Eli5: How do electrical devices know how much battery they have left?

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Basically the title.
How do devices measure the amount of energy stored in them? What do they measure to say that I have 87% battery left?

In: 3

3 Answers

Anonymous 0 Comments

They count the current that leaves or enters the battery and estimate a percentage. I.E. last time we counted 3000mAh from 100% to 0%, then we charged 1500mAh and used 500mAh of it so our percentage must be 33%

And yes that means it can be inaccurate if you didn’t do a full charge cycle in a while

Anonymous 0 Comments

The voltage of the most common battery chemistries is proportional to the charge level. The battery loses strength as it’s getting discharged. The device has a regulator in it that draws more current to allow it to operate normally. With other types of cells like NiMH it doesn’t work. The charger could place a constant dummy load on it, and measure the time until the battery is flat.

Anonymous 0 Comments

Every electrical device has a upper and lower limit in terms of voltage where it can correctly operate. For example your RC car might be designed to use a 12v battery(the battery is at 12v when fully charged), but if for example the battery drops to 8v because of use the car it will not be able to run due to lack of power. In this example you have 4v of power you can “use” which corresponds to a percentage. Batteries can track consumption non-linearly which means 10v might not be 50% but instead 15%.

Note: The current(amps) of a battery doesn’t change because it measures the quantity of electrons in the battery. If will not change unless electricity is being routed the wrong way to the ground instead back to the battery.