DC-to-DC charger: charging my LifePO4 trailer batteries from the towing vehicle's alternator

[John Frum]

Think about the f150 power-on-board as an example- isn't that the same concept?

Same concept as what?

 

[eric_h]

I guess you could end up with dc return over the ac ground or neutral.
Which would make a ground loop with the 7-pin ground.
If both the tow vehicle and trailer side are floating it should be ok.
@eric_h is the tow vehicle inverter floating?
Is the trailer house system floating?
Actually if either side is floating it should be ok.

The RV ground is connected to the chassis.
TV does not originally have ground.

 

[eric_h]

Same concept as what?

As what I'm looking to do.

 

[John Frum]

The RV ground is connected to the chassis.
TV does not originally have ground.

The word originally in this context makes no sense to me.
Is the tow vehicle inverter ground lug connected bonded to the chassis?

 

[John Frum]

As what I'm looking to do.

I would guess the f-150 power on board inverter is bonded to the chassis.

 

[eric_h]

I'd be curious if your trailer lights still work when you have the 120V from the truck connected. If it impacts the trailer ground, you could end up impacting the trailer lights or brakes.
IF something were to happen and you lost lights or trailer brakes, it would come with HUGE liability in an accident, your carrier might even disclaim that portion of the case based on the alterations. I'm not saying it's likely, just saying if something happened, you would be in a really bad spot.

Honestly, I'd just charge off the trailer plug to get what you can get on the road, and leave it all alone. Converting 12v to 120v in the truck, then using 120v to charge 12v in the trailer across a separate connection seems like a lot of conversion losses and potential failure points while going down the road.

Charging it while it's parked, do whatever you decide makes sense, but on the road, be careful.

Thank you. Why would I lose lights on the trailer?
7 way is 12v and on a different circuit. If something were to happen to the 120v it would trip. Please elaborate how it could affect the 7 way.
7 way cable could be harmed in so many ways, btw. I have had mine rip once while driving when it got pinched by the chains. Not a happy moment to rewire the extra one (that good that I thought to carry) on the side of the road...

 

[eric_h]

The word originally in this context makes no sense to me.
Is the tow vehicle inverter ground lug connected bonded to the chassis?

I don't know yet. Just had the idea last night and haven't had the chance to check
Will do soon, but wanted to know if I'm completely off, before I unscrewed anything...

 

[John Frum]

I don't know yet. Just had the idea last night and haven't had the chance to check
Will do soon, but wanted to know if I'm completely off, before I unscrewed anything...

That gives me a idea how to easily check this.

Step 1. Isolate the charge wire in the 7 pin on the trailer side.
This way you can only charge via inverter not the tow cable.
This step should be done regardless.

Step 2. Isolate the return wire in the 7 pin on the trailer side.
This way if the tail lights work than we know that the dc return is via the extension cord between the inverters.
Then you need to think about isolating the inverters.

Are we on the same page?

Actually how is the breakaway battery being charged?

 

[rickst29]

I guess you could end up with dc return over the ac ground or neutral.
Which would make a ground loop with the 7-pin ground.
If both the tow vehicle and trailer side are floating it should be ok.
@eric_h is the tow vehicle inverter floating?
Is the trailer house system floating?
Actually if either side is floating it should be ok.

smoothJoey, His concept doesn't mess up the 7-pin with anything related to 120-VAC, there's an entirely separate cord. The TV's 120-VAC should definitely be generating that 60Hz power with the approximated "sine wave" centered around chassis ground (as zero voltage). The "neutral", supporting return current in the same 60Hz sine wave pattern, must *never* be interconnected with Trailer or TV ground - ground is only connected with the green "safety grounding" wire.

My 1st concern would be with the quality of "modified sine wave versus so-called "pure sine wave" coming out of the Inverter, and the receiving trailer power converter ability to handle that somewhat distorted. The Converter is expecting "clean" power from a utility, and maybe the THD coming out from the Inverter will cause the trailer's converter to die young.

My second concern would be the safety of the 120-V cord, for the possible case of a "hot" disconnect.

 

[John Frum]

smoothJoey, His concept doesn't mess up the 7-pin with anything related to 120-VAC, there's an entirely separate cord. The TV's 120-VAC should definitely be generating that 60Hz power with the approximated "sine wave" centered around chassis ground (as zero voltage). The "neutral", supporting return current in the same 60Hz sine wave pattern, must *never* be interconnected with Trailer or TV ground - ground is only connected with the green "safety grounding" wire.

At the risk of repeating myself if both the inverter's grounding lugs are bonded to the chassis then dc return currrent from the 7-pin could have a new path back to source, no?
The test I just described in the previous post should satisfy my curiosity.

My 1st concern would be with the quality of "modified sine wave versus so-called "pure sine wave" coming out of the Inverter, and the receiving trailer power converter ability to handle that somewhat distorted.

I must have missed that.
@eric_h is the tow vehicle inverter modified sign wave?

The Converter is expecting "clean" power from a utility, and maybe the THD coming out from the Inverter will cause the trailer's converter to die young.

