At what BATTERY PERCENTAGE SHOULD I replace battery?
At what percentage should I replace my phone battery?
iPhone 13 Pro Max
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Go to this web site for information about how to request battery service for your iPhone: iPhone Battery Repair & Replacement - Apple Support
For cost and availability, enter your model information in the "Get an estimate" box at the bottom. (I will let you do this since the answer will depend upon the country where you are located.)
See this excerpt from: Batteries - Maximizing Performance - Apple
Apple says: "A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge."
From that I would infer that if it is below 80% it is not "normal" and may merit replacement.