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huguley3 03-06-2009 05:20 PM

Alternator wiring to batteries
I am putting in a Xantrex combo charger/inverter. I don't really plan to use the inverter except to allow the occasional guest to charge their cell phone. I mostly want it to charge up the batteries when I am plugged into shore power.

right now I have 2 AGM batteries in 2 banks. The alternator is connected via a switch so it can charge both or just one of the batteries. Thats pretty normal except that most people have the house and starter batteries in the 2 banks as opposed to just 2 of the same batteries.

My question is this. Can I combine both batteries into one bank and connect the alternator directly to them? Will the alternator draw some of the power off when it is not running and that is why it is switched? The switch is not one where you can switch the banks with the engine running so to charge both batteries it needs to be set to all or stop the engine and switch it. It seems kind of silly to 2 identical batteries split into banks the way they are.

I realize the general idea of having splitting batteries into 2 banks is for safety so that you can always bridge to the other bank and get the engine started. My engine does have a manual hand crank so that is an option if the batteries are dead. If the batteries are dead i am going to be really crabby anyway so hand cranking a diesel will really put me over the edge. :)

I am going to add another post asking about battery monitors.

badsanta 03-07-2009 02:08 AM

just to address your inverter. If you are just charging phones and such all you need is the little inverter that plugs into your cigerett lighter plug. I have been using one for years with no problem.

badsanta 03-07-2009 02:11 AM

on your batteries do a search of this site, there is alot already posted. Sorry I dont have the time tonight to go over it

pvanv1 03-07-2009 06:20 AM

One bank (in your case, with both batteries combined) is always simplest. Of course, you do run the risk of accidentally draining the battery with accessories. The alternator should not drain the battery when it's not running, if everything is working properly. Just like a car. That happens to be the setup we have on our boat. We sized the bank so that it's big enough that we don't deplete it. That would be impractical on a larger vessel.

Since you already have the battery switch, you could leave it setup as now. Just run the aux on "both" -- and when not motoring, switch to whichever bank you want to call "house". The next time you're going to fire up the aux, again switch to "both" (first) and everything should be OK. You could even alternate which battery you call "house", say, batt 1 on odd-numbered days, and batt 2 on even-numbered days. Since your switch isn't the make-before-break type with a field connection, just remember to never move the switch while the alternator is running.

A shore-fed inverter/charger shouldn't be an issue. They are typically setup to handle 2 banks for charging, so you can feed to each bank, and the switch position during a shore charge would be irrelevant, as long as the charger allows combed outputs. Ours does. We have a Guest 2-bank charger, but combine both outputs into one bank. Works "swimmingly", as the British might say.

The better chargers on the market now, and I think Xantrex is one, can proportionally charge whichever battery needs all the amps. In other words, say you have a 2-bank "nominal 10 plus 10 amp" charger, wired to 2 banks, and one bank is already topped up, these kinds of chargers can direct the full amperage (in this case, 20 amps) into the bank that needs it. Very nice.

As mentioned, if your inverter needs are only for an occasional cell phone charge, you could just use a "pocket" inverter plugged into a "cigar lighter" power outlet, up to about 75 watts or so. However, a sophisticated inverter/charger is a great convenience, since the AC outlets can be live even when the shore power is disconnected.

huguley3 03-07-2009 07:46 AM

Thanks for the info. My main goal was to make it simpler so that whenever I was plugged into shore power the batteries would get topped up without having to do anything. I don't really mind having the 2 banks but I generally run off both and charge both so I don't use them separately.

My charger will only charge 1 bank its an Xantrex HF 1000.

Now that I am getting over my cold and beginning to think a little better I think I will keep both banks and connect the charger positive to the DC main switch where the alternator connects. Then I can keep the 2 banks and the wiring is simpler. The only more complicated thing is an extra step to flip the switch to both banks when I connect to shore power. Another bonus with that is that if I get one battery that is misbehaving it won't bring the other down with it. I can more easily monitor what is going on with each battery without having to disconnect them from each other.

I guess if I had to do it over again I would have just gotten 2 chargers and forgotten the inverter. Then all I would have had to do is to add another switch on the ac panel and it would have been a lot less messing around to accomplish the same goal.

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