DC to DC Charger with MPPT controller + lithium iron phosphate batteries - Fiberglass RV
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Old 11-17-2020, 07:08 PM   #1
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Name: Ian
Trailer: Escape
British Columbia
Posts: 4
DC to DC Charger with MPPT controller + lithium iron phosphate batteries

Hello all, I have a 2013 17ft. escape trailer. I am putting in 2 X12V 80Ah lithium phosphate batteries in parallel. I am disconnecting the battery connections to the WFCO 8900 distribution center. I am installing a 20 Amp. Renogy DC-DC charger. I have a 95W Go Power solar panel on the roof that I am pairing with a
20Amp Renogy MPPT controller. So my question is: will there be conflict between the DC-DC charger and the solar input with both going to the batteries?? Mostly I wonder if the DC-DC charger might damage the controller.
Any feedback would be appreciated as I am trying to figure this out as best I can
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Old 11-17-2020, 08:08 PM   #2
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Name: Eddie
Trailer: 2014 Escape 21, Lil Joe
Florida
Posts: 1,749
Red face

I was trying to figure out the DC to DC charger issue with 200W of solar panels and not wanting to upgrade my tow vehicle wiring. I am using a 100A lithium battery. The CTEK 250SE DC to DC charger was recommended and several others agreed. It is DC to DC charger combined with a MPPT controller. I have not installed it yet in my trailer.
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Old 11-17-2020, 08:25 PM   #3
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Name: Carl
Trailer: LiL Hauley
Syracuse, NY
Posts: 442
I wouldn't anticipate any issues. I have 200 watts of PV, a 20 amp Renogy MPPT and a Victron 9 amp DC-DC charger between the TV and trailer, and it works fine. Set the MPPT and DC-DC chargers to the same voltage. I can provide some theory explaining this if you like. ALSO, run a dedicated fuse and wire from the TV battery to the where the trailer connector is, a dedicated return wire is also advisable. I am not sure of the maximum current you can put thru the 7 pin connector so you need to figure that out. And you will have to make sure you have wire that can handle the maximum current in your trailer, including the return wire.

The TV wiring and trailer wiring from the connector to the DC-DC will have to be able to handle in greater than 20 amps of current. This is due to the different voltages, efficiencies, and line losses. Volts x amps = power. If your LFP battery is at 14 volts and drawing 20 amps the DC-DC converter will need to supply 14x20= 280 watts of power. The TV will have to supply that, PLUS the efficiency losses in the DC-DC converter, PLUS the line loss in the TV and trailer wiring to the DC-DC converter. Assuming 85% efficiency of the DC-DC converter, I could easily see your TV having to supply 280/.85=330 watts of power at the input of the DC-DC converter. If you assume 50 feet of 10 awg wire you will have approximately .05 ohms of resistance. Which means your TV will have to supply an additional .05 x 27 amps squared = 36 watts of power. It adds up to approximately 330 + 36 or 366 watts. If your TV alternator puts out 13.8 volts it will have to supply 366 watts/12.5 volts (at the DC-DC converter) = 29 amps. The current draw from the TV is actually a differential function and I am not going to go there so I estimated the current at 27 amps. I made some assumptions with these calculations which you should verify based on your design and converter efficiencies.

My 9 amp DC-DC converter draws about 13 amps when operating.

Hope this is not to confusing.
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Old 11-17-2020, 09:03 PM   #4
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Name: Ian
Trailer: Escape
British Columbia
Posts: 4
The Wiring

