I have one phone that once got into a reboot loop: its battery was flat, so I’d provide it with a USB power source, it’d charge the battery for about five seconds, then it’d try to start up automatically (without me telling it to), but that would stop it charging the battery from which is all it seems to be willing to draw from at that stage of boot, so after another about five seconds it’d die of a flat battery again, then after another few seconds it’d recognise the power source and start the loop again. The entire cycle took about 17 seconds, like clockwork. Not sure quite why it went like that; historically it had been willing to charge from flat, and since then I think it is too, though I haven’t used it much for the last couple of years, and I can’t remember whether it normally powers on automatically in this case. I tried almost all combinations of four USB cables and five power sources (Surface Book, car with 12V → USB adapter, Surface Book AC adapter which has an additional USB outlet, Surface Dock, and a wall charger) before finding some combination that worked, not triggering the untimely and defective power-on.