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Old 06-01-2023, 06:58 PM   #1
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Ext Cord to Truck while driving

Okay so I've been doing some research and I'm frustrated that I can't fully charge my trailer batteries while driving.

I'm en-route from UT to AK and driving a lot. Like a lot a lot. I don't pay for campsites and therefore have little to no chance to plug in to shore power.

I don't want to run my genny if I don't need to, and I feel like driving a diesel 6 to 10 hours a day is producing plenty of power.

So... my thought is to just use my trucks ac plug by running an extension cord through the trailer across to the truck to be on "shore power" while the truck is driving. Surely someone else has done this, any reason not to?

I'm pretty new to the RV life as I just sort of inherited this huge trailer a few months ago. I do have some limited electrical knowledge though, and I don't see why this wouldn't work, aside from the added risk of dragging the cord on the rd, but I'll zip tie it in place.

Thoughts?
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Old 06-01-2023, 07:12 PM   #2
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I wouldn’t hesitate to run a cord. I am not sure what legalities may prohibit this action, but I would ask for forgiveness if I was caught.
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Old 06-01-2023, 07:17 PM   #3
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You should be OK. If the cord somehow shorts, the truck side should be fused to protect the truck. Ou just need to set it up so it can be easily unplugged when/if you disconnect the trailer from the truck - you don’t want to be having to cut a bunch of zip-ties.
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Old 06-01-2023, 07:19 PM   #4
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Originally Posted by DarkFrost View Post
Okay so I've been doing some research and I'm frustrated that I can't fully charge my trailer batteries while driving.

I'm en-route from UT to AK and driving a lot. Like a lot a lot. I don't pay for campsites and therefore have little to no chance to plug in to shore power.

I don't want to run my genny if I don't need to, and I feel like driving a diesel 6 to 10 hours a day is producing plenty of power.

So... my thought is to just use my trucks ac plug by running an extension cord through the trailer across to the truck to be on "shore power" while the truck is driving. Surely someone else has done this, any reason not to?

I'm pretty new to the RV life as I just sort of inherited this huge trailer a few months ago. I do have some limited electrical knowledge though, and I don't see why this wouldn't work, aside from the added risk of dragging the cord on the rd, but I'll zip tie it in place.

Thoughts?
What is the voltage on the charging line in the 7-way plug from the truck? We run our Ram 3500 and we can charge our batteries with no issues.
Another thought is how old are the batteries in the rig?
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Old 06-02-2023, 07:33 AM   #5
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The proposed electrical arrangement should work fine.

Bulk charging depends mostly on capacity of the charger. In this case there would be two chargers. The engine alternator and the built in travel trailer converter/charger.

Alternator charging capacity through the 7 pin connector varies a lot. Some use 10 AWG wire from tow vehicle battery to 7 pin and again 10 AWG to TT battery. Charging can be 10 to 20 amps when TT battery is discharged. It would rarely reach 30 amps. It depends on lots of things.

I modified mine to use 8 AWG wire for both positive and ground 12v connections.

Plugging the shore power cable into the tow vehicle may overload the tow vehicle inverter. If not, I would expect another 30 to 50 amps of charging.

Four hours of charging with both sources should get an 80% state of charge.

An additional 10 hours of charge would be required to get 100% state of charge. It is in the lead acid battery chemistry.

A full 14 to 18 hour charge is required for long service life of the batteries. Possibly once per week.

Also, lead acid batteries must be stored fully charged for 14 to 18 hours before putting into storage. Fully recharge before terminal voltage drops below 12.4 volts. Again, it is in the chemistry.
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Old 06-02-2023, 07:40 AM   #6
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Why would you invert to ac then convert back to dc? Total waste of power. Run a wire back big enough to handle the amperage you want to charge at from your chassis side, (alternator)
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Old 06-02-2023, 08:17 AM   #7
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Have you measured DC Voltage on the 7 pin charge line?

You mention Generator........are you using it at all?


AC Power from your truck......what is the outlet/inverter rated for?
How long of an extension cord would you need to use?
*Where is our RV receptacle...at rear, middle, front of trailer

I have dual receptacles....one at rear and one at front
Gen is mounted in front of truck bed and I have plugged it's cord into front receptacle when temps are HOT to power the 5th wheel and run A/C while traveling.
Nothing 'illegal' about it.......just have to make sure cord is not interfering with anything or catching on anything
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Old 06-02-2023, 09:48 AM   #8
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Its perfectly fine to do as long as the cord is properly secured. Be careful how many watts you have for the truck outlet and how many your battery charger pulls. Typical AC outlets in vehicles are only 250w or 400w.

Obviously this is the simple cheap method. The TT should be charging off the 7 pin at least 10 amps so that helps but long term the best option is some DC to DC 12V charger pulling how many amps you comfortably can from the alternator.

