Welcome aboard Pkanev. Obviously the Raymarine drops off due to voltage drop but without knowing the actual wiring
setup, it is too hard to say where the drop is occuring.
I suspect the problem is a combined voltage drop in both the wiring
and the batteries. The voltage drop in the batteries is probably "normal" when using the winches but added to the drop in the wiring, the Raymarine turns off. When you parallel up the staring battery
, you are bumping up the battery supply voltage enough for the wiring drop to be less obvious.
Best way to be sure is to measure the battery volts right at the terminals when using the winch
and also right at the chart plotter (or as close as you can get). Then you will have real figures to work with and see where the major problem is.
Again I suspect it will be in the wiring, switches, fuses
, connectors, terminals rather than just not enough batteries.