Interesting idea - I looked around and you can get a lead-acid backup battery for about $15. Looking around some more for li-ion backup batteries (lighter) I found this 12v 6.8 Ah for $18 on Ebay (
http://www.ebay.com/itm/New-Designe...228?pt=LH_DefaultDomain_0&hash=item27d8f32634).
Thinking further about batteries, I wonder if you can strap four of these together (27.2 Ah total), wire them in parallel, and use it to replace the Leaf's 12v AGM battery? I looked at li-ion 12v replacement starter batteries and they are very expensive, mainly because they are designed to deliver a high power burst to energize a starter, which is NOT needed for the Leaf. Basically the Leaf is like a laptop - I don't know what the energy draw is on start-up, but it is nowhere near what's needed for a starter. The weight saving would be considerable as well.
I am not a EE, so this is a question for someone more learned than I.