Powering a stepper motor
How did you set the amps on the http://www.schmalzhaus.com/EasyDriver/ to 0.5A?   What was the voltage at the test point?  The EasyDriver v4.5 schematic says the formula for coil current is A = V/6.  So rearranging, V=A*6, and to get 0.5A, the voltage should be 0.5*6=3V at the motor driver's test point.

The linked motor spec is 0.6A max per coil, and if things went wrong, you could have overloaded a motor.  The 18V battery could have easily provided 1.8 amps to the 10ohm coil in the motor for a bit, before 3x the rated current of the coils would burn it out, or the easydriver went into thermal shutdown or something.  

The easydriver looks like it would be capable of controlling the current adequately, but if misconfigured, could easily overcurrent the motor.