What's this 3rd battery do??

[Umpyman]

OK, a 1994 290DA with twin V6 Mercs...has three batteries, all group 27 brand new put in last year when we bought it. One on the port side (just aft of the black water holding tank) and two on the starboard side (just starboard of the fuel tank, as far forward in the bilge as they can go, one behind the other). I need to figure out which one does what...the wiring is done very nicely in sealed looms, so tracking where the cables go is not entirely feasible, so I disconnected one at a time.

When I disconnect the port one (which I THOUGHT would have been a house battery), I have no juice to the port engine. When I disconnect the aft-most starboard battery, no juice to the starboard engine. OK, figured that part out. But the port also appears to run the house power (cig lighter, lighting), and the Clarion radio drops out when EITHER are disconnected. And the big mystery? When I disconnect the forward starboard battery, I can't find ANYTHING that lost power...what's it doing?

Ideally I'd like to have the third battery, the one not starting either engine, power all the electronics so there's no danger of hours of DVD movies and video games on the hook don't strand us....can I use that mystery 3rd one and run new wiring to it?

 

[wadebmadden]

Perhaps instead of disconecting one at a time and see what dosnt work (you still have 2 connected remember)

try unhooking 2 at a time and see what still works on the 1 battery. MAKE SURE YOU REHOOK THE SAME WAY THEY WERE REMOVED!!!!!!

Nice neat wire bundles are a blessing unless you are trying to trace wires,,, I understand.

Are you sure there isnt a battery switch somewhere?

West Marine offers a book on marine electronics,,, havnt read it, but I know its there.

Not saying this is the norm, but my 3batt setup works like this,,,

I have a switch with 4 different positions. Bank1, Bank2, All, and Off.
Bank 1 just has one battery, Bank2 has 2. I can sit and run the microwave, blender tv whatever off either bank one or 2 and if I run the battery down I can switch over and still start the engine,,,, I can use all 3 and if I drain them Im dead in the water.... I Like this setup as it provides a very big safety net. Just my 2cents


wade