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Charge Batteries from Tow Vehicle, using a Solar Controller.

rickst29
Explorer
Explorer
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.
18 REPLIES 18

rickst29
Explorer
Explorer
MrWizard wrote:
your still using the T/V alternator
switches, relays, voltage booster, solar controller

Why not 'new wire, booster, (controller optional)
mount booster at TT, set output to 14.4v for charge {same as solar controller}
less conversion steps, less parts

Good question!
The #1 reason is: Setting a constant 14.4V output, rather than running through a nice charge controller, will pretty quickly overcharge and boil the batteries. So the Solar Controller is not optional: You need to have the Voltage adjusted according to SOC.

The #2 reason is: Voltage Drop and Power loss along the Bargeman path. If the Boost doesn't happen until the Trailer, you might be pulling fairly high current along that path. {The "+" Path is: The "Trailer Battery Charge" wire within the TV to the Bargeman; then going through the connectors; through the Trailer<->TV cable; then to the load center; and then back to the batteries. The "-" path is generally: Batteries to the Trailer frame (very short); into the Trailer<-->TV cable; going through the connectors; and then hooking into the Tow Vehicle frame (very short); back to the TV under-hood battery connector.} Boosting at the TV makes it run with half the current, all the way to the Solar Charge Controller.

The #3 reason is: More available power, with less chance for starting a fire. We all used a very short length of big wire (#8) and high-current Circuit Breakers (40A) to go into the Booster device. The current capacity of TV "Trailer Battery Charge +" wires is actually quite low - most vehicles are built with wire smaller than #12, even though the fuse is typically 30A. (My own 4Runner was built with #14.) Their excuse is "This is chassis wiring, within an electrical component system". I don't like that approach: By limiting the current to only 20A after a very short #8 wire, until we reach the Solar Control Battery Connections, we make the questionable wiring a bit more safe. (The neutral/ground return wire is sized larger within most Trailer cables, the problem is mainly with the +12V).

The #4 reason is: You need to consciously switch your Solar Controller from your hard-wired "Booster" input to your genuine panels, when you stop to camp. I feel that there's a very high risk of forgetting to do that. This way, it automatically switching back to Solar panels whenever your turn off the Tow Vehicle (for a long lunch, for camping, or for anything else). And anyway, you probably want to make that manual switch-back using switches - I just replace them with Relays, and add the "24V controller Relay" to make it automatic.

I'm definitely using the TV alternator to supply power into the Booster. But it doesn't typically run at full capacity, and if you're concerned you can simply avoid switching it "ON" until you're at speed , running fairly high RPMs. But most half-decent mid-size SUVs (4Runners, Traverses, Explorers) have no problem running pretty high loads, even at idle speeds. ("Pretty High Load" meaning 200-300 Watts.)

MrWizard
Moderator
Moderator
your still using the T/V alternator
switches, relays, voltage booster, solar controller

why not 'new wire, booster, (controller optional)
mount booster at TT, set output to 14.4v for charge {same as solar controller}

less conversion steps, less parts
I can explain it to you.
But I Can Not understand it for you !

....

Connected using T-Mobile Home internet and Visible Phone service
1997 F53 Bounder 36s

rickst29
Explorer
Explorer
time2roll wrote:
If I need that much power I am pulling 120vac back to the trailer.
I am certainly not feeding an expensive MPPT controller with some makeshift 24v supply system.
If you have a good MPPT you already have plenty of solar to charge while driving.
JMHO, good luck.

"Genuine" Solar panels don't work under clouds (or late in the evening, or early in the morning). This "Fake Solar" system is now in use by multiple Trailer owners, and we all use the "fake" whenever we're towing. It's more dependable than the "real" Solar system as weather conditions and sun angle change through the day, and it's totally reliable. Your characterization "makeshift" seems unfair: This is well-designed, and utterly incapable of damaging an MPPT Controller *or* the Tow Vehicle.

rickst29
Explorer
Explorer
DanNJanice wrote:
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.

Re-read carefully - I use a $30 "Voltage Doubler" Boost Converter inside the TV, under the control of a dashboard switch. Switch Off = Towing a "normal" Trailer, or not Towing at all. Switch On = only when towing a Trailer with the Mod, capable of recognizing and handling high input Voltages via a Solar Controller. With the Boost Converter in the TV, the input to the MPPT COntroller is typically 26-29V.

rickst29
Explorer
Explorer

GordonThree
Explorer
Explorer
smkettner wrote:
If I need that much power I am pulling 120vac back to the trailer.


that was my solution... bought a 12/3 outdoor cord and a bunch of that plastic wire loom tubing. ran it inside the box section of my trailer frame with the extra coiled up at the front.

boondock or wallydock in quiet, time to hit the road, plug into a bed mounted generator and charge up while I'm driving. might be hard to do with a FW covering the bed, but it works good with my bumper pull 🙂
2013 KZ Sportsmen Classic 200, 20 ft TT
2020 RAM 1500, 5.7 4x4, 8 speed

time2roll
Nomad
Nomad
If I need that much power I am pulling 120vac back to the trailer.
I am certainly not feeding an expensive MPPT controller with some makeshift 24v supply system.
If you have a good MPPT you already have plenty of solar to charge while driving.
JMHO, good luck.

rickst29
Explorer
Explorer
Bob B wrote:
How much current will the 24V boost device provide?

