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Old 02-23-2021, 03:21 PM   #21
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TT battery charging

I have a dedicated 6 GA wire from the battery, through a solenoid and using a dash on/off switch, direct to the dual TT batteries. I have a 110v converter that runs a Residential 3 door SS refrigerator. My setup will run the refrigerator going down the road plus keeps both batteries fully charged. The small wire in the 7 way plug is way too light to do the job. I pull yearly from Arkansas to Washington State. Also, I have a 60 amp fuse on the charge line at the vehicle battery and another one at the trailer batteries to protect the circuit if it gets shorted out anywhere along the way! I shut off the dash switch just long enough to refuel.
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Old 02-23-2021, 03:46 PM   #22
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Does the battery voltage between TT and TV battery level off to the same voltage when the TT is plugged in? My guess would be no. The way an alternator works AFAIK is it has a sensor wire from the battery to the regulator. The regulator applies more or less current to the fields to regulate how much charge is put in the battery. If the alternator doesn't read the TT battery voltage then it is controlled by the TV battery. Does that seem correct?
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Old 02-23-2021, 04:35 PM   #23
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Dunno how much charge the TT's batteries may get from the TV, but sure know it'll "borrow" charge from the TV's batteries given the chance. A few years ago we found that out the hard way. TT's fridge was on DC power (bad decision). We stopped for the night at a motel. Next morning the TV wouldn't start. Had to wait for AAA to come jump us before we could continue. Now we make sure the fridge is on propane - or turned off.

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Old 02-24-2021, 07:12 AM   #24
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The answer to the OP's question is, It Depends.

Im not sure of the 17 and up GM Pickups but prior to then the mechanism/wiring exists on the truck to power the 12V pin on the 7way connector but it IS NOT connected at the factory.

The lead is located below the Master Cylinder taped to other wiring under the hood. What has to be done is to cut it loose and bring it up to the #2post on the positive side of the under hood fuse box and bolt it on.

That lead will then be fed by a 30Amp fuse in that fuse box.

Unless you also install an inline relay it will also drain the vehicle batteries unless you disconnect the trailer from the truck when you stop overnight. A relay, actuated by a switched circuit from that fuse box will eliminate this problem .
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Old 02-24-2021, 08:48 AM   #25
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Originally Posted by DouglasReid View Post
The answer to the OP's question is, It Depends.

Im not sure of the 17 and up GM Pickups but prior to then the mechanism/wiring exists on the truck to power the 12V pin on the 7way connector but it IS NOT connected at the factory.

The lead is located below the Master Cylinder taped to other wiring under the hood. What has to be done is to cut it loose and bring it up to the #2post on the positive side of the under hood fuse box and bolt it on.

That lead will then be fed by a 30Amp fuse in that fuse box.

Unless you also install an inline relay it will also drain the vehicle batteries unless you disconnect the trailer from the truck when you stop overnight. A relay, actuated by a switched circuit from that fuse box will eliminate this problem .

Good response! " It depends" should the opening response on all replies to these types of questions.
On my used 2013 Silveraldo I do get the 2-4 amp trickle charge when running the engine, but I have left the trailer hooked up for days with the ignition off with no apparent drain on the TT battery. So I assume that the connection from battery to battery is predicated the ignition switch being left on, right?
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Old 02-24-2021, 09:31 AM   #26
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Originally Posted by PhilFromMaine View Post
Good response! " It depends" should the opening response on all replies to these types of questions.
On my used 2013 Silveraldo I do get the 2-4 amp trickle charge when running the engine, but I have left the trailer hooked up for days with the ignition off with no apparent drain on the TT battery. So I assume that the connection from battery to battery is predicated the ignition switch being left on, right?
Hi Phil,
I have attached an image that shows the 7 way connector at the bumper. With the ignition off use a multi meter from the 12V battery (top right) to the ground (bottom left). If you have 12 volts between them it is supplying power to and from your truck.

If no power, turn on the ignition and try again. If you have 12V then has a solenoid controlling it.

If still no power look about 8" directly below your master cylinder. See if there is a wire there, coiled on top of another wire and taped to it.

