Battery Charging Wire? - Page 2 - Fiberglass RV

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Old 08-13-2006, 09:08 AM   #15
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Trailer: 13 ft Boler
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Just a word of caution, I recently added a seven pin plug to my Boler so I could charge the battery while travelling. I was leary of hooking up the isolator system myself on my 2003 Dodge Dakota so took it to an RV dealer and had it done. Cost me $85.00 including parts, cost them $650 to have a Chrysler dealer diagnose and repair the ABS warning light was on after they did the work. They had accidentially shorted out a $350 dollar part.
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Old 08-13-2006, 09:55 PM   #16
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Trailer: Scamp 13 ft
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These ain't old pick-up trucks, any more!

I'm going to take my Nissan back to the dealership and get someone who really knows there stuff to explain things to me. Until then, I'll just keep the battery disconnected.
I'll also ask them about the isolator system, too. However, it's been my experience that dealers get nervous about giving advice on adding after-market stuff that they don't sell.

Thanks for the tip.

Can anyone make a guess on how much amperage a battery depleted by half will draw once it's connected in parallel with TV battery/charging system?

Also I looked for a specific fuse for the + battery connection to the trailer and couldn't find one. It's not mentioned in the manual, either. Maybe there's a fuseable link buried deep somewhere in the wire harness.

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Old 08-22-2006, 04:40 AM   #17
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I too have upgraded my tow vehicle ('06 Nissan Frontier) and installed the factory wiring harness for towing and here's my situation. Before I had a '99 Chevy Astro with the old style 4-pin wiring harness so this 7-pin stuff is new to me. I'm confused about function of the pin on the TV marked "+ Battery". I metered the pin and found 0 volts when parked, 12.7 V with the key in the ignition turned to ACC, and 14.1 V with the engine running. I traced the mating wire through the harness of my Scamp and found that it splices into the trailer's main battery wire. The wiring looks correct according to Scamp's schematic.

Any ideas would much appreciated.


The voltages you report are about right. On a good digital volt meter, a fully charged 12V battery (at rest after 24 hours or so) should read 12.72 volts. A typical automotive alternator will charge that same battery at 14.1 volts because the voltage regulator built into the alternator is set to cut out at that voltage. That voltage is called the 'float' voltage ... if the battery was a little flat, say the resting voltage was 12.5 or so, then the alternator should kick the voltage up to 14.3 or so until the battery voltage reaches 14.1 then it will reduce the charging voltage to 14.1.

This system is intended to recharge the vehicle starting battery, after starting the machine, by putting back the charge just removed by the start-up process. That job is different than charging a camper's deep cycle battery where the battery may be run down to 50% charge (maybe 12.3 or 12.4 volts) before recharging.

Generally the wiring of your tow vehicle won't be heavy enough to support charging the RV battery efficiently. Also the two batteries will attempt to equalize charge states and given enough time a flat RV battery could drain the TV battery. That's why folks put isolator diodes between the batteries. They allow current to flow only one way ... but they also cause about a .5 volt drop in charging voltage.

There is no perfect solution here but your TV charging circuit should provide enough juice to eventually bring your RV battery back up without much risk of overcharging. 14.1 volts will recharge the RV battery but it will take a while maybe 6 or 8 hours of driving or even more depending on the discharge level of the RV battery. 14.1 volts is also below the threshold where the electrolyte will boil off... it takes 14.3 or more to get the battery fluid boiling. That also explains some of why folks go to auxiliary charging systems like generators or solar panels to recharge the RV battery. A good RV charging system could replenish a 50% discharged deep cycle battery in an hour ... but that's an entirely different subject.

Just don't leave the RV plugged into the TV for very long without the engine running and make sure to check battery fluid levels.
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Old 08-22-2006, 10:34 PM   #18
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In Fred's case, it should be noted that since he gets 0 VDC on the charge wire with the truck key off, he has an isolation system already functioning (apparently part of the factory package) so there's no risk of running down the truck battery from the trailer's electrical system.
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Old 08-23-2006, 11:06 PM   #19
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Hey guys, thanks for the input.

