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My old 8 plus after 2 years and 3 months of usage is at 84% battery health and perfectly usable.
Your 8+ battery holds up better than my old 6s's one. I've used the 6s for 21 months and for the past 3 (almost 4) months mainly my parents have been using it since it's now theirs. The battery health is 84%. It too is perfectly usable. A9 devices while far from the fastest are still quite good, even in late 2019/early 2020.
 
I have a x a 8+ and a pro max all at 100 percent. I use them all day and charge them at night.
 
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