design. I pop the battery half out for storage every time now.
It’s a terrible design. If they removed that dumb always on feature and used a proper physical power button the battery would last basically forever.
The problem would be solved if these scales would take a AAA battery which has a lot more capacity than a tiny cell.
I find spending a bit more on batteries goes along way. Although the nominal voltage and size may be the same, better batteries have lower internal resistance, ie provide the same current with less voltage sag. This prevent the low bat detection from tripping prematurely.
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Eventually, the battery drops lower than the device needs. So no, none of which is fixed by a voltage regulator.
Absolutely, My kitchen scale remains accurate and can use the same battery for years.
No, not voltage, current.
Which in this case means: you need a constant supply voltage