I am running 10AWG X2 silicone from the 4runner battery to the DC-DC charger located in the front area of my trailer under the bed. Then running 10AWG X2 wire from the DC-DC charger back to the batteries. I will put a 40 amp fuse at the 4runner battery and at the LiFePO4 batteries. Putting connector for this where my trailer electrical hookup, at the tongue, is located. I realize in my original post that any potential conflict between DC-DC charger and MPPT controller is kind of an isolated issue, in that the DC-DC charger is only working as I drive. The reason for disconnecting the WFCO center from the batteries is that it will not recognize the lithium... and may mess with them.
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Old 11-18-2020, 02:25 AM   #5
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Name: Tom
Trailer: BigFoot 25B25RT
Massachusetts
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Quote:
Originally Posted by IanHall View Post
I am running 10AWG X2 silicone from the 4runner battery to the DC-DC charger located in the front area of my trailer under the bed. Then running 10AWG X2 wire from the DC-DC charger back to the batteries. I will put a 40 amp fuse at the 4runner battery and at the LiFePO4 batteries. Putting connector for this where my trailer electrical hookup, at the tongue, is located. I realize in my original post that any potential conflict between DC-DC charger and MPPT controller is kind of an isolated issue, in that the DC-DC charger is only working as I drive. The reason for disconnecting the WFCO center from the batteries is that it will not recognize the lithium... and may mess with them.
I have a Renogy DCC50S 12V 50A DC-DC On-Board Battery Charger with MPPT. It is all one unit and the instructions specified 4 awg wire. I have the truck wired but I am in the process with the camper.
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Old 11-18-2020, 11:55 AM   #6
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Name: Russ
Trailer: Northern Lite
British Columbia
Posts: 1
I have the Renogy 20A DC-DC charger. It draws 30 amps at full output (20A output). That amount of current found a weak spot in my stock 10 ga wiring - I experienced a failure in the umbilical cord (when I took it apart I found a very sloppy connection in the female plug, causing a bit of a melt). Anyway, the charger has a half-current setting which I enabled, (after repairing the plug) so it now draws 12 - 13 amps and outputs 10 amps.
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Old 11-18-2020, 12:14 PM   #7
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Name: Ian
Trailer: Escape
British Columbia
Posts: 4
DC-DC Charger and Wiring

Thanks for all the input... it actually looks like I can't disconnect the WFCO Distribution Center from charging the battery... as I would then have no 12V source to power it . We never stay in campgrounds so don't have a 110V plugin on our trips.
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Old 11-18-2020, 12:21 PM   #8
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Name: John
Trailer: Black Series HQ19
Smith Valley, Nevada
Posts: 2,239
An Anderson plug mounted on the rear bumper is an excellent way to easily connect a dedicated 50 amp source to the trailer. This one uses #6 wire and I have a 50 amp circuit breaker next to the tow battery.

To hook up, I plug in the conventional seven pin, and the separate Anderson plug that runs alongside it. In my system, the Anderson is always live on the truck side. When stopping for the night, I don't bother to unplug it. In sunny weather, the solar peaks out at just over 250 watts to the trailer batteries, and would, if needed send some to the truck as well. I have four 100 AH AGM batteries in the trailer. The truck has an alternator that brings everything up to 14 volts, and then drops to 13 volts as a float charge. The solar controller is an MPPT and ties the panels directly to the batteries at the same point as the Anderson wires. This system is simple, has no conflict with different power sources, sends a powerful charge to the house batteries from the truck, and the Anderson plug on the trailer can be used a a power source to run a 12volt compressor, electric/hydraulic jack (for tire changing), or as a point to plug in a suitcase solar system. It can also be used to charge the truck if I get a dead battery. In my case, there is a junction box on the tongue where the #6 house battery charging wires connect to the Anderson pigtail that plugs into the truck. That junction box is where I get power for the emergency breakaway switch. It can also be used as a source for an electric tongue jack, if desired, but I prefer a manual jack.
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Old 11-18-2020, 12:31 PM   #9
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Name: Carl
Trailer: LiL Hauley
Syracuse, NY
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Here is another way to look at it. If you look at the specifications, the Renogy 20-20 charger will operate down to 8 volts on the input. Assuming 14 volts on the output at 20 amps, the charger will output 20 x 14 or 280 watts. Lets call the efficiency 95% this time. The input power will be 280/.95 or 295 watts. If the voltage drop in the wiring from the TV is such that the voltage at the charger is 8 volts, the charger will draw 295/8 = 37 amps! You have to design for that value. As mentioned above, one bad connection and you can have melt down. The smaller the wire the larger the current. Also remember that copper has a positive temperature coefficient, which means the hotter it gets the higher the resistance. In this situation that means it will have greater voltage drop causing the charger to draw more current which will only heat the wire more. Positive feedback as the say! This may not seem intuitive but that's the way current sources work.
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Old 11-18-2020, 12:40 PM   #10
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Name: Ian
Trailer: Escape
British Columbia
Posts: 4
DC-DC Charger and Wiring

