Charging while towing

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Re: Charging while towing

Postby Bogo » Sat Oct 27, 2012 4:28 pm

That's a bummer on the Sure Power battery separators. They used to be good. All the units I've dealt with in the past have worked flawlessly, but we're talking about ones made well over a decade ago. Looks like they changed parts suppliers or the design.

Oh, on a similar vein, Optima batteries. A few years back they changed manufacturing processes and then manufacturing plants. They are no longer the high quality USA made battery they once were. The newer ones aren't handling high current draws like from winches. Also they are near impossible to recharge or get to keep a charge if they ever get nearly fully discharged. They've gotten a bad name in the off roading communities.
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Re: Charging while towing

Postby ntsqd » Fri Sep 13, 2013 10:31 pm

I read thru all of this and while I can appreciate the idea that all you need to do is to unplug the trailer while camped I know that eventually I would forget to do that. Or I'd forget to plug it back in when leaving. So.... that option is out for me. Early-on there was mention of "how the RV folks do it." I've seen some of their work, and not just the so-called "service centers" but a couple factories. Count me out of that. Most RV specific electrical parts are such that as a mfg I'd be ashamed to sell them. I go right past all of that and look at marine parts. They're more expensive in general, but how many times do you want to buy something for a single job? I want to buy it once and a failure within this decade is too soon. I consider something 10 years old "just broke-in."

Search out the "HandyBob" blog. It is a long and somewhat painful read about full timing on just solar panels, but his lessons learned (& shared!) are worth digesting as some of it pertains to vehicle electrical in general. The biggest thing that I took away from his blog is that voltage drop (VD) should be avoided at all costs. You don't really have 12 volts to play with. Fully charged most types of batteries sit around 12.6 volts. A 50% discharged battery has a standing voltage of about 11 volts (% discharge is defined as remaining amp-hrs, not voltage) and that is as low as you will want to go if the battery is to have a long life. So you have ~1.5 volts to play with. Suddenly the 5% voltage drop that used to be acceptable isn't very acceptable any more as that 5% VD just dropped you from 12.5 volts down to 11.9 volts or 42% of your margin. Or consider this in the TV to TD charging wire. Most automotive regulators top out at around 14.4 volts. Only the wire going to the TD has a 5% VD in it. So now that 14.4 volts is 13.7 volts at the TD's battery. 13.6 volts is about the minimum possible charging voltage, so you're barely there. Meaning that the system will barely be able to charge the TD's battery(ies). The higher the charging voltage, to a point of damaging the battery(ies), the greater the amps that can be 'stuffed' into the battery. (I'm grossly simplifying a lot of nuances about the care & feeding of batteries.)

At any given state of charge and charging voltage the battery will accept a finite charging rate. It will accept less, but it won't accept more unless the charging voltage is increased. When considering voltage drop the grounding length needs to be included in the total length of the circuit. Looking at Ancor Marine's chart for sizing wires for a given voltage drop (I'll use their 3% VD chart) over a set distance (say 15' battery to battery, so 30' overall) with a specific charging rate (arbitrarily picking 20 amps) I find that a 6AWG wire is called for. Using all of the same numbers, but looking at their 10% VD chart only a 12AWG wire is called for. Quite a difference in wires.

Another thing that I noticed in some of the wiring diagrams posted earlier in this thread was that the trailer frame was used for the ground path. My experience has been that this is an all too frequent cause of trouble with trailer lighting. I don't live in a particularly wet climate, officially I live in a "Coastal Desert", but even here rust and corrosion plays havoc with those grounds. In the future I will ground to the trailer frame, but all lights will have their own ground wire connected to the main ground wire and not rely on the trailer frame to carry the ground for any distance.
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Re: Charging while towing

Postby Shadow Catcher » Sat Sep 14, 2013 5:32 am

I have the conventional seven wire trailer connector, lights brakes etc. the wire through that for power to the battery is 10 ga and a line runs to the battery that is that.
One thing often not taken into effect when calculating voltage drop is alternator voltage. The Subaru voltage at the battery is 14.2 volts I have less than a 15' run but will use that for calculations. This gives a voltage drop of .60 volts or 13.6 V at the trailer. However there is the 185W 66 cell high voltage panel stuck to the top of the trailer that will back feed into the TV battery if I were to forget to unplug.
One of the things Handy Bob emphasizes is monitoring battery state of charge and I went with a Victron battery monitor because of its small size and easy set up. Monitoring battery condition by voltage alone does not tell the story, or at least not easily. My feeling is that while you will not get the most charging through the trailer connector how often would you need to. If I need to get it in fast I have a set of 000 welding cable jumper cables that I can run from the TV battery to the trailer battery.
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Re: Charging while towing

Postby bdosborn » Sat Sep 14, 2013 2:08 pm

Shadow Catcher wrote:I have the conventional seven wire trailer connector, lights brakes etc. the wire through that for power to the battery is 10 ga and a line runs to the battery that is that.


My truck with the factory towing package had a #16 wire back to the 7 way connector. :O It was basically useless due to voltage drop.

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Re: Charging while towing

Postby ntsqd » Sat Sep 14, 2013 3:16 pm

15' or 30' in your calcs? The ground path needs to be included or the calcs are off.

X2 on a real battery monitor and not just a voltmeter. I went with the Trimetric, but I'm still in the process of setting it up (other things got in the way). After pricing out those T-125's that came with our pop-top slide-in camper I realized that if I didn't want to semi-frequently spend money on replacing them that I needed to step up my monitoring ability.

There is more to this than simply the rate of charge as a low charging voltage will never fully top off the TD's battery. Which will result in poor performance and life span from that battery. BT, DT. IMO adding solar with a quality solar charge controller is the best mod since it will top off the battery regardless of how long you drive or what the wire gauge is between the TD's battery and the TV's alternator. wire it right and you may not even need the TV to TD charging wire.

On our camper I went with 6 ga. charge and ground wires for a total run length of about 24 feet. I did that not for charge rate, but for voltage drop.
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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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