This is a link to the thread I was part of when got it working on my truck. Im 407Driver on that forum. Read Ron Neilson's post and mine directly below it.

https://www.duramaxforum.com/threads.../post-12298617
Attached Files
File Type: pdf Camper Vehicle side 7-Way.pdf (1.42 MB, 52 views)
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Old 02-24-2021, 11:25 AM   #27
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Originally Posted by CHICKDOE View Post
i am certainly not an expert. but i have wondered how the dc to dc charger can charge the battery better when nothing changes on the tow vehicle. i understand the concept of trading amps for volts. but i wonder if there is more to it.

in a stock setup the tow vehicle alternator / battery are directly connected to the trailer battery. there is resistance in the circuit composed of the cabling itself and the internal resistance of the trailer battery. that combined resistance will limit the amps that will flow on the circuit.

now what happens if you place a dc to dc charger into the circuit? is it just that the battery is now getting charged at a higher voltage with less amps (trade amps for voltage). or does the dc to dc charger have a lower resistance in the charging circuit from the town vehicle as compared to directly connecting to the battery? if so, would this now lower resistance in the charge circuit allow more amps to flow through it? if this is the case could the dc to dc charger actually provide more amps at a higher voltage as compared wiring direct to the trailer battery?

i truly don't know. it's just something i have been wondering about.
I am still studying the subject also. I usually think I know a lot until I get into these discussions, but here goes. Isn't the voltage a given? Plus or minus 12-13 volts for a 12 volt system? I would surmise that it is the increase in amperage predicated on the much thicker gauge of wire used with a DC-DC charger and its built in control systems. On my DC-DC system I use 4 gauge wire. On the wire sizing charts I could have even gone up to a 2 gauge. I haven't measured it, but I don't think the 12 volt auxiliary wire at the 7 pin connector is any where near that.

I have read (ETrailer.com) that the 7 pin connector, if it is set up (as per other posters here) only contributes 2-4 amps of charging. There are a lot of variables here including wire sizing, DC-DC chargers (or lack of), and the type of battery. Lithium (LIFEPO4) batteries can be charged at a much faster rate than conventional batteries and will suck up as much power as you will give it, up to the limits of the battery.
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Old 02-24-2021, 01:09 PM   #28
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The charging wire for the 7 pin is smaller - 8ga. I still haven't heard from anyone if the voltage between TT and TV battery levels out when plugged in? I know the TV battery will discharge if you leave it plugged in. Like I have said the alternator has a feed from the TV battery that regulates the fields on the alternator to increase the amperage output based on battery voltage. If the wire size limits the amperage to the TT then it doesn't get as much from the alternator as the TV so the TV battery would acheive max voltage sooner and the alternator output would be reduced. I am just not sure if the voltage between TT and TV levels out when plugged in.
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Old 02-24-2021, 01:53 PM   #29
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Originally Posted by pedwards2932 View Post
The charging wire for the 7 pin is smaller - 8ga. I still haven't heard from anyone if the voltage between TT and TV battery levels out when plugged in? I know the TV battery will discharge if you leave it plugged in. Like I have said the alternator has a feed from the TV battery that regulates the fields on the alternator to increase the amperage output based on battery voltage. If the wire size limits the amperage to the TT then it doesn't get as much from the alternator as the TV so the TV battery would achieve max voltage sooner and the alternator output would be reduced. I am just not sure if the voltage between TT and TV levels out when plugged in.
8AWG wire is a fairly large wire. Large enough such that there wouldn't be a significant voltage drop given the load for an average trailer including the devices and battery charging. The voltage as measured for the trailer battery while connected to the tow vehicle should always read at least slightly above 12V to upwards of 14V or so. 14V is the range to adequately charge a lead-acid battery. This will drop to a level lower once it's charged. (If not then there's a fault in your electrical system.) The resistance of an 8AWG wire even up to 30 ft. would be insignificant (milliohms.) In other words no significant current limitation. 8AWG copper wire can handle upwards of 50 Amps for a few feet but even at 15 ft it will handle 30A (see chart in the following post.) I was throwing numbers off the top of my head so thanks for the chart. Most truck/SUV alternators have enough current capacity to handle the extra load of the trailer with no issue.
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Old 02-24-2021, 02:28 PM   #30
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Old 02-24-2021, 02:33 PM   #31
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Originally Posted by pedwards2932 View Post
The charging wire for the 7 pin is smaller - 8ga. I still haven't heard from anyone if the voltage between TT and TV battery levels out when plugged in? I know the TV battery will discharge if you leave it plugged in. Like I have said the alternator has a feed from the TV battery that regulates the fields on the alternator to increase the amperage output based on battery voltage. If the wire size limits the amperage to the TT then it doesn't get as much from the alternator as the TV so the TV battery would acheive max voltage sooner and the alternator output would be reduced. I am just not sure if the voltage between TT and TV levels out when plugged in.
This is going to totally depend on whether the charge pin is kept live from TV at all times. Many, if not most, today are shut off when engine is not running so there is no complete circuit from TT Converter/charger when TV ignition is off.

If the pin is live all the time you run the risk of a dead TV battery after sitting long periods without running the TV.
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Old 02-24-2021, 02:42 PM   #32
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Originally Posted by DouglasReid View Post
Hi Phil,
I have attached an image that shows the 7 way connector at the bumper. With the ignition off use a multi meter from the 12V battery (top right) to the ground (bottom left). If you have 12 volts between them it is supplying power to and from your truck.

