Charge Batteries from Tow Vehicle, using a Solar Controller.

[rickst29]

This modification occurs in both the Tow Vehicle AND the Trailer. A Solar controller is required (with or without existing Panels).

In the Tow Vehicle (hereafter "TV"), a set of switch-Controlled Relays allow the Driver to choose between "24V" and "12V" operation on the Bargman "Trailer Battery Charge" wire. (The "12V" option is kept for the situation of towing other, unmodified Trailers.) When the "Mode Switch" is activated, the Relays switch the two legs of the Bargman "12V Battery Charge wire" (battery->fuse->Relay#1, Relay#2->Bargman Connector) so that a cheap 12V-to-24V DC Voltage Booster is engaged between the Relays.

In the Travel Trailer, another set of Relays does two main jobs. The first Relay detects that 24V is present, and switches a "coil input control" circuit for another Relay. The second Relay chooses between two power Sources for the Solar Controller: When 24V is not detected, the wire from Solar Panels is connected to the "Solar +" terminal. But, when 24V is detected on the Bargman, then the wire from the Bargman is connected instead. (Optional: a 3rd Relay, allowing a "12V" Bargman connection to connect the "Battery +" terminal. But I'll explain why you probably shouldn't do this.)

Cost with pre-existing Solar Panels and Controller (MPPT strongly preferred): $40 Booster device, four or five 5-pin automotive Relays @ $5 each, a "Mode Switch" for the TV dashboard ($10-20); and a "special" 4-pin Relay with 24V coil ($10). Total: $75-90.

[rickst29]

Why "12V operation" is never desirable in this configuration:

If your "genuine Solar" is performing well, and your Controller has determined that your batteries and loads need "Bulk" or "Boost" charging, the the Controller attempts to keep the battery charging Voltage fairly high: 14.2-14.4 Volts.

But, you have connected the Trailer's +12VDC power distribution to the Tow Vehicle, via the Bargman. The Tow Vehicle has probably "decided" the alternator in a way which yields 13.9V (and maybe even less). Your Controller's attempt to run at "Bulk Charging Voltage" leaks power into the Tow Vehicle - and the "thirsty" batteries get much less.

Whenever the Solar conditions aren't great, you should flick the switch and run in 24V mode. And, if Solar conditions are great, you should probably leave the Bargman "Trailer Battery Charge" wire isolated, disconnected from the Trailer 24V system. ("12V mode" in the Tow Vehicle = 12V with other Trailers, but nothing in a properly configured "24V" Trailer charging.)

[DanNJanice]

GordonThree wrote:
Is 24v standard for all diesels, otherwise where does it come from?

Good question, it is unclear to me what the OP is talking about. If you had a 24V system I could see using the solar controller as described. I just don't know what vehicles have 24V electrical systems.

[GordonThree]

Is 24v standard for all diesels, otherwise where does it come from?

[rickst29]

In my configuration, an MPPT Solar Controller can receive power from either the "genuine Solar Panels" (rated ~ 330W max, 5A @ 67V under prefect conditions) or the Bargman Input (rated ~ 290W max, 12A @ 24V under any conditions). Maximum power draw on my MPPT, in "Boost" mode, also corresponds to ~ 290W.

Under Good Sun conditions, I leave it running "genuine Solar". But, under clouds or sub-optimal conditions (night, early morning, dusk), I flick the switch - and it disconnects the "genuine" Solar Panels, switching over to the fake "24V Solar Panel" instead.

The Solar Controller monitors Battery State and adjusts output Voltage accordingly. A PWM controller could perhaps be used, but wouldn't take more than 14.4V (average) from the Voltage Booster, and might attempt to draw more than 12A while connected. That would be a 'Bad Thing'(tm), and blow a fuse inside the sealed Voltage Booster.