You do not have to have the backup battery to tinker. Pull the fuel pump fuse and turn on the master.
In fact, to do software updates you will have to have the master on, AND the avionics master on (to power the transponder and ADSB box if you have it) AND the AP switch on (to power the servos) because a software update sends new code to all those things. The backup battery will not power those things. It only powers the Skyview box, the EMS, the ADAHRS, and the GPS receiver.