If no power, turn on the ignition and try again. If you have 12V then has a solenoid controlling it.

If still no power look about 8" directly below your master cylinder. See if there is a wire there, coiled on top of another wire and taped to it.

This is a link to the thread I was part of when got it working on my truck. Im 407Driver on that forum. Read Ron Neilson's post and mine directly below it.

https://www.duramaxforum.com/threads.../post-12298617
Thanks Doug!

Hmmm.................... Have power with the key off. So, I am guessing that both batteries were nearly evenly charged and no, or very little, drainage took place. Come Spring when the snow is gone, I will experiment some more.
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Old 02-24-2021, 02:59 PM   #33
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Simple question: When your travel trailer is connected to the tow vehicle, does it charge the trailer's battery? And if so, will that drain the battery of the tow vehicle if it remains connected while the vehicle is off, or does it pause the charging?
I tow with a Hyundai (Kia rebadge) minivan that was wired for towing by my dealer. I get about 10 amps max through the 7 pin charging pin. More than 10 amps, the voltage drop prevents my trailer batteries from charging. The 10A is sufficient to run the fridge on DC, but the trailer batteries will not charge, and will discharge to run the fridge while stopped. I have never left the minivan connected to the trailer for more than 2-3 hours without the engine running. And I have large batteries in both the minivan and the camper (2 210AH GC-2). So I have not experienced a dead minivan battery from the camper drawing it down.

If running the fridge on propane, the trailer batteries charge reasonably well in a 4hr drive. Not quite as good as the converter plugged in, but good enough that I'm back to 12.5 or 12.6V after arrival at the next campground without power.

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Old 02-24-2021, 04:17 PM   #34
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My 3500 Ram Diesel has two large batteries under the hood. I use a constant duty rated solenoid controlled by a dash switch and pilot light to charge the tt batteries. I can shut off the circuit to eat ant a food store or when refueling. When I do get to the campground, the charge wizard shows full charge within 1 to 2 hours of remote 50 amp service connection. Also note that I use a truck winch quick connect to supply the 12 volt power through the 6 ga wire. Do not use the 7way! Not heavy duty enough!
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Old 02-24-2021, 04:24 PM   #35
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This is going to totally depend on whether the charge pin is kept live from TV at all times. Many, if not most, today are shut off when engine is not running so there is no complete circuit from TT Converter/charger when TV ignition is off.

If the pin is live all the time you run the risk of a dead TV battery after sitting long periods without running the TV.
On my 08 Silverado the 12V pin in the 7 way receptacle is live even with the ignition off. It appears to be a #8 wire, direct from Post #2 in the under hood fuse box to the rear bumper, fed through a 30 amp fuse.

I have been unplugging the 5er cable but sooner or later I will forget so I am gong to install a solenoid that is rated for Continuous duty.
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Old 03-01-2021, 07:11 PM   #36
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The simple answer is yes, it charges, but it also drains. As new TT owners we found out the hard way during our first trip. Leaving the trailer attached to our truck in a parking lot for a few days while we stayed with friends in their home. We came back to the rig 3 days later and found that the trailer had not only drained the house battery, but also the truck battery. So now EVERYTHING was dead, and we could not leave until the truck battery was charged. Lesson learned, disconnect the 7 pin from the truck when leaving the rig for a few days.
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Old 03-01-2021, 07:25 PM   #37
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My Renogy DC2DC gives me 38.9A at the trailer batteries. I run 6AWG from the battery through a 60A breaker to an Anderson connector mounted to the bumper. DC2DC is mounted in the trailer and is turned on with the trailer marker lights (which I need on for the backup camera anyway)

Works great so far. Isolated the tow rig's batteries, though mine is newer so its cutoff from the 7-way regardless.

35 minute drive took my SOC from 90.1% to 99.8% for 400Ah of lithiums.
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Old 03-01-2021, 07:32 PM   #38
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Sure does! 36 amps at 14.4 volts = about 500 watts or better. Victron Dc to Dc charger works a treat. 4 gauge wire all the way.
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Old 03-01-2021, 07:52 PM   #39
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My tow vehicules (Old Toyota Venza, Current Highlander) do not naturally charge the battery of the trailer. I did add a relay to connect the Tow Vehicle to the +12 on the trailer.

If the engine is off, the relay disconnects the +12V. Otherwise your trailer will kill you car starting battery.

If I don't run the fridge on +12V, the charging while towing is enough to keep battery charged enough for us (fan, pump, lights).

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Old 03-01-2021, 08:08 PM   #40
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TT to TV

Don't remember all the Gory details, however this did work for me on a cross-country trip. My 2015 GMC offered a hotspot through OnStar. I left the key on in the first position and the TT charge the vehicle battery while I was using the vehiles hotspot. Just thought somebody might be interested in knowing that.
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