Taza wrote:
guys
The boat has only one battery and the selector switch is for a single battery,the selector switch is mounted inside a metal black box also fitted is a fuse holder for the bilge pump,its the origanal as the wiring is part of the loome and is on plug n sockets.
It doesn't look OEM, with the clear plastic part and the double sided tape holding it on ( also what are the 2 holes for ?). My 1999 Sundowner, with one battery didn't have that diode. The later dual battery set up had 2 diodes connecting the two battery terminals at the switch together, that provide power to the bilge pump (so always powered by both batteries, but one duff battery could not flatten the other).
Maybe what you have is a remnant of a non OEM dual battery set up ( but not using a dual battery selector switch) that was removed before you got the boat ; maybe connecting a leisure type battery not used for starting, or a small 12v domestic alarm type battery for radio memory retention; the diode would still allow them to be charged. I have such a diode set up but using a small domestic alarm 12V battery at the helm for my chartplotter, which is very sensitive to the very voltage drop when the engine is started . The small battery maintains the voltage to the chartplotter when the engine is started but the starter cannot draw any current from it because of the diode. Despite the voltage drop across the diode, the small battery maintains a pretty high level of charge from the alternator.