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2021 Sierra 3500 Danali Dually, Crew Cab, Long Bed (ordered, but has not yet arrived)
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I'm stepping up. Have 2020 Silverado 1500 RST, but about 6 weeks ago I ordered a 2021 Duramax Sierra 3500 Danali Dually, specifically for 5th wheel towing.

So, maybe I'll get to spend some time visiting this forum, with you fine people, to pick your brains, and maybe answer some things too down the road after my truck gets here, of course.

The Sierra has dual alternators, so I was wondering about setting up a high amp cable off a solenoid, that runs to the bed, to plug directly to the RV batteries for a pretty quick charge whenever the truck is connected and running.

Pros? Cons? Is there more I need to consider?
 

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I would be pretty shy about doing that. I would want some sort of auto detection in the loop to make sure there was voltage on both sides before closing that circuit.
I have the product below on my boat that has separate circuits for the engine battery and the house (electronics) battery. It auto detects the higher voltage on the engine side and then closes.
Relay
 
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I'm stepping up. Have 2020 Silverado 1500 RST, but about 6 weeks ago I ordered a 2021 Duramax Sierra 3500 Danali Dually, specifically for 5th wheel towing.

So, maybe I'll get to spend some time visiting this forum, with you fine people, to pick your brains, and maybe answer some things too down the road after my truck gets here, of course.

The Sierra has dual alternators, so I was wondering about setting up a high amp cable off a solenoid, that runs to the bed, to plug directly to the RV batteries for a pretty quick charge whenever the truck is connected and running.

Pros? Cons? Is there more I need to consider?
I would think a better solution would be power to a DC to AC converter and run a bank charger on the RV batteries. Some of the better ones out there will do 10 amps a bank which would be great for topping off the RV batteries. Would not be very hard to find a decent DC to AC converter from the truck electrical to power that. Dean
 

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JD Warren has a few nice writeups on some work he did on his 2017 that should help get you where you want to be

 
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I amend my original post. I read some write-ups on etrailer since I'll one day own a TT. You should use a DC to DC charger. You should isolate the truck from the trailer. Proceed carefully, do your homework. The computers in these new trucks are nothing to mess around with.

Etrailer
 
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From the manual on my 2020 which is dual alt:

When the ignition is on, power is
supplied to the outlets after the DC/
AC switch is pressed. A green
indicator light on the DC/AC switch
indicates when the DC/AC operation
is active. One power outlet can be
used with electrical equipment that
uses a maximum of 400 watts.
If both outlets are being used, 400
watts will be shared between the
outlets. Ensure that all connected
devices do not exceed 400 watts.
The power outlet can be turned off
by pressing the DC/AC switch. The
power outlet can be turned back on
after 10 seconds, during which the
indicator light on the switch will
flash.
An indicator light on the outlet
illuminates when power is provided
to the outlet and no system fault is
detected. The outlets will not
operate when the ignition is off, the
DC/AC switch is not pressed, or the
plug is not fully seated into the
outlet.
If equipment is connected using
more than 400 watts or a system
fault is detected, a protection circuit
shuts off the power supply and the
indicator light turns off.
Do not use a power outlet with a
missing or damaged cover.
The power outlet is not designed for
the following, and may not work
properly if they are plugged in:
. Equipment with high initial peak
wattage, such as
compressor-driven refrigerators
and electric power tools
. Other equipment requiring an
extremely stable power supply,
such as
microcomputer-controlled
electric blankets and touch
sensor lamps
. Medical equipment
 

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Something to be aware of, there is actually truck DC power already at the trailer 7-pin plug at your hitch (pin 4) - it's not enough to really get a good absorption charge on fully dead batteries, but it definitely provides 12 volt truck power to the RV battery circuit. I think there is a maxi fuse for like 40 amps near the battery for this circuit. I know that when my trailer is unplugged the power jack runs MUCH slower than when I plug the 7-pin cable in. If I run the slide on my trailer while boondocking, I always make sure to plug the truck in first or else the slide system goes into fault for low voltage (after a few days off the grid)

Biggest issue will be voltage/current loss over long distance - for example, if you've got 20 ft of heavy gauge cable routing to the back of the truck, then another 20 feet to get to your battery bank, depending on whatever the charge voltage requirements are on the battery bank, you could see a few volts less at the battery string under charge. Batteries are really quite sensitive to charge voltage - even being 1 volt off can mean the difference between absorption and float charge states.

I did this on my 2004 Powerstroke, but I ran 2/0 cable from the battery on the truck (with a 200 amp fuse) to a quick connect coupler by the trailer hitch (it was in the bed, as it was a fifth wheel) And was able to get probably %80 of a full charge on the RV batteries on a 3-4 hour drive (that was from a totally discharged state)

If you do it, run good heavy gauge (fine wire) cables, make sure to use a fuse right at the battery (to prevent a truck fire).

1083598
 

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I'm stepping up. Have 2020 Silverado 1500 RST, but about 6 weeks ago I ordered a 2021 Duramax Sierra 3500 Danali Dually, specifically for 5th wheel towing.

