7 Pin for charging + 4 pin for driving?

[shade]

Air is a dielectric/conductor just like the insulation on wires. However when air becomes plasma due to an arc or other event it conducts extremely well. Arcs in DC systems are much more difficult to extinguish than AC systems. You also have to deal with the chance of a finger or some other object bridging the gap.

There is also surface contamination and aerosols to deal with.

Here's a 500kv arc when a line wasn't de-energized prior to disconnect.

I used to have a Jacob's Ladder made from a neon sign transformer. Fun, but I gave it to Dad so he could be the one to get zapped.

 

[shade]

Sorry I dont have ability to post a drawing, Visually my caution would make perfect sense.
Anyway,
My caution maybe a moot point, but its based on a couple unknowns of your specific PV controller and 36V powersupply.
Many PV controllers are so called "positive ground". That means all its input & output positive terminals are internally common to each other. Its negative side does the voltage regulation.
Using this type on a vehicle this is normally not a problem as long as its PV input does not have a negative grounded conductor. (As well its load terminals do not supply any negative grounded loads.)
If your proposed 36V powersupply (I assume powered from your car ?) has its output negative common with the vehicle negative, that effectively bypasses the voltage regulation of the PV controller.
It will apply 36V direct to battery.

 

[Buddha.]

Sorry I dont have ability to post a drawing, Visually my caution would make perfect sense.
Anyway,
My caution maybe a moot point, but its based on a couple unknowns of your specific PV controller and 36V powersupply. You need to know what type they are.

Many PV controllers are so called "positive ground". That means all its input & output positive terminals are internally common to each other. Its negative side does the voltage regulation and if used, load control.

Using this type on a vehicle is normally ok as long as its PV input does not have a negative grounded conductor. (As well its PV load terminals do not supply any negative grounded loads.)

If your proposed 36V powersupply (I assume powered from your car ?) has its output negative common with its input negative/vehicle negative, connecting that to PV controller input effectively bypasses voltage regulation of the PV controller to apply 36V direct to battery.

Yes it's powered from my tow vehicle.
I think I understand what you're saying and I appreciate the help. Is there anyway to know if these products will work together without "bypasses voltage regulation of the PV controller"?

SmartSolar MPPT 100/30 & 100/50 - Victron Energy

Maximize the energy-harvest, driving it intelligently to achieve full charge in the shortest possible time using the SmartSolar MPPT. Learn more now.

www.victronenergy.com

DC 12V Step Up To DC 36V 15A 540W Boost Power Converter Regulator Module New | eBay

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Or should I scrap the idea and just buy one of these to charge the trailer's batteries from my tow rig?

Amazon.com: Renogy Board 20A DC to DC Battery Charger Using Multi-stage Charging: Automotive

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[luthj]

Most solar controllers are negative ground these days. You can check the service documentation or ask Victron for details. Only a small number of grid tie type units are positive ground.

Grounding MPPT 100/50 - Victron Community

community.victronenergy.com

Do not ground the positive or negative of the PV array: the PV negative input of the MPPT is not isolated from the negative output. Grounding the PV will therefore result in ground currents

99% of electronics DC converters are negative ground plane, or floating/isolated. Either will work fine.

 

[shade]

Yes it's powered from my tow vehicle.
I think I understand what you're saying and I appreciate the help. Is there anyway to know if these products will work together without "bypasses voltage regulation of the PV controller"?

SmartSolar MPPT 100/30 & 100/50 - Victron Energy

Maximize the energy-harvest, driving it intelligently to achieve full charge in the shortest possible time using the SmartSolar MPPT. Learn more now.

www.victronenergy.com

https://www.victronenergy.com/upload/documents/Manual-SmartSolar-charge-controller-MPPT-100-30---100-50-EN-NL-FR-DE-ES-SE.pdf

3.2 Grounding

● Battery grounding: the charger can be installed in a positive or negative grounded system. Note: apply a single ground connection (preferably close to the battery) to prevent malfunctioning of the system.

● Chassis grounding: A separate earth path for the chassis ground is permitted because it is isolated from the positive and negative terminal.

● The USA National Electrical Code (NEC) requires the use of an external ground fault protection device (GFPD). These MPPT chargers do not have internal ground fault protection. The system electrical negative should be bonded through a GFPD to earth ground at one (and only one) location.

● The charger must not be connected with grounded PV arrays (one ground connection only)

 

[DaveInDenver]

Don't get wrapped around the axle regarding positive ground. Whether it's "most", "many" or just "some" solar controllers use positive ground just make sure you get one that is intended for negative ground or is floating. Read the specs and the manual and wire appropriately.

Referencing the positive side of a circuit to Earth ground is a carryover from the telephone system. Early on they figured out that by doing that they could reduce the corrosion with anything in contact with the soil. This way copper in the ground rod tends to migrate to the negative wire rather than away from the positive as would happen with a negative ground.

So there's still benefit to doing that and since solar is often used in fixed installations you'll see it. But with 12V systems designed for vehicles it's not necessarily common. Higher voltages like 36V or definitely 48V (telecom uses -48V) it certainly would be possible.