Tow Vehicle to Trailer Charge Booster

[Mike Magee]

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?

[Thomas G.]

Originally Posted by Mike Magee

...... 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.

[CraziFuzzy]

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.

[Thomas G.]

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

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.

[Bob Miller]

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)

[Thomas G.]

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.

[Raz]

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

[Thomas G.]

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.

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

[Byron Kinnaman]

Originally Posted by Thomas G.

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.

Alternator - Wikipedia, the free encyclopedia

Thomas,
There's physical laws the come into play. Two that come directly to mind are Ohm's Law and the Power Law. Contrary to what somebody said earlier power is power, watts is watts. Meaning watts at the generator are equal to watts at the trailer minus IR loss in watts. When using a DC-DC voltage booster you reduce the current available by the same percentage you increase the voltage minus the losses in the wire and the efficiency of the DC-DC voltage booster. This how the law of physics work. The physics police won't allow you to violate these laws.

[Carol H]

Originally Posted by Thomas G.

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.

]

In Canada the car needs it lights on in the daytime (the law) - there is actually no way of shutting them off on most newer cars.

So based on the above amp estimates out of 40 amps I would only have 11 amps left to run the DVD or radio for music up front, gps and the Movie player in the rear for kids to watch a movie, oh and the older kid probable isn't interested in what their little siblings are watching or what music the parents are listening to so they will probable have their MP3 play plugged in or playing on their Ipad. Either Dad or Mom's cellphone is probable recharging up front. Darn its started its cold out the rear window is frosting up - turn on the rear window defroster - there goes another 20 amps.... gezzz why isnt that working!

Edit: Forgot the most important electronics of all - the brake controller - dont know what its pulling in amps but due to the digital reads out etc its got to be using up something

[Thomas G.]

Originally Posted by Byron Kinnaman

Thomas,
There's physical laws the come into play. Two that come directly to mind are Ohm's Law and the Power Law. Contrary to what somebody said earlier power is power, watts is watts. Meaning watts at the generator are equal to watts at the trailer minus IR loss in watts. When using a DC-DC voltage booster you reduce the current available by the same percentage you increase the voltage minus the losses in the wire and the efficiency of the DC-DC voltage booster. This how the law of physics work. The physics police won't allow you to violate these laws.

I guess I still don't see the issue. My thinking: If due to voltage drop, you are only getting 12 volts to the trailer battery, it isn't going to fully charge (due to internal resistance) no matter how much current is available. However, if you can increase the voltage at the trailer battery to 13.5 to 14.5 volts, the battery will charge. Granted, it will charge at a lower rate than if it was mounted next to the alternator, but the charge rate should be significantly higher than without the DC-DC voltage booster (at 12 volts).

Look at the chart below. Assuming a charge rate of C/10 which is 10 amps for a 100 amp hour battery, look at the battery charge at 12.5 volts vs 14.5 volts. Huge difference.

So where is my logic breaking down?

http://www.scubaengineer.com/documen...ing_graphs.pdf

[Jon Vermilye]

Originally Posted by Byron Kinnaman

Thomas,
There's physical laws the come into play. Two that come directly to mind are Ohm's Law and the Power Law. Contrary to what somebody said earlier power is power, watts is watts. Meaning watts at the generator are equal to watts at the trailer minus IR loss in watts. When using a DC-DC voltage booster you reduce the current available by the same percentage you increase the voltage minus the losses in the wire and the efficiency of the DC-DC voltage booster. This how the law of physics work. The physics police won't allow you to violate these laws.

Yes, but the IsquaredR losses will be less at the higher voltage. Take two cases - moving 1000 watts from one place to another. At 1 volt & 1000 amps it would take a huge wire to avoid unacceptable losses. On the other hand, at 1000 volts & 1 amp, you could use a very small diameter wire & still have acceptable losses. That's why the utilities transmit energy at as high a voltage as practical.

