What my battery going down?
My battery maximum power going fast down
iPhone 11 Pro Max
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You're talking about the battery health?
I believe it's covered under warranty if it drops below 80% in the first year.
Other than that, a battery is a consumable item and will degrade over time.
My advice would be to charge your phone overnight, every night.. top off during the day as needed.. don't let it go below ~20%