I have BMW's Battery Tender, which uses the 12v socket. I just run the cable through the bottom of the door where the black plastic shelf is. I have a garage, so the tender sits behind the car and plugged into the wall. Anytime I am away, or notdriving for three weeks, I plug the Elise in.
I actually need a couple more battery tenders for my scooters and MINI...
2013 Porsche Boxster (in silver and red, home at last!)
2011 BMW 128i cabrio, ///M Sport
2005 Elise in Racing Green with the touring pack (sold 8/6/12)
2007 MINI Cooper S in Sparkling Silver (sold)
Founder of the Racing Green Confederate