Tow Vehicle to 5th Wheel charging... inverter-to-ac to onboard inverter to battey bank??

rsg1963 · December 2015

Hi all, I did a search and did not find anything specific like I am asking here, so please forgive if in fact it does exist.
Wondering how feasible it would be to run an inverter off my TV battery(s) then use that converted AC power to run my inverter with built-in 125amp charger in the 5th wheel.
I know this is not efficient power-wise, but I think it may be efficient simplicity-wise. And I am really hoping to get some real world experience or better ideas here. I thank you in advance for even reading this drivel.

My setup:
I have a 2016 RAM with twin 220 Amp alternators and of course twin batteries.
I also have a 2016 5th wheel with residential refer. Unit came from the factory with twin automotive style marine batteries (I will NOT call them deep cycle as I do not think they qualify) and a small Magnum inverter that is ONLY hooked to the refer circuit and battery bank.
I swapped out the twin 12v batteries for '4' 6v Trojan L-16H-AC's in series and parallel for 870ah total.
I am swapping out the inverter for a Magnum 2812 with 125 amp charger and 4/0 welding cable to the batteries one of these coming weekends.

BB. · December 2015

I am not quite sure I understand your question...

If you want to run an inverter off of your truck electrical system and run that back to the RV (120 VAC) to run a battery charger/inverter-charger (appropriately sized)--Yes, that can be a very good solution. Much more accurate charging, and usually higher charging current that can be done with 12 VDC direct connection from the truck to the RV with relatively small gauge cable.

Trying to run 12 VDC (or 24 VDC) from the truck to the trailer is tough--And if you want more than 10-20 Amperes of DC Direct--You would need to run some very heavy welding cable+quick connect between the two. Running heavy 12 volt current over long(er) cable runs is difficult.

-Bill

oil pan 4 · December 2015

10 gauge wire and MC4 solar panel connector would get you 20 to 30 amps from the vehicle electrical system to the camper.

The MC4 connector would give you a water tight easy to remove wire connector.

That battery charger may not like being powered with modified sine wave power.

BB. · December 2015

I would suggest that 10 AWG cable running 20 feet from vehicle to RV batteries @ 30 amps is too much voltage drop to be useful for RV bank charging. Using a generic voltage drop calculator:

http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=12&phase=ac&noofconductor=1&distance=20&distanceunit=feet&amperes=30&x=51&y=17

You get about 1.2 volts drop.. If your vehicle charger is set for ~14.2 volts, that would give you ~13.0 volts at the RV battery... Certainly enough voltage to run DC loads--But not near enough to have any sort of high current battery charging... If you were OK with 14.0 volts charging, or 0.2 volt drop, then that would be ~5 amps of charging current (or ~60 Watts of charging power).

Sending lots of 12 volt current any distance is a difficult task--The cabling resistance generates too much voltage drop even with fairly heavy copper cabling. Certainly, if you put panels on a trailer and connect them in series/parallel (for higher Vmp-array voltage) to a MPPT charge controller on the main battery bank--That would be nice--But don't use MC4 for the trailer to tow vehicle connection.

MC4 connectors are not good for repetitive connecting/disconnecting or making/breaking under load. From an older thread:

http://forum.solar-electric.com/discussion/comment/288477/#Comment_288477

BB. said:

Re: ❀ Vacation: Leave batteries hooked up or no???

But the MC4 is a high pressure contact with flash (gold?) plating. They are not designed to be taken apart very often, and certainly not to be disassembled when under load.

http://www.multi-contact.com/AcroFiles/Catalogues/PV_Solar_(de-en)_hi.pdf

Plugging cycles
The maximum life of the PV connectors is 100 plugging cycles.

Unplugging under load
PV plug connections must not be unplugged while under load.
Plugging and unplugging while under tension is permitted.

Protection against weather
Sealing caps must be used to protect unplugged PV-connectors
from moisture and dirt.

Positioning of the junction box
The junction box must be fixed on the PV module in such a
position that the cable outlets of the junction box point downwards
when in use.

-Bill

-Bill

oil pan 4 · December 2015

How long would it take you to hook and unhook the trailer 100 times?

So what would make for a good trailer connector that is water tight and can handle some amps and doesn't cost a lot?

You can get a pile of MC4 connectors for $20.

Use it 100 times, cut it off and replace. Don't connect or disconnect under load.

zoneblue · December 2015

Anderson powerpoles for the connectors.

oil pan 4 · December 2015

Anderson power poles don't appear to be the least bit water tight.

techntrek · December 2015

Bargeman trailer connectors aren't really water tight either. I would use Anderson over MC4. As for the OP's question, while there are disadvantages to converting DC-AC-DC using a larger inverter and the converter built-in to the camper (power loss for example), Bill's points are valid. Plus you can keep your fridge cold. With twin 220 amp alternators you have the power to spare.

Tow Vehicle to 5th Wheel charging... inverter-to-ac to onboard inverter to battey bank??

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