boot loop after battery replacement

Ew. That shop needs to replace that battery out and do a proper volt test on it. Like Ben said before, it's a bad battery and might be part of a bad crop or by itself.

I don’t know if this will get added to this discussion, but I had a similar problem. I replaced a bad iphone 6s battery with one purchased through Amazon. It is a little tricky getting the old battery out and this is where my problem was. I unfortunately lost a couple tiny screws in the replacement process. The critical screw is the one holding the metal cover tight over the connection between the battery and iphone. I reinstalled without that and the connection was not tight (enough). The phone would start and be fine….for a couple minutes and then reboot. I opened the cover and scavenged another screw (needs to be the slightly longer one) and tightened that cover down well. No more problem with restarting.