So, maybe I'll get to spend some time visiting this forum, with you fine people, to pick your brains, and maybe answer some things too down the road after my truck gets here, of course.

The Sierra has dual alternators, so I was wondering about setting up a high amp cable off a solenoid, that runs to the bed, to plug directly to the RV batteries for a pretty quick charge whenever the truck is connected and running.

Pros? Cons? Is there more I need to consider?
Question: how is your trailer typically used? I ask because if you're generally plugged into shore power then there's really no need to worry about topping off batteries because it'll always be charging from the grid, and the 12V AUX conductor in the trailer plug can supplement that as well when you're in transit. And if you're boondocking you probably already have an inverter-charger in the trailer and a set of not-inexpensive batteries that need to be charged according to a specific charging scheme that an automotive alternator can't provide anyway. I don't mean to patronize; I just want to make sure that this doesn't become a solution looking for a problem.

For a variety of reasons, some of which are extremely technical, the best way to do this is with a large DC-AC inverter (see either the link in my sig or the link in @1Blue78's post; they go to the same thread). This effectively turns your truck into a generator. Then you can figure out how to get the AC power connection to the trailer in a way that is convenient and safe to keep connected while driving. I did this on both my old TT and my current FW. With the TT I just mounted an outlet box to the frame by where the power connection came out so I could plug the shore power cable into it while driving. The outlet box was connected to a power cable secured along the frame rail up to the trailer tongue which I kept plugged into an auxiliary 30A outlet in the bed of the truck. On the FW I had to relocate the power inlet from the side of the trailer to the front under the bedroom overhang so that the protruding power plug and cable wouldn't add to the width or slap against the trailer while I drive.

If you do it this way you can use your power converter/charger to keep the batteries charged; it will operate just like it would if you were at the campground. DC charging schemes need to be matched to the type of batteries used, so you're always going to be better off converting to AC and then using an AC-DC battery charger compatible with your particular batteries. DC-DC chargers can be found but they are finicky so I don't recommend going that route if you need high rates of charge.

It's the same principle used in long distance power transmission. Step the voltage up (and current down) between the power plant and the destination to minimize line losses and conductor costs and then step the voltage back down for the consumer.

Edit: regarding solenoids, you need to make sure that whatever you use is rated for continuous duty and is well ventilated. I decided against a solenoid for my inverter install for simplicity's sake.
 

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I like jdwarren's solution - it also lets you run the fridge on AC power while driving, saving you propane (on a 3-way fridge)
The truck alternator control really isn't setup to charge deep cycle batteries effectively. Will it do it? Yes. Is it the most efficient way to get a good charge on the RV batteries - absolutely not.

Part of why I didn't consider dual alternators on my truck - I had them on my 2018 Cummins and never found a need for them.
 

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...it also lets you run the fridge on AC power while driving...
And if you go big enough on the inverter (~3kw) it lets you run the hot water heater off electricity as well. Not to mention being able to quickly microwave up some lunch or fire up a Keurig in a rest area rather than have to walk from the truck parking spaces all the way over to the building and wait in lines, deal with covid stuff, etc. And of course now that you have 120 VAC on board you can run a lot of stuff like full size air compressors, etc.
 

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I know that the standard plug in the bed will charge the batteries. When the axle on my unit failed and I spent 5 days in a hotel, the refer was draining the battery every other day to the point the refer would shut off. I would pull up to the camper, plug in the 7 way, kick on the PTO idle (high idle) and let it run for about 30-45 minutes. This would get me through 24 hrs with out ref tripping. I suspect the fan on the refer was drawing the battery down faster than just the pilot sensor. On the trip home after putting new axles on unit, it was fully charged when we got home. I would imagine if left plugged in longer it would fully charge the batteries and then could get away with a 30-45 min top off daily.
 

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An additional note for @Trouble. If rapid charging isn't necessary but you want to make sure that the batteries are able to receive a maximum charge when connected to the truck, then the DC-DC converter can be effective. This device should be installed on the trailer and programmed for your specific battery chemistry.

In my post above I alluded to some "very technical" reasons for my preference for a DC-AC inverter; what follows is a description of those technical issues.

Some DC-DC chargers will attempt to maintain a constant output wattage; for example, a unit labeled "30A" may actually be programmed to maintain a constant 360 watt output (30A * 12V == 360W). This is where the problems start. While the output of the DC-DC charger may in fact be, say, 14.4V and 25A, the voltage on the input side will be much lower because of the voltage drop between the truck's charging system and the DC-DC charger. For example, if the alternator is putting out 14.4V and there's 50 feet of 10 AWG wire between the alternator and the DC-DC charger (in terms of resistance, this is probably about right due to the truck's wiring plus the wire between the trailer's power plug and the batteries, and also remember there is added resistance from all of the terminals and the plug/receptacle connection), you'd be down to 11.9V on the input side of the charger given 25A output.

