How to add the battery maximum

Hello mamadou99, 

It sounds as though you’re wanting to view the Maximum Capacity percentage on your iPhone 6. You’ve come to a great place for help! Battery Health was introduced in iOS 11.3 which is compatible with your iPhone 6. If you haven’t updated to iOS 11.3 or later you would want to do so. You can find the steps for updating in the following article. 

Update your iPhone, iPad, or iPod touch

After updating you can follow the related iPhone Battery and Performance article.

If you're asking how to increase the Maximum Capacity percentage that isn't possible unless you replace the iPhone battery, especially if the Maximum Capacity is below 80%. The iPhone battery is a consumable component and your will notice the battery health decrease over time. If you have concerns about the battery life of your iPhone you can find helpful information in the following article that may help with maximizing the battery life. 

Maximizing Battery Life and Lifespan

If the Maximum Capacity is under 80% and you would like to look into having the battery replaced you can contact Apple via the Get Support website, or you can visit a location which can be found via the Find Locations website. 

Take care!