Tow Vehicle to Trailer Charge Booster - Page 4 - Fiberglass RV


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Old 04-17-2013, 08:16 PM   #43
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OK, thanks for all the comments on the DC-DC booster.

Over and out.
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Old 04-18-2013, 05:25 AM   #44
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The perpetual motion crack (me) comes from the fact that in theory your DC to DC converter will have an increased output voltage which you are hoping will speed up charging. But that only happens by increasing the current. You can't have both increased voltage and current..

In practice, as Byron points out, as soon as you connect up the trailer battery to the converter, the output will drop. The battery is a hugh energy source/sink. It determines the voltage across its terminals.

I can't seem to find this graph anywhere else and the article you referenced has no bibliography nor biography of the author. Have you seen it anywhere else? Raz
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Old 04-18-2013, 06:14 PM   #45
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I thought we were having a good informative discussion here. Apparently I was wrong??. Most unfortunate as there were a number of misconceptions that seem to occur over and over. Raz
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Old 04-18-2013, 07:18 PM   #46
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As my brother likes to say "there are a plethora of infinite variables".
In my opinion, (as an electrical engineering tech) Tom is basically correct. To illustrate why I say this I will use a hypothetical example. If two batteries of equal state of charge, and voltage are connected in parallel, then there should be no current flow. If a DC to DC converter is put between them, then the one on the low side of the converter will be supplying power to the battery on the high side, till another state of equilibrium is reached.
The problem of charging a battery in the trailer is a bit more complex. We have to consider voltage losses in the wire between the tow vehicle charging system and the trailer battery. Something that must be understood, is that unless there is current flow, there is no loss. The voltage at either end of that wire will be the same until current flows. The voltage loss in the wire can be calculated by V=I^2 * R. Where R is the resistance in the wire, and I is the current. This voltage drop will limit the current flow. But my point is this, it will not stop it. If a DC to DC converter is used at the trailer end, the additional current draw, to compensate for the increased voltage will further increase the voltage drop in the wire between the charging system, and the trailer. If the converter is at the charging system end, then the additional voltage would reduce the current required to transfer the same amount of power. Since we are talking about a squared proportion, a reduction of half in the current will result in 1/4 the losses in voltage. BTW, the voltage at the battery will change due to charging current. Using the same formula, the internal resistance of the battery will cause any charging current to cause a voltage rise at the battery.
This totally ignores the problem of total power consumption and the generating capacity of the alternator. But the battery would charge faster with a suitably sized DC to DC converter. This would work even better if a vehicle mounted, 120VAC inverter was supplying AC to a multi stage battery charger on the trailer. But I am sure that there are regulations against using 120VAC between the trailer and the tow vehicle.
Just using a real big wire between the trailer and the vehicle charging system is still probably the best, (least expensive) way to charge the trailer.
Just my thoughts, no actual experiments were preformed.
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Old 04-18-2013, 07:35 PM   #47
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Maybe I'm missing something. In the original post, Tom suggested using the DC to DC converter to raise the voltage & connect, not directly to the battery, but to a solar controller (which would act as another converter. It still seems to me like that would provide an advantage if the charge line was too small. It is doing the same thing a pair of transformers would do to ship AC more efficiently.

Now, if the efficiencies of the converters are so poor the I squared R losses are not as bad as the converter losses, that is a different story, but as to the concept - why wouldn't it work?
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Old 04-19-2013, 09:11 AM   #48
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Folks don't understand the relationship between the charge line resistance and the trailer battery voltage. Any time someone says that the charging voltage is too low (because of the voltage drop) to charge the battery, they don't understand the circuit. The batteries cause the voltage drop and the charge current is the resultant. No matter what the resistance of the charge line, the trailer battery will always charge.

Tom thinks that this voltage drop across the charge wire needs to be "overcome" and by boosting the voltage with a DC to DC converter he can charge the battery faster. In the ideal case boosting to 14v. will increase the charging current because the voltage drop across the wire has been increased. The problem is that the output of the DC to DC converter is an electronic circuit not a power source. In reality what will happen is the DC to DC converter output will load, that is drop to about the trailer battery voltage. If you had enough energy out of the converter then yes the increase current will charge the battery faster assuming the power in is adequate. That's where the system fails. You can't get more out than is available.

