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Old 06-15-2006, 11:35 AM   #1
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Charging 12V using TV and jumper cables?

What is the argument against using the TV and jumper cables to top off the 12V every few days when boondocking? I'm sure it would end in plague, pestulance, and bad karma, and I am sure the engineers are against it; what I dont know is why. Thanks
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Old 06-15-2006, 12:31 PM   #2
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I'm no engineer, but one (probably obvious) issue is the enormous amount of "energy" you'd be expending to do something that proportionately takes a very small amount of "energy" (charging a 12v battery) - like using a firehose to get a drink of water.
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Old 06-15-2006, 12:49 PM   #3
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i'm probably cornfused about the proposition, but i read it as:

* hook one end of jumper cables to TV battery.

* hook other end of jumper cables to AS battery.

* a) wait OR
b) start engine and wait

so if i just do "a" and wait, i risk transferring so much energy from the TV battery that i can't start my TV, thus extending my boondock indefinitely.

but if i just do "b" and wait, i don't need the jumper cables (at least on my AS), cause the alternator can charge the AS battery through the umbilical cord. with or without jumper cables, i've just turned my TV is just a spectacularly inefficient generator.

both sound unappetizing to me.

but then i've probably misinterpreted the proposition entirely, so

if so,

ignore me.

jon


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Old 06-15-2006, 12:54 PM   #4
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Inefficient for sure...

Im sure it would be inefficent. I was thinking using the cables might transfer the juice a bit faster what with the increased cable thickness and all. Im just wondering if it would be a workable stop-gap untill I can go the $1K for a generator.
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Old 06-15-2006, 01:38 PM   #5
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oh, i see what you're saying!

at the risk of really making a fool of myself, i'll put on my dirtball EE hat, and do an undoubtedly flawed analysis.

4 ga wire, commonly used in jumper cables, has a resistance of .000292 ohms per foot, according to some chart i found on the web. it's so low, that i'll happily assume that it's zero, even over a 12-foot length.

the trailer cord could be as thin as 16 ga, which at .00473 ohms per foot, comes out to .057 ohms in a 12-foot run.

now, assuming alternator output of 13.7 volts, a 50% discharged house battery at 12.5 volts, and a charging rate of 10 Amps (which is perhaps a little optimistic given my experience), the battery's internal resistance is (at least initially) .12 ohms. i'm using our magic jumpers with 0 resistance here.

ok, so now i take off the jumpers, hook up the umbilical, and i've got my trailer cable with a resistance of .057 ohms in series with the battery at .12 ohms.
Charge rate drops to around 6.7 Amps, and nearly 1/3 of the power I'm transferring is expended heating up the wire.

i'll therefore conclude that using the jumper cables is a boffo idea.

now, a real EE can come along and destroy me....

i'm bent down with my head on the block....

jon
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Old 06-15-2006, 01:46 PM   #6
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hi dad and disarray.....

great post toasty....

sure hope to camp with toasty's family again and soon......

you nailed 'em...

understand the need for stopgap charging of the coach battery rodney....

issues i imagine....
-how many amps does your alternator produce at idle?
-without the coach to control charging the battery, will it boil?
-how often will you need to add water?
-might it leak adding insult to disarray?
-what will it do to the life of the truck charging parts?

how about a stationary cycle at the campsite..
that charges the battery while you pedal..
think of the legs you will grow!

or here's another idea....
how about getting 2 coach batteries....
and put one inside the truck so it is charging when ever you drive around and without completely wasting fuel...
since driving is gonna happen anyway?

or get one of those little tiny trick charge solar things....and hook it up to the spare coach battery.....it might add a few amps....or not.

or take the spare coach battery to work
and juice it in the office using instutional power.....
not unlike borrowing a roll of toilet paper...
which i never did during the lean years....
of campus life....
no sir...

cheers
2air'
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Old 06-15-2006, 02:04 PM   #7
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Quote:
Originally Posted by 2airishuman
....
or here's another idea....
how about getting 2 coach batteries....
and put one inside the truck so it is charging when ever you drive around and without completely wasting fuel...
since driving is gonna happen anyway...........

cheers
2air'
Thats an interesting idea. I really just need to be sure I can keep the fridge going and the laptop in business if I persue the idea of doing my writing out west next month instead of staying here in southern Illinois
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Old 06-15-2006, 02:25 PM   #8
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From my 9100 Intellicharger "cheat sheet":

55 Amp Intellipower 9100 Converter using a fully discharged 125 amp hour battery and taking it to 100%.

Boost Mode takes about 18 hours

Normal Mode takes about 78 hours

Storage Mode takes about 105 hours

I am reading this off of a charge curve chart.

