Charge TT battery from tow vehicle.

Vladivar · 02-23-2018, 10:40 AM

Just got confirmation from my tech that the 7 pin connection on my TV (Ram 2500 CTD) has a 12v pin to charge the TT battery while connected. I am pretty sure it wont be a heavy enough gauge wire to do much more than a trickle charge, but I’m wondering what others experiences have been with his sort of set up and how much charge I could expect to pump into the TT battery during a normal day of towing (say on average 4 to 5 hours).

Kaadk · 02-23-2018, 10:51 AM

My experience is your charging will be practically none. The power provided by the 7 pin (at least for me) tends to offset the parasitic drain of things like the radio, CO/Propane detector, fridge, etc.., so my battery doesn't get drained during the drive, but I haven't had it ever bring a low battery back up at the same time.

kfergiez · 02-23-2018, 11:03 AM

My experience is similar...

The output rating on my alternator is around 80 amps. I'm thinking that most of that power is recharging the battery after startup for the first 30 minutes, then just maintaining power for the car. But that can't take more than 10-15 amps (TOTAL guess).

I'm guessing there is some sort of restriction to limit the amps that actually get supplied to the 7-pin. On mine there is a 10 amp fuse to that pin, so it can't go over than limit and I'm assuming that's just to protect it from a short. I doubt 10 amps EVER gets to the TT battery.

acastillo74 · 02-23-2018, 11:08 AM

I have had the opposite experience. The 7 pin connector charged my 2 batteries from 1/2 charge to full charge after a 8 hour trip.

babock · 02-23-2018, 11:29 AM

The problem is the voltage drop in the small gauge wire that goes from the alternator to the hitch plug. If the vehicle battery is fully charged, the alternator voltage will be somewhat low and any current traveling between the alternator and the TT plug will have enough voltage drop that the TT battery won't be able to accept much of a charge.

Of course, if you have a battery monitor with current shunt, you would know exactly how much current the battery is charging. Even a clamp meter, although super inaccurate, would give you some idea how much current the battery is accepting.

**ependydad** · 02-23-2018, 11:57 AM

Quote:

Originally Posted by Kaadk

My experience is your charging will be practically none. The power provided by the 7 pin (at least for me) tends to offset the parasitic drain of things like the radio, CO/Propane detector, fridge, etc.., so my battery doesn't get drained during the drive, but I haven't had it ever bring a low battery back up at the same time.

My experience is the same. My Victron battery monitor confirms it.

Witch Doctor · 02-23-2018, 12:51 PM

Quote:

Originally Posted by Kaadk

My experience is your charging will be practically none. The power provided by the 7 pin (at least for me) tends to offset the parasitic drain of things like the radio, CO/Propane detector, fridge, etc.., so my battery doesn't get drained during the drive, but I haven't had it ever bring a low battery back up at the same time.

This is also what I have experienced, your alternator will produce only what it reads from your TV battery, so it will drop down fast depending on the state of the TV. Also remember the size of the wire running to your plug, your would have to change that size wire, but your still will be limited. Do not depend on that charging a battery, at most just a trickle charge, but I could be wrong, but don't think so....☻

NMWildcat · 02-23-2018, 12:55 PM

It depends entirely on how your truck system is set up. Research your own truck and use a multimeter to figure it out.

BandJCarm · 02-23-2018, 12:59 PM

On mine, it only slows the drain a bit.

bob213 · 02-23-2018, 01:11 PM

I think on mine, if I could drive from left coast to right coast in a day I would just maintain the batteries from the refrig. being on propane and control board using DC.

RussPastuch · 02-23-2018, 01:18 PM

Same.

Only way it would work is if you picked up one of the boost regulators off eBay and got things up by a few volts. Then things would charge.

**Flybob** · 02-23-2018, 01:20 PM

Unless the wiring in the TV is upgraded, you can generally expect the that pin to maintain the charge on a fully charged battery while traveling.

bigred123 · 02-23-2018, 01:22 PM

I ran a fused 6 ga wire from my truck battery to my rear bumper ...then 50 amp connector fused again to the batteries(2- 100 amp hr) with a shunt meter....charges at 40 amps to a full charge from my 4runner alternator in about 4 hrs...same as the converter charger. never had an issue.

