S21 Ultra bootlooping after screen replacement

I'll try and replace the battery ASAP. It's strange because it'll bootloop for hours until the battery finally dies. Could that really be the problem? Ribbon cable for it is fine and not loose, otherwise wouldn't turn on at all.

@spiritedaway, yeah, it's entirely possible to get a battery with marginal electronics. I'm not an expert on Samsung phones, but it's pretty common on Apple phones for them to bootloop when they can't read a sensor on the battery and even occasionally on the screen.

So unless we get someone with more intimate knowledge of your phone to chime in, it kind of sounds like the replacement battery to me, but it's also possible the new screen could be contributing.

As @strongbow mentioned, if you still have the old battery it would be worth your while to put it in and see if the problem persists. If it does, then I'd try swapping out the display with the old one if possible, even if it's only partially functional.