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


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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I dunno about phones, but I have worked on several things that don’t get their power from an external source in any fashion. They pull power directly from the battery and the battery pulls its power from the external source. Could just be the way it’s wired internally.

Some can but more and more newer devices can’t because of a few reasons (all of them being true in some devices).

Companies are starting to demand a reply back from the battery to even complete the basic boot cycle. If it doesn’t get an OK back from the battery it won’t boot at all.

To piggyback off that, more and more logic is being added inside of the battery itself. All lipo batteries are just that but now a lot have logic inside (on boards) that are there to not just manage charging but answer back to a bootrom or such. Some kinda take over for the old CMOS batteries that allow hold data like time and such. In this they incorporate that into the general operation of the device instead of just being a mindless battery. So the battery cells can be good but if that internal logic board is bad the whole battery is bad. This happens a lot in Nintendo Switch batteries where, in my anecdotal experience in repair, about half the batteries themselves are fine but the boards died.

The general answer, IMO, is planned obsolescence. At the end of the day I think it’s a bonus to add more failure points for them to fix at a fee or have you buy a another of their devices (APPLE).

Some cases it might be a safeguard against unsafe operation, if your phone is missing a battery, chances are something is not ok.

Also batteries provide a known consistent current, whereas a DC charger could theoretically be outputting any amount of volts at any given amperage. So having the battery as a buffer is safer. Now why? Well because now the technology to detect whether or not a battery is in the device is much cheaper and easy to implement .

There also might not be a bypass implemented, so the power isn’t ever going directly from the wall to the phone, always going through the battery first.

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.

Some circuits are designed to only feed off the battery. So even when plugged in all the circuits rely on the one battery feed. This is a hardware design choice, but not one to be taken lightly. Allowing both external power and battery power runs the risk of the user removing the external power with no battery reserves. This can cause irreversible data loss. Apple devises for example feed off both, but when you plug a dead apple devise into the wall it won’t power on right away. The software requires a margin of error so the battery has enough for a safe power down.