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Old 08-14-2022, 04:24 PM   #1
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LiFePO4 and B2B Charger Issues

Hi - Having recently converted my Flagstaff T12RB PUP to a LiFePO4 battery I'm learning the hard way that my tow vehicle (2016 Highlander) will not fully charge this battery. So, bottom line: I need to install a Battery-to-Battery charger to better manage the 13VDC output from the tow vehicle and bump it up as needed to meet charging requirements for the LiFePO4 while on the road. I would really appreciate some help with this since Renogy's instructions are specific to "inhouse" (i.e. vans and motorhomes), and Forest River will not provide schematics (of course). My questions are:
1. Since I'm mounting the B2B charger (a Renogy 20A) in the trailer, can I (or should I) redirect the "Battery Charger" lead (Pin 4) from the 7-way connector so that it feeds the +DC input of the B2B exclusively?
2. Can I assume that the same ground can be used (shared) on the -DC Input and -DC Output sides of the charger?
3. The B2B unit will be mounted next to my solar controller (which has existing 3' 8AWG hardwired on the output side directly to the battery), so can I piggyback the B2B outputs onto the existing wiring from the solar controller (wired in parallel, of course), and thus avoid additional wire runs to the battery?
4. By design, my Prodigy brake controller does not pass current between the tow vehicle and the trailer on Pin 4 (the "Battery Charger" lead) unless tow vehicle's ignition is "On". If I feed +DC input of the B2B unit with this lead, can I also use it for D+ Ignition connection to the B2B?

Sorry for the wordy post; hope somebody who's figured out this puzzle and found it interesting can share some words of wisdom.
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Old 08-14-2022, 06:49 PM   #2
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I installed my Renogy DC-DC charger in my trailer. I added some #8 awg marine type duplex (2 conductor) wire from truck battery to rear bumper and terminated it with a 50 amp Anderson connector.

Ran same type wire from input of DC-DC charger back to the 50 amp connector on the rear of my truck. Put 50 amp ANL fuse next to truck battery to protect wire if shorted.

Disconnected the charge wire from truck where connected at j-box on trailer frame and extended to DC-DC charger "D-Plus" terminal which now switches on the DC-DC charger when engine is running.

Only difference in Renogy's instructions which appear van or motorhome specific is that a travel trailer will merely need a connection between tow vehicle and trailer. The 50 amp Anderson connectors do the job and allow an easy connect/disconnect when connecting/disconnecting for tow.

Just zip tie duplex wire to the 7-wire umbilical cord from trailer. Wire in tow vehicle can be zip tied to trailer harnes wiring which runs typically under vehicle from battery to trailer wiring receptacle.
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Old 08-14-2022, 07:38 PM   #3
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LiFePO4 and B2B Charger Issues

Thanks Mike. Lots of good info here. Is your tow connector a 7-pin and if so, I'm wondering why you steered away from using the "Battery Charger" lead on Pin 4. This is what I'm considering since it might avoid adding another vehicle-to-tow connector + wiring, but not sure what the down sides might be.
Steve
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Old 08-15-2022, 07:56 AM   #4
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Quote:
Originally Posted by sbjenni View Post
Thanks Mike. Lots of good info here. Is your tow connector a 7-pin and if so, I'm wondering why you steered away from using the "Battery Charger" lead on Pin 4. This is what I'm considering since it might avoid adding another vehicle-to-tow connector + wiring, but not sure what the down sides might be.
Steve
In most situations the 7 pin connector wire is too thin for a dc-dc connector. You need to do as Mike says and run a separate wire of an appropriate gauge from the truck battery to the back of the pickup with an Anderson type connector. From there you can run wires back to the trailer battery.

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Old 08-15-2022, 07:23 PM   #5
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LiFePO4 and B2B Charger Issues

Thanks Phil. Checked into it further and you're right - the available amps on the "Battery Charger" lead (Pin 4) seems to top out at about 7 when hoked to my half-charged LiFePO4. Somehow I thought this lead was coming straight from the starter battery and all I needed was the right B2B charger to make it work. Guess it's going to be Anderson Connectors, ANL fuses and 8AWG wire runs after all (and thanks again for the M&P Mike!).
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Old 08-17-2022, 11:09 AM   #6
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It looks like you are located in California... would adding a solar panel (or two) to the top of your PUP do the trick?

You should be able to charge your battery fairly quickly and there will be less to remove and rewire should you decide to change your tow vehicle.
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Old 08-17-2022, 12:57 PM   #7
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LiFePO4 and B2B Charger Issues

Thanks Rob and yes, that's an idea I've begun to look into. I'm a little concerned about adding permanent weight to a roof panel that's already somewhat difficult to raise and lock into place. We do carry 460 watts of portable solar, but these are foldable "soft" panels that we deploy on site.
Probably I'll end up with Mike's/Phil's idea for wiring in a B2B charger to keep the battery topped up while towing.
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