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Old 04-15-2016, 09:22 AM   #1
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How to get more charging while towing?

We boondock a lot and have 2 6v golf cart batteries that run down when no sun is available to charge them. I'm always disappointed at how few amp-hrs are put back in the batteries while towing. When starting off, the charge rate is 25 amps but this soon tapers off to just a few amps as the truck (2012 Chevy 1500) battery recharges and reduces alternator output. I can plan on just 25-30 amp-hrs recharge per tow because of this reduction instead of 25 amp-hrs per hour of towing that I'd like to see.

Is there any way to boost charging of the trailer battery and not fry the tow vehicle battery?


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Old 04-15-2016, 10:40 AM   #2
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In essense, no. You can increase the wire size from the tow vehicle to the trailer, which will help a little, especially if you put a separate plug on the system to carry the charge current only (the normal single wire in the 7 wire plug is just too small for the job).

However, the fundamental problem is that you are trying to charge one set of maybe highly discharged batteries in the RV which is 25 to 35 feet away from another battery which is very lightly discharged and is near the charge source which is optimized to charge it only (the one in the tow vehicle). Those two batteries have different needs and cannot be charged the same way.

A few years ago, here on the forums, there was a guy in Texas who built an elaborate charging system to do what you want to do. As I recall it used very complex and expensive components (I recall from England, but not sure now) to monitor the needs of both the RV and TV batteries, and send the proper charge to both. As I recall the cost of what he did was in the couple of thousand dollar range. I have seen no posts from him in several years, so no idea how it held up in the long run.

If I were to come up with a relatively simple system to do what you want to do, here is what I would do: I would put an inexpensive but relatively large inverter in the tow vehicle, near the front batteries. It might be in the 1000 watt range. Then I would run 120 volts back to the trailer with a separate cord and plug combination and power the existing converter/charger in the trailer with it. I assume you already have a 3 stage converter/charger upgrade. That would let the tow vehicle system do it's thing, and the RV system operate independently, as if it were hooked up to grid power.

That would probably be the easiest and cheapest way to have a larger charge when towing.

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Old 04-15-2016, 10:45 AM   #3
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I'm not sure how difficult this would be, but I would "think" you could rewire the truck to have an extra set of wires coming out of the alternator, then tie those directly into the charge line running back to the trailer. Not sure about the OEM converter, but would think that one like a Progressive Dynamics with their "Charge Wizard" would happily accept the higher current coming back and then feed that into your batteries, tapering it off only once they start to come back to full charge.

Not sure how this would work with the voltage regulator on your truck. And if the voltage regulator is integral to the alternator, then I think this idea goes out the window. But, I would do some research into something along these lines. If the alternator is separate from the voltage regulator, then I don't see why you couldn't run two outputs (like a Y connector) off the alternator and one feeds the truck, one feeds the trailer.

It might be easier to modify the trailer harness and just run a Honda generator in the bed of the truck as you go down the road....

Best of luck,
- Jim
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Old 04-15-2016, 11:35 AM   #4
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It is a simple problem of voltage drop along the wire from the alternator to the battery bank. You need about a 2/0 cable.
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Old 04-15-2016, 11:54 AM   #5
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An alternative way is to install a second alternator and and a second smart alternator. Drive belting and bracket would take a little figuring out. The alternator should be at least 100 amps, and the wire should be a #2 or larger. That way your trailer would not be drawing down the batteries of the tow vehicle.

I think the cheapest way to solve the problem is a cheap inverter generator. Simple and cost $500-600.
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Old 04-15-2016, 12:30 PM   #6
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Sorry, should have said a second alternator with a second dedicated smart voltage regulator. Sorry for the confusion.
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Old 04-15-2016, 01:03 PM   #7
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I used to worry about the low charging. Found no real solutions. Now I carry a generator just in case. That was easier for me than rewiring the trailer for solar or re wiring the truck to get at best a very marginal improvement. I sleep with a CPAP. It does not draw a lot but I like to know I can keep it running.
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Old 04-15-2016, 01:53 PM   #8
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Old racers trick. Back in my racing days we used two modified Corvette alternators in the truck to charge batteries in the car hauler. The Vette alternators were high output beast and was the hot ticket in trucks and race cars.

