86% is fine.
restart and drain the charge to zero then leave it charging for as long as it takes to reach 100%.
overheating would be caused by having too many apps open at the same time - possibly causing your freeze-ups also.
go to settings/battery.
scroll down to ‘usage’ and have a look-see.
‘last 10 days’ tab.
if battery usage bars are all green then battery’s normal.
if some red spikes, then it ain’t.
scroll further and you’ll see see which app is using what %.
(screen can be off yet the apps are still running)
and by tapping ’show battery usage’ more stats are revealed
so, for example, my VPN has been running in the background for 8.5 hours, Fox news for 6 hours since last restart even though i only viewed it for 5 minutes tops.
if you lift and slide slowly from bottom of whatever page you're open at, flick up successive apps to effectively close them, then you’re only using the remaining open app.
that’s to say, closing an app doesn’t actually close it.
sounds irish i know.
i had the same issue with our 12 minis and old mate at the local apple auth repair centre showed me the foregoing and the minis have run ‘normal’ ever since
