How does vehicle 12 volt power connect from the 7-Pin to RV batteries (3740 BH-R)

[Ron R]

Greetings to all and what a great forum,

I have been trying to determine how the truck 12 volt power is connected to the house batteries on our 3740BH-R. GD tells me it goes through the converter and an examination of the device only has 12 volt output and 120 volt input. I also checked the inverter and saw no 12 volt through put as well. I asked GD for a schematic and was told none are available. (Does anyone have a schematic?)

The reason I am asking is to keep the house batteries charged on travel days since we have a residential refrigerator and a 2000 watt inverter. I have a lot of available power on the truck with dual 220 amp alternators, however, the 7-pin will be the restriction due to wire and connector size. I may have to run larger wiring back to the RV and use a DC to DC charger to control charging.

Does anyone know if the truck 12 volt power through the 7-Pin will keep the house batteries charged on a travel day with the Samsung 15 CF refrigerator operating? (Maybe I don't need to worry)

Thank you for your assistance,

Sarasota Ron

2020 3740BH-R 2021 Ram 3500, Cummins, CC, 8' bed

GREAT ANSWERS AND THANK YOU! Thank you for the truck schematic - gold!

It is as I suspected, the RV house battery is wired in as a parallel to the truck batteries(s). I Can see a future issue with a dead battery or bad cell overdrawing the fuse to the #5 pin on the 7-Pin connector. Will add spare truck fuses to the load-out list (good reminder, need anyway).

My future plan is to run heavy gauge wires from the truck battery, fuses on both sides, to the RV, install a DC-DC charger. Have a connector outside and unplug when stopped.

Now....that's the right way. The jury-rig way is to place a trickle charger at the house batteries and plug the cord into my rear bed 110 volt , 400 watt outlet for the time being.

I feel sorry for anyone entering this life with no mechanical or electrical general knowledge or handy with tools (better have a fat checkbook and a lot of patience, loose schedule...right). It is the expedition life!

Thank you again for all who took time to answer. I love the GD community, great folks!

Sarasota Ron

[Jkwilson]

It goes directly to the battery, possibly via the breakaway switch.

As you suspect, not a lot of power gets through to the trailer. Maybe 7A or so.

[gbkims]

Greetings to all and what a great forum,

I have been trying to determine how the truck 12 volt power is connected to the house batteries on our 3740BH-R. GD tells me it goes through the converter and an examination of the device only has 12 volt output and 120 volt input. I also checked the inverter and saw no 12 volt through put as well. I asked GD for a schematic and was told none are available. (Does anyone have a schematic?)

The reason I am asking is to keep the house batteries charged on travel days since we have a residential refrigerator and a 2000 watt inverter. I have a lot of available power on the truck with dual 220 amp alternators, however, the 7-pin will be the restriction due to wire and connector size. I may have to run larger wiring back to the RV and use a DC to DC charger to control charging.

Does anyone know if the truck 12 volt power through the 7-Pin will keep the house batteries charged on a travel day with the Samsung 15 CF refrigerator operating? (Maybe I don't need to worry)

Thank you for your assistance,

Sarasota Ron

Hi Ron,

Most have the truck's 12V running thru the junction box at the pinbox and then to front compartment copper bus bar with the auto reset circuit breakers via 10 Awg wire, often Black color.

Trailer 7-pin Junction Box 2021-03-26.pdf

[ncitro]

You’ve hit the nail on the head, for a variety of reasons, the most you’ll ever see out of the stock configuration is about 7A. A few of us have installed the Renogy 40A DC to DC charger, which works great (I think we all have lithium, not sure about lead acid), but requires upgraded wiring to the truck battery. I know at least one person on here has used the 20A version with the factory wiring with good success, if you’re staying with lead acid that may be enough.

[Jlawles2]

I would check the wiring package to see what they have listed.
https://www.ramtrucks.com/BodyBuilde...4XObqxE4%2F%0A
Not certain, but pages 2&3 SHOW 21A Trailer battery continuous power.

