I need to take out the battery and charge it. I can't charge it in situ. I've done this many times and it is a bit of a chore - making sure the alarm doesn't sound on removal, re-programming the radio code, the windows and the sunroof on replacement. I assume the Elite seats retain their memory or will I have to re-store them as well?
Anyway what I'm thinking of is a small 12v battery (the 20a/h one built into my jump starter) plugged in to the rear cigar lighter socket, positive to positive and neg to neg. So it's connected in parallel with the main battery. Then take out the main battery and let the 20ah one keep all my settings while I swap it immediately for a spare main I have or even overnight while I charge the flat main. Are there any disadvantages? Obviously I won't run anything big like the starter or the HRW on the small batt.
After that I need to identify what the drain is, with my shiny new DC amps clamp meter

. I do know the batt wasn't self draining.