LiFe, why?
Only thing the LiFe batteries have going for them now are that they're more resistant to overcharging and neglect, more compact form, and the cell voltage matches up to a 12V (14.4V nominal) charging system.
Lipo's have them beat hands down in all other aspects (charge and discharge rates, less weight, less voltage sag, cheaper, etc.), they're even making them in 18650 sizes to replace the heavy LiFe counterparts.
Lipo's are getting 130C discharge rates now with a 15C charge rate...that means you can theoretically discharge the entire battery in 30 seconds and then recharge it in 4 minutes without harming the battery.
LiFe batteries just can't get close to that.

If you're planning on a charging system then LiFe is the way to go, a normal lipo will catch fire under a 14.4V charge (yeah, I've tried it!).
Another tip...if you have a 3S lipo, the peak voltage is 12.69V, whereas a lead acid car battery is 12.6V.
That means there's no possible way to overcharge a lipo hooked directly up to a lead acid battery so long as the car isn't running.
The current will flow in at max charge rate, and keep in mind that the lead acid isn't pushing the amperage in...the lipo is leeching it out of the lead acid.
Therefore, it will only take in what it can handle until it's fully charged (or right under 1/10V of full charge).
I can fully charge a 4,000mah battery in about 10min...(it runs at 40A until about 75% charge, then it drops rapidly to 10A or less)which would mean that I'd need a lipo charger capable of pushing a minimum of 40A.
The best part about it...I can also hook several batteries up at a time.
My batteries charge up long before I discharge them playing around with RC toys.

Keeping that in mind, if you had 4 batteries (one to run the lights, one as a spare to take with you, two recharging at camp) you'd never have to wait on charging batteries.
Might need to use an isolated spare battery to avoid draining you truck down during an all night ride, or stop for a half hour and let the truck idle without the lipos connected.
That's what I'm planning on doing, soon as I convince myself that an LED lightbar of any size is worth the premium they're asking for it.


What's your plan for the lightbar?
Seems like The XM-L would be a better choice given its efficiency and doesn't need a huge reflector.
Those big chips need big reflectors (I can imagine 6 toilet plunger reflectors all mounted to the handlebars LOL), unless you're going for more of a flood beam which is probably better for a quad anyway.
12 XM-L emitters should give you 10-12,000 lumens, pulling a max of 36A at 2.8V.
You could run something like that for 1/2 hour on a 4,000mah battery
I wonder what the LED lightbar manufacturers are using on their stuff?


Oh, and if you could figure out how to get 120V power...here's a 3,000 lumen single module complete with heat sink!:
http://www.cree.com/led-components-and-modules/products/modules/non-integrated/lmh2