Tow Vehicle to Trailer Charge Booster

[Thomas G.]

I have been kind of fascinated with the new availability of inexpensive DC-DC converters and their application to trailers.

A common problem with trailers is that if you just use a light wire to charge the trailer battery from the tow vehicle, you don't end up with enough voltage to fully charge the trailer battery. The typical alternative is to use huge honking wire to try to beat the voltage drop.

I came across this 10 amp capacity DC DC converter on eBay which can boost the voltage to anywhere you want it. You can also buy inexpensive solar controllers that act like smart chargers and take a wide range of input voltages and give the battery just the right voltage to keep it fully charged.

My idea is to feed the dc dc converter off a relatively light wire from the tow vehicle, bump the voltage to about 18 volts, feed this into the solar controller which then charges the battery. Input current is limited to 10 amps by a circuit breaker. (see attachment)

NEW 150W Mobile Power Supply Boost /step-up DC-DC Module Adjustable For laptop | eBay

20A 12V/24V PWM Solar Street Light Panel Charge Controller Regulator Auto switch | eBay

I'd appreciate thoughts from anyone with more electronics experience as to the pitfalls of such a device.

[David Tilston]

Well, big wire is not a bad thing. Your DC to DC converter will still draw current. This will cause voltage drop. More current, more drop. You are installing the equivalent of a lift pump, but with the same sized pipe. At some point the pipe will be the limiting factor.

[Ron in BC]

Originally Posted by David Tilston

Well, big wire is not a bad thing. .

Couldn't agree more. I ran a 10 gage charge wire from my tug battery to my trailer battery. Just tried it out on a trip to Portland and back. Ran the fridge on 12 volts. At the end of the day my fridge was cold and the trailer battery was charged. A simple system, but works for me. Why over complicate things

Ron

[evanwilliams]

The voltage isn't the issue, it's the amps. I don't think you can do anything to remedy the issue of sending a lot of amps through a wire (to charge your trailer battery) other than to use a thicker wire. The thicker the wire the more amps it can carry (without melting!) and the faster your battery will charge.

Sorry to say it but I don't think your idea will work. Whatever you do, be sure to fuse all your lines running from your battery.

EW

[David Tilston]

Originally Posted by evanwilliams

The voltage isn't the issue, it's the amps. I don't think you can do anything to remedy the issue of sending a lot of amps through a wire (to charge your trailer battery) other than to use a thicker wire. The thicker the wire the more amps it can carry (without melting!) and the faster your battery will charge.

Sorry to say it but I don't think your idea will work. Whatever you do, be sure to fuse all your lines running from your battery.

EW

Normally I don't split hairs, but power is amps X volts, (X power factor). If the DC to DC converter was at the TV battery, and was set to output around twenty volts, the voltage at the input to the solar charger might still be sixteen or so. If he is using #14, he could send about 15 amps, or about 240W. If, without the DC to DC converter, he has had 10 volts at 15 amps, that would only be 150W. In both cases he is loosing about 60W in the wire. My point is that increasing voltage also increases power. This is how a little #8 steel wire can power a subdivision.
If he puts the DC to DC converter in the trailer, the power from the TV will still be limited to 150W or so, but at least his voltage would be sufficient to charge the battery.
Now that I think about it. won't the solar controller do this on its own?

[Thomas G.]

Originally Posted by David Tilston

Well, big wire is not a bad thing. Your DC to DC converter will still draw current. This will cause voltage drop. More current, more drop. You are installing the equivalent of a lift pump, but with the same sized pipe. At some point the pipe will be the limiting factor.

The limiting factor is the 10 amp circuit breaker. My objective is to charge just below 10 amps.

Originally Posted by Ron in BC

Couldn't agree more. I ran a 10 gauge charge wire from my tug battery to my trailer battery. Just tried it out on a trip to Portland and back. Ran the fridge on 12 volts. At the end of the day my fridge was cold and the trailer battery was charged. A simple system, but works for me. Why over complicate things

Ron

Do a search and you will find that many have not been so lucky as you. In the UHaul for instance, the battery is all the way to the rear of the trailer. Mouse (Brooke), as an example, could not get her trailer to charge with the fridge running.

Originally Posted by evanwilliams

The voltage isn't the issue, it's the amps..............EW

I respectfully disagree. If you don't get a full 14.5 volts to the battery, you can't fully charge it. A 12 gauge wire will handle 20 amps., but it has a significant voltage drop over 25 feet.

[lamimartin]

This so called "trailer charge booster" is advertized without any specification to use for laptops. Producing enough power to recharge a laptop, recharge a 12V RV battery and to power a RV fridge is a completely different challenge.

Laws of physics will prevail, no matter what. No guizmo can create miraculous power boost exceeding the wiring and fuses capacity. Laptop power supply need less than 6A at 12V, a RV battery will recharge only if you provide more Amps than your fridge need (generally 12-15A). I have a 20A line on my Trillium to recharge the RV battery from the car's alternator, trough a standard 7 way connector, using standard wiring specifications you can find at Trailer Wiring Diagrams | etrailer.com

[Thomas G.]

Ok, I was hoping to get some opinions from the EEs, but I plan to build it and mess around with it to see what it is capable of.

[itlives]

I'm not a wizz on this stuff but I don't see how it would/could work. I'd be interested in a follow up on this.
That said, I wouldn't buy the one you linked to because I can't make heads or tails of this (taken from the link)-
system-level power supply before, when you do a project when the input 10-18V, when the system board and you also need about 24V power supply and its large, with the general DC-DC power module too small, then you choose this module we will be your best choice, do not debug directly on the machine can work easily achieve efficient high-power boost.
Can anybody say "Engrish" ?

