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Old 03-12-2010, 09:47   #1
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How to Wire a Battery Isolator with a Three Wire Alternator ?

I now how to wire a battery isolator to a dual battery bank with a single wire alternator, but my alternator is three wire and I am confused how to best wire it. If you can help I would be very grateful.
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Old 03-12-2010, 11:20   #2
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The large gauge Red wire ("BATT"), that goes to the Battery Positive in a single battery set-up, goes (instead) to the Common Terminal of the Battery Isolator.

Wiring up the GM 10SI Alternator
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Old 03-12-2010, 11:29   #3
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The large gauge Red wire ("BATT"), that goes to the Battery Positive in a single battery set-up, goes (instead) to the Common Terminal of the Battery Isolator.

Wiring up the GM 10SI Alternator
But what about the two other connections, the field and sensing? I ended up with the a alternator voltage of about 15.5 volts which my Freedom inverter/charger didn't like?
The other thing with using a isolator and a two way battery switch is that one diode is shorted by the switch whilst the other will get less because of the volt drop across the diode in the isolator.
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Old 03-12-2010, 17:37   #4
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bazzer, what kind and model of alternator do you have? What are the 3 wires? Are you talking by chance of a dual output alternator?
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Old 04-12-2010, 10:29   #5
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bazzer, what kind and model of alternator do you have? What are the 3 wires? Are you talking by chance of a dual output alternator?
To be honest I don't don't know the make, but it isn't a dual output. Its on a Volvo MD17, the three wires are likely 1. the output 2. the voltage sense 3. the field excite or maybe it goes to the lamp on the control panel

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Old 04-12-2010, 10:39   #6
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You have to put the voltage sense wire on the load side (house side) of the isolator to compensate for the voltage drop across an isolator. All your loads must also be on that side of the isolator otherwise your loads will be receiving a higher than necessary or possibly too high a voltage that might cause damage to your battery or electronics.

The other wire is your field wire which controls your alternators field which controls its output. Don't do anything to this wire, just let it function as is.

The third wire is probably your ignition on wire. The ignition wire is turned on at the key and provides voltage to your regulator so it can create a field for your alternator.

Another wire is going to be your alternators primary output wire....the largest red wire.

Another wire might be a ground for your alternator, if you have this is, it is usually wired to the engine block or back to a secure ground wire going back to your battery.

You may also have a ground wire for the regulator which provides a secure ground.

Most alternators now have internal regulators which means that the wiring arrangement that you might see is different from alternators with external regulators.
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Old 04-12-2010, 15:05   #7
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OP you're somewhat confused. Normally a battery isolator ( unless fitted with field disconnect) doesn't go anywhere near the charging circuit. There's two schools of thought either wire the output of the alternator directly to the battery bank using a diode splitter to allow you to charge the banks. Fuse this wire near the battery. That way you can never inadvertently disconnect the alternator when running.

However this method does not allow full battery isolation. Some people just want to charge whatever battery bank is selected by the 2-1-both isolator switch hence the charging wire is connected to the + common of the isolator. Of course switching off that switch when the engine is running can kill the alternator

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Old 05-12-2010, 13:47   #8
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As Gord said "The large gauge Red wire ("BATT"), that goes to the Battery Positive in a single battery set-up, goes (instead) to the Common Terminal of the Battery Isolator."

What he didn't say is that a typical isolator will then have two "battery" terminals and two isolation diodes, one connected to each. You connect one battery to each terminal.

The problem is that the battery SENSE lead must now also be switched when the batteries are switched. This can be most easily accomplished if you have a battery switch with a second set of terminals for "field" isolation, and you use them to switch over the sense lead instead. (Then you lose field lead protection, but some alternators have that built in anyway.)

If you don't switch over the sense lead, what some folks do is put a third matching diode (same type used in the isolator) and they tie back the sense lead directly to the alternator charging output. That kinda defeats the purpose of a sense lead--but there's no easy and perfect solution here. With multiple banks, either you rig someone up to switch the sense lead--or you kludge it out.
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