armchair obsession on wire size - Fiberglass RV


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Old 04-07-2014, 11:24 PM   #1
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armchair obsession on wire size

This past January I had my tow wired for my Scamp 13. Since I moved the battery to the back of the Scamp I was concerned that on-the-road charging from the tow would be inefficient, especially since the shop used #12 wire, some 25 feet in all to the battery. Surely voltage drop would be a problem, right?

Not really. On our recent 8000 mile trip the battery was fully charged after two hours on the highway each travel day. So all my armchair worries about charging were wasted. Seems a #12 conductor can carry quite a few electrons.

My armchair musings get me in trouble on occasion, this time they just added some useless worries.

Cheers, john
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Old 04-08-2014, 07:16 AM   #2
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Originally Posted by John Linck View Post
This past January I had my tow wired for my Scamp 13. Since I moved the battery to the back of the Scamp I was concerned that on-the-road charging from the tow would be inefficient, especially since the shop used #12 wire, some 25 feet in all to the battery. Surely voltage drop would be a problem, right?

Not really. On our recent 8000 mile trip the battery was fully charged after two hours on the highway each travel day. So all my armchair worries about charging were wasted. Seems a #12 conductor can carry quite a few electrons.

My armchair musings get me in trouble on occasion, this time they just added some useless worries.

Cheers, john
Hello John. I'm an EE and I get on here and ramble about this from time to time and it's nice to have someone confirming what I say. Folks get confused between running a skill saw where the voltage drop needs to be minimized verses charging a battery. With the saw any current flowing causes a voltage drop in the wire which subtracts from the supply. What ever is left is what the saw has to run on. Too little and the saw won't run. You have what is called a voltage divider.

When you connect two batteries together, the voltage drop has to be the difference between the two battery voltages. There is no voltage divider. The wire resistance determines the charging current and as such the charging time. Had you used 14 ga. wire, the battery would have still charged, it just would take a little longer.

When you hear someone concerned his battery won't charge because of voltage drop, he's treating his battery like a skill saw.

Thanks for giving me the chance to ramble again , Raz
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Old 04-08-2014, 08:40 AM   #3
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It would be interesting on how far down your battery was at the start of your journey, and how you determined that it was "Fully Charged".

Left out of the consideration is that battery charging is also voltage dependent and, if you start out with 13.5 volts at the alternator, and drop even .5 volts in the charging line, it will not only reduce the charging rate, but can prevent ever getting to a full charge as the battery voltage rises.

I many years of working on FGRV mini-motor homes, many an owner learned that no matter how big the alternator was, that the wires running to the coach battery often became the limiting factor.

In the case of TV's, the connection at the 7 pin plug is often the biggest source of voltage drop.

BTW: This is why also running the refrigerator on 12VDC while under way often leads to a dead battery. The Voltage drop from the additional current flow reduces the charging voltage to below that of the coach battery, resulting in a no charging condition.

Along with wearing a belt and suspenders, I always use a #8 stranded wire for my charging line, and I have also been known to include an extra bonding wire for ground between the tv and trailer.
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Old 04-08-2014, 09:01 AM   #4
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Seems a #12 conductor can carry quite a few electrons.

My armchair musings get me in trouble on occasion, this time they just added some useless worries.

Cheers, john
Stick around John you will find there are a few other topics regarding wiring, batteries and solar etc that a lot of folks muse over that at the end of the day are not going to be the big deal breaker some think they are.

Glad your trip went well.
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Old 04-08-2014, 09:34 AM   #5
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On a related topic...
What size hose do you use to fill your water tank with a constant source pressure of 13.5 PSI?
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Old 04-08-2014, 09:35 AM   #6
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I use a Number 7 size hose......
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Old 04-08-2014, 10:43 AM   #7
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........drop even .5 volts in the charging line,....

The voltage across the charge line is always the difference between the two battery voltages.

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?...The Voltage drop from the additional current flow reduces the charging voltage to below that of the coach battery, resulting in a no charging condition.
Thats physically impossible. The end of the charge line is connected to the trailer battery. How can it be two different voltages?
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Old 04-08-2014, 11:17 AM   #8
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Hello John. I'm an EE and I get on here and ramble about this from time to time and it's nice to have someone confirming what I say. Folks get confused between running a skill saw where the voltage drop needs to be minimized verses charging a battery. With the saw any current flowing causes a voltage drop in the wire which subtracts from the supply. What ever is left is what the saw has to run on. Too little and the saw won't run. You have what is called a voltage divider.

