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

[eric_h]

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.

I love the idea. No dedicated wiring, should be fairly easy to assemble.
I assume a dedicated mppt charger would be in order, of course, as the panels operate on a different voltage, but a basic one should do.

 

[eric_h]

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.

Yes, it is, and yes, I'd need the inverter to be on. Which I've done already, trying to power the fridge from solar and a full battery for an hour.
Was an interesting exercise... ?

 

[John Frum]

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?

@rickst29 do you not think that dc current will flow on the ac ground wire if the above criteria are met?

 

[eric_h]

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.

What about the breakaway system? The powered jack? These are 12v and currently connected to the house battery, thus on the same line with the 7 way charge wire.
I'd probably need to to run a separate wire for the 36v charge wire into the charger (upstream only), and leave the current "charge" wire in place to feed the breakaway and jack with 12v from the battery (downstream only).

 

[rickst29]

I love the idea. No dedicated wiring, should be fairly easy to assemble.
I assume a dedicated mppt charger would be in order, of course, as the panels operate on a different voltage, but a basic one should do.

In my case, the MPPT solar controller "sees" a sudden voltage shift from ~85V down to 36V, but handles it and re-tunes extremely quickly - with no problems. Starting out with an "optimal voltage" of 85V, it receives nothing at all when the switch occurs - and finds 36V to be optimal on the following sweep, which occurs almost instantly. Within the Trailer, however, another set of Relays is required to manage the dual switches of TBC (12v system versus the "Solar Input Switcher" Relay) and the MPPT input itself ("Virtual Panel" 36-volt from the Truck, versus ~85 volt "genuine panels" on the roof of the Trailer).

I have my single MPPT controller supporting both arrays, but never at the same time. It handles the switch with a new Voltage Sweep, almost instantly, and switches to the corresponding correct operating voltage immediately. My solar controller is an EP-Ever "Tracer" BN3215, I have no idea how other MPPT controllers might handle such a switch.

It is important that the maximum "demand" from the MPPT be less than the capacity of the Boost Converter under the hood. If the Real Solar Array is a lot more powerful than the Boost Converter, and the MPPT output amps allow nearly all of that array power to be sent out, it will try to pull "as much as it can get" from the Truck - and 'magic blue smoke' will be released from under the hood of the truck.

 

[rickst29]

What about the breakaway system? The powered jack? These are 12v and currently connected to the house battery, thus on the same line with the 7 way charge wire.
I'd probably need to to run a separate wire for the 36v charge wire into the charger (upstream only), and leave the current "charge" wire in place to feed the breakaway and jack with 12v from the battery (downstream only).

Not really. Breakaway is also powered by the Trailer's own battery bank at all times. (Keep in mind that the whole concept of "breakaway" is handling a total disconnect of the bargman, including its TBC.) While the TV is still running and the bargman is still connected, both are fed from the MPPT controller (in addition to the battery bank). if you need that extra 'Tow Vehicle' power to run the jack, just leave the tow vehicle running while you raise it.

 

[rickst29]

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?

Great question, very much worth repeating! Some REALLY TERRIBLE 120-vac inverters are wired with "neutral" bonded to safety grounding at the Inverter - that's totally wrong for a Trailer and a Tow Vehicle (riding on rubber tires, with no "true" grounding provided. If that wiring can't be broken apart, then IMO the Inverter needs to be thrown away.

Some Ebay reviewers have said that their EDECOA Inverters were wired with this major design defect. I don't know if current EDECOA inverters are built correctly, but it's an issue which should be checked when an Inverter is purchased. (These instructions are obvious to you, but maybe not everybody): With the Inverter powered off, verify that resistance between the safety grounding BOTTOM, ROUNDED port of the 120v sockets and the chassis grounding lugs is zero. Then verify that the resistance between the TALL VERTICAL SLOT (the current carrying neutral) and the chassis grounding lugs is INFINITY.

