Hi there
For Battery Drain issue, Please provide Screenshots of Battery Stats and SOT.
Follow the steps :
Let Your Battery drain till it is switched OFF and charge it to Full in Switched OFF Condition. It will reset your battery stats. Then use your Device Normally like you used before till battery is drained below 10%. Now take Screenshots of Battery Usage and SOT details and Share here.

For Battery Graph : Goto Security--Battery Usage--Battery Use--Click on the Clock like Icon on top right corner--Take Screenshot.
For SOT : Goto Security--Battery Usage--Battery Use--Under Hardware Tab, Click on Screen--Take Screenshot.

Also refer this thread for getting good battery backup : https://c.mi.com/thread-39970-1-0.html
Try these and let me know the results.