all i was trying to do was solder a ground wire to
a ground strap i had installed in the truck, trivial job for any
soldering iron.

No, not quite. The gauge on a ground strap is well beyond what any simple
soldering iron would handle. Warming up THAT much metal for it to take the
solder isn't gonna happen with a cordless job.

I've got one of those 'cool heat' units and it works well for soldering
something small like a 18-24ga wire. But it's not good for any sort of work
requiring constant heat.

the tool would melt a bead of solder which would
promptly roll off the ground wire like a little BB, but it wouldn't
melt the glob that i was trying to attach the wire to.

Bad soldering techniques won't work with even the best of irons.