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Old 12-31-2008, 11:33 AM   #1
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tow vehicle charge line to trailer battery..

I was wondering because the trailer battery is so far from the engine alternator and tow vehicle battery, would a battery separator or a battery isolator be a good idea? Because.. once the vehicle battery is charged and the other battery is so far away, how does it ( alternator) know that the battery in the trailer needs charging beyond the needs of the primary vehicle battery? Because.. I am running the refrigerator in gas mode which still uses some 12 volt electric while traveling. Thanks for any feed back.
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Old 12-31-2008, 11:46 AM   #2
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Safari, happy birthday. Welcome to old age.

My not too great understanding is that recent model trucks with a tow package should have a battery isolator, heavy duty alternator and larger battery. I believe (and surely will be corrected if I'm wrong) is the isolator charges the truck battery first and when it is fully charged, charges the trailer batteries. Reddy Kilowatt's little brother may be inside the isolator making that decision, but more likely it's transistors and capacitors.

Since you have an older truck, check with the service dept at a dealer and they should be able to tell you exactly what you have from all those strange numbers in the VIN. If you bought the vehicle used, someone may have added after market equipment and either the dealer or a reputable RV shop could check for these items. Though the tow vehicle doesn't really provide all that much to charge the trailer batteries, it's plenty for the igniter and electronic stuff in the refrigerator and other accessories (such as the propane detector) that are used while traveling.

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Old 12-31-2008, 12:38 PM   #3
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I tow with a 99 Dodge Dakota that did not come equipped with a tow package. I had to add the isolator for the trailer battery along with the brake wiring, etc.
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Old 12-31-2008, 02:25 PM   #4
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I know I will get disputed on this, but here goes....you really don't need an isolator. All the isolator does is not allow the trailer to discharge the truck battery, or vice versa. When the truck and trailer batteries are tied together in operation, the alternator sees one load, and the battery with the lower voltage, gets the most charge current until it equals the other battery, and then they get the same current until the alternator regulator cuts back. If you do not have an isolator, it is a good idea to disconnect the trailer electrical from the truck when you park for the night.
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Old 12-31-2008, 02:36 PM   #5
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What happens is that it just takes longer to bring the RV battery up to full charge.

The vehicle charging system senses the chassis battery and often has a separate lead for this. Once that battery is up to a proper float voltage, the system keeps it there.

So the situation is that the charging source quickly gets the chassis battery back to a float voltage yet the RV battery is still discharged because current to it has a lot of resistance to get through. The RV battery will then continue to charge because it is below the charging system's float voltage. The charge current will be limited by the resistance in the feed line plus the fact that the source voltage is not very high. That is most of the reason why it can take quite a while on the road to get the RV battery charged up.

When the battery gets up to a full charge, it will no longer accept current at the float voltage level and the very low currents nearing that point mean very low loss in the charging line. So it gets there, it just takes a while.

Note that the ground return on the RV battery is often a good source of resistance itself and a common source of problems.

A properly wired vehicle charging system that supports both RV and chassis batteries will have precautions against draining the chassis battery with RV loads. This can be a relay that will close only after the engine is running and the charging system functioning or it might be a diode isolator. The diodes have fallen out of favor because they impose a voltage drop that slows charging and prevents full charging.
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Old 12-31-2008, 03:00 PM   #6
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I put the isolator in to protect the tow vehicle charging system in the event that something goes amiss with the trailer, I also installed a circuit breaker downstream of the isolator for extra protection. Call me paranoid
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Old 12-31-2008, 04:00 PM   #7
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What is the gauge of the charge line?

Quote:
Originally Posted by safari25-05 View Post
I was wondering because the trailer battery is so far from the engine alternator and tow vehicle battery, would a battery separator or a battery isolator be a good idea? ....
Interesting that you should bring this up - the charge rate/voltage available at the trailer battery is HUGELY dependant on the size of the wire between the Tow Vehicle alternator and the trailer battery.