While they are a bit more expensive than the version Tom tried, and while most are still using traditional transformers and AC to increase voltages, some utilities are using DC to DC voltage boosters to feed 500KV or higher voltages for power distribution lines. At higher voltages, there is less radiated loss with DC.

[Raz]

Originally Posted by Thomas G.

Raz, I know that we've chewed this bone before, but I'm still convinced that the alternator is not the weak link.

If the energy is not there, nothing else matters.

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.

Remember, most charging occurs when your engine is turning 2000 rpm. What's the alternator output there?

I guess I still don't see the issue. My thinking: If due to voltage drop, you are only getting 12 volts to the trailer battery

This is your problem. As I said before. Cause and effect. Because there are two batteries involved the voltage drop is not caused by the current. The charging current is the result of voltage difference between the two battery voltages. What you are trying to do is make the trailer battery voltage whats left after you subtract the wire resistance voltage drop rather than the wire resistance voltage drop being what's left after you subtract the two battery voltages. Read it again . Draw the circuit. Two batteries, one larger than the other. Negative leads connected. A resistor between the two positive leads. Solve for current. Raz

[Thomas G.]

Originally Posted by P. Raz

If the energy is not there, nothing else matters.

True

Remember, most charging occurs when your engine is turning 2000 rpm. What's the alternator output there?

Alternator output is very nonlinear with rpm, which is why we had to put such big alternators in trucks to get enough charge at 550 rpm idle. See the chart for a Honda Odyssey below.

This is your problem. As I said before. Cause and effect. Because there are two batteries involved the voltage drop is not caused by the current. The charging current is the result of voltage difference between the two battery voltages. What you are trying to do is make the trailer battery voltage whats left after you subtract the wire resistance voltage drop rather than the wire resistance voltage drop being what's left after you subtract the two battery voltages. Read it again . Draw the circuit. Two batteries, one larger than the other. Negative leads connected. A resistor between the two positive leads. Solve for current. Raz

OK, I'm a little slow. How can you ignore the effect of the DC-DC converter that bumps up the voltage to 14.5 volts to the second (trailer) battery?

[Byron Kinnaman]

Originally Posted by Thomas G.

OK, I'm a little slow. How can you ignore the effect of the DC-DC converter that bumps up the voltage to 14.5 volts to the second (trailer) battery?

It won't bump the voltage up to 14.5, that's "open circuit" voltage. Once connected to the battery the voltage will drop to close to the battery level. It doesn't matter as far as voltage is concerned if you something else running or not, the 12 volt trailer system cannot go any higher than the battery voltage. The result of having something else on is the battery will simply take longer to get up to the DC-DC converter voltage, provided the combination can supply enough current to run the fridge with some left over. You'll also burn up some the energy supplied by the tow in the DC-DC converter.

If the tow cannot supply enough current to fridge and have some left over, the battery will supply what the fridge wants until an equilibrium is reached where all the available current is going to the fridge and none to or from the battery. The DC-DC converter at this point is working pretty hard to raise the voltage and thus using up energy transformed to heat.

As I've said before the laws of physics are the laws of physics. Perpetual motion machines haven't been invented yet.

[Raz]

Originally Posted by Thomas G.

True



OK, I'm a little slow. How can you ignore the effect of the DC-DC converter that bumps up the voltage to 14.5 volts to the second (trailer) battery?

Tom, I'm not dealing with your DC to DC converter, I'm still back at basic DC analysis. Cause and effect. Read it again. As far as the DC to DC converter ( can you spell perpetual motion), you are stuck on the notion that the charging voltage has to be 14 volts to charge the battery. As long as the tow vehicle voltage is larger than the trailer battery voltage, the trailer battery will charge. It may take a long time but it will charge. Making the charge line resistance smaller increases the charging current and speeds up the charging. Now, reread Byron posts. Raz

[Raz]

Originally Posted by Carol H

In Canada the car needs it lights on in the daytime (the law) - there is actually no way of shutting them off on most newer cars.

