What is the electrical reason some devices like mobile phones require a working battery even during times when they are plugged into a wall?

Question

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?

In: 18

Answer 1

Cell phone batteries power the phone. When you plug in a phone, it doesn’t supply power directly to the device. It supplies power to the charging circuit that controls when to send power to the battery to charge it.

If you wanted to be able to bypass the battery and get power to the phone by plugging it in, the phone would need additional circuitry to bypass the battery and manage the flow of power which adds to space requirements and heat generation.

For a device that is supposed to be mobile, adding a bunch of hardware to it so it works when it’s not mobile is kind of missing the point.