Q: Magnetic watch band closure

No it won't.

You're welcome!

If you have not yet done so, it may help to update the software on both your iPhone and watch, starting with your iPhone:

• Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support
• Update the software on your Apple Watch - Apple Support

Otherwise, if you feel that battery life has recently decreased significantly and without your usage having changed, the following steps may help:

• Restart both your iPhone and your watch:
  • Close all open apps on your iPhone:
    • Double-click the Home button, then swipe up on each app preview to close it.
  • Turn both devices off together, then restart your iPhone first;
    • Restart your iPhone, iPad, or iPod touch - Apple Support
    • Restart your Apple Watch - Apple Support

• Unpair and re-pair your watch:
  • The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app. Choose to Restore from Backup during set up. Most data and settings will be restored, with some exceptions (eg Apple Pay cards, passcode).
  • Unpair your Apple Watch and iPhone - Apple Support
  • Set up your Apple Watch - Apple Support

For the best performance from your watch, be sure to keep both Wi-Fi and Bluetooth permanently enabled on your iPhone.

You are most welcome!