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Old 06-15-2021, 09:16 AM   #1
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Using LiFePo4 - 7-pin safety

Hi - I'm planning to upgrade to a LiFePo4 battery, and will keep the stock WFCO-8755 and use solar to top up the charge. I'm wondering how switching to this battery chemistry will affect the 12v connection to my Tow Vehicle through the 7-pin connection.
I always considered that a LI-BIM was necessary only for Class A, B, or C type coaches with integrated engines, and that a DC-DC charger would only be if you wanted to charge from the alternator.
But how necessary is one of these devices for a trailer with a drop-in Lithium battery? I wouldn't want to damage my alternator or blow the 30a fuse for the 7-pin 12v wire.
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Old 06-15-2021, 09:44 AM   #2
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If you want total peace of mind the best option would be to install a basic DC-DC Charger like the Renogy DCC 1212-20. it's a 20 amp unit that can be installed in one of two configurations.

If you want to merely protect the Alternator and wiring/circuit breaker from the higher current draw LiFePo3 batteries can draw, install it and activate the "Current Limiting" feature on the DC-DC charger. This is as simple as a jumper from the current limiting connection to a 12 volt source that is live when connected to the tow vehicle. In this mode the output is 50% of the max for the model installed.. On a 20 amp DC-DC charger the output would be limited to 10 amp which is fine for the existing wiring on a 7-pin connection.

Going this route would provide both isolation of house (lithium) batteries from TV (Lead Acid) and low enough current that no wire upgrade would be necessary.

The other advantage of installing this unit is that should you want more charge power, just add larger wiring from battery to TT.

Right now the Renogy DCC 1212-20 is selling for $110.49 on Renogy's website. Regularly $129.
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Old 06-15-2021, 10:25 AM   #3
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But how necessary is one of these devices for a trailer with a drop-in Lithium battery? I wouldn't want to damage my alternator or blow the 30a fuse for the 7-pin 12v wire.
Not necessary. I dropped in 4 lithiums 4 years ago and it's been fine in both trucks now.
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Old 06-15-2021, 10:44 AM   #4
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Not necessary. I dropped in 4 lithiums 4 years ago and it's been fine in both trucks now.
Wouldn't it depend on the SOC of the Lithium battery? Have you ever driven with the house batteries at a low SOC?
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Old 06-15-2021, 11:17 AM   #5
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Wouldn't it depend on the SOC of the Lithium battery? Have you ever driven with the house batteries at a low SOC?
I did a "drop in" with two Battleborn's three years ago and just tried charging from Tow Vehicle "as is". Found that charging current was extremely low which is caused by the voltage drop across the factory wiring. This, in itself, is a current limiting factor.

I installed the DC-DC charger in order to increase charging while driving. Did feel that the factory wiring was a little strained so added a large wire (pair) from starting battery to TT where DC-DC charger is located. If I had merely activated the current limiting feature I could have avoided the need to install the larger wire.

I too had no issues with the original wiring but one's mileage may vary on that. I just don't like using wire resistance (and resulting heat) to regulate current.
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Old 06-15-2021, 11:41 AM   #6
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Wouldn't it depend on the SOC of the Lithium battery? Have you ever driven with the house batteries at a low SOC?
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This is the only one I have handy but we were at about 50% state of charge when we pulled away from the Disneyland Hotel after the camper sat for a week in the parking lot with the fridge on.
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Old 06-15-2021, 01:18 PM   #7
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lithium batteries with built in management systems (BMS) are direct swaps. nothing to worry about since they will take care of themselves.
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Old 06-16-2021, 11:03 AM   #8
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I installed the DC-DC charger in order to increase charging while driving. Did feel that the factory wiring was a little strained so added a large wire (pair) from starting battery to TT where DC-DC charger is located. If I had merely activated the current limiting feature I could have avoided the need to install the larger wire.
Just curious how you practically did this - My 7-pin cable goes to a junction box under the front of the trailer, is this where you tapped into the 12v connection for your DC-DC charger?

