Just another observation here.....
- the original poster has only space for a battery
- material cost for a second deep cycle battery, wires, terms, batt management system
- some labor time
OR
Get one of these and be done with it.....
http://www.odysseybatteries.com/battery/pc2150.htm
* 2150 cranking amps for 5 seconds
* 1985 cranking amps for 10 seconds
* 1695 cranking amps for 20 seconds
* Short circuit current over 5000A
* 100 amp hours
* 200 minute reserve capacity with 25amp load
* 3/8 in. (9.5 mm) corrosion-free stud terminal
* Length 13.0"
* Width 6.80"
* Height 9.40"
* Weighs 75 lbs
* Rugged military construction
* Can be mounted in any orientation
* Limited 4 year full warranty
* Container and cover made from oil resistant plastic
* US DOT and IATA certified non-spillable
* Shipped fully charged - just install and go!
Sure this thing is heavy but not much more than two full size batteries...
Sure this thing is expensive but add up the cost of what I mentioned earlier
You STILL get better performance than any two Yellow Tops
Sure this thing is big but you can make it fit
Ali