Rick's "Charge from the TV" solution for owners of MPPT-charge Solar in the Trailer

[rickst29]

I have posted comments which describe my "Charge LFP batteries from the TV" solution in another Thread already, but that Thread's Title does not do a good job of explaining how this gets put together, and why it's typically better than using a DC-->DC charger "solution". This repeats that information, with just another picture or two. But by separating this scheme into a Thread of its own, I hope to answer questions about THIS scheme exclusively, and avoid this design getting lost among a ton of store-bought DC-->DC implementations.

In Charging from a typical "Tow Vehicle", typically running at around 13.2-13.5 Volts after the starting battery is "charged, a high-powered "DC->DC charging solution" must dry high current (at low Tow Vehicle Voltage) through a long cable, which must either be (a) fairly large, not a Bargman cable; or (b) running at only very low power. The DC-->DC charger is responsible for matching battery requirements.

But if you already have solar panels and an MPPT SCC within the Trailer, the Battery Charge Voltage management which the DC--DC chargers Provide is REDUNDANT with the MPPT Solar Controller. I simply use a cheaper and larger Boost Converter/Regulator under the hood, sending only lower current into the MMPT through a Bargman connector at slightly more than 13A. My parts cost about $80, and reach up to about 440 maximum watts delivered from your existing MPPT into your battery string (30A @ 14.5V maximum charge Voltage). But you must limit your MMPT to that maximum current value, or else you will burn out the Boost Converte/Regulator which I use.

My system also has the limitation of forcing you to choose between "real Solar Panels" and the TV " virtual Solar Panel", they can't run at the same time. This is under the control of a dashboard switch, "enabling" 36.0 Volts on the Bargman "Trailer Battery Charge" wire. The hard part of an installation has always been adding that dashboard switch. The Trailer wiring is pretty simple, and the "normal versus boosted" circuit under the hood of the TV is fairly simple as well. With this design, the TV defaults to using "12-volt" battery charge - allowing you to tow normal trailers at any time. The Trailer automatically detects and switches, according to the presence of about 24 Volts (requiring a minimum of about 18 Volts to maintain the "hold-in" on the Detector Relay.) The "load terminal" current handling on the Detector is irrelevant, because it will only carry tiny amperage at 12V to drive the coills of other 12V Relays. (But those "Switcher" Relays do need to be capable of handling considerable current at moderately high Voltage, the "PV +" and "Virtual Panel +" inputs are both switched on the power terminals.

My "Detector" Relay is this one: https://www.amazon.com/Ehdis-Motor-Relay-40amp-Model/dp/B01J5CLJ0I/. It uses a 24V coil (a 28V or 32V coil would be more "optimal", but none of these have blown up while subjected to "36.0V" coil voltage in the trailer of myself and my friends.

 

[rickst29]

This implementation diagram shows mods in the Trailer (enabling the auto-switch capabilility). In "Low Voltage Mode" as shown, the "Detector Relay is open -- and neither of the "Switching" Relays get any Voltage on their coils. In this mode, Trailer Battery Charge "TBC" is connected directly to the "12V" Trailer power bus, along with other things. The genuine Solar panel "PV +" goes through its safety fuse, and is connected inside the Relay to the "NO" port on the "MPPT Input Switcher" Relay, going directly to the MPPT.

But when more than 24V (and subsequently, at least 18V) is present on "TBC", the "Detector Relay" closes the circuit for it's power pins. One of the power pins comes from the Trailer "12V" power bus. The other pin goes through the Coils of the "Switcher" Relays and activates their TBC connections. "Power" from the detector Relay is only 12v, with tiny current. The other two Relays have automtive-type 12V coils, but must be wired to handle the full Voltage and Amperage of the "TBC" and "Genuine Solar" power connectors. My own "Genuine Solar" runs at almost 80 volts, but I have found these "el-cheapo" high-current Relays to work fine in this relatively low-current application (multiple Travel Trailers, multiple years each). I chose Relays with Indicator lights, so that I can see what's going on without using a DVM. Nowadays, my newer MPPT monitor shows the PV side pretty decently - but my older "big iron" machine didn't show that so clearly. Trailer Diagram:

 

[rickst29]

Here's the diagram for the Tow Vehicle. The hardest part is wiring the ignition controlled dashboard switch, through the firewall. Although I have used 5-pin Relays to choose between the "high-voltage" and "normal Voltage" current paths, I recommend against using automotive 5-pin Relays as I did: Even with high-power Relays and bigger power pins, it's extremely difficult to build an automotive socket base with #10 wire coming in. #8 is nearly impossible. Smaller wires (pre-built "high-current" sockets mostly use #14 on their power terminals) are absolutely no-go, this is a very high-current application from the battery into the power-boost device. My own is currently built with 8-AWG wire through hand-built sockets, but if it ever fails -- I will upgrade to use a 4-pin Relay, with genuine high-power screw post terminals, as my first "power path" Relay.