Example - for a 20 amp charge rate a typical charge line (both positive AND negative connections) would require a #4 wire - decrease that size to #6 if you would be happy with a 15 amp rate.

As mentioned earlier, there would be a sever amperage blockage at the coupler unless an additional (properly sized) negative jumper line were installed between the TV and the trailer.

Note that most typical 7 pin trailer/TV jumper lines are woefully inadequate to carry much of a useful amperage load.
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Old 12-19-2013, 08:16 AM   #8
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I had my trailer at the factory for about a week. I drove it home (~4 hours) yesterday with the battery in the disconnected mode. I flipped it on to check and it was low according to the monitor. I have a feeling I'm not charging from my tow vehicle.

I'm prepping to drive to Houston for Christmas and I *really* need to be able to charge on the road. Any advice before I head out with a multimeter?

Thanks.

--kyler
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Old 12-19-2013, 08:23 AM   #9
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We had our trailer parked for 4 days at a friends a few years back. I had left the refrig on gas during that time. I thought the batteries would have carried it but when we picked up and hit the road it was dead, dead, dead. So I guess they couldn't carry the refrig for the 4 days. Anyhow, hooked up and on the road it took about 4 hrs of travel before the batteries re-couped enough to operate the refrig. So, as had been said before there are many factors that determine how much charge gets to the trailer batteries and unless you modify and upgrade those links the factory wiring charges at a very low rate. However, it does charge.

We are now careful to remove the stuff that needs refrigeration so we can shut it down when stored for more than a day or two.
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Old 12-19-2013, 10:42 AM   #10
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Thanks, Roger. I got dirty with the wiring and poked around the owner's manual. It looks like it should charge from the tow vehicle. The diagram shows that it charges through the +12V line, not the center (reverse) pin. "The 7 way wire is spliced to the main harness in the area of the 12-volt distribution panel in front of the trailer." I haven't found it yet. There's a 30A breaker from the 10 gauge charge line. 30A? That sounds like a decent charge current to me.
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Old 12-19-2013, 10:54 AM   #11
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get out a test light or meter. does the charge line have power at the 7-way?

was the furnace on?
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Old 12-19-2013, 01:52 PM   #12
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I believe that when the TV battery is fully charged, getting a charge rate of more then 1 amp to the trailer would be optimistic on most stock vehicles.

The system was only designed to keep a backup battery for a breakaway switch for electric breaks charged, NOT recharge a battery for a TT. That's not to say that it shouldn't be able to keep the battery charged and power the 12v circuit board in the fridge, that much should be easy.
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Old 12-19-2013, 02:02 PM   #13
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Quote:
Originally Posted by Wazbro View Post
The system was only designed to keep a backup battery for a breakaway switch for electric breaks charged, NOT recharge a battery for a TT.
Probably true, but - if I understood the conclusions above correctly - getting more power (say 10 to 20 amps) to the camper for actual charging purposes shouldn't be too much harder than running heavy wires (appropriately fused) to the connector both on the tow vehicle and the trailer. Did I follow that conclusion correctly?
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Old 12-19-2013, 03:11 PM   #14
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Quote:
Originally Posted by kylerlaird View Post
I had my trailer at the factory for about a week. I drove it home (~4 hours) yesterday with the battery in the disconnected mode. I flipped it on to check and it was low according to the monitor. I have a feeling I'm not charging from my tow vehicle.

I'm prepping to drive to Houston for Christmas and I *really* need to be able to charge on the road. Any advice before I head out with a multimeter?

Thanks.

--kyler
1) It varies from trailer to trailer but some will not charge the battery from the charge line if the switch is in the "store" or "disconnected" position

2) A clamp-on dc ammeter will help diagnose things. I have a cheap mechanical one which works well enough.

3) You may have a loose wire at the Airstream fuse block

4) on some tow vehicles the charge line is not connected as the vehicle is shipped by the factory and you have to move a wire or add a fuse to get it to work.

Good luck
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