I've always used sealed motorcycle batteries in project cars so when I bought a street bike / track bike I looked into other batteries and I found alot of people were making custom batteries from lithium iron phosphate batteries that you can only get in large quantities apparently or by scavaging them from new dewalt power tools as made by a123.

heres a link how to

http://www.tppacks.com/documents/10-...structions.pdf

Anyways there are per gram more powerful energy source than any other batteries dont require a tender for under 3 months between use and you can deep cycle them.

Long story short they allowed me to replace a factory 8 pound 130 dollar bike battery with a custom made 45 dollar 11 oz battery for street use.

I am using 4 cells however for your applications in cars probably 8/10 is required thats still only 2 pounds and you can mount them anywhere.

A company called speedcell makes them for 160 for 4 cells and 240 for 8

here is a thread on the company

http://www.600rr.net/vb/showthread.php?t=122171

here is it on my bike

 

That's really cool - lithium technology is far above your typical stuff out there.

I've been using a lawnmower battery for a couple years but will look into this. It just needs enough CCA (looks they do) and then be weather/heat proof. The only concern is fire - lithium will burn the car to the ground if something happens.