New Alternator wiring Question

Noah · Post by **Noah** » Fri Sep 30, 2005 10:06 am

Ok, I got a new Balmar 70 amp alternator to replace the old 50 that wasn't working in my boat. The engine is a universal diesel with factory wiring harness. The positive output on the alternator originally went through the factory Ammeter with 10 gauge wire, and back to the battery.

The boat now has a house bank and a seperate starting bank so the wiring is a bit more complex, but not much.

Basically both battery banks go through a guest A/B/Both Switch to a big distribution bus. This bus has a lead for the electrical panel, and the starter, as well as a couple of other things.

Should I run the new alternator positive output straight to this bus bar? It's the shortest wire run, and would seem to be the perfect place send it's output. I could also run the pos output straight to the feed terminal on the Guest switch, but it's a longer wiring run.

I understand that I will loose the Ammeter function if I go this route, but it's an Internal Shunt (direct reading), and I don't want to run 20ft of 2 guage wire. Perhaps this winter I will try to retrofit an external shunt Ammeter, maybe even one of the Xantrax Link battery monitors.

My copy of "Sailboat electrics Simplified" is on it's way, but not here yet, and I would love to get this job done today.

Thanks,

Noah

Figment · Post by **Figment** » Fri Sep 30, 2005 2:39 pm

Don't bother running the charge circuit back through the 1-Both-2-Off switch. Especially if the house bank and starting bank are different battery sizes.

Run the charge to the ammeter, as before, but then run it to a battery combiner (I'm 99% sure that West Marine makes one big enough for your application), then directly to the postive terminals on the batteries.

The battery combiner will allow both batteries to recieve charge whenever the alternator is charging, but will not allow the batteries to draw from one another while under load or at rest.

Noah · Post by **Noah** » Fri Sep 30, 2005 2:58 pm

I had thought of going with the battery combiner, but in this case the boat is very nearly sold, so every doller I put in I don't get back. It would be around $80 for the combiner, then 3 new "off/on" switches (So I can turn everything off, but still parallel them) which would run around $80, then a whole bunch of cables made up. Plus I don't really have the room to mount it all.

Would my simple solution work for the time being? Perhaps if I still have the boat next spring I will keep working on the system over the winter.

Thanks,

Noah

BTW, the house and starting bank are the same flooded gell-cell deep cycle batteries, except the house bank is 2 in parallel. House amperage is 220, start is 110.

Figment · Post by **Figment** » Fri Sep 30, 2005 3:06 pm

Even though the banks are made up of the same batteries, wiring two of them in parallel to make the house bank means that the two banks will draw and charge very differently. The battery combiner is smart enough to manage this difference.

The battery combiner is also about the size of a pack of gum. Mounting space is a non-issue.

You shouldn't need to modify your on-off switching at all. The difference is that those switches would now only affect which battery you draw from, and have zero effect on which battery is being charged at any given time.

Yeah, of course your proposed solution will be fine in the short term. The battery combiner thing is in the interest of the long-term health of the system.

Noah · Post by **Noah** » Fri Sep 30, 2005 3:43 pm

Ok, I'm starting to understand...I would just connect the ACR between the 1 and 2 terminals of the switch. )1 would be start battery) This way I can start the engine on 1 or both, and charge under power on 1. What happens when I put the switch to both? Particularly when I'm under power, but also using the house bank? Or just going out for a day sail, when it's fine to use both banks. Just because I don't want to flip a switch to turn on the engine at the end of a sail after using the radio, etc all day...

Sorry I'm asking a lot of dumb questions...

Figment · Post by **Figment** » Fri Sep 30, 2005 5:25 pm

Or you could just connect the alternator's charge wire to the hot terminal of the starter. This is functionally the same as connecting to the hot-output terminal of the battery switch, but would be a shorter cable run.

In this condition, the alternator charges whatever the battery switch indicates. If drawing from 1, you're charging 1. If drawing from 2, charging 2. If both, both.

The one HUGE caveat here is to never never EVER turn the battery switch through the "off" position with the engine running. This leaves the current produced by the alternator nowhere to go, and will instantly kill the alternator. I suspect you already knew this, as this is was also true under your original wiring configuration.
(wiring direct to the batteries eliminates this possibility)