Because you may now buy 2912-type relays with 36 volt DC coil voltage, (5 pin automotive type relays) on Amazon, wiring diagrams which I used in earlier Threads can be simplified. Here is that product:
https://www.amazon.com/dp/B07QLB9CZM.
You will need just ONE under the hood of the tow vehicle, plus another "controller relay" which directs power from the battery to the voltage booster when the 12v coil of the controller relay is switched ON by a dashboard switch. That relay must be cable of very high current, 60-80 Amps, and you must not use a pre-built socket (with skinny wires, 12-AWG and even less). Youwill need to build and solder your own connectors from bigger wire for the path from the battery into voltage converter. Before reaching the relay, that wire should be fused (I have a 40A "maxi" fuse inline.)
Within the TM, one or two identical "36 volt" relays direct the "trailer battery charge" wire connection into an MPPT solar controller, disconnecting any genuine panel wire at the same time - and optionally directing "trailer battery charge" current directly to the 12v system whenever high voltage is not present on the wire.
Here is the under-the hood voltage booster, rated for up to 540 watts:
https://www.ebay.com/itm/335017564224. At my MTTP Solar controller, I limit the maximum draw from the booster to about 460 watts. (That limit is imposed indirectly, via an output currrant limit of 30 amps.)
- - -
When activated, high-voltage power is supplied on the TM "trailer battery charge" wire through the unmodified bargeman cable. The TM is modified to use one or two of the same 36-volt relays, directing the power into an MPPT solar controller when high voltage is present. A second relay can be used to connect the TBC wire directly into TM "12v" electric service when high voltage is not present, but I didn't bother installing that relay. (I might add that capability next spring, but my own TM is never towed by another vehicle for substantial distances, and doesn't need low voltage "trailer battery charge" at all.)
In comparison to the use of regular DC->DC chargers, which can cause excessive current to flow through small wires (within the Tow Vehicle, within the bargeman cable, and within the TM), this reduces the amount of current on the TBC wire path by a factor of about 3x, preventing wire failures (and possible fires) from excess current.
An MPPT solar controller is mandatory for this to work, you may already have one. It will run regular solar panels, whenever it is
not being used by the high-voltaqe connection from the Tow Vehicle. (When towing on sunny days, you can take your pick of MPPT input sources via the dashboard switch, on the fly).
Stockton's "DC power from Tow Vehicle" (re-visited)
Linear Mode