[Orlen Wolf]

Tom

The system you propose won't increase the voltage on the line from the front to rear of your car enough to do you any good. And you have two controllers in the system. For the same, or less, money you can install a much better and simpler system.

Most vehicles with a factory installed trailer charge line use a number 14 wire, way too small to charge the battery at more than a trickle, one amp or so. If you have a charge line "professionally" installed they will do the same thing. In addition, if you have a 12V refrigerator you will exceed the 10 amp rating of the voltage booster.

If you boosted the voltage to, say 48 volts, you would have a better chance of getting a good charge but then you would need a higher voltage booster and solar controller to drop the voltage back to about 14 volts. The cost and complication increases dramatically.

The simple answer is to replace the vehicle wire with a larger wire. I use a pair of number 8 wires in parallel. That is equivalent to a number 4 wire and is much easier to route than a number 4. You can buy parallel number 8 wire from places that cater to the high power mobile stereo crowd and amateur radio equipment dealers. Cost is about 1.50 per foot. Do not use house wire. It has far fewer strands and will eventually break. Make sure all connections are properly crimped or soldered (there is a lot of debate on which one is better) and sealed. Add an in-line relay connected to an accessory power line so trailer power is disconnected when you shut down the engine (not a diode isolator - they have too much voltage drop) and fuse it at 30 or 40 amps and you will be ready to go.

I've used this configuration in many vehicles dating back to '72 and have always had good results.

Once you have the system in operation, check the end to end system voltage drop at a specific current with the engine running at high idle and make a note of it. I use a couple of old car headlights with both high and low beam connected in parallel at the bumper plug to give a 20 amp load. If you experience trailer battery charging problems later down the line you can repeat the check to see if your system is deteriorating.

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.

[GeorgeR]

Your proposed system would likely be more expensive than a higher gauge wire. Simple solar controller would simply chop off 18V down to 13.6V losing about 25% from the get go. In order to reduce this loss you would need an MPTT solar charge controller which would accommodate high input voltage.

To achieve the objective of 4 stage control charge from the vehicle alternator you could consider alternator DC to 4 stage charge controller such as these: Advanced Alternator Regulator This solution would not change original TV to TT wiring requirements.

One fellow on the Sprinter forum is using 120VAC inverter close to the engine bay powering his inverter/charger. In lieu of inverter charger 120VAC you could connect to a 4 stage convertor. Unfortunately, well thought out solution for a van camper would require TV to TT connection for 120VAC but your wiring size requirement would go tenfold down.

Personally, if I would go with higher gauge wire if my TV and TT batteries are the same (flooded) but in case of flooded TV and AGM TT batteries I would consider DC to 4 stage DC controller. For a van application such as Sprinter I would simply match house with vehicle battery such as AGM. Sprinter’s alternator controller has sufficient smarts to control AGM batteries.

George.

[Raz]

To avoid I squared R (wire) losses, the power company does exactly that. Since (in most cases) it is AC, they use transformers. Large transmission lines are at hundreds of kilovolts. The line in front of your house is around 10 kilovolts. It goes through a transformer (picture) reducing it to 220 v. Power in = Power out (+ losses). For transmission you have high voltage, low current and low line losses. At your home you have low voltage and high current but the wire losses are negligible because of the short length.

Unfortunately in this case the losses by the DC-DC converter and controller would be greater than what you would gain by the reduced current. Raz

[ericw]

Since you asked to hear from the EE's....

I largely agree with Orlen. I think your idea is theoretically correct at a first level in that it will help with the wire power loss issue and works similar to an AC power transmission line. However, in your specific situation I would emphasize that the losses in the converter on both sides may well negate most of your power savings. Converter efficiencies run ~75-95% depending on the design. The increased cost of all the electronics vs. a heavier duty wire is another factor.

It is still a fun test to run if money is not too big a deal. You will want to run at as high a voltage/current as you can (staying within specs of both converters on each end), as long a wire as is realistic as you see in the field, and see what setup gets the most power through. You could also do a rough theoretical prediction before hand based on all the losses of each setup.

[Thomas G.]

Originally Posted by ericw

Since you asked to hear from the EE's....

I largely agree with Orlen. I think your idea is theoretically correct at a first level in that it will help with the wire power loss issue and works similar to an AC power transmission line. However, in your specific situation I would emphasize that the losses in the converter on both sides may well negate most of your power savings. Converter efficiencies run ~75-95% depending on the design. The increased cost of all the electronics vs. a heavier duty wire is another factor.

It is still a fun test to run if money is not too big a deal. You will want to run at as high a voltage/current as you can (staying within specs of both converters on each end), as long a wire as is realistic as you see in the field, and see what setup gets the most power through. You could also do a rough theoretical prediction before hand based on all the losses of each setup.

Thanks for the reply. I don't know the efficiencies of the inverter or the controller, but at $8 and $15 respectively, I'm willing to buy them and play with them. If it doesn't work out, I'll use them for other projects.

I wasn't clear in my initial post. The intent is not to run a refrigerator off the inverter nor to boost the voltage leaving the tow vehicle, but rather to boost the voltage right at the trailer battery to attain that necessary top off voltage.

The function that I am trying to replicate is similar to this:

DC Input Battery Chargers for 12 volt Lead Acid and SLA Batteries useful for electric golf trolley

[Thomas G.]

Originally Posted by Orlen Wolf

............
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.

[ericw]

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!

[rabbit]

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?

jack

[Thomas G.]

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.

[RogerDat]

Originally Posted by Thomas G.

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.

[Thomas G.]

Originally Posted by RogerDat

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.