I run two fish finder units on my boat. a Humminbird 595 in front and a 597 at the console. My problem is with the front unit, after being on the water for awhile the unit will start showing low voltage input and keep dropping until the unit turns off. The unit at the console has no drop in voltage and continues to work. If both units are working off the same battery why is this happening? Any advice of correcting this problem would be appreciated. Thanks