Along the same lines you could power an inverter to increase the voltage, run the AC back to a DC power supply and charge the battery that way too... Raz
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Old 04-19-2013, 10:46 AM   #49
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Quote:
Originally Posted by David Tilston View Post
V=I^2 * R.
OK, as Raz pointed out, I am full of it. V=IxR, there is no squared proportion. Ohms law, Kirchhoff's Law, I am going to go take my meds and be quiet.
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Old 04-19-2013, 01:06 PM   #50
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Quote:
Originally Posted by David Tilston View Post
OK, as Raz pointed out, I am full of it. V=IxR, there is no squared proportion. Ohms law, Kirchhoff's Law, I am going to go take my meds and be quiet.

Probably most important dealing with changes in voltage levels is the power law. Power stays the same minus losses. There's always some losses, sometimes they're not significant sometime they are. No matter how much you change voltage or current you can't change power. I^2xR = P Power in watts. That's a derived formula the basic power formula is P = IxE.
If a source can only provide x number watts into a system or circuit that's all that going to be available at the other end. Raising I will lower E, raising E will lower I.

The losses are key to what Thomas has been talking about. That's why I mentioned the perpetual motion machine. The reason perpetual machines won't work is because of energy losses, in mechanical terms the losses are friction, which so far has not been totally eliminated.
A form of friction electricity, resistance, creates the same effect in electrical stuff. In both the mechanical and the electrical, energy is transformed into heat which is lost as far as doing work is concerned.

This also goes back to Einstein - "Energy cannot be created or destroyed, it can only be changed from one form to another." Mechanical change motion energy to heat via friction, electrical changes electrical energy to heat via resistance.

I hope this helps a bit in understanding.
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Old 04-19-2013, 05:51 PM   #51
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Sillyness

I hope you all realize that this all a bunch of sillyness. Here's why.
Assumption: Wire length from tow battery to trailer battery 21', I think that generous.

Wire resistance for 12 AWG wire .00521 ohms per meter, approx. 3'.
Total resistance = 7 (21' =approx. 7m) * .00521 = .03647 ohms.
Not much, eh?
Next assumption.. Initial charging current 10 Amps.
Voltage drop at 10 Amps = .03647/10 = .3647 Volts. Hmmm Not much.
Current drops as the trailer battery gets closer to charge voltage provided by the tow.
Next assumption: Charge current drops to and maintains close to 1 amp.
Voltage drop across the wire .03647/1 = .03647 volts. Wow.... So what are we trying to gain? 36mVolts of increased charge above what the alternator provides?



Even at
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Old 04-19-2013, 08:06 PM   #52
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A glimpse into automobile charging/battery circuitry: A good/new car battery at it's full charge has a voltage across it's terminals roughly b/t 12.5 to 12.6DCV. When a car battery is charging something, let's say another battery or converter...the smaller gauge of wire will draw less current, other way it will take longer time to charge another battery to it's full charge and vice versa. Do not see the big gauge of wire in car's battery to starter motor to fool you. About starting car motor, it is another story and it DO NEED HUGE current...i.e...bigger gauge of wire. To mention ab separarated charging circuitry from a car battery to other components...let's put this way: A huge tank of 12gallons of water(battery) with pipe(wire) leading to another tank of 1gallon of water(2nd battery), there is NO WAY the charging 2nd battery would have higher voltage than a "mother" battery-a closed circuitry of battery supply and 2nd battery(load) regardless of line resistance(resistor) on electrical terminology/analysis. So bottom line is...to charge a 2nd battery with a higher voltage of a car battery is...non existence. Just my thought...
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Old 04-19-2013, 09:35 PM   #53
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Originally Posted by Thinh View Post
A glimpse into automobile charging/battery circuitry: A good/new car battery at it's full charge has a voltage across it's terminals roughly b/t 12.5 to 12.6DCV. When a car battery is charging something, let's say another battery or converter...the smaller gauge of wire will draw less current, other way it will take longer time to charge another battery to it's full charge and vice versa. Do not see the big gauge of wire in car's battery to starter motor to fool you. About starting car motor, it is another story and it DO NEED HUGE current...i.e...bigger gauge of wire. To mention ab separarated charging circuitry from a car battery to other components...let's put this way: A huge tank of 12gallons of water(battery) with pipe(wire) leading to another tank of 1gallon of water(2nd battery), there is NO WAY the charging 2nd battery would have higher voltage than a "mother" battery-a closed circuitry of battery supply and 2nd battery(load) regardless of line resistance(resistor) on electrical terminology/analysis. So bottom line is...to charge a 2nd battery with a higher voltage of a car battery is...non existence. Just my thought...

