[XCD]

Hello Everyone,

I am a new member, just took delivery on my 2022 3500HD Denali last week.

I ordered the truck with dual alternators thinking that I would have increased current to charge batteries in the RV. However, I think that the RV charging pin is limited to 40 amps by a fuse in the truck. So I have the following questions for the group:

1. Is the charging current to the truck's RV electrical connector limited to 40 amps with the dual alternator configuration?
2. Can the 40 amp limit be increased to approximately 100 amps and if so how? I don't consider simply increasing the value of the fuse to 100 amps to be a safe option.

The reason I'm asking is because I'm installing a 24V battery array in the RV to power a whole house inverter solar system and I will need to supply 70-90 amps to the 12-to-24 volt battery charger that will be charging the 24V battery array in the trailer when the truck is connected. If the RV batteries go low and the charger draws in excess of 40 amps I could end up blowing the 40 amp fuse on the RV charging line.

Cheers,
Mark

 

[jdwarren]

Hello Everyone,

I am a new member, just took delivery on my 2022 3500HD Denali last week.

I ordered the truck with dual alternators thinking that I would have increased current to charge batteries in the RV. However, I think that the RV charging pin is limited to 40 amps by a fuse in the truck. So I have the following questions for the group:

1. Is the charging current to the truck's RV electrical connector limited to 40 amps with the dual alternator configuration?
2. Can the 40 amp limit be increased to approximately 100 amps and if so how? I don't consider simply increasing the value of the fuse to 100 amps to be a safe option.

The reason I'm asking is because I'm installing a 24V battery array in the RV to power a whole house inverter solar system and I will need to supply 70-90 amps to the 12-to-24 volt battery charger that will be charging the 24V battery array in the trailer when the truck is connected. If the RV batteries go low and the charger draws in excess of 40 amps I could end up blowing the 40 amp fuse on the RV charging line.

Cheers,
Mark

Nope. The wire is 10 AWG and can only handle 30A. But if you do the math on the voltage drop for 30A through about 30 feet of 10 AWG, well...it ain't pretty. That connection is really only good for 15A or so, in practice.

If your trailer inverter is also a charger the way to send large amounts of power to the trailer is with an inverter on the truck side to provide 120 VAC (or even 120/240 VAC split phase) and connect that to the power inlet on the trailer. There's a link in my signature with some details of how I did this on my truck. When we are en route with our fifth wheel I have the 120 VAC input on the trailer energized from an auxiliary outlet in my truck bed, fed by my inverter. I had to relocate the power inlet on the trailer from the side to just under the front overhang though.

If you really need high current DC from the truck and can't use an inverter, you can look at the connectors used for snowplows, although I think if you want 100A over 30-40 feet within acceptable levels of voltage drop you'll probably need 2/0 AWG welding cable and most snow plow connections are 4 or 2 AWG. A quick look at Amazon reveals there are connectors compatible with 2/0 though, so it is possible. But I'm partial to the inverter solution for a variety of reasons.

Update: I don't know what kind of DC/DC charger you're using, but if it's anything like my Victron stuff, there's an "interesting" aspect of its behavior of which you ought to be aware. I have an 18A, 12/12 VDC Victron charger that takes input from the 12V AUX circuit on the trailer's 7-pin connector and is connected to my lithium battery array in the trailer. It's there just in case I have to tow the trailer with another truck and want to be able to trickle-charge the batteries if it's nighttime and the solar isn't an option, etc. What I found is that the internal logic in the charger is set to provide constant output power of about 220 watts; this is regardless of the input voltage. The problem is that if I sized the fuse for 18A assuming that the charger will see 12V on its input terminals, then if the voltage drop is such that the charger sees less than 12V it will have to increase the current to achieve 220w and hence will blow the fuse instantly. This is exactly what happened when I originally tried to use their 30A 12/12 charger which is programmed to deliver a constant 360w. The implied current at 12V input is 30A, and at 30A the input voltage drops to about 11.8V (assuming 14V at the alternators). 360w/11.8V = 30.5A, and since the current just increased, the voltage drop will also increase, and it becomes a vicious cycle where all of a sudden it's trying to pull 50-60A and only seeing about 7 VDC on the input terminals and that's when the fuse blows (this happens in seconds). Even the 18A charger is a problem because 220w is just too much to push through that little 10AWG for the distance between the alternators and the charger (which includes at least 10 feet of wire from the receptacle to where I mounted the charger).

