how to restore full battery health

You cannot restore lost battery health without buying a brand new battery.

Are you sure you need a new battery?

Referring to the "Your battery's maximum capacity" section of the support article: iPhone Battery and Performance - Apple Support

"A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions." Even rechargeable batteries get worn down and 1-2% health loss per month is perfectly normal. According to iPhone Battery and Performance - Apple Support , Apple apparently only considers it unusual (and covered by warranty) if a battery drops below 80% maximum capacity in less than a year after you receive it. Ergo, some storage loss as you use it is anticipated and acceptable. A possible 20% drop over the first 12 months equates to 1-2% drop per month being within the range of what Apple finds acceptable. "The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge." In other words, if maximum capacity is below 80%, the battery is below normal performance and worth replacing. A notification will appear about this. You can still use a device with a lower health percentage than 80% but for optimal usage you may wish to consider having it changed when it reaches 80% battery health.

So:

- If battery health drops a few percentage per month that is normal.

- If battery health drops below 80% before the end of the first year, contact Apple about a warranty replacement. (Apple will test it to determine its actual eligibility.)

- If battery health drops below 80% (you will see a warning message) after the first year, consider getting the battery replaced but anticipate paying for it.

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.)

For instructions on how to get your iPhone, iPad, or iPod touch ready for service click here --> Get your iPhone, iPad, or iPod touch ready for service - Apple Support

You can't without getting the battery replaced; they wear out as they age.

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