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Old 03-15-2023, 10:15 AM   #11
rickst29
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Default The fridge will pull from the most 'attractive' power source.

Long explanation first. The running tow vehicle (SUV or Truck) provides maybe 13.6 volts, but power losses in the wiring will create voltage drop along this path when significant current (and power) begins to be pulled from that power source.

The "+12v" wiring to the fridge comes from the load center. From there, voltage is supplied by either your lithium batteries (at 12.8v) on short and large wires, or the tow vehicle. The fridge wants to pull in about 11-13A (varying in a linear way with voltage available across the resistance coil heater).

The tow vehicle is much longer distance, and contains a lot of smaller wires. Within the vehicle itself, "12v" from the engine compartment might b e using a surprisingly small wire (the original wire in my 2007 toyota 4runner seemed to be smaller than awg-14), and my be about 10 feet long. The actual size of the "Trailer Battery Charge" wire in the cable itself may also be smaller than you expect, that cable comes up into the TM behind the fridge for a total length of perhaps 25 feet and from there, a better wire (AWG-10) runs into the load center.

The grounding return path is "better" in most vehicles, connecting to the vehicle frame after only a short distance of wire from the bargman connector. Within many bargman cables, the grounding wire also also a lot bigger than the TBC wire (I have seen this in a cable which I destroyed by overloading it on the TBC wire.) But the overall path will become "less attractive" than the batteries after the current load exceeds a small value - perhaps just 3-4 Amps out of the 11-13 Amps required by the fridge. Everything else is pulled from the batteries.

The voltage across the load center bus is always constant for al the wires which are attached to it, and your LFP batteries will maintain that voltage at 12.8V until they're less than 20% full. The LFP batteries "hold up" the voltage on the TBC wire, but the TBC wire will not provide more current - if you were "magically" able to pull more current from the Tow vehicle for a moment, it's voltage drop would increase, and your "magical current" would be offset by pulling from the batteries instead.
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TM='06 2619 w/5K axle, 15" Maxxis "E" tires. Plumbing protector. 630 watts solar. 450AH LiFePO4 batteries, 3500 watt inverter. CR-1110 E-F/S fridge (compressor).
TV = 2007 4runner sport, with a 36 volt "power boost".
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Old 03-15-2023, 10:51 AM   #12
rickst29
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Default One solution is a DC->DC boosting battery charger,

and the other solution (daytime only) is a couple of solar panels.

The best of the DC->DC boosters is this one, which can be monitored and programmed from a cellphone. I recommend this one - but be sure to get the "Smart" version, not the cheaper dumb one without cellphone control. If you go over 15A, your bargman cable might be at risk of damage due to overheating (I burned out my own original cable by doing that), but just 12a on the input side should support most of your fridge DC power requirement. That might be around 120 watts, maybe a bit more - and the fridge uses around 140-150 watts in your high-voltage situation.

https://www.amazon.com/Orion-Tr-12-3.../dp/B0851TPKV7

A couple of 100w solar panels (peaking at 200w at "noon" under perfect condition, you'll actually get less than that) would work for daytime, but not early AM or evening travel. They would possibly also help to re-charge the batteries in camp.
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TM='06 2619 w/5K axle, 15" Maxxis "E" tires. Plumbing protector. 630 watts solar. 450AH LiFePO4 batteries, 3500 watt inverter. CR-1110 E-F/S fridge (compressor).
TV = 2007 4runner sport, with a 36 volt "power boost".
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Old 03-15-2023, 03:32 PM   #13
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I'd like to get the DC-DC charger you linked, for the purpose of helping keep up with the fridge's DC draw during long drives (I'm planning on adding 100-200W solar and hopefully the combination will avoid getting to a dry campsite with depleted battery). I'm OK at least trying it with my existing tow vehicle wiring, understanding that the current will be relatively limited due to small gauge wires.

What is the best way to take my current bargman wire, and get it connected to the input side of the DC-DC charger (assuming I'll mount it up front near my tongue-mounted battery)? Specifically, do I re-route the 12V wire from the bargman to go to the input side of the DC-DC, instead of to the junction box with the rest of the bargman wires? How about the ground/negative wire for the input side of the DC-DC? I've seen lots of youtube videos of folks running new 4AWG wire from the tow vehicle battery to the DC-DC via an anderson connecter, but I'd like to avoid all that extra complication if not absolutely necessary.

Thanks for any guidance you can share!
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Old 03-15-2023, 04:24 PM   #14
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Default Put it either behind the power center (under the tub)

Quote:
Originally Posted by Diamond_D View Post
I'd like to get the DC-DC charger you linked, for the purpose of helping keep up with the fridge's DC draw during long drives (I'm planning on adding 100-200W solar and hopefully the combination will avoid getting to a dry campsite with depleted battery). I'm OK at least trying it with my existing tow vehicle wiring, understanding that the current will be relatively limited due to small gauge wires.
Behind the fridge, where the bargman cable terminated, gets to warm for this device. At that spot, the TBC wire goes into a 3-way junction which you don't need to disturb at all, the other wire supporting the disconnect switch. Don't even look at it, your installation occurs behind the load center.

The other wire in that "behind the fridge" junction goes to the load center's 12v fuse board. You disconnect that wire from the DC fuse board, and instead plug it into into the "input 12v" of the Victron DC->DC charger - while the tow vehicle is NOT connected. The Vcictron is an isolating device two grounding ports, but you will simply connect both of those (using AGW-10 white wires) into the 12v grounding bus connection block (you may have just one of those, or you might have two). Any open ports on those ose are fine to use.

This is NOT the current carrying "120V-AC" connector strip near the 120V-AC circuit breakers, the grounding blocks are normally behind the load center.

The connector on the 12v fuse board where you disconnected the TBC junction wire receives a new wire from the the Victron "battery charge output". This one also provides power to the device in its normal jumpered configuration. Download the App into your cellphone, turn on the devices, and connect via bluetooth. Then program your current limit, and program output voltage boost to only about 13.4 volts, so that the fridge doesn't try to run "too hard".
- - -
Just one solar panel of size 100 watts is not worth the trouble. At least 200 watts is enough to do a bit of charging, but most people with serious solar configurations have at least 300 watts of panels. The sum of rated power on my own panels is about 480 watts.
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TM='06 2619 w/5K axle, 15" Maxxis "E" tires. Plumbing protector. 630 watts solar. 450AH LiFePO4 batteries, 3500 watt inverter. CR-1110 E-F/S fridge (compressor).
TV = 2007 4runner sport, with a 36 volt "power boost".
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Old 03-20-2023, 08:11 PM   #15
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Thank you for the detailed response, this is very helpful!
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