My own current draw is about 46A (maximum input to the Boost Converter), when the LFP batteries are at low SOC. My alternator, and yours, has to handle this load decently. The switch named "36 Volt control switch" is running at only 12V, with both the coil and load connections. Note that the lower right label is wrong: It's "13.x versus 36.0 Volts, not 12 versus 24). When I created this diagram, I used only a 24V boost Voltage.

 

[rickst29]

My own "36 Volt Control Switch" (labeled "Zombie Lights", in the dashboard:

 

[rickst29]

Here's a shot of my current "mess under the Hood", with the probably unneeded 3rd Relay also included. It's got bigger #8 wiring for the high current Path, and smaller #10 wiring on the low-current paths:

 

[rickst29]

My Voltage Booster unit is this one. It is CRITICAL that your MPPT-SCC be configured with moderately low power output, to avoid overrunning the 'Voltage Booster' and maybe blowing it up (or at least overheating it). There are higher-rated sellers of the same thing, at slightly higher prices: https://www.ebay.com/itm/Well-DC-12...Power-Converter-Regulator-Module/124482025367

 

[rickst29]

At a significantly higher price, this "bigger" Converter is rated for up to 20A (720W), and would probably work pretty well with a power draw of up to 600W from the MPPT. That could allow for up to 40A output from MMPT (@ 14.6 Volts or less), and many MPPT SCCs are built with 40A maximum current capability. You'd just have a higher risk of burning out the Bargman cable, since you'd now be pulling almost 20A through the Bargman TBC. https://www.alibaba.com/product-detail/High-efficiency-step-up-voltage-transformer_60821677736.html

 

[chrisski]

I probably missed it, but how many charging amps or watts do you see to your battery using this system?

Also with this output you mention, is this idle, and if not, what is idle output? I ask because I think of an alternator at idle as having enough power to keep the DC systems in the car running with very little amperage left over to charge a battery.

I never thought a significant amount of charging amps could be sent to the tow vehicle from the truck. You’ve built it, and I’ve only thought about it. Looking at your block diagrams, appears the power is not sent to the trailer through the seven pin but an additional breakaway.

 

[rickst29]

I probably missed it, but how many charging amps or watts do you see to your battery using this system?

Also with this output you mention, is this idle, and if not, what is idle output? I ask because I think of an alternator at idle as having enough power to keep the DC systems in the car running with very little amperage left over to charge a battery.

I never thought a significant amount of charging amps could be sent to the tow vehicle from the truck. You’ve built it, and I’ve only thought about it. Looking at your block diagrams, appears the power is not sent to the trailer through the seven pin but an additional breakaway.

Chrisski, those are both GREAT questions.

Idle Speed: My 4Runner ECM (it is a 2007 model) pulls up the idle speed in order to maintain "adequate" TV Voltage when I leave the system active at idle for traffic lights, while pulling the RV through towns. (Idle without the "switch" disabled is only about 600 RPM, when the air conditioner is off. But when the switch is "enabled" and the batteries want to be charged, it will instead hang around 1100 RPMs. With just the Air Conditioner compressor running at a red light on a hot day, the minimum RPMs are right around 900. My normal habit, though, is to only "enable" the 36V switch while driving at highway speeds, and to turn it off when I go through towns. At 1900 RPMs and higher, the alternator creates plenty of spare energy. It's still original, I've not yet burned it out - although I only use the RV charger for a few weeks each year, and it's only been in there since 2015. (Originally with SLA batteries, now with LFP.)

Power Through the 7-pin Bargman Cable: The whole advantage of my scheme is that it DOES send power through the 7-pin, at much lower current and much higher Voltage. No additional cable is required for the ~435 Watt limit which my "small" 30A MPPT puts into the batteries (limited by the maximum output current of the MPPT, only 30A in my own implementation.) With MMPT consuming 5-7% of the input power while doing that Voltage conversion, the power draw against the Bargman TBC (at the MPPT) is about 465 Watts. Only a tiny bit of power is lost via "Voltage Drop" along the TV-to-Bargman current path, because the current is quite low (it's around 13A).

