How to back 100 percent battery health?

There is no way that is a new iPhone, Apple hasn’t sold it for at least 6 years. Take it back where you bought it.

What is your iPhone Model?

Settings > General > About

just want to note that if you bought a 'new' iPhone, but it's really an older model.. the battery might be quite old.. it may have been sitting on a shelf somewhere with a depleted battery, which is terrible for the battery.

ahh, yes.. that iPhone model was released about 5.5 years ago and then discontinued about 2.5 years ago.

So the age of your battery could be somewhere in that range.

Having bad battery health would then make a lot of sense.

Have a new battery installed.

You can't without replacing it; they wear out as they age.

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Contact Apple Support or an authorized service provider.

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Go to Settings > Battery > Health, take a screen shot and post it here.

What model iPhone is this and where did you buy it. Niel is correct, you need to take it to Apple or an authorized service provider and get it replaced.