I do not think you would accomplish much by trying to charge off of jumper cables, unless you want to run your tv all day long.
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Old 06-15-2006, 02:59 PM   #9
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I keep 2 deep cycle batteries in a box on the back of the truck. They can be hooked up to the truck for charging, when out touring around. When back at the trailer they can be pluged in, by way of #4 wire, to the trailer. This has worked for years for us, in campgrounds like Ocala Forest with no electric. We can stay as long as we like and have charged batteries all the time. Much cheaper then solar or generators as we already have the truck.
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Old 06-15-2006, 04:06 PM   #10
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Yet another idea

Sell some crap on eBay to fund the purchase of a generator.

"Ones man's trash is another man's treasure"

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Old 06-15-2006, 05:36 PM   #11
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Quote:
Originally Posted by Toasty's Dad
oh, i see what you're saying!

at the risk of really making a fool of myself, i'll put on my dirtball EE hat, and do an undoubtedly flawed analysis.

4 ga wire, commonly used in jumper cables, has a resistance of .000292 ohms per foot, according to some chart i found on the web. it's so low, that i'll happily assume that it's zero, even over a 12-foot length.

the trailer cord could be as thin as 16 ga, which at .00473 ohms per foot, comes out to .057 ohms in a 12-foot run.

now, assuming alternator output of 13.7 volts, a 50% discharged house battery at 12.5 volts, and a charging rate of 10 Amps (which is perhaps a little optimistic given my experience), the battery's internal resistance is (at least initially) .12 ohms. i'm using our magic jumpers with 0 resistance here.

ok, so now i take off the jumpers, hook up the umbilical, and i've got my trailer cable with a resistance of .057 ohms in series with the battery at .12 ohms.
Charge rate drops to around 6.7 Amps, and nearly 1/3 of the power I'm transferring is expended heating up the wire.

i'll therefore conclude that using the jumper cables is a boffo idea.

now, a real EE can come along and destroy me....

i'm bent down with my head on the block....

jon

I agree with what he says - I don't understand but that usually means the other guy is right.
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Old 06-15-2006, 06:03 PM   #12
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You can hook up jumper cables to do this, but, as when I was learning nuclear engineering, it is an expensive way to boil water. A modern vehicle's charging system is designed to keep a fully charged battery fully charged, not charge a discharged one. Newer vehicles have high draw loads on the tow vehicle all the time, and alternators, even 135 amp ones, will overheat from the effort of trying to charge the dead trailer battery while supplying the power needs of the tow vehicle, necessitating an early replacement of the (several hundred dollar) alternator. Getting an alternator replaced on , for example, a 2004 Suburban could cost as much as $350, and for another $250, you could buy a new Honda E1000 to charge the battery on the coach, and use a gallon of gas a day doing it.
You can probably charge the trailer battery several times, and get away with it, but eventually the law of averages will catch up with you, probably at the most inopportune time.
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Old 06-15-2006, 06:17 PM   #13
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Quote:
Originally Posted by Shacksman
I keep 2 deep cycle batteries in a box on the back of the truck. They can be hooked up to the truck for charging, when out touring around. When back at the trailer they can be pluged in, by way of #4 wire, to the trailer. This has worked for years for us, in campgrounds like Ocala Forest with no electric. We can stay as long as we like and have charged batteries all the time. Much cheaper then solar or generators as we already have the truck.
I use to do the same thing with my old pop-up . The only trouble I had was when I thought I would be smart and keep the batteries charged between camping trips . Dry cells don't like consant charge like a vehicle battery , they will get fried if you leave them hooked up for a week or two.
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Old 06-16-2006, 12:29 AM   #14
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Rodney,
I would not do it. I recently picked up a Honda EU1000 (1 month old with 2 hours of run time) for $500. I use it to charge my battery when I am out for several days.
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Old 06-16-2006, 05:56 AM   #15
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Why not just plug in your pig-tail and charge it the you "supposed to"!?
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Old 06-16-2006, 06:24 AM   #16
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Using jumper cables and the truck alternator will produce a high rate of charge. For an occasional trip this might be okay, although the battery life will be shortened due to the rapid charge rate. You will spend a lot on gas idling your truck for several hours to charge the battery... but for a few trips you will still be nowhere near the $900 to buy a Honda generator.

Consider a used, cheap Honda generator. You should be able to find an older 500-1000 watt model for ~$200. When you are ready to upgrade, you can sell it for what you paid for it.
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Old 06-16-2006, 07:41 AM   #17
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So, last year, when the power was out statewide, there were No generators to buy. There was only one gas station within 10 miles in operation, can you say "gas line?" 4 days without power gives you time to experiment.

The first setup was use the pigtail, and as stated earlier, was 6.7 amps. That means whenever the heater kicks in, the house batteries discharged. After 3 hours of pigtail charging the batteries gained 20 % charge. Not enough to run the heater thru the night.

