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Old 06-17-2014, 11:42 PM   #1
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Charging coach batteries from tow vehicle

I am trying to find out if there is anyone that has wired a tow vehicle so that coach batteries will charge while towing. We have boondocked on occasion and found that even towing for 6-8 hours did not significantly charge the coach batteries.

The 12 gauge wire at the 7-pin plug is apparently not up to the task. We had a Class A and the connection from the engine to coach batteries looked like 6 or 8 gauge. It would be very helpful if the drive from campsite to campsite could more than productive just a long drive.
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Old 06-18-2014, 10:39 AM   #2
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You are correct - it takes a fair sized wire from the vehicle battery to the trailer to have any any chance of charging. Probably at a minimum 10 gauge if not 8 gauge (the bigger the better).

The second problem is that the vehicle battery is in the charging circuit. If it has a full charge the voltage regulator will limit the charge current to the auxiliary battery in essence giving you only a "trickle" charge.

However if you stay plugged to the tow vehicle you will have the additional capacity of the tow vehicle battery but run the risk of discharging it to a point where the vehicle won't start. In most motorhomes there is a solenoid that isolates the vehicle battery from the coach batteries when stopped.
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Old 06-18-2014, 05:30 PM   #3
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For (at least) newer GM trucks, if you run in tow haul mode or with the headlamps turned on (not in DRL mode), the output from the alternator will increase and help to charge the trailers battery. I would guess it is true for all other vehicles equipped to tow.
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Old 06-18-2014, 07:04 PM   #4
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I am trying to find out if there is anyone that has wired a tow vehicle so that coach batteries will charge while towing. We have boondocked on occasion and found that even towing for 6-8 hours did not significantly charge the coach batteries.
What is your TV? Some Chevy/GMC trucks require to connect a wire under the hood to energize the tow connector power.
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Old 06-18-2014, 07:12 PM   #5
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I have a 2008 Tundra with tow package. I am getting voltage at the plug, but apparently not enough amperage.

I am thinking about running an 8-10 ha wire from battery to spoof voltage regulator into charging both TV and TT batteries.

Ron
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Old 06-18-2014, 07:16 PM   #6
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Double check tightness of connections in the current path from under truck hood to trailer battery. A loose one could be dropping the voltage and reducing current.
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Old 06-18-2014, 07:26 PM   #7
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I have 2/0 battery cable both power and ground running from my battery to the rear of the truck. There is a 200 amp circuit breaker for when I use it charge my trailer battery on my goose neck for the winch and for my recovery winch. I plan to use it to charge the battery on my trailer I'm waiting to be delivered. So that will give me 4 truck batteries for dry camping as I will put 2 on the trailer
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Old 06-18-2014, 10:29 PM   #8
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Voltage regulators work on demand. Once you turn on your air conditioner it sees the demand and provides the power to compensate.

You must assume your vehicles battery is usually fully charged so the alternators primary function is keeping up with the demand of the vehicles electrical system. You auxiliary battery, although it is probably slightly lower voltage than the vehicle battery is not seen as a big demand. Hence you cannot expect to see a 30 amp charge going to you auxiliary battery. The charging systems attempt is to equalize the two batteries so both have 12.7 volts.

Large wire assist but not necessarily improve charging capacity.
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Old 06-19-2014, 12:07 AM   #9
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Couple of things. First, the Japanese were the pioneers of ECM controlled charging voltage. It's amazing the number of alternators that get replaced because they don't charge at idle and won't until system voltage gets down to 12.35V in some instances.

That means that your RV battery that is 12.2V when it gets hooked up and the system detects that the battery is healthy and never gets above 12.4V or so your RV battery loses and never get's charged up.

Second, the factory charging lead is only good for 15-20 amps. So a 100 amp battery at 50% charge will take nearly 5 hours to charge on the road. It's not meant to recharge, it's meant to top up and maintain on the road.

If you go with a larger cable rig then I'd go with a bumper mount jumper cable setup.

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Old 06-19-2014, 04:07 PM   #10
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Couple of things. First, the Japanese were the pioneers of ECM controlled charging voltage. It's amazing the number of alternators that get replaced because they don't charge at idle and won't until system voltage gets down to 12.35V in some instances.

That means that your RV battery that is 12.2V when it gets hooked up and the system detects that the battery is healthy and never gets above 12.4V or so your RV battery loses and never get's charged up.

Second, the factory charging lead is only good for 15-20 amps. So a 100 amp battery at 50% charge will take nearly 5 hours to charge on the road. It's not meant to recharge, it's meant to top up and maintain on the road.

If you go with a larger cable rig then I'd go with a bumper mount jumper cable setup.

this is the setup I use but higher amp connection due to my winch. works great
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