$10 DC-DC charger for charging LiFePo4 via 7pin trailer cable?

[BretS]

My plan is just to run larger wire from the TV battery to a Victron DC-DC charger in my trailer. It will have a separate fuse at the TV battery. Seems like a simpler way to go with more charging ability...

 

[rickst29]

My plan is just to run larger wire from the TV battery to a Victron DC-DC charger in my trailer. It will have a separate fuse at the TV battery. Seems like a simpler way to go with more charging ability...

Your plan avoids my complicated installation, but adds the job of dragging and connecting the "big" wire each time. It does not have more power, unless you obtain a model bigger and even more expensive than the $220 "360 watt" version. It would almost certainly support more than "360 watts", because you will have more than 13V on the input side while the Truck is running. The 30A limit (which I SWAG to be the actual operating limit) probably allows for a bit more than 400 watts on the input side, with only a small loss in the conversion.

But my design, through the Bargman cable, is allowed to pull about 450W into the MPPT, and the MPPT Voltage conversion cost is also a mere 3-6 percent. My implementation avoids the extra cables, generates MORE power, and costs much less in purchased hardware. Instead, it costs time and effort installing the parts - just once.

Your method probably calls for a 2-conductor cable, rather than just a single 8-AWG wire. While my method pulls only 12-13A along the Bargman path, yours will pull 30A -- in both directions. Within a Bargman cable, the return ground wire is always built bigger than the TBC wire, but it might not be "good" for a full 30A continuous load (I usually see #12-AWG, and even #10-AWG would be at its design limit). The total thermal insulation value of the cable is very high: Ground wire insulation, PLUS the insulation on the surrounding wires, PLUS the outermost foam wrapping and cord cover. When these cables fail, it is often "voltage drop" power loss (heat) creating high temps within the conductor, travelling all the way down the cable to the plastic end assembly, and melting the end assembly. The wires don't often fail in the middle - when overloaded, they fail by melting the 7-pin plug assembly.

Your design is simple, and otherwise sounds great. We share the use of a separate "big wire" battery connection, with its own fuse. (Mine is 50A, although yours can be just 40A, because less power is being provided).

 

[BretS]

Your plan avoids my complicated installation, but adds the job of dragging and connecting the "big" wire each time. It does not have more power, unless you obtain a model bigger and even more expensive than the $220 "360 watt" version. It would almost certainly support more than "360 watts", because you will have more than 13V on the input side while the Truck is running. The 30A limit (which I SWAG to be the actual operating limit) probably allows for a bit more than 400 watts on the input side, with only a small loss in the conversion.

But my design, through the Bargman cable, is allowed to pull about 450W into the MPPT, and the MPPT Voltage conversion cost is also a mere 3-6 percent. My implementation avoids the extra cables, generates MORE power, and costs much less in purchased hardware. Instead, it costs time and effort installing the parts - just once.

Your method probably calls for a 2-conductor cable, rather than just a single 8-AWG wire. While my method pulls only 12-13A along the Bargman path, yours will pull 30A -- in both directions. Within a Bargman cable, the return ground wire is always built bigger than the TBC wire, but it might not be "good" for a full 30A continuous load (I usually see #12-AWG, and even #10-AWG would be at its design limit). The total thermal insulation value of the cable is very high: Ground wire insulation, PLUS the insulation on the surrounding wires, PLUS the outermost foam wrapping and cord cover. When these cables fail, it is often "voltage drop" power loss (heat) creating high temps within the conductor, travelling all the way down the cable to the plastic end assembly, and melting the end assembly. The wires don't often fail in the middle - when overloaded, they fail by melting the 7-pin plug assembly.

Your design is simple, and otherwise sounds great. We share the use of a separate "big wire" battery connection, with its own fuse. (Mine is 50A, although yours can be just 40A, because less power is being provided).

Simple is best for me... I'm already concerned about burning my 5th wheel up...?
I planned to use a quick connect plug on the big wires in my bed right by my 7 pin plug. I was also planning to use a Victron Orion 360W, and a 40A fuse like you mentioned.

 

[rickst29]

New and Simplified "Trailer Picture" of my scheme (with 3 Relays, which also supports 12V towing). The diagram shows "low Voltage" towing with the Detector Relay not yet closed (by Coil Voltage at or Above 24v, it lets go below about 18v). In this mode the Bargman input goes almost directly into the "+12v" bus bar, through the "coil is not pulled in" position of the "Bargman Switching" Relay #1.

But, when the Detector coil DOES get pulled in by the presence of 24V (and they can handle 36V with no problems), its power pin circuit gets closed. The power pins only carry "+12v bus" Voltage into the COILS of "Bargman Switching Relay #1" and "PV Switching Relay #2".

Withe Coils on the "Bargman Switching Relay" and "PV Switching Relay" get pulled in, the "Bargman Switching Relay" move the connection from the "+12v bus", to feed into a switched power port on the "PV switching Relay". (The Bargman Switcher needs to handle more than 36 volts on the POWER pins, although it's COIL is only 12v.) The "PV switching Relay" disconnects the common power leg (headed into the MPPT "PV +") from the "genuine" Solar Panels, and connects the switched-on input from the Bargman Switcher instead. The PV switching Relay may be required to handle live switching of even higher Voltage (My panel array runs at about 65V), but I've had no issues in using a "high current" standard 5-pin automotive Relay for this purpose, over many years of intermittent use.

 

[rickst29]

I've put all of that stuff into a new Thread, here

 

[sunrise]

Your plan avoids my complicated installation, but adds the job of dragging and connecting the "big" wire each time.

Exactly the reason I don't want that, every time I will have to plug/unplug one additional cable.

 

[ultrakiev]

Here is another brand for 12V batteries, 20A dc-dc charger from ThermoKing. Unfortunately its only Lead Acid and AGM batteries. But they are industrial grade waterproof units.