 

[eric_h]

Not really. Breakaway is also powered by the Trailer's own battery bank at all times. (Keep in mind that the whole concept of "breakaway" is handling a total disconnect of the bargman, including its TBC.) While the TV is still running and the bargman is still connected, both are fed from the MPPT controller (in addition to the battery bank). if you need that extra 'Tow Vehicle' power to run the jack, just leave the tow vehicle running while you raise it.

I don't have a separate battery for the breakaway system. I removed the wet cell battery and wired the breakaway and jack, to the house battery.
Sending 36v towards these two while the cable is connected will not be embraced fondly by them I suppose.
That's why I thought to isolate the charger wire coming from the B cable upstream into the charger controller, and leave the two connected to the house battery circuit, which is after the voltage has been stepped back down.
That means that either if the cable is connected or disconnected I get proper power to both.

 

[eric_h]

In my case, the MPPT solar controller "sees" a sudden voltage shift from ~85V down to 36V, but handles it and re-tunes extremely quickly - with no problems. Starting out with an "optimal voltage" of 85V, it receives nothing at all when the switch occurs - and finds 36V to be optimal on the following sweep, which occurs almost instantly. Within the Trailer, however, another set of Relays is required to manage the dual switches of TBC (12v system versus the "Solar Input Switcher" Relay) and the MPPT input itself ("Virtual Panel" 36-volt from the Truck, versus ~85 volt "genuine panels" on the roof of the Trailer).

I have my single MPPT controller supporting both arrays, but never at the same time. It handles the switch with a new Voltage Sweep, almost instantly, and switches to the corresponding correct operating voltage immediately. My solar controller is an EP-Ever "Tracer" BN3215, I have no idea how other MPPT controllers might handle such a switch.

It is important that the maximum "demand" from the MPPT be less than the capacity of the Boost Converter under the hood. If the Real Solar Array is a lot more powerful than the Boost Converter, and the MPPT output amps allow nearly all of that array power to be sent out, it will try to pull "as much as it can get" from the Truck - and 'magic blue smoke' will be released from under the hood of the truck.

I have a Renogy Rover 60 and not sure how it would behave in such case.

Also, my panels are 600w, 27.5v - 82.5v, and the above max is <400w, 36v draw from the alternator.

 

[rickst29]

I have a Renogy Rover 60 and not sure how it would behave in such case.

Also, my panels are 600w, 27.5v - 82.5v, and the above max is <400w, 36v draw from the alternator.

The maximum (if bargman limited at 15A) for a 36V under-the-hood power supply is actually 540 watts, and that is the rated limit of my boost converter. Since the output power of my "Tracer BN" @ 30A is 14.2V * 30 A, the MPPT output is current limited to 426 watts. By the controller isn't 100% efficient, and I don't want to run the booster at it's bleeding edge.

In practice, the MPPT is never running at full power (30A and 14.2V) from either that "490 watts" of panels mounted flat on the roof, or from the 470-480 watts theoretically available from the under-hood booster with my safety margin applied. Voltage drop and lower output voltage (in nearly all situations) reduces the demand from the MPPT, and the "490 watt" panels are mounted flat on the trailer roof - and have never generated more than about 420 watts at 1 PM (daylight time) on a perfect June day. The wire path, to and from the Truck booster, also consumes considerable power, making 426 watts a pretty appropriate safety margin for voltage drop which would be seen at the MPPT input terminals while pulling almost 15A.

I've never interrupted the TBC cable with a current meter for testing while the batteries are low. I've also never ordered the wife to "hold the gas pedal" for sustained RPMS while looking at the MPPT monitor in camp, with high draw on the batteries through the Trailer's Inverter, in order to see the voltage drop and resulting maximum input current result on my "30A" controller running at maximum power.

Depending on how much power you really need to obtain, while the Trailer's Inverter is only running the refrigerator, and the highest input power you've actually seen from your panels, it might be desirable to the Rover's output current "maximum" to keep it from overrunning your bargman cable and/or boost converter. Or, you could do a separate (smaller) MPPT for just the TBC - that would allow you to utilize both panel power and truck power at the same time. My configuration can't do that, because I'm a cheapskate.