I use an iPhone 8 Plus and replaced the battery twice at around 79%. The first was cheap—$29, I think, because Apple was caught throttling performance unbeknownst to users, so they had to make it cheap. The second upgrade was $49. Both def well worth it because performance & battery life were noticeably boosted, the phone still rocks, and it beats the hell out of having to buy a $1000+ every few years.