Charging Camper LiFePo4 Battery From Truck

[FlyboyTR]

Originally Posted by rich2468

Hey Travis,
I have Lithium Ion Phosphate batteries in my TM and have towed for two years without a DC-to-DC charger. My tow vehicle (TV) has a "smart" alternator and my TrailManor batteries are located at the rear storage of my 2922KB....so the Lithium batteries work out to be about 36-38 feet from my TV battery. (considering the routing through the tow vehicle, over the Bargman wiring, connecting through the TrailManor's power converter, the Lithium battery shunt, then finally to the Lithium battery bank).

Rich,
Thanks for the detail reply and info...appreciated. We have two tow vehicles, a Jeep Grand Cherokee Summit 4x4 Ecodiesel and a old 2008 Dodge Ram 2500 Megacab 4x4 Cummins. The old Dodge is just strong (500 HP on high tune)...NOT smart but has a 200 amp alternator! LOL We are using it for the upcoming two month trip. Lots of storage space in that one and it still gives us 16.5 MPG towing the TM!

I can see where upgrading the supply with a DC/DC converter would make things better. I will be planning on that upgrade after this trip. I will just watch things closely and go from there.

I look forward to seeing your solar panel upgrade. Impressive!

Thanks again!

Travis

[rickst29]

Originally Posted by FlyboyTR

....

I have pondered over this at great length. Our drive times after DRY camping will be typically 4-6 hours. It appears with a 10 amp load on the HOT terminal of my 7 pin connector on the truck, I am seeing about 13.8 volts. I was shocked at that. That said........

Will I harm the lithium battery OR the truck alternator by just connecting and hitting the road; without a DC to DC converter?

As always...you input is appreciated!

Travis

That is slightly higher than the running "12v" voltage of my older 4Runner (closer to 13.6V), but its not a significant difference. When running actual current over a long distance (including a substantial segment and through a very small wire "Trailer Battery Charge" within the TM's Bargman connector cord, you will have voltage drop (proportional to the amount of current SQUARED) on the path to the power center's fuse board. With "12v" LFP batteries typically operating around 12.8 volts over most of their SOC range, the PD load center (the fuse board) will prefer to pull more from the Tow Vehicle, and less from the battery, while running the fridge in 12v mode.

Although LFP battery packs are capable of pulling high charge current, the low initial voltage and increasing voltage drop along your long current path will almost certainly be effective for protecting the truck alternator, and the bargman cable as well. Without a boosting Dc-to-Dc battery charger, you CAN'T exceed the lower limit (burning the cable) when starting with only 13.8 volts at the Source. "plug it in, plug it in".

[rickst29]

Depending on the maximum allowed current limit of a DC-to-DC converter/battery charger, you could create a situation where high current could burn out the bargman cable TBC wire insulation, or the TBC wire connection to into the adapter.

Although the wires within the TM are AWG 10, and capable of up to 30A of DC currrent, the "TBC" wire in most cheap Bargman cables is barely 18-AWG, and WILL overheat if run at more than 14A for an appreciable amount of time. (I know that they're fused at 30A ... they shouldn't be.) The Bargman cable is heavily insulated by the sheathing, and doesn't dissipate heat well at all.
- - -
If your Converter is allowed to provide "12A" at up to 14.0 volts in "boost mode" charging, it will pull much more than 12A on the input side. Voltage drop can pull the ending voltage down from your under-the-hood "13.8" value down to about 12.5 volts, maybe even less, so the input current do do that "boost mode charging would be 14V/12.5V * 12A = 13.44 amps, that's getting very close to the 14A limit which I'd apply.

A "20A" Converter, if not programmed or configured for less current, is likely to to cause burn out. Do not even consider a DC->DC "battery charging" converter rated at 40A or higher.

[FlyboyTR]

Originally Posted by rickst29

Depending on the maximum allowed current limit of a DC-to-DC converter/battery charger, you could create a situation where high current could burn out the bargman cable TBC wire insulation, or the TBC wire connection to into the adapter.