Second setup, turn the truck around, and jumper cable. Toped the batteries up in 2 hours. I figured the charge rate about 30-40 amps/hour. I didn't feel that requiring my 135amp truck generator to produce 25% of it's rated capacity was dangerous or distructive. The lesser of two weavels, and the truck still runs fine.
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Old 06-18-2007, 11:26 AM   #18
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Quote:
Originally Posted by Shacksman
I keep 2 deep cycle batteries in a box on the back of the truck. They can be hooked up to the truck for charging, when out touring around. When back at the trailer they can be pluged in, by way of #4 wire, to the trailer. This has worked for years for us, in campgrounds like Ocala Forest with no electric. We can stay as long as we like and have charged batteries all the time. Much cheaper then solar or generators as we already have the truck.
I'd like to explore this idea a little.

We haven't done a lot of boondocking, but when we have, we've found it difficult to get through a 2 or 3 day weekend "comfortably", with only the single group 27 battery. (It would be difficult to add /fit a second one to our trailer). Maybe I've only had crappy batteries (sears). but I get the impression that our experience isn't far from the norm.

anyway...I don't think we boondock enough to justify the expense of a generator. but it would be nice to have a "backup" battery, "just in case" we run low before the end of a weekend.

so..couple questions:
I keep 2 deep cycle batteries in a box on the back of the truck.

are you just talkin' about any old plywood box, or are these regular battery storage boxes?

They can be hooked up to the truck for charging, when out touring around.

How?

When back at the trailer they can be pluged in, by way of #4 wire, to the trailer.

again, how? through the umbilical? do you have to take the batteries out of the back of the truck? connect them through some sort of jumper to the coach's battery?
I was thinking it would be neat to be able to say...just take a single battery out of the back of the pickup, and lay it down near the trailer tongue, and plug the umbilical into it. but the ubilical's charge line is pretty small. I know, it would take "days" to recharge a fully discharged coach battery w/ the tow vehicle, in large part, because of the size of this wire. But what I have found in the past, when the coach's battery is low, if I plug it into the tow vehicle, everything inside the trailer starts working normally. (lights brighten...modern car stereo doesn't keep shutting itself off when there's a momentary voltage drop, cuz someone switched on a light, etc).

thoughts?
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Old 06-18-2007, 02:22 PM   #19
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Quote:
Originally Posted by Chuck
I'd like to explore this idea a little.

We haven't done a lot of boondocking, but when we have, we've found it difficult to get through a 2 or 3 day weekend "comfortably", with only the single group 27 battery. (It would be difficult to add /fit a second one to our trailer). Maybe I've only had crappy batteries (sears). but I get the impression that our experience isn't far from the norm.

anyway...I don't think we boondock enough to justify the expense of a generator. but it would be nice to have a "backup" battery, "just in case" we run low before the end of a weekend.

so..couple questions:
I keep 2 deep cycle batteries in a box on the back of the truck.

are you just talkin' about any old plywood box, or are these regular battery storage boxes?

They can be hooked up to the truck for charging, when out touring around.

How?

When back at the trailer they can be pluged in, by way of #4 wire, to the trailer.

again, how? through the umbilical? do you have to take the batteries out of the back of the truck? connect them through some sort of jumper to the coach's battery?
I was thinking it would be neat to be able to say...just take a single battery out of the back of the pickup, and lay it down near the trailer tongue, and plug the umbilical into it. but the ubilical's charge line is pretty small. I know, it would take "days" to recharge a fully discharged coach battery w/ the tow vehicle, in large part, because of the size of this wire. But what I have found in the past, when the coach's battery is low, if I plug it into the tow vehicle, everything inside the trailer starts working normally. (lights brighten...modern car stereo doesn't keep shutting itself off when there's a momentary voltage drop, cuz someone switched on a light, etc).

thoughts?
I know that I'll be shot down here, but here's how it works for me. I use a plastic storage box from Walmart that fits 2 batterys. They are connected together so charge as one and I have about 5' of #4 cable with a blue battery connector from NAPA hooked to them. This rides and stays in the back of the truck. Coming from the truck battery, I ran a #4 cable through a 40 amp fuse and back to the back of the truck. I ran another piece of cable to the truck frame for ground. I put another connector on these cables. Now I can plug them together and keep the batterys in the back charged. I then have a pair of cables that run to the trailer battery with another connector that will plug into the batterys when we are back to the trailer. Sounds complicated but really works quite simply and gives me 2 charged batterys every time we go for a drive.
I know, I know, it's overkill to charge 2 batterys but I don't have to carry around a generator and it will be a long time using up the $1000 in gas to pay for one. (The gen. uses gas too.) Picture is power connector.
I can send more pictures latter if needed.
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Old 06-18-2007, 02:32 PM   #20
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Interesting. So..where is the battery in your trailer? mine is behind the wheel well...way aft. some are mounted up front...

so did you run the cables from the trailer battery so a spot that is accessible up near the a-frame/tongue?

how do you secure the auxilliary battery boxes in the back of the truck, so they don't tip over while driving?
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