Since updating my Galaxy A3 to Android 6.0.1 my battery drains very fast.

[AC Question]

I turn the phone COMPLETELY OFF overnight and the battery drains by 15% to 20%. How can this happen? Can I revert to the previous version of Android which had no problems?

 

[belodion]

If the phone is powered off, I don't see that it would be a software problem. Perhaps the battery is bad.

 

[Rukbat]

And yes, you can flash any compatible ROM (you can't just roll the ROM version back). See [Samsung] How to flash Stock ROM via ODIN.

A bad battery could be causing the problem (do you regularly drain it to less than 40% before recharging? That will do it in a few months.) So could a short on the motherboard.