I finally found someone at the Nissan dealership who knows something about this stuff. He told me that the aux. +12V power on the 7-pin connector is rated at 30A. There's a 30A fuse/relay block in the engine compartment fuse box. He defended the size of the wire for a 30A circuit even though it's not AWG 10, and said that the truck has several circuits rated at 30A using AWG 14 wire.

All is still good, however. I never intended to charge the trailer battery with the TV. I usually only go for 3 or 4 nights and a fully charged battery easily gives me all the juice that I need. I plan to run the fridge's 12V heater while driving and that only draws 8 amps. I now know more about what the TV is capable of and the rating and location of the aux. 12V circuit fuse.

I'm thinking that for now, I'll disconnect the trailer battery while driving so that only the fridge is drawing off the aux. circuit.

Thanks again.
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Old 08-24-2006, 11:18 PM   #20
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If one disconnects the battery while running, and has brakes, the breakaway brake switch likely won't turn on the brakes (unless one has a dedicated brake battery).

Also, if the ground/return wires are disconnected from the battery, the electric brakes may not work unless they are grounded to the frame.

If one looks at the wiring under the hood on a typical vehicle, one finds fairly small gauges of wire in the charging system, yet the alternators are typically capable of producing multiples of 30Amps -- The wiring is likely sufficient for the load in terms of safety, but the voltage drop may be enuf to prevent a 100% charge on the trailer battery.

Were it my truck and trailer, I wouldn't disconnect anything while running and would rely on the ignition-isolation to protect the starting battery from draining.
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Old 08-25-2006, 12:00 AM   #21
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My charge line is like Freds.. a 14g wire running from the positive terminal of my battery, to the 7 pin connector on the car socket, and then off to the trailer battery past a 30a fuse.

I left the trailer plugged in with the fridge running for a couple hours by accident. Car fired right up. I thought for sure I would need a jump

I always start the car with trailer plugged in. No problems.
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Old 08-25-2006, 07:30 AM   #22
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Suppose if the trailer battery was near dead or dead and tried to pull high current it would trip the breaker that is in the circuit....I`ve been using this system in RV use for 30 years without a problem.....Benny

The reason a battery "goes dead" is because its internal resistance increases to the point that very little current flows. It doesn't matter whether the current is going out, or coming in.

That is why you will notice, via your charger ammeter, that a dead, or near dead, battery won't take much charge when the charge is initiated. Then after a while, the ammeter will read higher and higher up to the limit of the charger output.

What this means in real life is the in-rush current when you first turn your ignition on would be very low with a fully discharged trailer battery. In fact, the worst case would be with a trailer battery that was just down a little.

Concerning the size of the charging wire -- I'm not convinced that you would ever want to jam anything like the full output of your tow vehicle alternator into a deep discharge trailer battery. For best battery life it is usually recommended to charge slower, say 6 to 10 amps maximum for a typical automotive lead-acid battery.

The reason for the large alternators on autos today is to power accessories, especially stereos that can suck huge amounts of power. Also accessory lighting. Many automotive batteries have charge limiting devices built-in to protect them from huge charge rates.

So, as was previously stated, a 10 guage charging wire should be about right. That's about as large as you can reasonably get onto a trailer plug anyway. Also, I bet there's a lot of vehicles running around with factory trailer wiring setups that just use 14 guage and call it good enough. And it probably is.
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Old 08-25-2006, 11:20 PM   #23
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I didn't mean to imply ?x30Amps heading into the battery, just that the under-hood wiring that supports all the automotive appliances is indeed deemed capable of handling the current by the designers.

There's a good set of charging curves for various battery types at WilliamDarden's Battery FAQ site:

It's showing currents up to 20Amps during the absortion stage for lead-acid batteries and slightly higher for other types.

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