The ONLY reason for the battery in the key is for the remote unlocking feature. You can take the battery out of the key completely and the truck will start fine, assuming the transponders are good. They require no power. If you are familiar with these tollway units that sit on top of your dash, which allow you to go through a tollgate without paying (charged to your account instead of paying cash at every stop) or these small units you "wave" at the pump to charge your gas, these keys have the same type of thing in them, 2 small transponders which like the tollbooth thing or the gas pump thing, requires no batteries.
The transponders can go bad though, and that's when the engine will no longer start. It will "attempt" a start (crank the engine) but if a transponder is bad, it won't "authorize" a start, ie no fuel injection.

Gilly