Had similar problem....my battery went dead when the car was sitting for a few days, so to find out if it was the battery that was gone, i charged it and disconnected it, check the voltage and wanted to check the voltage again after 24 hours to seee if it would have dropped.
The next day when i went to open the trunk, it would not open, still don't know if the lock was frozen or if i need power even with the key (since it was -20 Celcius here), anyway i connect another battery to a cigarette lighter adapter, plugged it in the car and open the trunk with the electric button in the dash.
I did not think it would have workked because the main power in the trunk was disconnected, but it worked fine and all the lights in the car went one.
Now i see my battery is good since the reading is 12.85 volts, now have to find out what drains it slowly...the same thing happen 2 years ago twice.