Charging while towing

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Re: Charging while towing

Postby capnTelescope » Tue Sep 24, 2013 2:31 am

I want to weigh in here, because I have successfully kept an auxiliary "camping" battery charged in my pickup's camper shell by a method that should transfer to Teardrops without any problem. Some folks have described this system above, but no circuit diagrams.

I wanted to keep a jumpstart/air compressor/12V supply charged in the back of the truck so it would be ready to use, if I needed it in the Boonies. Even if I only wanted to use the camper shell's dome light without draining the truck's battery. Or charge my phone. Or other small uses. It's a small battery, compared to a regular car/truck battery.

First consideration was that I expected the truck's charging system would not properly charge the jumpstart battery directly. Also, I did not want all the grief and short battery life that goes with paralleling dissimilar batteries.

Second, I wanted the shell system to be independent of the rest of the truck when the engine is turned off. I also wanted to be able to listen to the truck radio while sitting there with the engine off without any drain from in back.

Third, I didn't want to have to remember to flip a switch or do any other form of "operating."

But! The jumpstart has it's own built-in charger, which I assume is tailored to the battery's needs. It just needs a little bit of 110VAC.

The plan was to power a 12VDC inverter to supply the jumpstart with 110VAC. Without having to worry about remembering to turn the system on or off. All it has to do is just work.

The circuit diagram below shows how I did it. All it takes is 2 automotive relays, a couple of inline fuses, some crimp connectors, and someplace to mount the components. One relay and fuse goes in the tow vehicle and one each goes in the trailer. I wouldn't use less than 10 gauge wire coming off the TV battery. 8 gauge might be better for voltage drop. The "hot when running" wire can be 16 or 18 gauge, as it only has to carry a little current a short distance. Pick a good battery charger for the trailer, like one of those 3-stage ones.

To find the "hot when the TV is running" wire you will also need one of these: a wiring diagram for your tow vehicle, or a 12 volt test light, or a voltmeter, or your favorite mechanic if you are scared of electricity or technical stuff. WARNING: I strongly advise you to use a "hot when running" circuit that doesn't supply the engine computer, ignition system, fuel injectors or transmission. I did on the first try and got weird, annoying results. A power window circuit works really good. You are only powering a relay from this circuit, so it doesn't need to be heavy duty.

I've been running this system for about 3 years without any problems, and the jumpstart battery shows a full charge every time I use the built-in tester. The only problem I encountered was my original choice for the "hot when running" circuit was a bad one. I got check engine lights and funny transmission shifting. See the warnings in the paragraph above. When I switched to the power window circuit, the weirdness went away, and I was a happy camper.

Anyway, this worked so well that I will be using this approach in my soon-to-begin build. Another relay could add the feature of automatically running the charger from shore power when connected.

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Re: Charging while towing

Postby drgrumpus » Tue Sep 24, 2013 7:29 am

Cheezus, what a lot of info on this site. Nine pages on "charging while towing". I need to study all those posts fer sure.

Now, hee is my Newbie question: My new Forester has a power outlet in the back rated 120W at 12V (10A). It has a 20A fuse. Would this be satisfactory for charging the battery on my yet-to-be-built TD?

A follow-up question is what is the problem(s) with charging the trailer battery this way?

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Re: Charging while towing

Postby WhitneyK » Tue Sep 24, 2013 8:15 am

Excellent post er Capn :thumbsup:

Good thought, with your set up, it shouldn't take much of an inverter to operate the little transformers that charge the jump starter.

I have a battery maintainer (Schumaker 1.5amp) in my tongue box. I'll check the wattage requirement, may be an option for me :thinking:

Thanks for posting
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Re: Charging while towing

Postby capnTelescope » Tue Sep 24, 2013 10:21 am

drgrumpus wrote:Cheezus, what a lot of info on this site. Nine pages on "charging while towing". I need to study all those posts fer sure.

Now, hee is my Newbie question: My new Forester has a power outlet in the back rated 120W at 12V (10A). It has a 20A fuse. Would this be satisfactory for charging the battery on my yet-to-be-built TD?

A follow-up question is what is the problem(s) with charging the trailer battery this way?

