How Do I Wire Up An Extra Alternator and Batteries?

[charliehows]

I,ve removed an old refrigerator compressor, want to use the extra pulley space to put in a second alternator and an extra 12v battery or 2 just to run a smaller frig. - but i,m pretty hazy on how to wire it up. Ideally i'd like to have it as a completely separate system - ie no earthing through the engine or existing wiring system, but im not sure how to stop the batts discharging back through the alternator - would a diode or something be necessary?

[Dockhead]

Why don't you charge the start battery with the existing alternator, and install a larger alternator to charge your house bank, keeping them completely separate?

[40 South]

charlie
You need a special "above earth" alternator, one with + and - terminals and a field terminal. It can be internal or external regulator. Wire alternator + and - terminals directly to you new battery and your away. The alternator and battery are isolated from the rest of your electrical system.......that is until you wire it to the battery to get some electricity out, though you could wire one item (eg anchor winch) to this battery and it will still be separate system.

[S/V Alchemy]

You want to size the fuses and the switch correctly, and you want a way to monitor state of charge for the second alt and battery system. Lastly, you want a way to shunt power from that battery to the main panel (again, properly fused) or the house bank in an emergency, which I presume is part of why you are doing this.

There's a number of ways to set this up, of course. It depends on your end goal. I've decided (so far!) to have two alternators feeding into a large (1000 Ah) house bank, which in turn feeds via an Echo Charger to a start battery. I will have an entirely separately charged winch battery forward that will have a combination of switchable echo charging, and charging via genset...I'm doing this to avoid a long, expensive run of cable forward (it's a manual and electric winch).

The side benefit of this is that I will have a customarily fully charged battery that I can haul back to the engine bay if I have a catastrophic failure of the house bank.

40 South's suggestions are therefore of interest to me.

[charliehows]

thanks dockhead, i want to leave the existing sys. alone - 'if it aint broke...'
40 south - im thinking ill wire it to the frig. direct, just turn the thing on when im aboard. i'll check out the alt. you suggest. Internal regulator sounds good.
alchemy; if i have to route extra power to starter i'll just change the batt. (the other alt and a solar panel on the starter batt. keep that system pretty reliable) All i want this extra alt/batt. sys to do is run a small frig. while im aboard. I want to avoid having an expendable item like a frig. causing problems in the important system like lights and starter.

But i could be doing something a bit silly - has been known to happen...and a bit of constructive criticism at the planning stage might save me some trouble...