1083609


So while the alternator is putting out 360W, we're already down to 11.9A * 25A == 298W at the input of the charger (and this doesn't even account for losses within the charger itself!). If the charger is programmed to maintain a constant 360W output, it will compensate by trying to draw more current, but the truck's supply voltage can't increase. If we jump up to 30A the result looks like this:

1083610


Now we are at 11.4V * 30A == 342W, which is still less than the 360W the charger is programmed to supply. It will compensate by increasing the current demand even further, and since we're already at 30A, what happens next is it blows the fuse on the truck side (12V AUX is generally fused at 30A to protect the 10AWG wire).

I used a discrete jump from 25A to 30A for illustration purposes; of course the charger is probably incrementing more gradually but the point still stands. For you nerds, your homework is to formulate and solve the first-order differential equation whose solution as time -> infinity gives the steady-state DC-DC charger input voltage for this scenario. If you divide 360W by that number you get the amperage required to sustain 360W on the output side; I suspect the answer is in the ~40A range. Put simply, it's not going to happen given 10 AWG wire and a 30A fuse.

While solving ODEs may seem too theoretical, the result itself is not. The very expensive Victron Orion DC-DC chargers behave in exactly this fashion. I learned this the hard way when I installed an Orion 12/12-30 charger in my fifth wheel camper; there is no way to program an input current limit. I wound up returning it and using a 12/12-18 charger instead.

Just keep this in mind if you decide to use a DC-DC charger.
 

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Why not just put solar on the trailer? Batteries are ALWAYS charged...
 

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I'm stepping up. Have 2020 Silverado 1500 RST, but about 6 weeks ago I ordered a 2021 Duramax Sierra 3500 Danali Dually, specifically for 5th wheel towing.

So, maybe I'll get to spend some time visiting this forum, with you fine people, to pick your brains, and maybe answer some things too down the road after my truck gets here, of course.

The Sierra has dual alternators, so I was wondering about setting up a high amp cable off a solenoid, that runs to the bed, to plug directly to the RV batteries for a pretty quick charge whenever the truck is connected and running.

Pros? Cons? Is there more I need to consider?
FYI the 7 pin connector which you will use to connect the trailer to the truck has a power lead in it. with the dual alts, you should be able to charge just fine, but, check with the dealer for your 5th wheel.
 

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I suggest you go to an RV specific site to ask for guidance. The members on those sites deal with this type of question all the time.
The group I visit is Forest River Forums at Forest River Forums - Forest River Owners Community
There is a subgroup > Electrical, Charging Systems & Solar
If you say exactly what it is you want to do, I guarantee you will get some very specific answers. Be prepared to provide specifics - - exact model of truck, size of alternators, battery sizes, amp hours, wattages, etc..
Good luck
 

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If you checked out the set-up jdwarren put in his rig I'd pretty much go with what he recommends. Just because it is a "RV" forum doesn't mean the majority of those guys aren't talking out of their ass.
I suggest you go to an RV specific site to ask for guidance. The members on those sites deal with this type of question all the time.
The group I visit is Forest River Forums at Forest River Forums - Forest River Owners Community
There is a subgroup > Electrical, Charging Systems & Solar
If you say exactly what it is you want to do, I guarantee you will get some very specific answers. Be prepared to provide specifics - - exact model of truck, size of alternators, battery sizes, amp hours, wattages, etc..
Good luck
 

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"If you checked out the set-up jdwarren put in his rig I'd pretty much go with what he recommends. Just because it is a "RV" forum doesn't mean the majority of those guys aren't talking out of their ass."

You would not go with that statement if you spent any time on that forum. You would realize the stupidity of what you said.
Sure, there are some over there who don't know their ass from a hole in the ground - - just like here. But it is pretty easy to figure out who knows what they are talking about and who doesn't.
 

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"If you checked out the set-up jdwarren put in his rig I'd pretty much go with what he recommends. Just because it is a "RV" forum doesn't mean the majority of those guys aren't talking out of their ass."

You would not go with that statement if you spent any time on that forum. You would realize the stupidity of what you said.
 

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You thought about solar with an inverter for the trailer? Depending how you set it up, you might not ever need to plug it in again. One 100w Renogy panel on my toy hauler keeps the batts charged through a week long boondocking trip with furnace running every night, stereo playing all day etc. I wouldn’t mess with the electronics on the truck. Playing with fire if you ask me.
 

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I'm stepping up. Have 2020 Silverado 1500 RST, but about 6 weeks ago I ordered a 2021 Duramax Sierra 3500 Danali Dually, specifically for 5th wheel towing.

So, maybe I'll get to spend some time visiting this forum, with you fine people, to pick your brains, and maybe answer some things too down the road after my truck gets here, of course.

The Sierra has dual alternators, so I was wondering about setting up a high amp cable off a solenoid, that runs to the bed, to plug directly to the RV batteries for a pretty quick charge whenever the truck is connected and running.

Pros? Cons? Is there more I need to consider?
Greetings, I use my Truck with a dump trailer. I ran heavy high amp cable off the battery post connection bracket. Done it for years on Many trucks.... Used the same cable I had from my ‘17 Silverado on my ‘21 RST.... Use quality insulated cable and secure tightly. I use a welder lead connector off my bumper at Licence Plate area.
 
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