Peak Performance Capability tells you whether your battery can still supply sufficient power for the iPhone's most intense processing tasks. Normal peak performance is what you're aiming for.

If it says your iPhone has experienced an expected shutdown, that means your battery couldn't supply the needed power quickly enough, and the system shut down as a result.

Your iPhone will throttle its own performance—slowing itself down—in order to prevent further unexpected shutdowns. This can be disabled to manually enable peak performance.

Once you disable performance management, you cannot enable it again. It is automatically enabled after an unexpected shutdown.

Once your battery falls below a specific threshold (typically 80%), you will be notified that your battery's health is significantly degraded.

Follow this guide to replace your iPhone X battery, or click here for other iPhone models.