How do devices (smartphones / tablets) measure battery level?

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How do devices (smartphones / tablets) measure battery level?

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Battery voltage depends on how the charge level of the battery is, so by measuring the voltage of the battery you can know how full it is. It is not a perfect measurement because the voltage also depends on the current draw at the moment and temperature but I suspect it is considered good enough for most cellphones.

The precision if you do it this way is somewhat limited so if you need high accuracy you put a shut that is used to measure the current of one of the leads from the battery. By simply counting the current and time you can keep track of the charge level of the battery. This is used in electric cars, solar power systems, RVs, etc, I am not sure if cellphone do it.

If you measure the charge like this you do not just know how full the battery is but is capacity, battery capacity drops with use so a 100% new battery stores more energy than a 100% full battery a year later.