Tow Vehicle to Trailer Charge Booster - Page 3 - Fiberglass RV


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Old 04-17-2013, 12:45 PM   #29
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Originally Posted by Thomas G. View Post
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.

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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.
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Old 04-17-2013, 12:52 PM   #30
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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
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Old 04-17-2013, 12:56 PM   #31
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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
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Old 04-17-2013, 12:58 PM   #32
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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.
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Old 04-17-2013, 03:25 PM   #33
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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.
If the energy is not there, nothing else matters.

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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?

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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
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Old 04-17-2013, 04:12 PM   #34
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If the energy is not there, nothing else matters.
True

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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.

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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?
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Old 04-17-2013, 04:45 PM   #35
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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.
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Old 04-17-2013, 05:02 PM   #36
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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
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Old 04-17-2013, 05:10 PM   #37
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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..........
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Old 04-17-2013, 05:19 PM   #38
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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.

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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

Quote:
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.

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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.
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Old 04-17-2013, 05:34 PM   #39
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...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...
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Old 04-17-2013, 05:40 PM   #40
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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

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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.

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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.
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Old 04-17-2013, 05:44 PM   #41
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Can anyone tell me if the size of the trailer groundwire matters if charging via alternator-to-battery connection, and if so, how big should it be?

Thanks

Francesca
Since the ground side carries the same amount of current as the hot, technically, yes, it matters. Practically, the amount of steel in the tow vehicle's frame, hitch, the trailer hitch & frame probably is adequate to carry the current, although many, including myself, do not recommend using the vehicle steel as the sole current carrying ground.

One thing worth checking - if you are going to depend on the trailer frame as a current carrying conductor, be sure the wire between the frame & your converter (or where ever your DC circuits fuses are fed from) is large enough to carry the charge current. As to size, I'd suggest a minimum of #10, and, if you plan to run your refrigerator on 12V while traveling, larger.
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Old 04-17-2013, 05:46 PM   #42
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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.

Sounds right

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.

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.
In an electronic engineering degree. Electricity and how it works was my livelyhood for over 45 years. What you're trying to accomplish is the same as trying to invent the perpetual motion machine. Yes the voltage will jump, depending on how much current is being supplied to the battery, probably about the same without the DC-DC converter in the line. If something else like the fridge is running there's less current to charge the battery with. If the fridge requires more than the can be supplied with the DC-DC converter, it will get it from another source if it can, that other source is the battery you're trying to charge.

The whole point is you're going to wasting a lot of energy and taking longer to charge the battery than just going direct.
Also there's lots of stuff in your tow that draws current, enough that a 60 amp alternator is considered quite small, my little Dakota has 150 amp alternator. So once again you're attempting to get the impossible perpetual motion machine.
Good luck. If you manage you'll be a real hero.
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