When you connect two batteries together, the voltage drop has to be the difference between the two battery voltages. There is no voltage divider. The wire resistance determines the charging current and as such the charging time. Had you used 14 ga. wire, the battery would have still charged, it just would take a little longer.

When you hear someone concerned his battery won't charge because of voltage drop, he's treating his battery like a skill saw.

Thanks for giving me the chance to ramble again , Raz
At what rate do you wish to charge your battery ? A #8 wire has an ampacity of 40 amps but do you want to charge your battery at that rate . The reason I use #12 Awg or #10 awg is more for mechanical strength versus a #14 AWG and I have about 5000 ft of #12-#10 sitting around the house
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Old 04-08-2014, 11:19 AM   #9
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Because the charging line is a series resistive load and will drop voltage in direct proportion to current flow, as well as dissipate power. With no current flow the voltage "Potential" will be the same, but as soon as a load is applied and current flows, there will be voltage drop. Add in a few plug connections, wire splices and terminal ends and you can get a substantial total series resistance.

Some alternator systems compensate for this by providing a sensing line who's sole purpose is to sense voltage potential at the load end and increase output voltage to compensate for losses.
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Old 04-08-2014, 11:35 AM   #10
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Because the charging line is a series resistive load and will drop voltage in direct proportion to current flow
Your cause and effect is backwards. In the case of a charge line the voltage drop across the resistance ( wires, connections, etc. ) is the difference of the two batteries and the current is the resultant.

The charge current does not cause the voltage drop, the voltage drop causes the charge current.
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Old 04-08-2014, 11:42 AM   #11
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It would be interesting on how far down your battery was at the start of your journey, and how you determined that it was "Fully Charged".
.
Fully charged basically meant 12.7+ volts after some water pumping, etc. at lunch stops. I know the battery seldom gets drawn down below 12.3 volts overnight. And now my solar panel usually tops it off the next day even if I don't drive. But the solar panel was not hooked up for the first month of the trip and the tow charged the battery just fine on driving days. ("Not hooked up" is not exactly correct. Wires were connected but no charging occurred. Seems polarity is important in wiring charge controllers. So embarrassing. Luckily no damage occurred. Must have been those Chinese instructions.)

To more fully grasp the entire situation I need to persuade my dear wife to ride in the Scamp and relay the charging amps and voltage to me via walkie talkie. And if you think that is likely to happen I have a bridge to sell ya. ;-)
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Old 04-08-2014, 11:47 AM   #12
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Hmmmm I guess that I x R = E must not apply? and that all the design and money put into sensing lines for alternators is wasted?

The only point I am trying to make is that there is a voltage drop across the charging line and it can be enough to prevent satisfactory charging of the trailers battery.

Without being a E.E. (I only have a 18th grade education) I can site numerous examples where replacing a charging line in motorhomes, with a larger wire, and free of splices and unnecessary connection points, resulted in improved coach battery charging.
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Old 04-08-2014, 11:49 AM   #13
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At what rate do you wish to charge your battery ? A #8 wire has an ampacity of 40 amps but do you want to charge your battery at that rate . The reason I use #12 Awg or #10 awg is more for mechanical strength versus a #14 AWG and I have about 5000 ft of #12-#10 sitting around the house
People use large wire because they fear the current will cause such a large voltage drop that there won't be any left to charge the battery. That shows a clear misunderstanding of the circuit. There are some cases where 14 ga. will work fine and others where 6 ga. is needed. As long as you are just charging the battery no matter what gauge you use, sooner or later the battery will charge. You pick the size based on battery discharge, the charge rate and the time you drive.
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Old 04-08-2014, 11:55 AM   #14
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Hmmmm I guess that I x R = E must not apply?.

I most certainly does. But in his case I = E/R. The current is the resultant. Draw the circuit. Calculate the current. Notice how you do it. Subtract the two voltages and divide by the resistance. No matter what resistance the charge line is you always have a current flowing.
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