Unable to activate Optimized Charging on my iPhone

Open Settings ➡️ Battery ➡️ Charging ➡️ Optimized Battery Charging: Are you able to set this switch to ON?

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If you are not able to set the above metioned switch to the ON position, then Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:

• Press and quickly release Volume UP button
• Press and quickly release Volume DOWN button
• Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF). 

Axel F.

If you have charge limit set between 80-95, optimised charging will be disabled.

it will only be enabled if 100% limit is selected.