My 'Trailer Battery Charge' wire within the TV body, leading from the Booster to the 7-pin socket connector, is #10-AWG, and so is the wire path within the Trailer. Only the cord itself uses a smaller wire. TBC within the Bargman Cord is probably about #14-AWG; the return Bargman Ground is bigger - at least #12, and possibly even #10. Voltage drop on that path is less than 3% - the MPPT Controller "sees" 34.9 Volts at peak power (instead of seeing the original 36 volts), and it raises input current from about 12.9A (the 36.0 Volt value) up to about 13.3A (the 34.9 Volt value for the same 465 watts of input). Up at the TV, the "15A" rated Voltage Converter is running at about 89% capacity, generating about 480 Watts of power into the Bargman TBC Circuit at the TV. (15-20 Watts is wasted on "Voltage Drop".) With the "30A maximum Output Current" limit at the MPPT, the Converter is never pushed harder than that, and my "15A 540 Watt+-rated Converter has held up with no problems since I installed it.

 

[chrisski]

I think this write up is stickie worthy.
==========================
So this definitely seems like a way to charge through the 7 pin, at least while traveling. A couple more questions.

1) Can you charge your battery bank while parked, such as after a cloudy day or two? I see you have a high idle mod, so it seems you should be able to.

2) Since this pushes 36 volts through the 7 pin, is there anything special you need to do if you hook a different trailer up to it?

I had been worried about recharging my batteries during a cloudy day, but the overcast days I've been camping, I still got around 15 amps a couple hours before and after 1200. I added a couple more panels, so I expect I will be getting 17-20 amps. If I need more, I use the generator. Something like this would be good.

 

[rickst29]

(1) Yes, I have used the 4Runner as a mini-generator while parked in a clouded-over campground, more than once. I've idled for up to 40 minutes at a time, although many other vehicles disallow "extended idling" (more than 10-15 minutes) because their catalytic converters (and maybe some other "etc." exhaust components) are prone to overheat when air isn't in motion underneath the SUV/Truck.

(2) Because 36-Volt mode is only enabled by flicking the dashboard switch "ON", you may tow other "normal" Trailers by simply leaving the switch turned off. Bargman TBC Voltage stays down at "normal" under-the-hood Voltage, except when the Switch is used to enable 36V mode.

(3) Using the 3-Relay scheme in the diagram above, the Trailer can also be used with "normal" Tow Vehicles, accepting normal Voltage and leaving TBC attached directly to the Bus for "+12V" loads. The "TBC" doesn't get connected through the Solar Panel until the "detector Relay" his pulled in (by Voltage above 24V/36V, depending on the Detector), and it "lets go" as soon as TBC Voltage falls below about 18V/29V for those respective "Detector" coil sizes.

 

[fratermus]

Clever solution, and nicely written up.

 

[rickst29]

Clever solution, and nicely written up.

Thank you. I wish I could change the Thread Title, to instead emphasize that it can pull 470 watts through the Bargman without an extra cable.

 

[Kennedyma]

This is a fantastic idea! Great write-up.

 

[John Frum]

Great idea!
What is the voltage rating for the 7 pin connector?
Also what is the volt and amp rating for the aux power pin and from the return pin?

 

[rickst29]

Great idea!
What is the voltage rating for the 7 pin connector?
Also what is the volt and amp rating for the aux power pin and from the return pin?

smoothJ,

nearly all "12V" wiring and automotive-type Relays can handle at least 40V on the power contacts and wires. My scheme uses 36.0V when the booster is turned on, and stays at only ~13.3V when the booster control switch has not been activated.

The more important of your two questions concerns current. Within many Tow Vehicles, the "Trailer Battery Charge" wire path contains some thin wires. My Toyota 4Runner (from 2007) used #14, even though the factory fuse board allowed up to 30A. (This is the wire path to reach the 7-pin socket at the bumper, rather than the wire "TBC" wire inside the Bargman Cable. Some Tow vehicles even allow 50A, although that large fuse size should be reduced whenever a "normal" Bargman cable might be used to carry the load.

