Originally Posted by
GeoffryHawk
It's likely not the firmware, you said the phone got really hot and then has been randomly shutting down? That's a battery issue, specifically due to the Lithium Ion nature of it; it's completely discharged which means it's not holding more than 10% of it's battery charge and not very efficiently at that.
This used to happen all the time with Fast Chargers and iPhone 4-6s; unfortunately there's really nothing you can do to fix it with software, it's a hardware problem, and will require a replacement battery or a new phone if it's melted any circuits.
Also I'd be careful, we all saw what happened to those Samsung phones.
Take it to an apple store or talk with their customer support, this was a known issue back then and they'll be pretty understanding considering they were to my mother when she called them about her iphone 6 doing the same exact thing.
It's just how Lithium Ion batteries work, if the hardware switch that tells the phone to stop charging doesn't it overcharges, and then will barely charge again. It's the trouble with Fast Chargers and cordless chargers.