Although the wires within the TM are AWG 10, and capable of up to 30A of DC currrent, the "TBC" wire in most cheap Bargman cables is barely 18-AWG, and WILL overheat if run at more than 14A for an appreciable amount of time. (I know that they're fused at 30A ... they shouldn't be.) The Bargman cable is heavily insulated by the sheathing, and doesn't dissipate heat well at all.
- - -
If your Converter is allowed to provide "12A" at up to 14.0 volts in "boost mode" charging, it will pull much more than 12A on the input side. Voltage drop can pull the ending voltage down from your under-the-hood "13.8" value down to about 12.5 volts, maybe even less, so the input current do do that "boost mode charging would be 14V/12.5V * 12A = 13.44 amps, that's getting very close to the 14A limit which I'd apply.

A "20A" Converter, if not programmed or configured for less current, is likely to to cause burn out. Do not even consider a DC->DC "battery charging" converter rated at 40A or higher.

Rick,
Great info! Sincerely appreciated. I Had originally looked at a 20 amp DC/DC converter. Your point is well made!

I believe for this two month trip, I will leave it as it is with the option of killing the hot feed from the truck to the camper (I'm running out of time to get things ready!). After that... I will plan to upgrade the wiring from the truck battery to the camper battery. I think that would be best!

Thanks again!

Travis

[rickst29]

In my 4Runner, under the hood, there are LARGE cables from the battery into a DC-to-DC "boost" converter. It is not a battery charger, it converts from running voltage (around 13.5 - 13.6 Volts) to 36.0 volts, sending high power at high voltage (and low current) down the 'TBC' wire in the Bargman cable.

These prevents the Bargman cable burnout risk. In a slightly less complicated TM modification, the TBC wire no longer goes to the load center - it is split, near my MPPT solar controller, to become a "switched" alternate input leg for the SCC AND the coil sense "+" lead on an automotive-type relay.

That relay is actually actually a 5-pin Over-the-Road "big truck" relay, those trucks run at 24v and their Relays have 24V coils. My $6 Relay can handle 36.0 Volts on the coil connection with no issues. When the magnet is pulled in, the genuine solar panels on the roof are disconnected from the MPPT and the "Trailer Battery Charge" power connection becomes the MPPT solar controller "Solar +" input lead.
- - -
The tow vehicle appears as a single 36 volt solar panel, always running under "great" sunlight while the engine is ON and the magic dashboard switch is enabled. I have that MPPT configured from a maximum of 28 Amps output, at voltage never to exceed 14V (and usually operating at 13.5 Volts). That's up to 392 output watts. The MPPT wastes about 5% in conversion, so the input side is consuming around 415 watts maximum, at a voltage somewhat less than 36 Volts. (When run hard, just in testing, my operating voltage is around 34.6 volts). The amperage is right around 12.0 amps --- that's the point, this scheme allows very high power through the bargman cable by boosting the voltage nearly 3x.

Working backwards, the under-the-hood boost converter is putting out 12.0 amps at the full 36.0 volts, about 430 watts. That booster wastes about 8% in conversion, it's pulling around 470 watts of power on the input leads.

470 watts @ 13.6 volts = 34.5 amps on each input leg. My biggest problem was constructing a high-current relay socket (using short lengths of high-thread-count AWG-8, to insert a controlling relay on the input connection. My second biggest problem was pulling the relay leads through the firewall to a dashboard switch. Total time in the tow vehicle, with no practice or photos to work from (after acquiring the parts and special wire) was about 3 hours. Time in the TM was about 1-1/2 hours, and testing took an additional hour.

[rickst29]

Originally Posted by FlyboyTR

Rick,
Great info! Sincerely appreciated. I Had originally looked at a 20 amp DC/DC converter. Your point is well made!

I believe for this two month trip, I will leave it as it is with the option of killing the hot feed from the truck to the camper (I'm running out of time to get things ready!). After that... I will plan to upgrade the wiring from the truck battery to the camper battery. I think that would be best!

Thanks again!

Travis

I recommend against crazy schemes to hook "bigger wires" along the bargman doing the same thing, they all become hazards in an accident - and they're just more things to connect and disconnect, over and over. Have a great trip to MT!

I recommend that you consider this one (built by Victron and cellphone programmable, vastly better than the Renogy alternatives). Use the cellphone app to limit the current.
https://www.amazon.com/Victron-Orion.../dp/B0851TPKV7 Just be sure that you['re buying the 'smart' version, not the older and cheaper dumb one.

