Tow Vehicle to Trailer Charge Booster - Page 2 - Fiberglass RV


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Old 08-24-2012, 07:58 AM   #15
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Originally Posted by Orlen Wolf View Post
............
BTW - A friend dropped by the other day with a new Dodge diesel truck with the "heavy duty trailering package" that included a built in brake controller and a trailer charge line. I checked it out and found it had a number 14 wire. Not so "heavy duty" in my book. It also didn't have a charge line disconnect so battery drain would continue when you turn off the engine. Maybe some day vehicle manufacturers will see the light.
You have nailed the issue exactly. My tinkering is an attempt to see if there is an inexpensive way to get a reasonable charge off a small wire to avoid the hassle of doing a rewire. This is a frequent question here.

On my own TV/ trailer I have two 4 gauge fine stranded wires that I picked up at a recycler. It works great, but it is heavy and frankly a little crude.
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Old 08-24-2012, 11:37 AM   #16
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Tom,

Thanks for the kind reminder to that I needed to read everything more carefully. You were correct that I was mistakenly assuming the convertor was close to the battery on the tow vehicle. I believe you are saying the converter, solar charger, and trailer battery are all closer together.

In that configuration you are proposing, I don't think you are going to gain anything. If the problem you were trying to solve is "get higher voltage to the trailer battery" I think it would work. However, I think your problem is actually "get more power to the trailer battery" and a smaller wire is always going to constrain that problem no matter what you put on the trailer side.

I'll take a shot at an explanation. Taking a typical setup (such as on most trailers), let's assume at the tow vehicle you had 10A going into the wire at 14V. This is 140W of power going into the thin wire running to the trailer. If we measure at the trailer we would likely have a voltage loss due to the thin wire, so lets assume 12V at 10A (120W). This will charge the battery when it is really low, but as the battery does charge, the trailer battery voltage will come up. As the voltage comes up, there is less voltage difference between the two batteries and the current goes down so the voltage at the trailer battery gets higher yet. It is a very convenient cycle!

In the setup you described you will still have 12V and 10A (120W) at the convertor. Into the solar charger you will have 18V and 6A (108W) which assumes a 90% efficiency (realistic tending toward optimistic depending on how good the converter is). If the solar charger is also 90% efficient, they it will have 14V and 7A (98W) going to the battery. I think you would rather have the 120W of the typical setup. My efficiencies are assumed but you will have < 100% efficiency which means less power to the battery.

Back to the problem statement, if it was true that batteries charge faster on higher voltage/less power rather than lower voltage/more power, your circuit would help. However, I don't think that it is actually true and the traditional setup is preferred.

If you do run tests, I would be interested in the results. (I don't mind being wrong - I usually learn the most in those circumstances!) If you want to discuss the test methodology/procedure, I would be open to that discussion as well. If more or a different explanation would help, feel free to ask.

Have a great week!
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Old 08-24-2012, 12:27 PM   #17
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Not much on theory but . . 6awg welding cable is relatively cheap, quite flexible, and provides a big pipe without sistering separate conductors together. Would it be big enuf to avoid significant voltage drop in Tom's system or would he need #4?

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Old 08-25-2012, 06:42 AM   #18
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Thanks to all for humoring me. I've ordered the components and I'm going to have some fun playing Dr. Science.

I'm also reading up on flooded cell battery charging theory, as I still need to wrap my head around the interaction between charging voltage and battery state of charge.

As I've mentioned earlier, I wired my TV and trailer with #4 wire and it works, but not elegantly. Messing around with DC-DC inverters in an intellectual exercise and if I figure out something that can help me or someone else in the future, great. If not, my Chinese friends are $25 richer.
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Old 08-25-2012, 11:15 AM   #19
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Thanks to all for humoring me. I've ordered the components and I'm going to have some fun playing Dr. Science.

I'm also reading up on flooded cell battery charging theory, as I still need to wrap my head around the interaction between charging voltage and battery state of charge.

As I've mentioned earlier, I wired my TV and trailer with #4 wire and it works, but not elegantly. Messing around with DC-DC inverters in an intellectual exercise and if I figure out something that can help me or someone else in the future, great. If not, my Chinese friends are $25 richer.
I see lots of references to "power" but if I understand what you are going to be attempting with your experiment is to increase the voltage to the point that it is greater than the battery voltage so that whatever "power" you do have will feed into battery. Yes you will lose "power" but....

Seem to recall you must have greater line voltage than battery voltage for any available amps to flow into battery.

This device might be of use to supply that voltage. Would be especially interesting if it could be hooked in back at the trailer. Making it so trailer could get some charge even if TV had pretty lame 12 or 14 gauge wiring.
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Old 08-25-2012, 12:07 PM   #20
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I see lots of references to "power" but if I understand what you are going to be attempting with your experiment is to increase the voltage to the point that it is greater than the battery voltage so that whatever "power" you do have will feed into battery. Yes you will lose "power" but....

Seem to recall you must have greater line voltage than battery voltage for any available amps to flow into battery.

This device might be of use to supply that voltage. Would be especially interesting if it could be hooked in back at the trailer. Making it so trailer could get some charge even if TV had pretty lame 12 or 14 gauge wiring.
Roger, I think you are following my logic. I think of a battery like a pressure vessel. If you want to inflate it to 12 PSI, you really need more than 12 PSI at your air hose, especially if you have to overcome some restriction (think internal battery resistance).

