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Answer: They don’t, but the battery in them does.
The typical battery for a cell phone will last about 3 years with slight performance drop as the battery ages. Here’s the zinger. If you get a spare battery for the phone when you get the phone itself, and just keep it in a drawer, unused until needed, it will be at the end of its life just like the battery in the phone. It really needs to be new not sitting on a shelf in storage.
The new phones for the big names don’t let you in to the battery anymore. No spare battery to buy.