Hi @toncopy! .You may not be using it for a long time when watching YOUTUBE. But take into consideration the other applications that are running using data or while connected to wifi (e.g. skype, viber, and other unclosed apps also getting to consume your battery life. Are you using wifi connection or data connection? Data connection consumes battery faster than using wifi connection.
If you're on data connection, then running other apps, the running of video will makes the battery heats ups due to its reaction while in use by multi applications.
Here are some additional tips that may help you to save your battery from draining that fast.
1. Do a complete charging for your phone (minimum 2.5 hours).
2. After a partial charge, the battery level indicator may not be accurate. Wait for at least 20 minutes after removing the charger to obtain an exact indication.
3. Adjust the brightness of screen as appropriate.
4. Extend the email auto-check interval.
5. Update news and weather information manually, or reduce the frequency for auto-check.
6. Exit background-running applications if they are not being used for a long time.
7. Deactivate Bluetooth, Wi-Fi, or GPS when they are not in use.
I hope this given info tips will be able to help you with your concern. Thank you so much.