If you’re not using your cellular on your android tablet/phone but instead use it only offline or as a WIFI device, the phone is still wasting battery on the phone modem.

With no sim card inserted, the phone is constantly looking for a emergency connection and is wasting battery.

To fix this, you could use your terminal emulator and enter this: (Root required)

su
setprop persist.radio.noril 1

To enable it again, use this:
noril 0
also set flight mode