Charging while towing

Anything electric, AC or DC

Postby chukar » Sat Sep 25, 2010 5:41 pm

mezz wrote:If in a pinch, could you charge your trailer battery with jumper cables off your vehicle?


I think so. Anyone??
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Postby Von Pook » Sat Sep 25, 2010 8:12 pm

though dangerous, yes it would work very well. Just remember, jumper cables can create a spark, and well, to quote bugs bunny, "BANG!!!!" :?
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Postby weeeee » Tue Nov 16, 2010 3:37 pm

Using the Tow vehicle to charge the RV batteries during towing, what keeps the RV batteries from being overcharged?
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Postby absolutsnwbrdr » Tue Nov 16, 2010 4:13 pm

The same voltage regulator that keeps your tow vehicle battery from being overcharged
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Postby weeeee » Tue Nov 16, 2010 4:20 pm

Thats what I suspected, but I was concerned that the batteries are different type.

I am planning on using 2-6 volt Golf cart batteries in series.
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Postby jimqpublic » Thu Nov 18, 2010 2:27 pm

Although the "standard" is 10AWG charge cable, the RV forums are full of statements that you can't really expect much charging while towing.

I submit that 10AWG is way undersized for most systems, especially if you want to push much current (recharging vs. maintaining charge). The tow vehicle battery acts as a buffer and "tricks" the alternator's voltage regulator into dropping voltage. Also the voltage drop over a long cable run is significant enough to slow charging way down.

Going up to 8 AWG will halve the voltage drop.

But then there's overkill past the point of reasonableness. That's what I did when wiring up our Subaru Forester. It does a great job of charging the batteries- We have camped in the snow for four nights, then driven about 2-1/2 hours and camped three more nights with sleet and freezing rain and always had plenty of battery power to run the furnace. On the other hand, I spent over $200 in wire alone. Connectors, relay, fuses, heat shrink etc. were probably another $200. I should have just bought a good ultra-quiet Yamaha generator for $800 instead and charge the battery whenever I wanted.

My charge system is basically:

Car:
Alternator + output lug (90 amp Mitsubishi alternator with 2:1 pulley ratio, rated 50 amps at idle)
6AWG cable (approx 4')
40 amp self-resetting circuit breaker
85 amp continuous duty solenoid switched by accessory circuit.
6AWG cable (approx 15')
7 Pin female trailer connector.

Trailer:
7 Pin male trailer connector end on 7 wire trailer cord
10 AWG wire in trailer cord (approx 10')
Connection at standard RV power converter.
60 amp fuse (low resistance car stereo fuse holder & fuse)
4AWG cable (approx 18', from RV converter to rear-mounted batteries)
100 amp fuse (low resistance car stereo fuse holder & fuse)
Dual 6 volt GC batteries in series (220 AH rating)

negative side has cable to parallel the positive side

I went overboard on the fuses- but I wanted protection against a major short in the system while limiting voltage drop as much as possible. The 6AWG cable in the car is tinned marine cable (Current price is $92 for a 25' roll!). The trailer-side 4AWG battery cable was much cheaper at $1/foot. Most connections are crimped, soldered, and covered with adhesive-lined heat shrink. Fuse block connections are bolted and greased to inhibit corrosion. I ran parallel ground wires both in car and trailer instead of relying on high-resistance frame ground points.

The standard 7 pin connector and 10AWG wire in it are my weak links. I should really add a redundant charge + ground connection between the heavy wires on car and trailer.

Trailer running lighting is powered by the charge cable with a multi-relay trailer light controller (so it only draws signal current from the car's running, turn, & backup lights)

In hindsight it was completely over the top. That said, slightly heavier cable is a good idea (Just not the $$$ stuff from West Marine)

PS-
If you do have a good charge setup, or are using jumper cables- charging a heavily discharged trailer battery with the car idling can overheat the alternator. It gets cooling from its built-in fan and airflow from driving or the engine fans. Idling it is putting out a lot of amps from fairly slow (relative) rotation speeds, plus heat soak from the engine. Normally the electric fan for the engine is off most of the time at idle. My solution is to open the hood and turn on the AC, which triggers the fans to run. So far, so good.
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Postby taurusSHO » Wed Jan 12, 2011 10:17 am

I didn't see a plain diagram for this yet, so I drew this up in mspaint:

Image
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Postby Tumbleweed_Tex » Wed Jan 12, 2011 11:38 am

len19070 wrote:
Minicamper42 wrote:If I understand what everyone here is saying, you are going to connect a wire to a second battery to your existing one to charge the trailer battery?...uh, shouldn't you guys be using a battery isolator?...sure the battery will get a charge, but at the same time your turning your cars electrical system into 24 volts!...even if you use a relay and fuses, you can ruin an ECM better known as a computer that runs your engine...

If I am the confused one here, please enlighten me on your reason to just run a hot wire from a second battery and connect it to the alternator so it can be charge...I understand the whole switching thing to separate the two batteries for charging, but why go through all the hassle when you can just bolt in an isolator and forget about it. :thinking:


Whoa, whoa there.

There is NO WAY if you run a hot wire from your TV's positive battery terminal back to the positive terminal on the trailer battery (and the same with a ground to ground connection) that you will make 24 volts and burn up things.




