This is just for an example if your motor pulled a 10 AMP load and you used 4 gauge wire the drop in voltage for the 18’ run would be about 1.3 volts. Which means if you started with 12 volts at the battery by the time it reached the motor you would have 10.7 volts. Now if you were running 2 gauge wire then drop in voltage would be just about ½ of that about .6 volts, so your motor would be getting about 11.4 volts. So what I guess I am saying is some larger gauge wire would help you out. If you would like check the gauge wire you have now and I will check the voltage drop you are getting now so you can compare. Hope this helps.

FIT