[FlyboyTR]

Originally Posted by rickst29

I recommend against crazy schemes to hook "bigger wires" along the bargman doing the same thing, they all become hazards in an accident - and they're just more things to connect and disconnect, over and over. Have a great trip to MT!

I recommend that you consider this one (built by Victron and cellphone programmable, vastly better than the Renogy alternatives). Use the cellphone app to limit the current.
https://www.amazon.com/Victron-Orion.../dp/B0851TPKV7 Just be sure that you['re buying the 'smart' version, not the older and cheaper dumb one.

Rick,
Thanks for the link and additional info. Y'all ARE teaching an old dog new tricks! ...and...it is appreciated!

[Bill]

I think a couple things in this discussion need to be re-thought.

When running actual current over a long distance (including a substantial segment and through a very small wire "Trailer Battery Charge" within the TM's Bargman connector cord, you will have voltage drop (proportional to the amount of current SQUARED)

Voltage drop through a resistance is proportional to current, not current squared. Remember, V=IR. It is power (watts) that is proportional to current squared, and watts are what heats things up.

a very small wire "Trailer Battery Charge" within the TM's Bargman connector cord

the "TBC" wire in most cheap Bargman cables is barely 18-AWG

The big black 7-way cord on the TM is what I call the Bargman cable. I’ve never seen a Bargman cable that didn’t have #10 wire, good for 30 amps on the power and ground wires. Even the cheapest 7-conductor cable I could find had 2 #10 wires for power and ground, plus 1 #12 (brakes) and 4 #14 wires (lights). I suppose there might be some smaller cords for smaller campers, but the TM cable power line is #10. I can’t imagine #18 wire anywhere, even on the small 4-pin boat-trailer connector.

The wire sizes in your tow vehicle may be a concern. I had two Ford Explorers (2002 and 2006) with the factory tow package. In each one, I traced the physical wire from the engine compartment to the 7-pin connector at the rear bumper. Most of it was #12, but there were a few feet of #14. No good! I disconnected the original wires at both ends and ran new #10 wires for power and ground, and it was much better.

Bill

[rickst29]

Originally Posted by Bill

I think a couple things in this discussion need to be re-thought.

Voltage drop through a resistance is proportional to current, not current squared. Remember, V=IR. It is power (watts) that is proportional to current squared, and watts are what heats things up.
Bill

You're right (as always ). But it is the wasted power (overheating the wire and insulation) which creates the risk - and that is proportional to current squared. In this particular case, going from 10A to 14A is virtually harmless, and going from 14A to 18A might be disastrous.

[rickst29]

Originally Posted by Bill

I think a couple things in this discussion need to be re-thought.

. . .

The big black 7-way cord on the TM is what I call the Bargman cable. I’ve never seen a Bargman cable that didn’t have #10 wire, good for 30 amps on the power and ground wires. Even the cheapest 7-conductor cable I could find had 2 #10 wires for power and ground, plus 1 #12 (brakes) and 4 #14 wires (lights). I suppose there might be some smaller cords for smaller campers, but the TM cable power line is #10. I can’t imagine #18 wire anywhere, even on the small 4-pin boat-trailer connector.

The wire sizes in your tow vehicle may be a concern. I had two Ford Explorers (2002 and 2006) with the factory tow package. In each one, I traced the physical wire from the engine compartment to the 7-pin connector at the rear bumper. Most of it was #12, but there were a few feet of #14. No good! I disconnected the original wires at both ends and ran new #10 wires for power and ground, and it was much better.

Bill

I had that situation in my 4Runner as well (the OEM Trailer Battery Charge wire was about AWG-14, and I replaced with #10). The Bargman Ground was connected to the frame quite close to the bargman connector: the wire distance was only about 8". I replaced that as well, but the short distance probably create a ton of "voltage drop", and the solid frame connection provided an endpoint to conduct heat out fo the wire anyway.

My original "Bargman" cable (which was perhaps not OEM, although I bought the TM brand new) was defective in design and construction. That 'original cable' TBC wire, which I *DID* burn out, was far smaller than the grounding cable in the middle of the cord. YMMV, but the autopsy on my original cord was just as upsetting as discovery of the tiny "12v" TBC wire running along the driver side bottom door frames in my 4runner.