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Old 04-10-2007, 07:23 AM   #1
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The more I seem to get into batteries the less I know. Thanks to this forum I've looked at several great sites on batteries. However, I can't find the answer to this question so please let me have your thoughts!

Why isn't using jumper cables from the vehicle battery to the trailers batteries a good way to recharge those batteries? Too slow? Over charge? What?

Bob Caesar

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Old 04-10-2007, 07:23 AM   #2
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The more I seem to get into batteries the less I know. Thanks to this forum I've looked at several great sites on batteries. However, I can't find the answer to this question so please let me have your thoughts!

Why isn't using jumper cables from the vehicle battery to the trailers batteries a good way to recharge those batteries? Too slow? Over charge? What?

Bob Caesar

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Old 04-10-2007, 08:37 AM   #3
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You'd do better to just buy some fairly heavy gauge wire and run it between your tow vehicle battery back to the battery while it's still attached to the TT. Just get some kind of plug on each side (I recommend commnon 12v outlets for safety) and use them to link the TT battery to the tow vehicle battery so while in tow your TT battery will be charged. This is assuming that your 7 way harness (the common RV plug) doesn't have keyed 12v already going through it and to the battery.

Although, the better way to do this would be to find a keyed 12v power source on your tow vehicle if possible (only supplies power when the key is inserted and turned) so when you're using the TT while hitched up, you don't risk draining your starting battery since the keyed 12v would isolate it from the trailer's power system.

I have a 67 Shasta and it has brakes that work but the past owner had disconnected them and just put a flat 4 way on it. I've since removed it and placed a round 6 way harness on it and my tow vehicle (I didn't need 7, and 6 was cheaper) and I went ahead and made sure that there was keyed 12v power going through the 6 way and leading to the battery so while I'm towing it will charge the battery off of the alternator. When I turn off the van and leave it all hitched up, I can use the trailer's 12v system without risking my starting battery.

Such a method would be perfectly safe and fine assuming your tow vehicle's alternator can handle charging two batteries. I'm pretty well sure mine can due to it being manufactored an incomplete vehicle cargo van (made incomplete so it could be finished as a conversion). SMaller vehicles may not have such a powerful alternator. Anyway, motorhomes (at least smaller ones) use this same type of method for helping to charge the "house" battery and use what's called an "isolator" to seperate the starting battery from the house battery. If you're towing a trailer you don't really need an isolator unless you just prefer to have one. Simple keyed 12v power going through the 6 or 7 way harness and going to the TT battery will be fine. An isolator needs keyed 12v anyway. Main advantage to it, I guess, it can handle heavier guage wire but it's not meant to have to run too awfully far since Class B/C batteries are close up front usually. But really all you need is keyed 12v from your tow vehicled hooked up to the TT battery.
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Old 04-11-2007, 03:54 AM   #4
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Bob, there's no reason you can't use jumper cables, as long as you're sure you have a good connection.

The real issue is the length of time you allow the battery/s to recharge. If your trailer battery/s are pretty much discharged and you use the tow vehicle's alternator to recharge it, or them, it will take a few hours to recharge the trailers battery/s. The typical deep cycle trailer 12-volt battery is around 100 amp hours, and if you have two 6-volt batteries these are 200 amp hour batteries. Now if your alternator only puts out 80 or 90 amps you can see it will take at least a couple of hours or more. Even heavy duty vehicle alternators are typically no more then 140 amps. And these outputs are when the vehicle engine is above idle. Plus there's the issue of heat. When the alternator is putting out full amperage it can really get hot, especially when the engines idling because your not moving much air over the alternator.

Break down and purchase a Solar Panel w/controller system and enjoy the "quiet" environmental friendly power of the sun to recharge your batteries.
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Old 04-11-2007, 06:24 PM   #5
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Bob, even though I have an Onan generator to charge the batteries when boondocking, I found its better to hook up the jumper cables and charge the bats off the Jeep for a couple of hours. The problem is that most trailers are not equipped with a hi output converter to charge the bats. I changed mine out for a 90 amp unit with the charge wizard that will charge the bats a lot quicker when running the generator so I don't have to use the cables. You do have to remember to check the size of the wire from the converter to the bats and make sure its big enough to handle the amps of the new converter.

But, the answer to your question is yes, you can use the tow vehicle to charge your Bats, either by cables or when towing using the connection in your power plug.
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Old 04-11-2007, 07:57 PM   #6
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Bob, Most of there converters only put out4-6 amps. Like Vette Racer said ,you can get better units. His charges to 90 amps Mine, a Intel charger has 80 to charge my bank with and Like his has about four different cycles to do it with and are about the same as you running your truck with jumper cables. You can do that but will use more gas then he and I when using generator.
Basically,apples and oranges. the only differentness is the usage of gas. If ya got a generator and a battery charger you would be still be better off because gas is not cheap. you get more bang for the buck using generator and a good battery charger.Save the gas and the money will pay itself back to ya in short order.
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Old 04-12-2007, 08:44 PM   #7
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JMO, but if you are boondocking and don't have a generator jumper cables are your least expensive choice to recharge your batteries. Plugging you trailer connector will take forever.

You can buy a "throttle stick" from most auto parts stores that go inbetween the gas pedal and the steering wheel so you can raise the idle speed of your engine. I would open the hood just a bit for added air circulation and by raising the RPMs to about 1500-1800 rpm the alternator will put out a lot more amps than at an idle plus the engine will cool better. Buy decent jumper cables, not the $10 set.

Don't wait until the batteries are dead to charge them so you don't need to run your vehicle for hours at a time. Draining your batteries down too low can damamge them.
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Old 04-14-2007, 02:45 PM   #8
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Yes it can be done, but it's illogical.
It will be much cheaper in the long term to buy a small genset with 12V outlet for charging batteries. You've got a big investment in your tow vehicle, plus the engine will drink a lot of gas used for a 200+HP battery charger.
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Old 04-16-2007, 06:55 AM   #9
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I agree with Ray. "200+HP battery charger" Cheap small generator will be perfect for you.
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Old 04-23-2007, 10:09 AM   #10
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I use a Yamaha EF2400 generator, charging through a Xantrex Freedom inverter/charger and it takes 1 1/2 to 2 hours to fully top-off a pair of 6V deep cycle batteries.
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Old 04-29-2007, 10:47 AM   #11
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I have heard tht most auto alternators are made to go full power for the short time it takes to recharge the start battery and could be burned out going full blast for extended periods to recharge a large battery bank.

It takes a long time to fully recharge batteries. For example, it took at least 8 hours for a 40 amp charger to recharge our 2, each, Group 27's (200 Amp Hours total) from about 80% discharged.

Of course the idea is to never go below 50% discharge but you are still looking at an extended period of time - at least an hour.

It sounds like common sense but make sure you turn off all accessoroies in your tow vehicle so that your alternator can dedicate all output to recharging.
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Old 05-10-2007, 10:03 AM   #12
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Bare with me here - Understanding that offshore deep water boats (not the racers) have a lot in common with camping, a magazine called Passagemaker has an excellent article on the "pitfalls" of standard engine alternators and deep discharge battery banks in this months issue.

Basically, the standard alt. is built for charging a lightly discharged starting battery and powering running rights and stuff.
when you get into our heavy battery charging, the alts. and regulators do NOT hold up well.

It is a well written article that if you are in the bookstore I really recommend looking at. Also good articles on diesel fuel filtering, and other items. The real difference is they float and we drive.


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