I believe he said the trailer has an inverter_charger but clean power will likely be required.

My second concern would be the safety of the 120-V cord, for the possible case of a "hot" disconnect.

I think @eric_h said the circuit is GFCI protected.

 

[eric_h]

smoothJoey, His concept doesn't mess up the 7-pin with anything related to 120-VAC, there's an entirely separate cord. The TV's 120-VAC should definitely be generating that 60Hz power with the approximated "sine wave" centered around chassis ground (as zero voltage). The "neutral", supporting return current in the same 60Hz sine wave pattern, must *never* be interconnected with Trailer or TV ground - ground is only connected with the green "safety grounding" wire.

My 1st concern would be with the quality of "modified sine wave versus so-called "pure sine wave" coming out of the Inverter, and the receiving trailer power converter ability to handle that somewhat distorted. The Converter is expecting "clean" power from a utility, and maybe the THD coming out from the Inverter will cause the trailer's converter to die young.

My second concern would be the safety of the 120-V cord, for the possible case of a "hot" disconnect.

Yes. The TV inverter is definitely not pure sine wave and I thought to possibly power only the fridge with it, bypassing the RV charger.

That said, this is becoming too complex, with safety concerns, that I'm abandoning the idea.

I might go with an idea I saw here to step up the 12v alternator to 48v to use a lighter wire and less drop, and step down in the RV, going into the circuit as charge, and connect them via the Phillips safe 2 pole sockets or Anderson plugs.

Just trying to feed the fridge and battery as I drive without running 60 feet of expensive cables between the two...

Thank you all!

 

[John Frum]

Yes. The TV inverter is definitely not pure sine wave and I thought to possibly power only the fridge with it, bypassing the RV charger.

That said, this is becoming too complex, with safety concerns, that I'm abandoning the idea.

I might go with an idea I saw here to step up the 12v alternator to 48v to use a lighter wire and less drop, and step down in the RV, going into the circuit as charge, and connect them via the Phillips safe 2 pole sockets or Anderson plugs.

Just trying to feed the fridge and battery as I drive without running 60 feet of expensive cables between the two...

Thank you all!

That was @rickst29 good idea if memory serves.

 

[John Frum]

@eric_h if you just want to power the fridge then the 7-pin should be adequate to the task.
You didn't respond to my question about how the breakaway system is powered.

 

[eric_h]

I must have missed that.
@eric_h is the tow vehicle inverter modified sign wave?

Most probably, will need to check, but I assume Ram did but go the extra mile here.

I believe he said the trailer has an inverter_charger but clean power will likely be required.

Indeed

I think @eric_h said the circuit is GFCI protected.

I was thinking to do that myself anyways.


But, as said above, it was a nice idea but I never marry ideas. Too many variables. Back to the drawing board.

Thank you!

 

[eric_h]

That was @rickst29 good idea if memory serves.

Definitely

 

[eric_h]

@eric_h if you just want to power the fridge then the 7-pin should be adequate to the task.
You didn't respond to my question about how the breakaway system is powered.

Fridge is 360w AC. 7 way is not even close, unfortunately.

Breakaway is connected to house battery, if you recall our long conversation in the matter a while ago

 

[John Frum]

Dang, I was curious to see the results of my tests.
Oh well life goes on.

 

[John Frum]

Fridge is 360w AC. 7 way is not even close, unfortunately.

Understood.

Breakaway is connected to house battery, if you recall our long conversation in the matter a while ago

I don't.
I help a lot of people.

 

[rickst29]

Yes. The TV inverter is definitely not pure sine wave and I thought to possibly power only the fridge with it, bypassing the RV charger.

That said, this is becoming too complex, with safety concerns, that I'm abandoning the idea.

I might go with an idea I saw here to step up the 12v alternator to 48v to use a lighter wire and less drop, and step down in the RV, going into the circuit as charge, and connect them via the Phillips safe 2 pole sockets or Anderson plugs.

Just trying to feed the fridge and battery as I drive without running 60 feet of expensive cables between the two...

Thank you all!

What you're now describing which could go through the "trailer battery charge" wire and return on the shared grounding wire of the Bargman cable (and plug) you already have. That's what I have, stepping up to 36V under the hood and running into a MPPT Solar controller (rather than a fixed voltage "step-down") - providing power to the entire Trailer (including battery charge), and not just dedicated to the fridge.

In my case, the SUV is switchable (so that I can tow other trailers), and defaults back to "standard voltage" whenever I turn the ignition off. In the Trailer, a 5-pin Relay detects the incoming TBC voltage and routes it into the SCC when it is "high".

Is that what you want to do? An MPPT Solar controller costs more than a dumb buck converter, but you wouldn't have to mess with switches when you finished towing. The Fridge is definitely wired to share +12 VDC for both the heater and the control board, the control board must also be powered up for AC operation at a powered campground.

My scheme is good for around 400 watts.

 

[rickst29]

Eric, that Fridge is 120-VAC only? If so, my scheme still works - but you need an Inverter inside the Trailer to convert from Truck-sourced DC power.