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Apple generally recommends replacing your battery when it dips below 80% maximum capacity. Keep in mind that it's not just the maximum battery life that might suffer if you don't swap out for a new battery -- the quality might degrade so much that the battery itself could swell and warp enough to damage sensors and internal parts of the iPhone -- which is rare.
Apple offers in-store battery replacement for $99 for the newest iPhone 15 models, which drops down to a minimum of $69 for the oldest iPhone SE and iPhone 5 models the company still supports. Third-party phone repair shops may offer their own rates for replacing batteries, and iFixit has a guide if you want to boldly order a new battery and attempt the swap yourself -- just keep in mind that it may violate any AppleCare agreement if you go outside Apple's repair ecosystem.
Source: cnet.com