If the battery fails on a mobile phone, why is it unusable even when it’s plugged in to the wall?
There are many older electrical gadgets that can work with either batteries or being plugged in. Take an old fashioned alarm clock or stereo. If there’s no batteries in it, and you plug it into the wall it will work. If you unplug it, it won’t work unless there are good batteries in it that are charged. So why can’t a mobile phone work when plugged into the wall when the battery has failed?
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Often a device’s circuitry is just solely connected to the battery, and when tou plug it into the wall, it becomes a situation of: the device is plugged into the battery, and the battery is plugged into the wall. So the wall has to fill up the battery a little bit first, before the device can use any power!
And sometimes it gets more complicated! Batteries change their voltage as they drain or are charged, with the voltage decreasing as it is less charged. And some circuitry might need a specific voltage minimum to function. Or charging AND draining a battery simultaneously when it’s low on charge may stress the battery unnecessarily, or cause the device to function unreliably, so it checks to make sure your batter has at least, say, 3% before it can turn on. These are just some reasons!
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