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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It depends on the device. Wall adapters for smaller portable devices nowadays deliver mostly 5VDC. Batteries are almost always based on lithium chemistry and often run between 4.2V and 3.xV. The power management circuits powered by the battery might not be designed to continuously work from a full 5.0V supply, that means you’d need a quite powerful additional voltage converter for a use case not really fitting for a portable device.
An other reason could be that the charging circuit works with lower powered adapters and doesn’t know what kind of adapter it is connected to, while the device itself possibly draws current peaks exceeding what the adapter can deliver. Not going out of your way to design a bypass feature saves you the headache of explaining all the device resets to the customers that don’t read all the disclaimers.
More pragmatically, having multiple power supply paths requires design efforts and additional circuitry to make all work seamlessly for the end user. Most of the time it will probably not be deemed worth it, as it is not a highly demanded feature for a mobile device and it would add to the cost.
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