Earlier quoted context omitted.
Good luck. I’ve tried to replace battery in mine 12 mini last week - with no success. I had to leave my phone for 4 hours or several days (if they brake screen during battery replacement, they will wait for replacement phone to be shipped overnight). Also representative was convincing me to buy a new phone - saying that battery replacement won’t help much because new ios versions has features which high battery usage…
So do the swap yourself. Part and kit: https://www.ifixit.com/products/iphone-12-mini-replacement-b... Steps: https://www.ifixit.com/Guide/iPhone+12+mini+Battery+Replacem...
I've read online and heard from Apple Store representative that iPhone 12 (all models) has tendency to crack the screen when phone is opened for repair or battery replacement and in that case Apple Store would replace the hole phone (this is were multi day repair process). So I would rather pay $90 to Apple that guarantees that I'll get a phone replacement in case when screen is broken during battery replacement. Without the phone I sill would be able to answer the cell calls from Apple Watch and with ipad over WiFi.