Anyway.

Just forget the auxiliary power circuit for anything over 15A; it's not going to happen. You're going to be doing some heavy duty wiring with ANL fuses and hydraulically-crimped lugs, etc. And make sure you do the math on the voltage drop before you embark on any of this because the materials won't be cheap.

 

[Mc4life27]

I wasn’t giving a hard time in my
Other post I was just stating some facts but just wondering why do you need or do
You just want 100 amp dc to the trailer? If it’s a real need in very curious on what needs that. Just wondering regardless it’s your truck do whatever you want to do with it. Just more
Wondering out of curiosity. Thanks

 

[jdwarren]

I wasn’t giving a hard time in my
Other post I was just stating some facts but just wondering why do you need or do
You just want 100 amp dc to the trailer? If it’s a real need in very curious on what needs that. Just wondering regardless it’s your truck do whatever you want to do with it. Just more
Wondering out of curiosity. Thanks

I don't know the OP's application but I can give you mine. If we're boondocking and the batteries (7.8 kw*hrs worth of lithium) are depleted toward the end of the day (I have 1200w solar but who knows what the state of charge will be toward the end of the day given demand, cloud cover, etc.), it's nice to be able to use the truck to charge up the system. I even run a 120 VAC connection between the truck and trailer while we're driving so that I can run the electric hot water heating element so that we have hot water at rest stops (I'm trying to reduce dependence on propane). Plus we use the Keurig and microwave sometimes when we pull over for a break and having the 120 VAC connection means I never have to worry about the batteries' state of charge when we get to our destination. Hell, I've even run the front bedroom air conditioner while en route to keep our pets cool on hot days. And that's aside from just generally finding it convenient to be able to use my truck as a 3kw generator.

I agree that 100A on 12 VDC over the distance from the truck's alternators to the trailer batteries is probably impractical, which is why I recommended the inverter solution. It's the same principle used in large scale power transmission; i.e., step up voltage for transmission over long distances and step it back down for consumption at the point of use.

 

[jlawles2]

In order to get 100 amps back to the camper and into the DC-DC charger like you want, you will need to install an auxiliary set of cables to the rear of the truck and another set of cables from the KP or hitch to the charger. This has been discussed on several RV sites.

Not a lot of people like running large inverters to charge the batteries in the RV from the TV because there are too many losses with the DC-AC-DC setups. DC - DC charging is more efficient and with the proper charger can profile charge the batteries as needed.

If you do this, be sure to use a relay to kill the power to whatever system you use to avoid possible massive battery drain when stopped.

 

[mizterwizard]

Some thoughts:

If you convert to 120V in the truck, wire sizes and lengths become almost a non-issue. You would only be looking at 10 amps. 14 AWG would to the trick and that is available every where. There might be some efficiency loss in doing a double conversion but there is a big efficiency gain in the wiring losses. The power company does this sort of thing by default.

The aux 12 volt feed to the trailer is not only inadequate on the truck side but on the trailer side too. What you could do is convert that circuit to 120V and it would handle the 10A/1200W easily. 12 volt wiring will handle 120 volts almost without exception. Of course that opens the possibility that something else would get plugged into your 120 volt system and be fried. Better to have a dedicated circuit for this.

You don't need to go all of the way to 3000W like JDWarren did. That would be nice but you only need 1500W. That would be a lot cheaper.

100A is on the order of the amount of current a starter draws. The wire lengths are about 20 times as long as a starter cable though. You can get away with a lot on a momentary draw system like a starter. For continuous draw you need bigger wires.

You could go with a 24V alternator mounted in the second position, with totally separate wiring of course. At least then you would only be dealing with 50A. When the trailer wasn't connected it would just be sitting there doing nothing. There is such a thing as a 24V regulator to put into a 12V alternator.

Trailer wiring connectors are notoriously prone to corrosion. You would need to be extra sure to have sealed connectors. I'd coat them with dielectric grease too.

When it comes to inverters, pure sign wave is a myth. Anything smoother than a square wave is called pure sign wave. In fact, I don't think "pure sine wave" means anything unless there is an oscilloscope trace to back it up. Mostly that doesn't matter but it might so best to be aware.

