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Old 10-05-2012, 02:15 PM   #1
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1976 31' Sovereign
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Charging AGM Batteries via Tow Vehicle Charge Wire

Folks.. the 1976 Airstream Sovereign remodel is coming along well, but I have an electrical question!

I have 2 AGM Lifeline 210amp-hour batteries run in parallel, producing 12v for the trailers electronics.

I would like have the option of adding charge to this battery bank using the tow vehicle alternator, BUT I'm afraid to just hookup the charge line from the tow vehicle directly to the battery bank (wrong voltage, wouldn't know when to stop charging) so I assume I need a charge controller for the battery bank?

If so, what size charge controller do I need.. 10 amp.. 30 amp? How does the tow vehicle know how much of the alternator's current it can 'spare' and send to the Airstream for charging purposes?

Thanks for everyone's help!
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Old 10-05-2012, 02:39 PM   #2
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You're trying to solve a problem that doesn't exist. Just hook up the wire.

Your 2001 Tahoe has the alternator output regulated by the powertrain control module, which uses a temperature-compensated algorithm. In my experience Chevy seems to drop the voltage slightly after about 2 hours of continuous driving, but I've never confirmed that with accurate measurements or in any publications from Chevy.

Anyway, the AGMs are nominally supposed to be charged about 0.2 volts higher than flooded cells, so overcharging isn't going to be a problem, especially with the resistance in the charge circuit.

I have a charge line set up and have used heavier than normal gauge wire, and have written here and there about my setup. I measure the current from time to time and my overall experience is that with the right setup you can get some useful charging, enough to make a real difference in certain situations particularly when dry camping overnight to break up a long trip on the road, and when cold temps require running the furnace while under way.

I don't know of anyone who has tried to use a solar-type charge controller to help matters. I'm not sure I would recommend it.

In practice the current flow is limited by the overall resistance of the circuit. It's hard to get more than 30a to flow. If you don't have the large-frame AD-244 alternator (or the cost-reduced DR-44G one which works just as well but isn't repairable) in your Tahoe then you might consider that as a possible upgrade especially if your existing alternator fails.
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Old 10-05-2012, 02:48 PM   #3
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Originally Posted by gtbzz85 View Post
Folks.. the 1976 Airstream Sovereign remodel is coming along well, but I have an electrical question!

I have 2 AGM Lifeline 210amp-hour batteries run in parallel, producing 12v for the trailers electronics.

I would like have the option of adding charge to this battery bank using the tow vehicle alternator, BUT I'm afraid to just hookup the charge line from the tow vehicle directly to the battery bank (wrong voltage, wouldn't know when to stop charging) so I assume I need a charge controller for the battery bank?

If so, what size charge controller do I need.. 10 amp.. 30 amp? How does the tow vehicle know how much of the alternator's current it can 'spare' and send to the Airstream for charging purposes?

Thanks for everyone's help!
I am not sure what you are planning.
Are you going to have the Vehicle be the only method of charging the batteries?

The tow Vehicle does not to need to "know" anything, The design of the alternator, the condition and state of charge of the TV battery and the TT batteries, the size and length of the wiring in both the TT and TV will determine how much of the alternator's output goes to what purpose.
The loads will be: (in no particular order)
Running the vehicle and whatever accessories are turned on
Running whatever things are turned on in the trailer
charging the TV batteries
charging the TT batteries
You cannot control the priority of these functions without changing the design or construction of the system in some way or another.

In any case a controller in the TT would not help in any way. If you tried to use one, it would only be able to lessen the ability to charge, It could not improve it.

If you are talking about charging the trailer while driving, then just hook it up and go. The same factors above will determine how much charge you are getting to the TV batteries.

Ken
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Old 10-05-2012, 06:52 PM   #4
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There is one somewhat realistic option other then the charge line. That is to not connect the charge line and instead use a battery charger or your converter plugged into an inverter in your TV. This would do a better job of charging your trailer batteries, just make sure your alternator has enough power and the wires to the inverter are heavy enough for their length.
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Old 10-05-2012, 08:30 PM   #5
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There is one somewhat realistic option other then the charge line. That is to not connect the charge line and instead use a battery charger or your converter plugged into an inverter in your TV. This would do a better job of charging your trailer batteries, just make sure your alternator has enough power and the wires to the inverter are heavy enough for their length.
Interesting idea. I wonder how the loses in the inverter and converter would compare to the lose in a decent size wire at 12 volts.

Ken
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Old 10-06-2012, 04:20 PM   #6
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Interesting idea. I wonder how the loses in the inverter and converter would compare to the lose in a decent size wire at 12 volts.

Ken
Not sure just thinking the converter would just charge the trailer batteries normally not dependent on the TV battery for part of how to charge. Haven't tried it since I don't do much boondocking I don't have much reason to try it.
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