Grumpus (alias Mike)


Great question, Mike.

The short answer is "yes." This is basically the same principle as what I wrote about in my post. Running an extension cord from the TV to the trailer helps solve a lot of the voltage drop problem. On the other end, You can only put 10 amps into the battery from that outlet. You can't make watts out of nowhere. An alternative would be to put a larger inverter under the hood and run from there. Those factory power outlets are good for laptops 'n stuff like that, but a little light for what we want to do here, IMHO.

The only problem I see other than the 120 watt limit is routing the extension cord so you don't get rain or dust leaks. Or sparks!
Last edited by capnTelescope on Tue Sep 24, 2013 10:38 am, edited 1 time in total.
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Re: Charging while towing

Postby capnTelescope » Tue Sep 24, 2013 10:37 am

WhitneyK wrote:Excellent post er Capn

Good thought, with your set up, it shouldn't take much of an inverter to operate the little transformers that charge the jump starter.

I have a battery maintainer (Schumaker 1.5amp) in my tongue box. I'll check the wattage requirement, may be an option for me :thinking:

Thanks for posting


Good question, Whitney.

I use that same battery maintainer for my lawn mower and TV. I think they're great for what they do, but I don't think they've got enough oomph for a charge-while-tow application. You could drive all day and not make up for what you used the day before. Keep that maintainer hooked up between camping trips to be charged and ready to go and for longer battery life. Then get a good (and bigger) charger to use while towing.

The inverter-charger approach should scale up nicely. My yet-to-be-built Tear will have a nice big deep-cycle battery to charge.
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Re: Charging while towing

Postby Shadow Catcher » Tue Sep 24, 2013 5:44 pm

We have a Subaru Outback which has the same power point. We have a Waeco refrigerator which resides in the back when traveling. I tried it with the on board power point in the rear and there were two problems, the refrigerator has a boost cycle to get it down to temperature quickly and it would not function in boost, and the power point is only active when the engine is on. While it is fused for 20A the wire is a bit light for that. I ran a 10ga wire from the battery with a 30A circuit breaker to the back. My bet is that the power point is not expected to carry a substantial current for any length of time.
Your best bet in my seldom humble opinion is to just run a fused wire to the battery.
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Re: Charging while towing

Postby GuitarPhotog » Tue Sep 24, 2013 5:54 pm

It seems to me that the "battery -> inverter -> charger -> battery" charging scheme is unnecessarily inefficient. I just charge the trailer battery directly from the tow vehicle battery with a "trailer disconnect" relay in the middle to keep me from killing the tow vehicle battery while parked.

I ran a 10ga wire from a fuse at the battery to the relay, and from there to the 7-pin trailer receptacle. A 12 ga wire (in the 7-conductor trailer cable) runs from the trailer plug to another fuse then to the trailer battery.

A couple of hours on the road recharges the battery to 100% from the usual 70-80% level after camping.

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Re: Charging while towing

Postby KennethW » Tue Sep 24, 2013 6:35 pm

On newer TV's your lights can be set to be on when you drive. How would it work to use the taillight circuit and a small relay. to turn the trailer charging on and off. If you drive with your lights on. You will be charging your TC battery. When you park and turn your lights off you also turn the charging relay off.
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Re: Charging while towing

Postby capnTelescope » Tue Sep 24, 2013 9:26 pm

@ShadowCatcher, Your assumption about the power point is correct. They aren't good for much more than a laptop or something lightweight. The over-rated 20 amp fuse is there to handle momentary surges without blowing. Connect that big 10-gauge to a bigger inverter, and I think your Waeco will boost just fine. Go for at least twice the watts of the boost mode rating.

The fused wire from the battery works for GuitarPhotog and others. If it works for you, you're good.

