I was actually asking them the same question before I purchased the phone and I got more or less the same answer. It would be cool to have this feature, but it may be possible that it’s just a hardware limitation. For example, during boot, the phone can hypothetically draw more than 20W, which is the maximum charging rate. You can try asking them for more technical details, maybe it would work just fine if they enabled it in firmware.
The only solution I can think of is to get a decent 3.85V power supply and connect it directly to the battery pins. And even then it might not work without BMS.
I’m fairly sure that this is pretty much the reason. Also, you cannot expect everybody to connect a 20W power supply. Worst case it would have to work with 5W or even less. And any interruption in the power supply would also effectively power cycle the phone. So as useful as this feature sounds, it it probably something no company wants to commit themselves to without very good reason. Running a phone without battery will not become mainstream, I’m afraid.
An additional problem: The phone will probably not even be able to draw more than 5W or even 2.5W before it is sufficiently far booted up to request more via some QC/USB-PD logic.
I can confirm this works (don’t remember exact voltage, probably I chose 3.7V or 3.8V) with Librem 5 devkit and Fairphone’s FP1U (I used some laboratory power supply for that, which can deliver a current of 2.5A)
Just replace the battery by a huge capacitor (eg: 1F or higher) which can hold at least 5V and plug your phone to a 10W charger or more.
The capacitor will act as a tiny battery and will allow the phone to boot and run.
ATTENTION: this kind of capacitor is often polarized
a) The use of non-official parts probably voids the warranty, even if it works without creating any problems.
b) My feelings are that this is not really practical in a ‘common’ user sense. The idea I have is that people want the phone to work from the power supply USB C when the battery dies for example, not to have another ‘module’ to replace the battery. Although it may suit the odd person or two