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I too had no issues with the original wiring but one's mileage may vary on that. I just don't like using wire resistance (and resulting heat) to regulate current.
I read this from another site: "The voltage concern is that LFP actually has LOWER voltage than lead-acid during the majority of its charge and can force an alternator to output high current for much longer than lead-acid, which can wear out an alternator."

I guess I won't know how my SUV behaves until I get the LFP cells connected, but since I'll be letting the WFCO manage the charge I suspect they may be hungry for whatever the alternator can dish out.
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Old 06-16-2021, 11:06 AM   #9
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This is the only one I have handy but we were at about 50% state of charge when we pulled away from the Disneyland Hotel after the camper sat for a week in the parking lot with the fridge on.
What kind of amps was the battery charging at when you plugged in the 7-pin?
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Old 06-16-2021, 11:21 AM   #10
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Just curious how you practically did this - My 7-pin cable goes to a junction box under the front of the trailer, is this where you tapped into the 12v connection for your DC-DC charger?I didn't make any connection at the J-box but ran a separate wire from battery to rear bumper. I used #8 awg Marine Type Duplex wire, connected directly to the battery of the truck and terminated with a 50 amp Andersen Connector. From there I continued on to the DC-DC Charger mounted in the trailer.



I read this from another site: "The voltage concern is that LFP actually has LOWER voltage than lead-acid during the majority of its charge and can force an alternator to output high current for much longer than lead-acid, which can wear out an alternator."

This statement is almost right. It's the LOWER IMPEDENDE (internal resistance) of the LiFePo4 battery that causes the higher charge current. The discharged voltage of a LiFePo4 battery is always higher than a Lead Acid battery at the same state of charge. Even at full charge of ~12.6 volts a Lead Acid Battery is barely above the voltage of a LiFePo4 battery that is almost fully discharged.

In short, a LiFePo4 battery will draw as much current as the alternator can deliver, only limited by the resistance of the wire connecting



I guess I won't know how my SUV behaves until I get the LFP cells connected, but since I'll be letting the WFCO manage the charge I suspect they may be hungry for the alternator can dish out.
The main issue, aside from current regulation, will be the finish voltage when batteries are fully charged. LiFePo4 batteries need 14.6 volts (most brands) to achieve a full charge and most importantly, achieve Cell Balance. Most Alternator regulators will drop the output voltage to around 13.6 volts after the starter battery is fully charged even though the voltage at initial stages of charging will be 14+ volts.

DC-DC Chargers will charge the LiFePo4 batteries using the correct profile
even though the Alternator is providing the proper charge profile for a Lead Acid battery.

Typically a WFCO converter, as well as any other brand, will only be in the picture when on shore power, never while driving down the road.
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Old 06-16-2021, 11:56 AM   #11
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What kind of amps was the battery charging at when you plugged in the 7-pin?
I've never seen more than 3-5 amps charge to the battery (but that's also offset by parasitic draws in the camper).

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Old 06-16-2021, 03:29 PM   #12
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I've never seen more than 3-5 amps charge to the battery (but that's also offset by parasitic draws in the camper).



Attachment 257153
You should be able to double that without having to add separate wiring. Add a Renogy 20 amp unit and just jumper D+ connection to the LC connection. Just need a switched source of 12v to the D◊ terminal but clearance light wires are good for that. 10 amps is probably more than enough to keep up with parasitic draw and at least make sure your batteries are totally full at your next off grid stop.

A simple and relatively inexpensive add-on.
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Old 06-16-2021, 03:40 PM   #13
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I didn't make any connection at the J-box but ran a separate wire from battery to rear bumper. I used #8 awg Marine Type Duplex wire, connected directly to the battery of the truck and terminated with a 50 amp Andersen Connector. From there I continued on to the DC-DC Charger mounted in the trailer.
When you run these wires, is it ok to leave the 7-pin wiring as is?

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Typically a WFCO converter, as well as any other brand, will only be in the picture when on shore power, never while driving down the road.
Understood - my concern is for when the solar hasn't charged the LFP (cloudy days, disconnected, etc) and they have only charged as full as the WFCO can take them while on shore. If I then connect the TV to the trailer for a trip, will too much demand be placed on the alternator.
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Old 06-16-2021, 03:44 PM   #14
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I've never seen more than 3-5 amps charge to the battery (but that's also offset by parasitic draws in the camper).