The one I used is limited to 12A (24V), while drawing around 25A on the "12V" side. More expensive models can go a lot higher. I've only got 2*90Ah batteries, and my MPPT is limited 20A output - so this "little one" is a nearly perfect fit.

Bob__B
Explorer
Explorer
How much current will the 24V boost device provide?
2007 Lance 1181, 2013 Chevy 3500 DRW

rickst29
Explorer
Explorer
GordonThree wrote:
So you're increasing the voltage, to more efficiently transfer power from the tow to the trailer? Where does the voltage doubler get hooked up, hopefully someplace close to the alternator or starting battery?
There is an improvement in "Voltage Drop" power losses, but that's not the main purpose: The main purpose of all this stuff is presenting a sufficient Charging Voltage at batteries. First, we boost to more than the batteries can handle - then we let the Solar Controller adapt the high-power, 24V "fake Solar Panel" output into the correct profile (Volts and Amps) which the batteries should be given, according to SOC.

Per the linked picture (#2 of 4), the Boost occurs under the hood - reducing current on all the wires before the Solar Controller, including the "Trailer Battery Charge" wiring from under the dash back to the Bargman connector.
GordonThree wrote:

My thought: Bypass the bargman. Run 4ga or heavier off the alternator to the rear bumper and use a PowerPole connector to mate with similar wiring on the trailer side, leading to the battery box. Question is; would a factory alternator handle the extra load?
Yes, it would. (Unless your Trailer batteries are huge and discharged, e.g. 500Ah at 40% SOC.) "Brute Force" puts all the batteries into a single string, and the Trailer Batteries will drag down the Voltage of the Starting Battery by absorbing power until they are all equal. And at the same time, "Battery Voltage Sense" under the hood sees the reduced battery Voltage as well - invoking a higher "Battery Charge" Voltage within the TV (by drawing more Voltage and Power from the Alternator.)

IMO, having done a little bit of both jobs (rewiring the 4Runner "Battery Charge Wire" and installing my new gadgets) - installing the gadgets was much easier, except for mounting the new dashboard "mode switch". And it avoids hanging an extra cable between TV and Trailer.

That durned "mode switch" took about 2 hours: I needed to trim the dash panel it in order to get into an "unused switch plug" which didn't really fit, and I also had to pull leads (12V, ground) from other wires under the dash. :M

The 4Runner's OEM "Battery Charge" wire was not bigger than 14-AWG, and actually looked closer to 16-AWG. I pulled a new #10, and the body panel disassembly/re-assembly was very time consuming (about 3 hours, total). In contrast, the "Relays-with-Booster Assembly" (at the TV) and the "Two Relays with a few wires coming out" were both assembled in the house (with beer and TV), plus about 1/2 hour installation in each vehicle.

GordonThree
Explorer
Explorer
So you're increasing the voltage, to more efficiently transfer power from the tow to the trailer? Where does the voltage doubler get hooked up, hopefully someplace close to the alternator or starting battery?

Nice work if it's working for you ... Too many moving parts for me, and I like complicated solutions to simple problems 🙂

My thought, not currently implemented: Bypass the bargman. Run 4ga or heavier off the alternator to the rear bumper and use a PowerPole connector to mate with similar wiring on the trailer side, leading to the battery box. Question is; would a factory alternator handle the extra load?
2013 KZ Sportsmen Classic 200, 20 ft TT
2020 RAM 1500, 5.7 4x4, 8 speed

rickst29
Explorer
Explorer
GordonThree wrote:
Is 24v standard for all diesels, otherwise where does it come from?
DanNJanice wrote:
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.

Sorry guys - I left out something important. I just edited the first post, to explain that a cheap Chinese "12V-to-24V" Voltage Booster gets inserted into the Bargman wire path when the Relays engage.

TV Relay Coils disengaged: 12V mode, bypassing the Booster.

TV Relay Coils engaged (powered by output from the controller switch on the dashboard): 24V mode, Voltage gets increased by running the connection through the Booster before reaching the TV's Bargman connector.

rickst29
Explorer
Explorer
https://goo.gl/photos/DBtjLwMrpZ8sjmLYA

Here's a note on the Tow Vehicle Picture: There are 3 Relays (rather than two), because I didn't have access to the original "Trailer Battery Charge wire" under the hood, after the fuse. It's hidden within a gigantic bundle.

So, before the two 12/24 "switching" Relays, there is (1) a direct battery connection with a new fuse (30A), and (2) an initial Relay, which switches "on" whenever the engine is actually running.(My new Trailer Battery Charge" circuit is not activated by merely turning on "accessories".) I pulled the fuse for the OEM circuit.
- - - -
Meanwhile, in the Trailer: the schematic shows 3 Relays, allowing for an automatic switch of "Bargman 12V" back into the "+12V" power distribution, whenever the 24V coil Relay is not activated. But per above, we never want this if:
  • The Trailer will never be pulled by a different, 12V-only Tow vehicle; and
  • You can always remember to flick the "24V mode" switch when pulling the Trailer, and sun conditions aren't perfect.

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