Random percentage and battery use problem

I agree with Daniel. If battery replacements haven’t helped, it’s likely board level and replacing any ICs requires a high level of skill and equipment such as a hot air rework station, flux, and a microscope. It would most likely be cheaper to just replace the phone as opposed to having the IC replaced by somebody.

A sort of off the wall thing you can try: turn off the % display in Settings.

It's been a while since I saw it, but I worked on a 5-series phone (I don't remember if it was an S or not) that was having weird battery issues and that tip actually improved the situation quite a bit. Charger (aka Tristar) IC and/or battery issues with 5-series phones were pretty common (unfortunately).

Just to add to what Josef and Daniel said - a professional replacement of this IC will probably run $100+, and the equipment to try and DiY is much more expensive than that. So unless you're looking into doing this professionally some day - in which case the investment might make sense - I'd just try and nurse this thing along. That said, if you're *really* interested to see what it looks like: https://www.youtube.com/watch?v=0v-wKlVh...

As a sidenote - I've seen different behavior with 4 and 5 series devices by charging the phone with an original Apple charging cord and/or block. I've never seen it effect the sort of issue you're describing, but at minimum if this device is on its last leg - it might help it keep going a little longer.