Attachment 257153
Strange that you would see so low a draw - I've been reading of others who find their fuses were being blown.

Hopefully my Durango has whatever secret sauce your Ram has...
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Old 06-16-2021, 04:57 PM   #15
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You should be able to double that without having to add separate wiring. Add a Renogy 20 amp unit and just jumper D+ connection to the LC connection. Just need a switched source of 12v to the D◊ terminal but clearance light wires are good for that. 10 amps is probably more than enough to keep up with parasitic draw and at least make sure your batteries are totally full at your next off grid stop.

A simple and relatively inexpensive add-on.
Honestly, if I keep this fifth wheel, I'll likely add the DC-to-DC charger and do the same wiring that you did. I'd like to get a couple of DC appliances switched over to Anderson connectors any way and that would be a step in the right direction.
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Old 06-16-2021, 04:58 PM   #16
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Strange that you would see so low a draw - I've been reading of others who find their fuses were being blown.

Hopefully my Durango has whatever secret sauce your Ram has...
I've monitored it from both a 2012 Ram and my 2018 Ram and it's about what I typically see.

But again, this is overall +/-. So there's a charge from the truck but it's offset by the fridge running and the various parasitic draws on the camper.

And if I happen to leave my inverter on which I have make all of my outlets hot, then I run at a deficit and actually discharge the batteries while I drive.
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Old 06-16-2021, 06:45 PM   #17
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When you run these wires, is it ok to leave the 7-pin wiring as is?

The 12v+ wire from 7-wire cable needs to be connected to the + input of the DC-DC charger. Also connect - terminal to white - wire of 7-wire cord but leave the connection to trailer ground intact. Only disconnect the positive wire and connect to charger. Connect output to batteries.

Leave all other wiring intact.
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Old 06-17-2021, 08:19 AM   #18
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The 12v+ wire from 7-wire cable needs to be connected to the + input of the DC-DC charger. Also connect - terminal to white - wire of 7-wire cord but leave the connection to trailer ground intact. Only disconnect the positive wire and connect to charger. Connect output to batteries.

Leave all other wiring intact.
I have the 12v+ wire on our 7-way only powering our TPMS signal booster.




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Old 06-17-2021, 03:47 PM   #19
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Honestly, if I keep this fifth wheel, I'll likely add the DC-to-DC charger and do the same wiring that you did. I'd like to get a couple of DC appliances switched over to Anderson connectors any way and that would be a step in the right direction.
I cut off the battery clamps on my FlowJet and my Air Compressor. Put Anderson connecters on both the lead to the Flow Jet and Compressor and the battery clamps I cut off.

This way I can plug them into my TV power lead. Also made an extension cord so I can use either farther from the back of the truck if necessary. Also did the same for my Fresh Water transfer pump.

Next addition is to add a connection to my TT batteries so I can connect these "appliances" if my truck isn't nearby. A lead from chassis ground and the existing battery cutoff switch under the tongue (That still has a connection to the batteries but only shuts off power to tongue jack and slide).

Anderson connectors are very inexpensive and Amazon sells in various size multi-packs.

My goal is to have anything that is portable and runs off 12 volt's capable of being run off one of my Anderson compatible sources.

No need to raise hood or "hope" the cigarette lighter plug won't burn up under the current draw.
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Old 06-17-2021, 03:50 PM   #20
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I have the 12v+ wire on our 7-way only powering our TPMS signal booster.




If one is using the existing wiring with a DC-DC Charger the 12v+ wire is needed to be the source of it's power.

On my setup with the direct starter battery connection I too use the 12v + wire to power my TPMS repeater and also use this source to turn on/off the DC-DC charger. On my TV the "charge wire" is only hot when the ignition is on so it's a perfect switching source.

BTW, I love your Diagrams. Have you ever considered selling them to FR?
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