WOW, what great feedback. Having a 17ft. trailer being pulled by a 4runner sport edition my wire distances are within the specs. put out by Renogy. I am using an anderson type plug at the tongue. My circuit breakers are actually
30amps.; not 40amps. as previously stated. I feel a lot better about putting this system together after having more experienced input. Thanks.
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Old 11-18-2020, 01:03 PM   #11
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Name: John
Trailer: Black Series HQ19
Smith Valley, Nevada
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Carl,

That is why #6 wire from the TV is a good idea. And why the 50 amp Anderson plug style is so good.

I have never tripped the 50 amp breaker next to the truck battery, but I have measured 24 amps feeding the trailer when the trailer batteries were nearly at full charge. So, I'm certain it is in the range of 24-50 amps being fed back to the trailer, depending on the state of charge. After stopping for the night somewhere, and we almost never have hookups, or even use them if available, I'll see the truck output at 14 volts for an hour or so, and then it drops to 13 for float. There is no tenths reading, so I don't know the exact voltage coming from the truck.

The MPPT solar takes the final battery voltage to 14.5 during its absorption phase. So the truck mainly gives the bulk charge, and some absorption. In poor weather, or shade, and after a couple of days of weak solar charging, I can idle the truck and give the trailer a bulk charge, for a half hour or so, of nearly 50 amps.

This setup is so simple and maintenance free, that I hardly think about it now. I just look at the battery voltage on the monitor, once in a while to see how it's doing. Every morning, I make coffee in the electric coffee maker, sometimes two pots. We charge the electronics, run the heater as needed, watch a movie or two, use all the lighting we want, and run the microwave to re-heat things. A very comfortable amount of power with almost no thought.
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Old 11-21-2020, 12:49 PM   #12
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Name: Mel
Trailer: aliner
Texas
Posts: 37
No DC-DC as primary LFP battery charging for me

DC-DC AS PRIMARY LFP BATTERY CHARGING
Early warnings when charging LFP batteries included NOT to continually charge to 100% or lifespan would be reduced. Confirming that guidance:
* Early Lithium chargers only put out 14.4-14.6vdc. Common user practice was to monitor SOC and disconnect charging when near 100%.
* Recent LFP chargers like WFCO start at 14.6V for 1 hr. to balance battery cells then, when monitored charging current drops, output voltage is reduced to 13.6v. Full description is in the operator manual referenced below.
Most Solar controllers perform the similar Bulk then Absorption charge voltages.

DC-DC CHARGING FROM TowV
TowV battery to my RV battery is about 50'. That's about 100' round trip for most OEM #10 wiring. Per West Marine site referenced below, I can get 10A @12vdc for 100' using #10AWG with 10% drop. That's consistent with my testing using a Victron Orion-Tr 12 12-9 isolated DC/DC converter.
During installs to charge LFP batteries, I:
* Set these to 14.1v output to avoid 100% LFP charging
* Know they'll be charging slowly at <9A only while driving (not full time).

MY TAKE: Avoid using DC-DC converters for primary 100% charging unless they or you can follow Bulk-Absorption charge levels like Solar and WFCO follow. These batteries are too expensive to accidentally cook.