So based on the above amp estimates out of 40 amps I would only have 11 amps left to run the DVD or radio for music up front, gps and the Movie player in the rear for kids to watch a movie, oh and the older kid probable isn't interested in what their little siblings are watching or what music the parents are listening to so they will probable have their MP3 play plugged in or playing on their Ipad. Either Dad or Mom's cellphone is probable recharging up front. Darn its started its cold out the rear window is frosting up - turn on the rear window defroster - there goes another 20 amps.... gezzz why isnt that working!

Edit: Forgot the most important electronics of all - the brake controller - dont know what its pulling in amps but due to the digital reads out etc its got to be using up something

Don't forget the 400 watt stereo with the sub woofer- boom da boom da boom..........

[Thomas G.]

Originally Posted by Byron Kinnaman

It won't bump the voltage up to 14.5, that's "open circuit" voltage. Once connected to the battery the voltage will drop to close to the battery level. It doesn't matter as far as voltage is concerned if you something else running or not, the 12 volt trailer system cannot go any higher than the battery voltage.

That wasn't my observation when I was messing around with a small inverter and the 12 volt jumper battery. With the inverter putting out more than 12 volts, the voltage across the battery terminals jumps to more than 12 v - in the low 13 volt range. Similarly, when I plug my trailer into a small charge/ maintainer the voltage jumps from 12.2 volts to the low 13 volt range. In my car, the battery voltage reads 12 volts with the ignition off and jumps to 14.5v when the engine is started.

The result of having something else on is the battery will simply take longer to get up to the DC-DC converter voltage, provided the combination can supply enough current to run the fridge with some left over. You'll also burn up some the energy supplied by the tow in the DC-DC converter.

Sounds right

If the tow cannot supply enough current to fridge and have some left over, the battery will supply what the fridge wants until an equilibrium is reached where all the available current is going to the fridge and none to or from the battery. The DC-DC converter at this point is working pretty hard to raise the voltage and thus using up energy transformed to heat.

I can see where there would be a minimum gauge of wire that would allow the fridge to be operated and the trailer battery to be charged at the same time.

As I've said before the laws of physics are the laws of physics. Perpetual motion machines haven't been invented yet.

I have a degree in mechanical engineering, so I'm pretty familiar with the perpetual motion idea. But if the concept of using a DC-DC converter to boost the charging of a remote battery is unsound, then the guys who made this device wasted an awful lot of time designing and manufacturing it.

[Thinh]

...One thing ab automobiles' alternators people should know: In most of cars and trucks, alternators are not just electrical genarators ONLY. They are actually AC generators, then this AC current goes through rectifier with high power-bridges of diods to rectify it and coming out as DC current. Before it gets out of alternator, it goes thru: REGULATOR. All of these components are located INSIDE the alternator. So, the bottom line is no matter how high of your rev, this voltage IS REGULATED...

[Thomas G.]

Originally Posted by P. Raz

Tom, I'm not dealing with your DC to DC converter, I'm still back at basic DC analysis. Cause and effect. Read it again. As far as the DC to DC converter ( can you spell perpetual motion),

I don't get the perpetural motion jibes. A DC-DC inverter raises voltage at the expense of current plus efficiency losses, as heat. No magic, it all balances out

you are stuck on the notion that the charging voltage has to be 14 volts to charge the battery. As long as the tow vehicle voltage is larger than the trailer battery voltage, the trailer battery will charge.

That is true and the charge voltage vs charge state graph above shows the relationship, if I understand correctly.

It may take a long time but it will charge. Making the charge line resistance smaller increases the charging current and speeds up the charging. Now, reread Byron posts. Raz

Right, time is the key. As the charging current to the trailer battery trickles to zero, the voltage drop through the charge wire falls toward zero. The whole idea of the DC-DC inverter is to speed up the process.