Witch Doctor · 02-23-2018, 01:27 PM

Quote:

Originally Posted by RussPastuch

Same.

Only way it would work is if you picked up one of the boost regulators off eBay and got things up by a few volts. Then things would charge.

I think you would still have to change the wire gauge, to a lot larger size, and more then likely the fuse size. But saying that on my FORD I have a 20 amp fuse which could handle it. But my wire is the ultimate limiting factor. My TT wire size is 8ga. For the plug, But my TV is 12 ga. ( I think) maybe 10ga but I don't think so.

dmchapman · 02-23-2018, 01:50 PM

I have found that with all of my tow vehicles I have been able to get pretty much a full recharge on a 2-3 hour drive home. My former Avalanche TV and current Silverado have high output alternators and when in tow/haul mode the truck circuitry bumps up the charge delivered to the 7-pin. I have never successfully recharged while dry camping by idling the truck. That would take too long.

rdolan5 · 02-23-2018, 02:05 PM

I have a full size SS three door residential refrigerator running a inverter in the trailer. What I did was mount a winch quick connect receptacle on the bumper, I ran a 6 ga wire from the tow vehicle battery to this connector on the bumper, note: there is a 50A automatic circuit breaker in series near the battery to protect this wire. I used 6 ga wire on the trailer end going from the other half of the winch quick connect to the trailer battery positive. I also added another automatic 50A automatic breaker mounted in series near the trailer battery. Both circuit breakers protect the batteries should a short occur at any location on the 6 ga wire. I do have a 160 A alternator on my diesel ram and never had an issue with the trailer battery getting low on the road or the refrigerator shutting down. We travel every year from Arkansas to Washington State. A real test for this setup! I can't see why this same setup wouldn't work for any tow setup! You will keep your TT battery fully charged when traveling!

UT_Grandpa · 02-23-2018, 02:59 PM

Two things you can do:
1. With the TV disconnected, measure the voltage at the battery. Plug in the TT and measure it again, most likely it will go up some if the TT has been disconnected for a while. Start the TV and measure again. If you see something greater that the other two measurements - you are charging.
2. But how much: To do this you need an Ampmeter capable of say 50-60 Amps. These are not common. Something like this: https://www.ebay.com/itm/CHIEFTAIN-A...dRiKdu&vxp=mtr

Temporarily connect it between the battery and the cable from the TV.

Wilco · 02-23-2018, 03:25 PM

The rate of charge is dependent on the voltage of your TT battery and the alternator output. At best you can expect a 2-3 amp charge. (I=E/R; Current = Voltage differential between alternator and TT battery divided by resistance of wiring) Assume alternator output is 14v and TT battery is 12v and the wiring has 1 ohm resistance, the charging current would by 2 amps.

Adding a larger cable would only minimally reduce the resistance. It does not raise the voltage differential.

A severely depleted battery would start out charging faster then slow down as it's voltage rises.

larry pw · 02-23-2018, 03:33 PM

Run a10 ga wire from the tow vehivke to the B+ pin of the 7 pin plug (with a fuse). Doing that i never had a problem keeping the batteries charged. Check the voltage at the truck battery with the engine at 2000 rpm and then at the trailer batteries with the engine at 2000 rpm. I had 13.9 at the truck and 13.6 v at the trailer bat

johntaylor · 02-23-2018, 04:11 PM

My Ram 1500 provides a 5 amp charge to the trailer batteries. That's as good as my Renogy 100 watt suitcase solar system, and a lot better than the crappy WFCO converter that's located over 20 feet from the batteries. I was talking to another Ram 1500 owner a few weeks ago, and he said the same thing.

The Ram's 12 volt pin is live the whole time (not controlled by the ignition switch), so be sure to disconnect the 7-pin plug if you camp somewhere for the night without unhitching, or you could run the truck battery down.

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