The accessory socket is dead with the car off, and you want it off to change the battery. A 9 volt battery connected directly to the battery cables might work. People have done what you want to do, so let's wait for one of them to post here.
2013 "Brilliant Silver" SV with Premium and no QC, a 2009 Vectrix VX-1 W/18 Leaf modules, and 3 EZIP E-bicycles.
PLEASE don't PM me with Leaf questions. Just post in the topic that seems most appropriate.