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Old 07-20-2011, 02:59 PM   #1
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Does the truck charge the trailer battery?

I was just wondering when you are ready to tow and your 5th wheel or TT are plugged into your truck will the truck charging circuit charge the trailer battery when driving?
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Old 07-20-2011, 03:03 PM   #2
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Yes
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Old 07-20-2011, 03:04 PM   #3
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Yes, the larger white wire at your plug on your truck should be the charging wire for the TT batteries. If it is plugged in to the truck and you are not running the truck it can pull your battery in the truck down. Want to know how I know(been there done that)?
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Old 07-20-2011, 03:13 PM   #4
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Quote:
Originally Posted by Iggy View Post
I was just wondering when you are ready to tow and your 5th wheel or TT are plugged into your truck will the truck charging circuit charge the trailer battery when driving?
Maybe but probably not much.
The reason is the distance (feet of wire) involved.
The problem is your truck engine battery is very close to the alternator
and it's internal voltage regulator.
The trailer battery(s) is/are several feet down stream.
Your engine battery gets a quick charge and the voltage peaks and the
voltage regulator drops back to a lower voltage.
The trailer battery does not get enough voltage to boost its charge much.

Some folks have run larger battery wire like 8 or even 6 gauge and it seems to help.

There are gismos that you can buy that boost 12v up to a charge voltage
of closer to 15 just for charging batteries while on the road but they're
kinda pricy.
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Old 07-20-2011, 03:15 PM   #5
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Yes. However, I was told by our dealer that Ford's only work when the key is on. So, if you are connected, but the truck isn't running, there is no drawl.
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Old 07-20-2011, 03:25 PM   #6
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This discussion comes up here and other RV groups all the time.

Facts
Some trucks have a relay that disconnects the RV battery lead when
the truck is not running.
Some don't.
If you have the relay you probably can't run down your truck battery
by leaving it plugged into your trailer at a camp ground.
MY truck does not have the relay and I have in fact gotten up in the
middle of a cold night and gone outside and plugged my truck back in
so I could get a few more hours out of my LP furnace when my trailer
batteries were dead.

MOST trucks do not have large enough RV battery lead at the plug to
allow much worthwhile charging of the trailer batteries while the
truck is on the road.
YMMV

My last 2 trucks would not charge much.
More than once I've left the camp ground with very low batteries-
driven home (2 hours on the interstate) and once I get home
I turn on lights inside the trailer to unload our gear.
Within a few minutes the lights are dim proving that the trailer
got almost no charge on the drive home.

Just because the engine and trailer batteries are connected via the RV
plug does not mean you will get a meaningful charge while on the road.

The long wire run from engine to trailer produces enough voltage drop
to prevent much of a charge.
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Old 07-20-2011, 03:31 PM   #7
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Quote:
Originally Posted by sfrye5853 View Post
Yes. However, I was told by our dealer that Ford's only work when the key is on. So, if you are connected, but the truck isn't running, there is no drawl.
IMHO, that is a good thing. Theoretically, 12 volt items running in the camper could pull your truck battery down to the point where your truck would not start.

That was more important when I had my Trailmanor, which had a 3-way fridge with a12 volt function. The heating element in that pulled some major amps, so without the Ford 12 volt isolation circuit, that could have gotten me in trouble.

I think some pop-ups still use a 3-way, 12 volt fridge for cooling while traveling, so they need to be aware of that.
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Old 07-20-2011, 03:45 PM   #8
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Quote:
Originally Posted by sfrye5853 View Post
Yes. However, I was told by our dealer that Ford's only work when the key is on. So, if you are connected, but the truck isn't running, there is no drawl.

This is what I think my Ford is doing. The truck battery is disconnected when the truck is not running. You see my trailer battery was being used a lot while I was hooked up to my truck and just wondered.
Thanks for the answer.
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Old 07-20-2011, 05:07 PM   #9
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I was told to unplug it from the truck if I stay hitched at the CG otherwise it could over charge the truck battery.
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Old 07-20-2011, 08:06 PM   #10
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I was told to unplug it from the truck if I stay hitched at the CG otherwise it could over charge the truck battery.

Yes this is try if you are plugged into 120 volt shore power.
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Old 07-21-2011, 07:11 AM   #11
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Quote:
Originally Posted by Terier View Post
I was told to unplug it from the truck if I stay hitched at the CG otherwise it could over charge the truck battery.
Think about it...... if the charger in your trailer won't over charge
your trailer batteries why would it over charge your truck battery???

There's probably no good reason to leave your truck plugged into the
trailer while camping so I say unplug it.
But it's the same as the original question-- the battery closest to the
charger is going to get the most charge.

Of course if you have a newer Ford and maybe some others with the isolation
relay that disconnects the battery from the RV plug when the engine is off
charging or discharging the engine battery via the trailer connection is
not going to happen.

When we're dry camping---
OTOH if I want to use my truck battery as an extra boost sometime I'm
glad I can plug in and get the extra power.
I gave the example of needing a few more hours of juice to run my furnace
on a cold fall camping trip.
Another example is I watch a little TV using a cigarette socket inverter.
The TV is fussy about voltage and will shut off if the voltage is the
slightest bit low. We have to shut off the water pump when
we're watching TV because every time the water pump cycles it shuts
off the TV

Occasionally I've been known to plug in the truck to get that little bit
extra needed to keep the TV on. I usually unplug the truck when we
get done watching the TV.
Much of this was back when we had just a single 12v battery on the trailer.
We now have dual 12v on the trailer and I don't need the truck boost very
often.
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Old 07-21-2011, 07:50 AM   #12
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yes.

i'm fairly certain that the ford disconnects when not running.
i would compare the charging to using a battery charger. the wire size isn't going to play a significant part in it..it is large enough. if ur battery is low enough to require 30 amps charging, u will blow the fuze in the truck.
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