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Old 11-28-2006, 08:30 AM   #1
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Should our TV have an Isolator Installed?

We have a new Suburban for our tow vehicle now and there is no isolator. We have one on the Expedition and I do not have to worry about disconnecting the charging cord to the Airstream. My husband is thinking about taking it a local RV shop to have it installed (after they inspect it to see if they can with the newer model--something about new computers etc.?) The Chevy dealer said in the past they have sent customers to this shop and they have had it done.

Is there a reason we do not want to do it? It was mentioned that if our coach batteries were dead we could hook-up to run lights. But wouldn't we want the engine on then anyway to not deaden the truck battery?

And as long as I am asking, we have room for a second truck battery. Can we add a second battery to be useful for our Airstream and charging?

Thanks, any comments or info would be appreciated.
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Old 11-28-2006, 08:47 AM   #2
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I don't recommend you use a diode-based isolator. It will lower the charging voltage to both your tow vehicle battery and the trailer. Look for something called a combiner or automatic charging relay (ACR) at marine stores. It has three wires. You cut the wire going back to the trailer plug and use two of the wires to connect to the ends. The third wire goes to the chassis ground. It does not effect the charging to the vehicle battery, but only adds the trailer batteries to the vehicle when the latter is running and the charging voltage sufficiently high. If your Expedition is anything like our F250, this is somewhat like what it uses, except it's activated by the ignition key instead of charging voltage.

A second tow vehicle battery won't help the Airstream batteries if it's wired on the tow vehicle side of the combiner. Even on the trailer side, it goes through too long of generally inadequate wiring where it gets too much voltage drop to be useful.
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Old 11-28-2006, 09:07 AM   #3
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Thanks Moe. I'm glad to have all this information and will direct Brad to it when he gets home. He has done some of our trailer wiring in the past. Now maybe he may decide to do it himself or at least he knows how to direct the techs.

He is also curious what the second truck battery would be used for.

Do you Chevy and GM owners make this modification or do you teach yourself to pull the cord? Odd how it's left off of them and not Fords. I used to be in the habit of pulling the cord when we had the three way fridge and ran it on 12volt.
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Old 11-28-2006, 09:46 AM   #4
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The second battery in a truck is usually there to assist the first with the heavy electrical load of glow plugs and turning the high-compression of a diesel engine with a starter motor. We have two in the F250.

A second battery is also used in emergency vehicles to help power lights, winch, pump, etc.
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Old 11-28-2006, 10:09 AM   #5
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Thanks for your input. It is so nice to have members such as yourself offering such thorough answers. The collective knowledge here is impressive. Brad could have exhausted himself asking through the normal channels in the "real" world and never ended up with half of what we can find here!
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Old 11-28-2006, 10:32 AM   #6
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Paula Ford (Foiled Again) messaged me after a recent thread on this topic. Her 2003 Suburban 2500 had its batteries drained after leaving it hitched via the umbilical.
Quote:
Originally Posted by RoadKingMoe
The second battery in a truck is usually there to assist the first with the heavy electrical load of glow plugs and turning the high-compression of a diesel engine with a starter motor. We have two in the F250. A second battery is also used in emergency vehicles to help power lights, winch, pump, etc.
We have a GMC Sierra 2500HD. It has the optional snowplow package which adds this second battery and a heavy duty alternator. Maurice, shouldn't the heavy duty alternator improve the normally slow coach battery recharge when underway?

Carol & Brad have a sweet ride! Isn't that the 'burb with the backup TV camera? What a way to hitch up!
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Old 11-28-2006, 11:18 AM   #7
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The current output of an alternator depends upon two things. The first is the engine speed driving it, and the second, and most influential, is the acceptance rate of the batteries, which is largely determined by the batteries' states of charge.

For a given regulated charging voltage, and assuming an engine speed fast enough to maintain that voltage at a certain current output, the batteries states of charge will be the primary factor, and the more charged the batteries are, the less charging current they will draw. At 50% charge, a flooded cell battery may accept 20-22% of its amp-hour rating, while an AGM may accept 40% or more. By 80% charge, this may be down to 10% or less and by 90% only a few per cent.

A high output alternator typically only gives a gain with batteries that are deeply discharged, and then only in the deeper discharge range. Keep in mind the charge wiring must also be larger to handle this higher current, especially with long charging wire runs.

[edit] I guess I should throw in here the regulated charging voltage has a lot to do with that. Alternator output may be as high as 14.4 volts or so, which is good for bulk (deep discharge) charging, but can overcharge charged batteries, which should only see about 13.2 volts float. Some regulators are set around 13.8 volts, which is more like what you'd use in the acceptance range (shallower discharge) and is safer for fully charged batteries. In the case of 13.8 volts, you may not see much charging gain from a high-output alternator. There are 3-stage regulators for boats that give the house batteries higher charging voltage, while keeping that for the starting battery lower. They're pretty expensive.
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