:)

I suppose stirring things up just comes natural for me, but this got me to thinking. (that’s a scary thought in and of itself)

Let’s suppose I get my rig all wired up and ready to roll, with the TV battery (A) hooked up in parallel with the trailer battery (B) so it will charge whilst I travel.

This means that the A+ battery post is connected to the B+ battery post, and the A- and B- posts are connected as well. Parallel circuit, so a 12v light bulb somewhere in this circuit is happy, receiving 12v from the two batteries.

Just before I pull out of the driveway, I decide to check the hookup one last time, so I raise the hood on the TV, and using the open end of my new ½ inch chromium plated Craftsman combination wrench, I attempt to tighten the clamp on battery post A+. Unfortunately, the wrench slips and shorts out against the fender of the TV. (I’m such a clutz)

Now, I have two batteries in parallel (sorta) but I also have two batteries in series (sorta also) cause A+ is connected to B- by the wrench, which has no fuse. I wonder, at this point in time, what is the voltage across the light bulb…and um…across the ECM?

Just curious…
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Postby Shadow Catcher » Wed Jan 12, 2011 12:37 pm

As we all know :thinking: the care and feeding of a deep cycle battery is a bit tricky and the voltage regulator from a tow vehicle is likely to be less than optimal for this purpose. Recently I stumbled on to a solution in the form of a Balmar regulator. these are built for marine use but are used in behemoth RV's.
The Digital DuoCharge is of particular interest, the big drawback is they are not inexpensive!$>
But then how much is it worth to do the charging right. :thumbsup:

They are programmable for different kinds of batteries. There are other manufacturers.
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Postby len19070 » Wed Jan 12, 2011 6:52 pm

Review first Page of this Post

KISS


Less parts=Less problems


These are not some hair brained solutions thought up by a few egg heads that just put down a Bong.

These are the ways the RV industry has done it for at least 5-6 decades.....with no problems. Millions and Millions of miles have been traveled with this method.

If you throw up your hands and take both vehicles to a Professional RV dealer, this is the way they will do it.

And I will guarantee that if you break down on the road with anything other than a "simple system" You will;

A. spend a lot of time and money to get it repaired

or

B. The Tech will return it to a "Simple System"

Not my Opinion...facts

Happy Trails

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Postby bbbrt76 » Wed Jan 12, 2011 7:14 pm

Tumbleweed_Tex wrote::)

I suppose stirring things up just comes natural for me, but this got me to thinking. (that’s a scary thought in and of itself)

Let’s suppose I get my rig all wired up and ready to roll, with the TV battery (A) hooked up in parallel with the trailer battery (B) so it will charge whilst I travel.

This means that the A+ battery post is connected to the B+ battery post, and the A- and B- posts are connected as well. Parallel circuit, so a 12v light bulb somewhere in this circuit is happy, receiving 12v from the two batteries.

Just before I pull out of the driveway, I decide to check the hookup one last time, so I raise the hood on the TV, and using the open end of my new ½ inch chromium plated Craftsman combination wrench, I attempt to tighten the clamp on battery post A+. Unfortunately, the wrench slips and shorts out against the fender of the TV. (I’m such a clutz)

Now, I have two batteries in parallel (sorta) but I also have two batteries in series (sorta also) cause A+ is connected to B- by the wrench, which has no fuse. I wonder, at this point in time, what is the voltage across the light bulb…and um…across the ECM?

Just curious…



what you have at that point is two batteries wired in parallel and two circuits coming off of those batteries, one of those circuits (the carelessly handled wrench) is a dead short, assuming you manage to hold the wrench there and not just momentarily bump it and make a few sparks, the massive current flow through the wrench will cause the voltage of the batteries to drop and all other circuits coming from those batteries will see a low voltage situation, not usually damaging to the components, although the burns on your hand and the melted wrench....
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Postby Von Pook » Wed Jan 12, 2011 10:00 pm

if you can hold the wrench there, without jumping backwards from fear, then the result would be, to quote Tweety Bird, "a big BANG".
Eric, the camping sound guy
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Postby Shadow Catcher » Thu Jan 13, 2011 7:24 am

I am a great fan of the KISS principle, unfortunately that may well not be the smartest most efficacious solution. Part of the education concerning the care and feeding of deep cycle batteries has come from those with solar experience. And part of that experience is you cannot effectively use mismatched batteries. As pointed out what you're doing with charging both the tow vehicle battery and your trailer battery is a parallel circuit. In a parallel circuit as I understand performance is set by the weakest battery. Because RV dealers have been doing it for years, does not mean they're doing it correctly. And in this case some/most use a two charging circuit system, one for the tow-starting vehicle and another for the house batteries. Some are in fact using the Balmar regulators.
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Postby Tumbleweed_Tex » Thu Jan 13, 2011 8:18 am

While reviewing the first post of this discussion, I searched and searched for the part that says “please provide details of the ONLY way to charge the TD battery from the TV.â€
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Postby eamarquardt » Fri Jan 14, 2011 12:45 am

len19070 wrote:Review first Page of this Post

KISS


Less parts=Less problems


Len


It is a "rare moment" when Len and I agree (as we have agreed to disagree on other issues).

We do however agree when it comes to this topic. Keep your load light (no inverters to power 120 volt appliances) and you'll do fine with a simple disconnectable connection between your tow vehicle and trailer battery.

Cheers,

Gus
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