I apologize for being late to comment. I was enjoying 70dF camping and hikes at Inks Lake in Texas hill country in November! I hope this helps.
Mel

https://www.westmarine.com/WestAdvis...e-And-Ampacity
http://wfcoelectronics.com/wp-conten...Ion-Manual.pdf
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Old 11-21-2020, 04:12 PM   #13
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Name: Carl
Trailer: LiL Hauley
Syracuse, NY
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Mel, I have a progressive dynamics, which Battleborn sells, in my camper. It is basically a constant current, constant voltage, 14.6 volt, 40 amp power supply. Per Battleborn, they say the equalization happens when the battery voltage is above about 14.2 volts. The BMS shuts off the battery when it is fully charged. I am not sure what LFP battery you have and maybe it reacts differently, but I can't imagine Battleborn selling a charger that is not compatible with their batteries which have a 10 year warranty.
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Old 11-21-2020, 05:59 PM   #14
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Name: Mel
Trailer: aliner
Texas
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Vendors can only sell the best available equipment at that time. Many of us bought those because those were the best available.
But instead of constant voltage, the current Progressive Dynamics Converters and most other manufacturers now offer or only sell variable voltage strategies similar to the WFCO I'd mentioned.

Many LFP vendors have long offered 10 year warranties regardless of the charger.
My thought is with the improved charge strategies, converter manufacturers now help us maximize our battery operation time and improve the number of measured cycles (that some batteries count and report).

I think constant voltage will work but it is not the latest battery maximizing technology. I believe that was worth mentioning when someone is considering a DC-DC converter that only put out a constant voltage.
Mel
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Old 11-25-2020, 02:10 PM   #15
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Name: David
Trailer: None
Florida
Posts: 5
I use a Victron 18A DC-DC converter to charge from the stock converter and to charge from the TV via the 7pin. I wired a relay so that the DC-DC converter is normally connected to the stock converter, and when it senses 12v on the tow harness the relay energizes and disconnects the stock converter and connects to the TV. (It’s a SPDT relay).

This did require a little bit of rewiring. The existing tow charge wire on the trailer side now only serves to supply 12v to the emergency trailer brake. I ran a new 8ga 12v from the tow harness inside the trailer back to the relay. Also disconnected the converter from the 12v DC distribution panel, and it now runs back to the relay. Added a high amperage fuse block to fuse the DC-DC charger, an inverter, PV charger, and then the existing trailer DC distribution panel.

Sounds confusing without a picture as I re-read this. However on the plus sign, LifePO4 is great and I get a ton more watts/AH in the same form factor and not worrying about drawing below 50% is great.
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Old 11-25-2020, 07:43 PM   #16
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Name: Douglas
Trailer: Lil Snoozy
MD
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I have a setup very similar but not exactly the same as Carl's: Victron 9 amp DC/DC converter, Victron MPPT 100/30 solar charge controller, etc. charging two 100 Ah Battleborn Batteries. I think the 4Runner stock wiring to the 7-pin connector is 12 ga and the Snoozy wiring to the trailer batteries is 12 ga. I estimate the 4Runner wiring at 15 feet and the Snoozy wiring at 20 feet. I doubled the trailer wiring to two 12 ga. The DC/DC converter is set to 14.2 volts and charges the batteries via a 12 ga wire and 10 amp fuse. The Victron MPPT solar controller is set to 14.6 volts absorption and 13.6 float per BattleBorn, and charges via a 10 ga wire and 30 amp fuse.

I have been running it with these settings for a couple of years and observe the DC/DC converter charging at or near 9 amps pretty regularly after boondocking. I don't think that I would be comfortable charging more from the TV with the stock wiring.

I did run a 10 ga wire from the TV battery to the rear of the 4Runner and it is pretty easy to do, but that runs a separate system of roof solar panel, 9 amp DC/DC converter, MPPT solar charge controller, and 50 Ah BattleBorn battery to keep a fridge running in the back of the 4Runner.

This is all just for your info and perspective. Good luck on your build, I think you will love the LiFePO4s.
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