Just because you have a new battery does not actually mean it is any good.
It may have sat uncharged on a shelf for many months.
Have the battery load tested.
If the battery is bad, then the altenator in the big motor could boost the voltage enough to allow your unit to function properly.
I guess there is also a very small chance that interference could be showing up as fish images, when the big motor is running.
Also, make sure you are not in simulator mode.