Bargman Cables which I have repaired use either 14-AWG wire (typical) or 12-AWG (unusual) for the TBC wire within the cable.If 30A is continuously pushed through a 14-AWG Bargman cable, the TBC wire insulation will absolutely fail. (The insulation and the 7-pin adapter Assembly are both prone to overheat, melt, and fail under high current.) The grounding return wire is usually bigger than the TBC, its typically #12 (but almost never as large as 10-AWG). More "voltage drop", and more lost power converted to heat, occurs on the TBC wire. So that's the one which fails first, when people put too much current through a Bargman cable. With 14-AWG, much too much heat begins to be generated at loads above 15A.

So that cable failure is almost inevitable with a typical '30A' or '40A' DC-->DC charger pulling high current along a TV-to-Trailer Bargman Connection. Everyone wisely uses a separate and large 2-conductor cable in those configurations. Even the small ones, such as the Renogy "20A" model, can possibly burn out the cable if they're connected through the Bargman, without a separate cable dedicated for the DC-->DC device.
- - -
My system provides MORE power into the batteries, while putting only a safe current load (a peak load of only about 13.3A) on the TBC wire. With my scheme, you get high power AND you don't have to string another "bigger" cable between the TV and the Trailer.

 

[time2roll]

I did similar 20 years ago with a pair of AGM on my pop-up trailer. Put a battery in the truck bed and connect to the 7-pin connector to charge while we drive around sightseeing for the day. Swap batteries every day or two. Actually worked quite well for what we needed. With the batteries and charge equipment we have today this is a slam dunk.

 

[rickst29]

I did similar 20 years ago with a pair of AGM on my pop-up trailer. Put a battery in the truck bed and connect to the 7-pin connector to charge while we drive around sightseeing for the day. Swap batteries every day or two. Actually worked quite well for what we needed. With the batteries and charge equipment we have today this is a slam dunk.

Not really the same thing, because you're not doing a voltage boost on the "trailer battery charge". But your scheme certainly allow you to charge a "Trailer" battery while driving around in sightseeing - and my scheme can only charge the Trailer batteries when the Trailer is actually attached. Your method also avoided a lot of distance along the Bargman cable, and allowed the Engine/Alternator "Voltage Control" circuit to better see that "The Battery" (actually being two batteries) "needs charging", with Voltage being drawn down by low Voltage on the whole system.

A shorter Bargman cable is always a great thing, and the truck's voltage controller kept it from overcharging your AGM. This was a good solution for your boondocking situation, limited only by the fact that you kept having to swap Trailer Batteries.

 

[John Frum]

Here is my math on the boost converter...
540 watts / .9 conversion efficiency / 14 volts = 42.857142857 input amps.
The wires look kinda small for 43 service amps.
Even worse for 50 fault amps.

 

[rickst29]

Here is my math on the boost converter...
540 watts / .9 conversion efficiency / 14 volts = 42.857142857 input amps.
The wires look kinda small for 43 service amps.
Even worse for 50 fault amps.

You've made an interesting question. Here's my answer, involving some relevant details - but I agree with you, I would rather have used AWG-8 on the Tow Vehicle "12V" side leading into the Boost Converter. The input lead to that Converter Box may be even smaller than 10-AWG, so I cut it fairly short before plugging into my relay output lead. My "automotive Relay" socket base couldn't handle anything larger than 10-AWG (and even that was a bit of struggle, I ordered custom bases to make them take #10 AWG on the 3 "power legs" in question, and I blew up one of the two I bases which I ordered while trying to squeeze in the wire).

But you're assuming that I attempt to pull the full RATED watts of the Boost Converter. I'm only pulling about 480 watts maximum for wire losses to reach the MPPT, MMTP inefficiency, and the MMPT's rated maximum output power 28A at about 14.4 Volts. 480W /13.4V = 35.82 Amps. 35.82 Amps / 0.90 efficiency yields 39.8 Amps total draw at maximum output, a figure slightly lower than your 43 amp calculation.

These wire distances are also short (heat can dissipate at the copper ends, not just through the insulation) and most of what goes on in here can arguably be called "chassis wiring". This is also THWN-2, for which I typically see ampacity ratings of 40A @ 90 degrees C. That's with conduit allowed, while the hood underside at this location (with wires in free air) has substantial airflow when driving at speed. But temperature is already a bit high in that location, somewhat reducing ampacity back down a bit.

The bottom line for wiring size in my scheme is whether it has performed OK in the real world, leaving its insulation undamaged. And for that question, I'm happy to report that I have seen no heat-related issues on the connectors, exposed insulation, or even the electric tape, over multiple years. Neither have the friends who use the same scheme.