So yes, you kick up the pressure (voltage) with a DC-DC converter, reduce the volume (amperage) proportionally - less electronic efficiency losses, but now you are now filling the vessel (battery). Until you reach a pressure (voltage) threshhold, no or little filling occurs.
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Old 08-25-2012, 04:57 PM   #21
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For the stated goal of raising voltage to achieve more complete battery charge top-off, it seems like it should work. However, a solar panel putting out 15v or higher, running into the solar controller, would do the same thing. Unless one does a lot of nighttime towing?
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Old 08-25-2012, 05:54 PM   #22
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...... However, a solar panel putting out 15v or higher, running into the solar controller, would do the same thing. Unless one does a lot of nighttime towing?
Definitely true - if you have a solar panel.
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Old 09-02-2012, 11:13 AM   #23
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Something to consider, is that the REASON the vehicles only have a 14 gauge wire running from the fuse block to the trailer connector, is to limit the current through the connector. Those trailer pins are only rated for about 15 Amps. Connecting a dead/low trailer battery at 11V to an already running truck at 14.4V with a larger wire would pull a LOT more amps, and likely create quite a spark. The resistance from the wire provides a much more limited current surge. Going to a larger wire is fine if you are also going to a larger plug designed for the higher current - however, I would not mess with it through the factory 7-pin.

If you DID want to maximize the power capability through the plug safely, you could run a larger power wire from battery to bumper, and build a current limiting circuit that would regulated the voltage to limit current to the rating of the plug (15A or so). Personally, I'd love to see this become a normal thing in the future on trucks with towing packages, because more and more trailers are using more power than just float charging house batteries while towing.

EDIT: I did just find some specs, and it appears that the connection itself IS rated for 40A, but only after it's fully inserted. If your tow vehicle has a relay to only allow charging when the engine is running, and you don't plug and unplug it when the vehicle IS running, it may be okay - however, you also have to upsize the trailer side wiring to the rated current as well.
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Old 04-17-2013, 09:19 AM   #24
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Update

Just to prove that I am as stubborn as I am crazy, I messed around with this DC-DC converter booster concept last winter using a 6/12 volt 10 amp battery charger as an input to charge a portable jumper battery.

What I determined is that the concept is sound, but the hardware I was using was just too small. (I suspect that it was originally designed as a car to laptop charger). The inverter can boost the input voltage back up to an adequate charging voltage, but of course the current falls even more than proportionately, so it was an inadequate charger. An analogy - pounding in a railroad spike with a hammer is a valid concept, but not if you use a tack hammer.

In researching it more lately, I see that a second commercial device is available for the same purpose, but the price still makes just installing huge wires between the vehicles look cheap.

Sure Power: DC-DC "Trail Charger" Converter (11020C01) ~ All Battery Sales and Service

Quote:
The Sure Power 11020C01 is a specially designed DC/DC converter that is used to charge a battery from a 12V source. An example is a battery that is mounted on the trailer of a vehicle. The distance between the alternator of the vehicle and the trailer-mounted battery makes it difficult to get adequate charging voltage to the battery. The Sure Power 11020C01 20 Amp Converter has a microprocessor on-board to measure the input voltage, output voltage and current, boost voltage, temperature sensors. An Intelligent Power Switch on the output is used to provide consistent, robust protection and control of the output current.
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Old 04-17-2013, 09:33 AM   #25
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To refresh: Charging voltage at the battery has to be above battery voltage for current flow to take place. Small wires drop voltage due to resistance. Low voltage won't charge the battery.

1) Installation instructions call for:
"Run 8-gauge battery cable the entire length of the trailer before connecting the fuse holder wire harness to the Aux. pin of 7-way connector at front of trailer."
2) Price is astronmical for travel trailer applications.
3) The wire alone will do the trick.

As mentioned many times, it's all about both current carrying capacity and keeping the charging voltage above battery voltage. Ya gotta have both.

Ya don't have to be an E.E. to understand Ohms Law. (No, that's not a new TV show about a Chi town law firm)
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Old 04-17-2013, 10:11 AM   #26
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The advantage of this device is that for tow vehicles already factory wired but with inadequate gauge wire, it saves a rewiring. I wouldn't pay this much, thus my experimentation with surplus electronics.
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Old 04-17-2013, 11:50 AM   #27
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Cause and effect. The batteries are the cause and the current is the effect. If all you are doing is charging the battery(nothing on in the trailer) then the wire resistance determines how long it will take. The battery voltage can never drop too low due to wire resistance. Draw the circuit. The two battery voltages subtract. The charging current is the difference between the two, divided by the wire resistance.



When you add a load ( fridge) in parallel with the trailer battery it shunts current away from the battery you are trying to charge. The available current is still the difference between the two battery voltages divided by the wire resistance. The wire resistance limits the current from the tow vehicle and the trailer battery fails to charge nor does the fridge work as expected. In this case lowering the wire resistance will provide an increase in the available current up to the next limiting factor, the alternator. If the total current exceeds the capability of the alternator, the wire resistance becomes irrelevant.


Now, if you remove the trailer battery, the wire resistance forms a voltage divider with the load(fridge). In this case the larger the wire resistance the lower the load voltage. Raz
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Old 04-17-2013, 12:25 PM   #28
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Raz, I know that we've chewed this bone before, but I'm still convinced that the alternator is not the weak link. Out of an output of say 60 amps, you would want 10 amps or so to charge a trailer battery, 10 or 12 amps for a refrigerator. That leaves 40 amps to run the blower motor (6 amps), an electric fuel pump (4 amps) and the engine electronics (5 amps??), which should be plenty. Headlights and tail lights would be about 14 amps, if used.

When I left the biz we were using alternators well over 100 amps, because the engine idle speeds had been slowed down so much that we needed an alternator big enough to cover all possible power needs at extended engine idle, where the alternator output is minimal.

Quote:
Typical passenger car and light truck alternators are rated around 50-70 A, though higher ratings are becoming more common, especially as there is more load on the vehicle's electrical system with air conditioning, electric power steering and other electrical systems.
Alternator - Wikipedia, the free encyclopedia
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