@KennethW, the relay hooked to the TV running lights is a good, easy solution to the problem of finding the wire that's hot while the motor is running. It does require "operation," so don't forget to turn the lights off (or on). Easy to remember if you arrive/leave in the dark! :thumbsup: I'm always forgetting something. :oops:

There's a lot of fuss made about voltage drops, dissimilar batteries hooked together, and so on. I haven't seen any perfect solution. I intend to use my solution because it's worry-free, proven to work, and is adaptable to occasional shore power. It only looks complicated.
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Re: Charging while towing

Postby KennethW » Tue Sep 24, 2013 10:38 pm

Hi capnTelescope My thoughts is on newer autos You can set your lights to always on when driving ( they turn off with the car). If a person wires the trailer with the running lights controlling your power wire. It should be petty easy to do because you have the control wire (running lights) on the trailer with any TV. When someone uses your camper you would only have to run a power wire to the Battery with a fuse to be safe.
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Re: Charging while towing

Postby capnTelescope » Wed Sep 25, 2013 11:17 am

@KennethW, :idea: or if the TV has front Daytime Running Lights (DRLs), which don't necessarily turn on the tail lights, you have an automatic on/off source at the headlights. This would be useful for almost any Charge While Towing scheme. Again this is newer vehicles, and not even all newer vehicles.

I think the DRL headlights are an even better solution as the wires are much easier to find and get to. Thanks for the idea, Kenneth.

We need some sort of light bulb smiley for "you gave me an :idea:".
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Re: Charging while towing

Postby ntsqd » Mon Sep 30, 2013 12:02 pm

While being a viable solution, the inverter > charger method is terribly inefficient. Whole lot of power losses going there that really don't need to happen for the TV to charge a camper/TD battery. I do not think that this is a good idea or a good solution, just one that does happen to work.

Much is made of not hooking different battery chemistries together. The regulator for the TV's alternator isn't that smart. It doesn't know what kind of battery or batteries it is connected to, and more importantly it can't change it's charging settings if it did. In short, they're pretty stupid. Since the alternator is the highest voltage source in the system (unless you have unregulated solar) it rules the system when everything is connected and the TV's engine is running. Once the TV's engine is not running and the alt is not charging those different chemistry batteries do need to be separated.

IMHO not enough concern/care/thought is given to voltage drop. There just isn't enough voltage delta available to carelessly give away any of it.
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Re: Charging while towing

Postby dean_petley » Sun Dec 22, 2013 6:46 am

i worked in a camping store for a few years and always got dragged out of the shop to see if products will fit in peoples set ups from push bikes to huge ex army equipment one of the best iders i ever seen was a trailer that had extra battery's but the charging was inboard instead of the normal free spinning wheels / hubs he ran a differential

and had the the input flange connected to an alternator that was mounted underneath and that charged his battery's i think he sayed he was photographer and he used a lot of power and this was the simplest way to charge as he went with out having to think
have funn
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Re: Charging while towing

Postby ntsqd » Sun Dec 22, 2013 2:01 pm

Interesting that this thread should pop back up. I just recently stumbled onto this connector pair:
http://www.delcity.net/store/Single-Pol ... ors/p_8595
It's 300A rating is overkill, but at least then you can use a wire gauge large enough to avoid any voltage drop.

I've been toying with putting a battery back on our TrailBlazer (took them off when the then Optima's were DOA) and have been pondering a charging method. I think that I'll move the single sensing VSR from the pop-top camper to the trailer and put a dual sensing VSR in the camper. That way the solar about to go on the camper will also charge the truck's batteries once the camper batteries are full. With the single sense VSR on the trailer I just need to supply it with wire from the alternator and I can do that by tapping into the camper's charge wire via the above plug and receptacle.
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Re: Charging while towing

Postby capnTelescope » Sun Dec 22, 2013 2:42 pm

@ntsqd -- If that one's not big enough, you really need to rethink :thinking: just how much juice you think you need. Or build your tear on the back of a mobile welder. :)

@ Dean -- Your photog friend's setup is pretty ingenious, but I bet it really impacts gas mileage. Something like the 4WD penalty you pay whether you use 4WD or not.

Some of us, and I'm only speaking for myself, wisht we could build a tear that's ultra light, has enough juice on board to light up a small town, and able to pump enough water to put out a forest fire. For cheap. You just gotta set your priorities.

:lightbulb: I don't think anyone in this thread has proposed running a small gas generator in your tear while towing. (Tongue mostly in cheek.)

It would look something like this: :vroom: :cigar:
I'll burn that bridge when I come to it.

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