[howson]

Sarasota Ron

Neil referenced this in his post--might be of interest: https://gdrvowners.com/forum/operati...e-battle-borns

[Guest]

I would check the wiring package to see what they have listed.
https://www.ramtrucks.com/BodyBuilde...4XObqxE4%2F%0A
Not certain, but pages 2&3 SHOW 21A Trailer battery continuous power.

My MY15 GMC wiring is capable of 35amps and is fused to 30amps. I power a 20amp Renogy DCDC from the trailer connector which has worked well, and the D+ or ignition circuit to turn on the Renogy is wired to the trailer marker lights.
The 7 way connector must be kept clean to efficiently power the Renogy where startup current can approach 25-27amps from the truck.

OEM wiring must be checked before adding the lower amperage 20amp Renogy to the 7 way trailer connector.

Jim

[Popga]

My MY15 GMC wiring is capable of 35amps and is fused to 30amps. I power a 20amp Renogy DCDC from the trailer connector which has worked well, and the D+ or ignition circuit to turn on the Renogy is wired to the trailer marker lights.
The 7 way connector must be kept clean to efficiently power the Renogy where startup current can approach 25-27amps from the truck.

OEM wiring must be checked before adding the lower amperage 20amp Renogy to the 7 way trailer connector.

Jim

I just installed 2 lithium batteries and am considering the 20 amp DCDC, on the OEM truck wiring. My concern is not as much with charging the batteries, as not frying my alternator (I will check OEM wiring size first). Is that a pretty basic install?
I did notice, when disconnecting the 7 pin from my truck yesterday, I dropped the cable of the 7 pin from the trailer and it went face down onto the frame of the trailer, creating quite a spark, as the 12V pin hit the frame. I was unaware that would be hot after disconnecting from from the truck....or is something messed up in my wiring???
Perhaps I've just never made the perfect drop onto the frame before....
Thanks,
Erik

[TedS]

The 12v trailer side pin is hot all the time.

[Guest]

X2 on Teds reply. The trailer connector is hot all the time.

Jim

[Popga]

TedS Guest
Thank you....can’t believe sparks have never flown before now...I will be more careful.
Erik

[Popga]

Guest
Jim, did you mount the DCDC charger in the TT pass-thru storage? Running a wire from 7 pin plug junction box to DCDC charger then back to junction box?
Erik

[Guest]

Guest
Jim, did you mount the DCDC charger in the TT pass-thru storage? Running a wire from 7 pin plug junction box to DCDC charger then back to junction box?
Erik

Yes the DCDC as well as a new ACDC charger was mounted in the passthrough. I disconnected the black charge wire from the TV at the trailer and ran it with with a dedicated 8AWG marine cable straight to the DCDC. Only one other circuit is connected to this line which is my tire minder where it will power up when the 7 way is connected to the truck since this circuit is now isolated from the TV lithium battery. Since the charge line is fused on the truck side, there was no need to add any additional protection. The D+ or ignition circuit to activate the DCDC is connected to the RV marker/running lights. I made up the panel and wired it up outside the rig since I cannot work in the passthrough. Three fused lower back disks and two more in the neck has a way of forcing some creative thinking. Lol.

Be sure your truck has the wiring in place to power the DCDC. I connected this out on the driveway to test it in advance and before wiring it into the RV. It will have a startup current up to 27amps from what I witnessed. Also the 7 way connect must be kept clean so it can pass this current through without excessive resistance.

Jim

[Popga]

Guest
Nicely done! Just what I need...
I mounted my PD9160AL in the space behind the AC DC panel on my rig. Its where the old charger was. Its a long way from the batteries but will have to do for now. One step at a time.
Erik

[Popga]

Is the panel with yellow arrow the “4th wall” of you box? Or is it attached right where it is?
Erik