Weird boot loop behaviour

Hi there,

I am trying to repair a first generation iPhone SE, and I struggle to make sense of its behaviour.

Even though the battery is most likely completely drained, when I plug it it instantly shows the battery charging image, and after only 3 seconds (maybe more, but no more than 10 seconds) it starts to boot, meaning the Apple logo appears. The Apple logo stays for a very short time too, no more than 10 seconds, and then the screen goes black, and the loop instantly starts again: battery charging image for a few seconds, Apple logo for a few seconds, back to black, etc. It can go on that way for hours.

It looks like a battery issue, but several things seem weird to me:

• first off, the previous owner told me the problem started after a fall. Indeed the screen is badly damaged. I don't see the connection between a fall and a battery issue
• second, if that was a battery issue, I don't see why it would start to boot prematurely

I tried disconnecting the screen and the battery and reconnecting everything, no change.

I don't have a spare battery to test, and would like to know whether that is the problem before ordering one.

Would anyone know what this is about?

Thanks a log in advance for your help.