Sometimes it really is just a bad battery
I have a secondary phone which doesn't get a whole lot of use. I bought it a couple of years ago before starting a full time employee gig with the pink mustache factory. I did this because my friends who were already there advised me that the company did not supply phones but did expect you to load a crapload of apps on to do your job.
I decided my way around messing up my "daily driver" with that much garbage was just to pick up the cheapest and simplest thing that would get the job done, and so I did: an iPhone 8. It got all of the "two factor", LastPass, Slack, Google apps gunk, Google mail stuff and more, and my "real" personal phone stayed nice and clean.
It also meant that I could put it away at the end of the day and all work-related stuff would stay out of sight and out of mind until the next morning.
Well, as previously described, I bailed out of that job right before the Twilight Zone started and we all went into lockdown early last year. That meant that phone had relatively little work to do. It just kind of hung around, and while I used it to help out other folks who reached out to me for ad-hoc consulting work, it's been pretty quiet overall.
So, overall, I was surprised when it started being really belligerent about battery statuses. I would find it at the end of the day somewhere down below 30% charge despite doing nothing but carrying it around in my purse all day. Then, it would stay there at some low value for a very long time while on the charger, and might leap from the mid-40s to 90+% with nothing in-between.
It was behaving VERY strangely. Not in fourteen years with iPhones had I ever seen one act like this in terms of battery life. Also, there was something else troubling which had started at the same time: one part of the screen now had this white blob of sorts, like something was making it flex. The phone itself hadn't been bent, and in fact had been in an Otterbox its whole life, so nothing had managed to hurt it.
The whole thing added up to "swollen battery that's pushing UP on the screen from underneath", and I just needed someone to swap it for me. Now, despite the obvious battery woes, the actual iOS battery health thing still said it was doing well enough - somewhere in the mid-90s of health, and still able to do peak performance, and all of this.
That's when I noticed that actually using it would cause it to just shut down, presumably from having the battery voltage collapse under load - you know, the exact thing the "peak performance" test is supposed to catch? The thing is, the phone won't let you disable that yourself. The thing has to detect it itself. But... it never did, despite crashing the phone multiple times by putting sufficient operational load on it.
The app let me book service, and it sent me to a nearby Best Buy. I went up there and wasted a bunch of time just to be told that "Apple won't let them replace the battery because it's not reporting any battery problem". Great. They also said the actual Apple store people have more leeway in these things.
The next day, I dragged it down to an actual Apple store. They looked at the battery status screen which was still reporting that everything was fine, and said "it's the logic board". Then, when it crashed by itself (again, most likely due to voltage collapse), the Apple person just doubled down on it and said it'd be a replacement, which for that phone would be the same as buying a new one, so why not just buy a new one?
At this point, I switched into "okay, nice doggy" mode, where I take whatever conversational path is going to let me politely end the transaction and get the hell out of there, and decided to take matters into my own hands. [If you don't know what this means, ask anyone in your life who has experience with not being taken seriously when attempting to obtain technical services. I'm sure they'll have any number of stories for you.]
I figured, okay, I'm going to try this. It costs, what, 30 or 40 bucks for the repair parts? If it fails, I'm out that money and my time, and will just have to buy a new phone. But, if it works, I can feel righteous AND I can write a post about it telling people about why the right to repair matters!
Now, I've said this multiple times: I am a software person, not a hardware person, and if you see me with a screwdriver, worry. With that in mind, I *still* went out to my usual haunt for repair supplies and bought an iPhone 8 battery replacement kit, which consisted of a new battery, a suction cup for pulling the screen, a glorified guitar pick for loosening things, a bunch of specialized screwdriver bits, and some other random doodads.
This shipped from the west coast via ground transit, so for a few days I just had to sit there and wait, but finally it turned up, and I went to work. It took some careful blow dryer action to make some of the parts "release", but I eventually got the old battery out, the new battery back in, and put all of the itty-bitty connectors and screws back in. (Oh, and I did this without starting any LiPo fires, too!)
Then I charged it up, discharged it completely (as per their advice) by streaming video at full brightness for like six hours, charged it again, then went back to using it as before. It's been over a week, and you'd never know anything had ever happened to the phone. It's doing its job just like before and looks fine.
Oh, and the white spot on the screen? It's gone, because, surprise, the bad battery is no longer there pushing up on the screen.
Bad logic board, my ass.