I don’t think that’s possible, nor a practical idea as the battery effectively ensures the voltage is within range and stable. It would be nice if phones were able to be run without the battery. They did a couple of decades ago.
See if there is a fairphoneangel near you who may have a battery you can use, they may also help you disassemble the phone if you are concerned about doing it yourself, although they are sold as DIY 
Each method or in total? Overall I would def. wait longer (at least 30min or even longer) before I try to start the FP when the battery was discharged.
?? I would give it a few hours not knowing how low the battery is.
Did you try another cable? You mention having tried different chargers but not different cables. Also, your no-name charger seems the best candidate for a charger failure.
Each method.
Because the phone is starting over and over again as described, while the battery and the charger are still cold. I’m concerned to damage the battery.
What if every reboot lowers the battery’s voltage even lower?
I did try a wall charger with a separate USB-A to USB-C Cable.
The other two were notebook charger with integrated cable.
If you don’t have the equipment to measure battery voltage you may be able to take the battery to a repair shop who will test the voltage. You are looking for a min of 3.7v to have the phone on for more than a few minutes. At 3.65 the state of charge maybe less than 5% Fairphone with have accurate data on this.
Apologies I did not notice the part about the Boot loop before. So what I would do if I would be you:
I’m currently travelling, but at home, I have access to an embedded lab.
Maybe the battery can be charged with a constant current source without any communication.
I’ll try as soon as I’m home.
Please don’t try to charge the battery by directly applying voltage to the terminals. That’s far too dangerous without the safety circuits of a dedicated charger.
Hi
I note @incanus warning however should you choose to charge with a known power supply ensure to be on the safe side it’s max voltage is 4.2V even though the battery states 4.4V this should also keep the current low.
If you can also limit the current keep it to 1A. This should charge to about 80% in less than 3 hours.
But ensure at the start the battery voltage is above 3.65 for a 1A input else reduce it for the first hour or so.
I didn’t charge it “by force” so far.
It’s the last option before I’ll trash the battery.
I ordered now a new battery and a new USB-Port module.
I’m an electrical engineer and have the appropriate equipment to safely charge a Li-* battery.
I left the phone charging with a notebook charger for ~48h.
After that it booted with 12% battery. I was not able to charge it further during operation.
Now it’s turned off but still plugged in. No sign of further loading.
I guess the connection between the charger and the phone must be pretty high impedance.
So that almost no current is flowing.
This could explain:
After the first successful loading period to 12%, I managed to port my 2-factor authentication, etc. to my notebook, so I’m fine right now.
The phone was turned off at 7% battery.
After two hours of charging (turned off), it has 9%, so it seems it’s still charging but very slowly.
Hopefully you will be able to establish the issue and see if either are at fault. Did you contact support at fairphone?
No I didn’t so far. Is there official support for this kind of issue?
Until it is resolved there is no knowing what it is, hardware or software.
The phone is under warranty for two years and it doesn’t seem as though you have done anything to void it so it is worth contacting support[at] fairphone dotcom and see what they have to say.
They can be a bit slow and ask you to do this and that but you should at least get an email acknowledging your request for support within a working day or two.
You will undoubtedly be asked to do a factory reset at some point so hopefully you don’t have important data on there that you can’t afford to loose.
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