Reasons your phone battery draining so fast from 70% to 20% within 10 minutes:

• It's an old phone.-An obsolete phone, on the other hand, would have an outdated battery, as we all know. As a result, if your phone is older than five years, you can no longer complain about the battery draining. It's time to upgrade your phone or get a new battery. This is how you can conserve energy.
• While Charging, You Can Use-This should not be done. If you use it while charging it all the time, the battery will overheat, which is one of the reasons why the phone battery won't last long. It will also cut the charging time in half.
• Examine the battery usage of your phone apps.-Open your phone's Settings > Battery > Go to Battery consumption > Choose Show entire device utilization in most Android versions. You can see which apps are draining your phone's battery the most, as well as the active and background duration.

• Turn off the auto-brightness feature.-Many smartphones include a feature called Auto-Brightness, which allows your Android phone to detect the light condition and automatically set the brightness level. So you can try to off Auto-Brightness and maintain it on a low setting.
• Disable Unwanted Notifications Although it appears that every app will immediately request to send notifications to your Android phone, some notifications are actually meaningless. Simply uncheck 'Show Notifications' in these apps in Settings > Apps.

Read More: What is the normal battery drain?