I see that his question is from a long time ago but, just in case it still gets attention, here are some thoughts based on running a small line follower:
Let's address the battery problem first:
I assume that the statement uXbot_move(MAX_SPEED + steer, MAX_SPEED); has parameters that map to PWM values - possibly by some scaling factor.
A fixed PWM value ...
this may be a bit late to answer, but i hope it may be of help for people learning ros in the future.
First, in the URDF model,
mu1 and mu2 is friction coefficient was set as 0.
If the wheel has no friction coefficient, it has no friction.
Hence, the wheel will not move, since it has no friction, ...