I've definetely considered/pondered it, but my depth of knowledge in electrical is not sufficient to feel confident going beyond pondering. I do not know if it advisable or doable or not. Not with those cheap converters at least. I am pretty sure (almost positive) Victron's converters (some of them at least) can be used in that manner, but at that point the price difference between a Victron DC-DC converter and Victron DC-DC charger (meant for alternator charging) is not that substantial.
If these cheap converters (from the link you posted) would work and don't introduce additional unacceptable risk, it would be a very cheap solution, and I actually think 13.8V (3.45Vpc) sounds like a good voltage for dumb fixed voltage (non 3 stage) charging.
My scheme, oriented towards travel trailers, uses a cheap boost/regulator converter under the TV hood and works great. (It has been implemented by several other persons over the years. My own has been working since 2016). It can send about
460 watts through the Bargman umbilical cable, at only about 13.3 Amps. I currently use and always recommend boosting to 36.0 Volts, using
this ultra-cheap device.
But I definitely DO NOT go straight into the Trailer Battery Bank. Within the Travel Trailer, a "Detector Relay" with a 36-VDC coil, is used to switch the "PV +" input power connector of an MPPT Solar Controller away from the genuine panel Array "+" wire, connecting the Bargman cable "TBC" into "PV +" instead. The SCC sees that the virtual panel has changed, and rapidly tunes the PV input Voltage to match the 36.0V input. In my own case, the "real" panels present about 80V under sunlight. My relatively cheap "Tracer BN" Series Controller, form EpEver, adjusts to the new max power point very quickly. It can also rapidly adjust from 0 power to the 36 Volt MMPT at night, with no ill effects.
In the Tow Vehicle dashboard, we have all added basic switch to "enable" the Boost output wire path, path on-the-fly. This switch controls a high current Relay into the Boost device "input +" terminal, connected to the battery on an 8-AWG wire with a 50A fuse (this is chassis wiring, and very short.) The Boost switch only gets power while the ignition is "on". Leaving the Boost Switch "off" allows you to tow other normal trailers, which can't handle 36V on TBC, and also allows you to turn it off when your engine is idling. (At idle speeds, relatively few SUV and Pickup Truck Alternators are "happy" to have the Trailer sucking in almost 40A of power).
The two disadvantages of my scheme are:
#1, "real" solar gets disconnected from the MPPT when the 36V boost voltage is present on Bargman TBC. You cannot run "real" Solar and the Boost scheme at the same time (although you could bypass this limitation, by using more than one MPPT controller). And
#2, the Boost Converter must NEVER be overloaded. Choose MPPT maximum output power by starting with a slightly conservative limit of utilization within the Boost device (perhaps using it at only 90% of its rated maximum). Make sure that the current through the Bargman cable won't exceed 15A. 36V at the under-the-hood Booster, travelling about 25 feet each way through the Bargman, will have considerably less Voltage at the MPPT input terminal (typically around 34.8V when running at full load). Available power at the MPPT terminals determines your maximum input power for the MPPT SCC, and the output limit (battery charging voltage * maximum amps allowed) must not exceed about 94% of the input power, due to power loss within the MPPT SCC.
EXAMPLE: In my own configuration, the MPPT charges at up to 14.5V. (A "12 volt" LFP battery bank). From under the hood, the device is rated for 540 watts, but I want to limit its use to only 85 percent of that figure - 460 watts. This would require about 12.8A, until we consider Voltage Drop along the way. That typically ends up around at about 34.8V, if the TV wiring of TBC from the Booster to the 7-pin socket is at least AWG-12, and the Trailer Wiring from the Bargman end is AWG-10. So instead of 460 watts we're down to about 440 watts. (For calculating the Voltage Drop through that output current path (Boost Device to 7-pin connector within the TV, Bargman wire, and some length from the Bargman termination to reach the SCC) you can note that the Bargman TBC is typically only 14-AWG, while the return ground wire is typically a bit larger (I've usually seen 12-AWG when repairing those cables). On the Return Path, only the Bargman length needs to be used (because the TV frame and Trailer Frame are capable of higher current).
We're now delivering only 440 watts as input to the SCC. My SCC is about 94% efficient in this conversion (34.8 input volts to 14.5 Volts "CV" into my LFP battery string), so the batteries can only receive 420 watts. At 14.5V, that's a bit less than 29A, and I have tuned down my "30A - Capable" Controller to limit itself at 29A.
- - -
It all works like a dream, as long as 420 Watts is sufficient charging energy for your battery bank.
If your battery bank is configured at "24V", this still runs great - just set your SCC output for "24V" batteries; the MPPT Input Voltage is plenty high enough to charge such a string (although the maximum MPPT output
current must be cut in half.) By switching on the fly, and only while already in motion at "adequate" RPMs, you avoid stressing the Alternator. You can tow other Trailers, just by leaving the "enable" switch turned off. And your Trailer sets itself back to default mode ("Trailer Battery Charge" being connected into the normal "12V" system, and the "genuine Solar panels reconnected to the Solar Controller) as soon as the Bargman is disconnected, OR the "enable" switch is turned off, or the SUV/truck ignition is turned off.
Best of all - no additional cable. You already have the Bargman connected for brake lights, brake power, turn signals, and running lights - you simply use the TBC wire within the Bargman cable differently, and more efficiently.
My existing Thread - written less well, and with a funny title (but with diagrams and pictures):
https://diysolarforum.com/threads/r...rs-of-mppt-charge-solar-in-the-trailer.20730/