Over 80% of battery suddenly drops to zero when I'm not using my phone?

[AC Question]

This has happened many times, very randomly. I fully charge my battery and use it for a while maybe to play games, then I close everything and leave my phone to be. When I come back in maybe a few hours the phone is turned off and battery at zero. Mobile data was turned off, like everything, only WiFi was connected.

This only happens when I'm not using my phone for a while. What could be causing it?

 

[belodion]

If your battery usage stats do not indicate anything to explain it - syncing and backups over a wifi connection for example, which might show as Google Play Services battery usage, among other possible users - the battery may need replacing.
Try switching off wifi as well as mobile data, diagnostically.