The bolded statement above is after 1/2 of rest. No charge current no discharge current.

My truck alternator/voltage regulator will charge the battery to temporary voltage of 14.4, measured by me. While running a typical automobile electrical system is around 13.8 volts. You look at the specifications for any mobile electronics the nominal voltage listed will be 13.8 volts. Commonly 13.8 volts 15%. Immediately after removing the charging source the battery voltage will read what ever the charging voltage was.

Another misconception, the charging current for trailer battery is NOT supplied by the tow battery, it's supplied by the alternator. The maximum voltage the batteries (both the tow and trailer batteries are charged at the same time) will see is dependent on the alternator and voltage regulator. On current vehicles the voltage regulator is built into the alternator vs days gone by when a generator was used.
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Old 04-20-2013, 05:06 AM   #54
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A 10 guage supply and a 10 guage ground return is simple and will work. Tom keep on playing with the gadgets because it's fun. Carol day time running lights are dimmer that when the main lights are, check this against a wall. Dodge has used marker lights as dtrl's since the 90's and Audi uses a sting of leds around the headlights so power usage is very low.
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Old 04-20-2013, 06:25 AM   #55
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My head hurts. I hooked up the charge wire the other day. The pickup side is 10gauge, but the trailer side is 12 gauge. It goes all the way down the side of the 19' scamp, around the back, and back up to the kitchen cabinet to tie in. Why? I didn't want to cut into the outside harness, so I hooked it up at the converter.

According to my fluke, it's 15 volts, even with 12 gauge, a junk trailer battery, and all that wire. What more do you want?
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Old 04-20-2013, 06:31 AM   #56
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Originally Posted by Byron Kinnaman View Post
The bolded statement above is after 1/2 of rest. No charge current no discharge current.

My truck alternator/voltage regulator will charge the battery to temporary voltage of 14.4, measured by me. While running a typical automobile electrical system is around 13.8 volts. You look at the specifications for any mobile electronics the nominal voltage listed will be 13.8 volts. Commonly 13.8 volts 15%. Immediately after removing the charging source the battery voltage will read what ever the charging voltage was.

Another misconception, the charging current for trailer battery is NOT supplied by the tow battery, it's supplied by the alternator. The maximum voltage the batteries (both the tow and trailer batteries are charged at the same time) will see is dependent on the alternator and voltage regulator. On current vehicles the voltage regulator is built into the alternator vs days gone by when a generator was used.
...IMO, I think you misunderstand my concept. What I meant is an analysis ab car battery charging other component alone w/o the alternator participation (or car in park with engine turn off). What I understand is 12.5-12.6volts voltage of car battery at full charge and is measured when it's engine is off. About current of charging, in other post of the forum, I already explained for other member ab his worrisome into the "over-kill" of his ceiling fan when running from a marine battery, and here was what I said.." An applied voltage into an appliance determines the amperage going thru it, not the amperage capacity of the power supply-in this case as marine battery"-Regulator and high power diodes with bridge rectifier built in car alternator I already mentioned in the previous post of this thread. Regarding to the current charging from car battery into a CONVERTER or 2nd battery, one should understand that the 2nd battery/converter doesn't play a role as a resistance load into this circuitry, so it will not follow the rule of V=IxR. Otherwise, one could hardly explain how a converter/battery charger is fed from houshold outlet 120VAC with a tiny gauge of wires / some case as ac-dc adapter...One more thing ab you said No charging current, no discharging current I somehow respectly disagree with that. A circuitry with 2 batteries alone one charging other IMO, the receiving battery couldn't have higher voltage than the "supplied battery". Otherwise the..reversed step will kicks in or the reverse charging will happen until they reach...equilibrium...
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