 

[jdwarren]

Some thoughts:

If you convert to 120V in the truck, wire sizes and lengths become almost a non-issue. You would only be looking at 10 amps. 14 AWG would to the trick and that is available every where. There might be some efficiency loss in doing a double conversion but there is a big efficiency gain in the wiring losses. The power company does this sort of thing by default.

The aux 12 volt feed to the trailer is not only inadequate on the truck side but on the trailer side too. What you could do is convert that circuit to 120V and it would handle the 10A/1200W easily. 12 volt wiring will handle 120 volts almost without exception. Of course that opens the possibility that something else would get plugged into your 120 volt system and be fried. Better to have a dedicated circuit for this.

You don't need to go all of the way to 3000W like JDWarren did. That would be nice but you only need 1500W. That would be a lot cheaper.

100A is on the order of the amount of current a starter draws. The wire lengths are about 20 times as long as a starter cable though. You can get away with a lot on a momentary draw system like a starter. For continuous draw you need bigger wires.

You could go with a 24V alternator mounted in the second position, with totally separate wiring of course. At least then you would only be dealing with 50A. When the trailer wasn't connected it would just be sitting there doing nothing. There is such a thing as a 24V regulator to put into a 12V alternator.

Trailer wiring connectors are notoriously prone to corrosion. You would need to be extra sure to have sealed connectors. I'd coat them with dielectric grease too.

When it comes to inverters, pure sign wave is a myth. Anything smoother than a square wave is called pure sign wave. In fact, I don't think "pure sine wave" means anything unless there is an oscilloscope trace to back it up. Mostly that doesn't matter but it might so best to be aware.

Welcome back from the abyss.

Among other reasons, a great reason not to run 120VAC over the AUX conductor is that you'd need a neutral anyway. If you use ground in lieu of neutral you could wind up energizing the entire truck/trailer at 120VAC in the case of bad grounding at the inverter, so a dedicated neutral is an absolute must. And since you're running the neutral, why not just run some new SOOW portable/flexible/stranded cable from the inverter to the bed outlets and keep everything neat, organized, and safe?

Another concern with running 120VAC through the AUX conductor is current induction in nearby DC wiring. That AUX conductor is usually packed inside wire loom with a couple dozen other conductors, and since you'd only have the hot side of the 120VAC circuit in there, there would be no neutral current to cancel out the oscillating magnetic field associated with the AC voltage. I would have to sit down and do the calculations to figure out if this effect is significant, but given the sensitivity of the BCM in these trucks (the 12V AUX conductor is inside the loom with the conductors leading to the BCM on my truck), I probably wouldn't want to risk it.

Anyway. My view is that the unforgivably awful 7-way RV connector is hardly acceptable even for transmitting 12VDC signals given its lack of moisture protection and undersized ground; the thought of energizing any of its conductors with 120VAC makes me cringe. The safe way to do this is to keep 12VDC and 120VAC separate all the time.

 

[66436]

Welcome back from the abyss.

Among other reasons, a great reason not to run 120VAC over the AUX conductor is that you'd need a neutral anyway. If you use ground in lieu of neutral you could wind up energizing the entire truck/trailer at 120VAC in the case of bad grounding at the inverter, so a dedicated neutral is an absolute must. And since you're running the neutral, why not just run some new SOOW portable/flexible/stranded cable from the inverter to the bed outlets and keep everything neat, organized, and safe?

Another concern with running 120VAC through the AUX conductor is current induction in nearby DC wiring. That AUX conductor is usually packed inside wire loom with a couple dozen other conductors, and since you'd only have the hot side of the 120VAC circuit in there, there would be no neutral current to cancel out the oscillating magnetic field associated with the AC voltage. I would have to sit down and do the calculations to figure out if this effect is significant, but given the sensitivity of the BCM in these trucks (the 12V AUX conductor is inside the loom with the conductors leading to the BCM on my truck), I probably wouldn't want to risk it.

Anyway. My view is that the unforgivably awful 7-way RV connector is hardly acceptable even for transmitting 12VDC signals given its lack of moisture protection and undersized ground; the thought of energizing any of its conductors with 120VAC makes me cringe. The safe way to do this is to keep 12VDC and 120VAC separate all the time.

Completely agree. Even if you were to get the wiring to work I would consider it a must to have a GFI up front. The chances of bleeding into the DC ground are there and inducing 60 hz noise into your truck are very real. We keep these circuits very isolated from each other in 28VDC aircraft electrical systems for this very reason. Dean