I've got this cheap battery level indicator that has 4 battery level segments, it is connected to a 12V-18AH lead acid battery that I to charge my laptop and my 1W light bulb.

I really can't seem to understand the behavior of it, as soon as I plug the charger to charge the battery it goes full (4 segments on) instantly, then goes back to original state as soon as I remove it. And when I start charging my laptop, it always drops a segment, then goes back as soon as i remove it as well. I feel it's more of a load indicator?!

I would appreciate an explanation for this please. Sorry for my english.


closed as off-topic by Andy aka, Michel Keijzers, PeterJ, Voltage Spike, Peter Smith Mar 11 '18 at 17:27

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  • \$\begingroup\$ Consumer electronics questions information \$\endgroup\$ – Andy aka Mar 8 '18 at 10:18
  • \$\begingroup\$ I need an explanation for how this battery indicator works, nothing of the not abouts. \$\endgroup\$ – Seraj Mar 8 '18 at 10:22
  • \$\begingroup\$ There is no generic battery indicator circuit hence this question relies on specific circuit knowledge of that consumer product's circuit. If you can provide a detailed circuit diagram then somebody will be able to help I'm sure. However, without that circuit this is just a question about some other piece of consumer electronics and should be closed. \$\endgroup\$ – Andy aka Mar 8 '18 at 10:33

This indicator is working by watching the voltage across the meter, and that's all it is doing. A battery's voltage decreases as it discharges, so the voltage gives you a rough measurement of the charge level.

However the voltage also decreases depending on the amount of current being taken from the battery. This effect is greater (more decrease) on smaller batteries.

A charger is like the opposite of a load. It pushes current the other way through the battery, which causes it to charge and which causes the voltage to rise above the normal battery voltage.

  • \$\begingroup\$ But why does voltage decrease when more current is drawn (more loads are being used)? \$\endgroup\$ – Seraj Mar 8 '18 at 10:32
  • \$\begingroup\$ @Seraj That's just how batteries are. The more electrons it has to push the less hard it can push them. \$\endgroup\$ – immibis Mar 8 '18 at 10:38
  • \$\begingroup\$ Ok, makes sense, is there anyway to have an accurate indication, or is there a way around this? \$\endgroup\$ – Seraj Mar 8 '18 at 10:40
  • \$\begingroup\$ @Seraj I would think the easiest ways are either: only measure it when the load and charger are off, or get a bigger battery so the effect is less, or get a more accurate meter so you can better compensate for it in your head when reading the meter. It's harder than you think to tell how charged a battery is. A lot of modern devices do it by measuring the current going in and coming out, but that needs a more complex meter and it has to be calibrated for the specific type of battery. \$\endgroup\$ – immibis Mar 8 '18 at 10:49

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