I have a dedicated 12V 300a alternator and use a WS500 to setup charge profiles so able to charge huge battery bank in a couple hours.
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Old 06-02-2023, 10:40 AM   #9
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Its perfectly fine to do as long as the cord is properly secured. Be careful how many watts you have for the truck outlet and how many your battery charger pulls. Typical AC outlets in vehicles are only 250w or 400w.

Obviously this is the simple cheap method. The TT should be charging off the 7 pin at least 10 amps so that helps but long term the best option is some DC to DC 12V charger pulling how many amps you comfortably can from the alternator.

I have a dedicated 12V 300a alternator and use a WS500 to setup charge profiles so able to charge huge battery bank in a couple hours.


Yeah so truck has a built in 110v 150w AC plug in the back seat. Running a cord from the back of the trailer underneath that ends at the nose, a second cord out the back window of the truck will plug in there 😆
Yes I have a generator but if I'm already burning diesel 6 to 10 hours a day why stop and run the genny for 2 more hours when I get to camp? At least that was my thought.
7 pin (from what I've read online) only charges trailer batteries when the alternator thinks the truck Batteries need charged. So driving this much, the truck batteries don't need charged often. I guess that's why my trailer batteries are totally dead after 4 solid days of driving.

Do you think the 150w is enough to add a bit of charge? Nothing will be in use in the trailer in this mode since I'll be driving...
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Old 06-02-2023, 10:55 AM   #10
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150 watts not going to get you very far. I don't think a battery charger will work on that plug with your battery being that low.
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Old 06-02-2023, 10:58 AM   #11
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Yeah so truck has a built in 110v 150w AC plug in the back seat. Running a cord from the back of the trailer underneath that ends at the nose, a second cord out the back window of the truck will plug in there 😆
Yes I have a generator but if I'm already burning diesel 6 to 10 hours a day why stop and run the genny for 2 more hours when I get to camp? At least that was my thought.
7 pin (from what I've read online) only charges trailer batteries when the alternator thinks the truck Batteries need charged. So driving this much, the truck batteries don't need charged often. I guess that's why my trailer batteries are totally dead after 4 solid days of driving.

Do you think the 150w is enough to add a bit of charge? Nothing will be in use in the trailer in this mode since I'll be driving...

150 watts will give you close to 10 amps of charging . You would be better off getting a DC-DC charger that connects to your trailer plug or a dedicated 12v line. Most of them will take 8 to 16 volts in and do a 3 stage charge on various types of batteries.
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Old 06-02-2023, 11:13 AM   #12
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150 watts will give you close to 10 amps of charging . You would be better off getting a DC-DC charger that connects to your trailer plug or a dedicated 12v line. Most of them will take 8 to 16 volts in and do a 3 stage charge on various types of batteries.
Right. I'll just run my 30a on the generator today to charge them up and use my extension cord to keep them up the next few days or until I can get a dc connection run.

Thanks everyone!
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Old 06-02-2023, 11:36 AM   #13
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Yeah so truck has a built in 110v 150w AC plug in the back seat. Running a cord from the back of the trailer underneath that ends at the nose, a second cord out the back window of the truck will plug in there 😆
Yes I have a generator but if I'm already burning diesel 6 to 10 hours a day why stop and run the genny for 2 more hours when I get to camp? At least that was my thought.
7 pin (from what I've read online) only charges trailer batteries when the alternator thinks the truck Batteries need charged. So driving this much, the truck batteries don't need charged often. I guess that's why my trailer batteries are totally dead after 4 solid days of driving.

Do you think the 150w is enough to add a bit of charge? Nothing will be in use in the trailer in this mode since I'll be driving...
You want a 10a charger MAX, anything more and you'll blow your breaker for the 110V. That's still 10amps per hour additional charge, so 60-100amps over your 6-10 hour drive. This will definitely help but depending on how many AH your battery bank is depends on what % they'll be added. You probably have 100AH batteries so it'll charge 1 battery or half of two. Regardless it helps and doesn't hurt anything at all.

If the 7 pin 12V wire is setup to charge your trailer's batteries it'll charge them regardless just not nearly as much. It basically connects your truck and trailer batteries together so they're both at the same voltage. The alternator turns up when there's an additional load and so having a DC to DC charger it'll force that load at whatever amperage there is regardless of your truck's battery.

Long term solution is to find out what amp fuse is on that 12V pin on your 7pin plug on the truck then find out where it goes in the trailer... if anywhere. Then connect a DC to DC charger in your trailer (under that amperage) and you'll be charging without anything additional to plug in. You can still combine your 120V if you want or add a 2nd wire with a larger DC to DC charger and charge as much as your alternator and batteries can take. likely 40-60amps.

I can tell you its SUPER nice to know when you park you always have a full 100% battery
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