Do you know what a learning experience is? A learning experience is one of those things that says "You know that thing that you just did? Don't do that."
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Old 04-16-2016, 10:30 AM   #9
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Get an HD alternator from DB Electric, that's what I did, a 200 amp plus their hd wiring kit. Huge difference.
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Old 04-16-2016, 10:32 AM   #10
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When starting off, the charge rate is 25 amps but this soon tapers off to just a few amps as the truck (2012 Chevy 1500) battery recharges and reduces alternator output. I can plan on just 25-30 amp-hrs recharge per tow because of this reduction instead of 25 amp-hrs per hour of towing that I'd like to see.
Just a few thoughts - you would only get the 25-30 amp hrs if your Chevy put out the 25 amps for at least an hour. More than likely, you are only getting 2-5 amps per hour back to the trailer. If you are checking the voltage on the trailer when you stop, it will likely be giving you an optimistic "surface voltage" reading which will diminish quickly, especially under a load.

There are some complicated and expensive solutions to the problem. (Either a completely separate charging system on the Chevy or a complicated device to measure the voltages of the truck and trailer separately and charge them accordingly. Requires a lot of rewiring as well.) Adding the extra gauge of wire will help, but not to the degree that you will need. Especially if the Chevy is only sending 2-5 amps.

The most reasonable solution I have found is additional solar panels. Also not a cheap alternative but very valuable to me. I ran out of real estate on the roof of my trailer for more panels so I have 150 watt panel that resides over the bed of my truck and is wired through the solar controller on the trailer. This panel is on a rack that allows for quick removal (so I can leave it charging at camp) and also allows for the rack to fold up against the back of the cab to allow for about 98% access to the bed of my truck. (I have a "Back Flip" cover over the bed.) The other advantage of the panel on the truck, is that when it is removed and setup near the trailer, I can better aim it for a 90 incident angle to the sun. This yields a significant increase in the solar generation for the times before and after the middle of the day. Even in low light (cloudy) conditions, I am able to boon dock without turning on my generator. (Sometimes, during winter, with the loss of efficiency on the batteries and greater usage from the heater, I'll need a few hours of help from the generators.) By the way, the low temperature impact on efficiency for both charging and discharging, is a HUGE issue if your batteries are not heated. But that is another whole subject ...

Good luck and happy boon docking.
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Old 04-16-2016, 10:52 AM   #11
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First I would determine what is the output of your alternator. For example, when I bought my F250 I had 3 options. The standard, a 200 amp, or dual 200 amps. Here is a website that may help you determine what you have:

Next upgrading the wire size to the trailer will help. I'd be surprised if the current wire is much more that 12 gauge. That's the cheapest, easiest, ad quickest fix. Pappy19 is saying the same thing.
The ability to follow instructions is highly underrated.
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Old 04-16-2016, 11:06 AM   #12
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In keeping with the connection between Airstreams and airplanes you could do what they did in the old days. They mounted a small generator with a propeller on it to provide power while in flight. They really existed. Here's a Google link:
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Old 04-16-2016, 04:08 PM   #13
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Or, there are plenty of small wind turbines that can generate 24hrs, all weather and can store for travel. But the most dependable/cost effective solution (after all possible amp trimming) is a small, quiet generator (like a Honda EU2000).
Good thread!
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Old 04-16-2016, 05:36 PM   #14
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GM Trucks charging circuit

Hi - I made my 2009 1/2 ton GMC into a dual battery rig, I followed the factory setup for the 3/4 ton EXACTLY. After completing the setup I noticed that exact same thing happening. In doing some research I found that the main battery has a sensor in the negative battery lead that monitors and controls the alternator output. This also affected my dual (2nd) battery in that while the 2nd battery relay is active (engine running) the 2nd battery is in parallel with the main battery. BUT negative lead is grounded directly to the frame. This ends up telling the sensor that only about half of the charge is happening. I ended up running the negative lead directly to the main battery negative at the battery itself to avoid "bypassing" the main battery charge circuit monitor (sensor).

One way to tell the charging circuit not to go to minimum (or even OFF) is to drive with the headlights in the manual on position. This tells the computer to stay a higher output. I seem to recall that there are other ways also. There is a very good description of this circuit in the GM service manual. This battery sensor circuits main purpose is the back off or even turn off the charging in order to up the gas mileage when the battery is charged.

Hope this helps.


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