5th wheel with factory installed magnum 2812 and 4 six volt deep cycle batteries--AGM OK?

Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset


  • rorr1821rorr1821 Posts: 1Registered Users
    rsg1963 said:
    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.

    I have ordered a 5th wheel with factory installed magnum 2812 and 4 six volt deep cycle batteries using a Ram 3500 with dual alternators. I have a choice of wet-cell batteries or AGM. It is not clear to me if the truck alternator charging system would work well with AGM batteries!  I will be taking delivery of the unit in about a week. Interested in knowing how well the truck charging system works with your set-up. Thanks.
  • BB.BB. Posts: 27,787Super Moderators admin
    You need to know the charging voltage of the alternators. In general, AGM should work very nicely as your starting batteries. They will (in general) work nicely as your RV batteries too (much cleaner, no adding water, etc. vs flooded cell).

    In general, it can be difficult to get "full charging current" from the alternator to the 5th wheel battery--Sending high current 12 volts the long distance from alternator to 5th wheel usually limits out around 10 amps or so.

    Do you plan on towing a lot with the RV? Or will you have other charging sources (solar, on-board genset, utility power)? If you have other charge controllers, then you want to make sure they they have an "AGM/Sealed" battery charging setting--Or are programmable for charging voltage. AGM's do not like high charging voltage/extended charging times (they are sealed and over charging can cause them to vent).

    Flooded Cell batteries are nice because you can use a hydrometer to check their state of charge--And are a bit more forgiving (you can always add more water if you over charge them a bit).


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • scrubjaysnestscrubjaysnest Posts: 174Solar Expert ✭✭✭
    Using an inverter between the tow vehicle and trailer has been discussed in several other RV forms. It seems to be A. questionable safety practice. B. not a good idea.

    For the Ram, ours is single alternator and the charge wiring from the factory was really really bad. I redid this with parallel 8 AWG wiring, additional plug besides the Bargman, and sure power solid state isolator between the cranking and trailer house batteries. With the refrig on DC, 11 amps, the truck in tow/haul the measured voltage at the batteries is 14.5 to 15 volts. Some people prefer a relay isolation system to the solid state. Since your set up is different then mine you may already have the isolation system and your charge wiring maybe larger.
  • BB.BB. Posts: 27,787Super Moderators admin
    Why would an inverter on the tow vehicle and AC to DC charger in the trailer be a questionable safety practice/bad idea?

    Other than added costs, I can see the system being more safe/stable vs DC from tow to towed vehicle. AC inverter lets you size the system for fixed maximum power transfer (i.e., the system will never supply more power/current than the AC inverter is capable of). And you can make the AC power transformer isolated with Ground Fault breaker. None of these are easy to do with DC power (cannot isolate power, and other than using a breaker, no easy way to limit current flow).

    Plus, in the end, you can have higher power transfer from tow to towed vehicle and use smaller gauge wiring (120 VAC vs 12 VDC).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • scrubjaysnestscrubjaysnest Posts: 174Solar Expert ✭✭✭
    I guess some were talking about extension cords between the tow vehicle and the 30/50 amp 120 vac shore power input. Me, I just like to keep things simple. Upgrading the 12 volt wiring is no hassle for me. For others it's a pain. Most often it turns out charging problems between the tow vehicle and the camper are just under sized wiring from the manufacturer. Even some RV manufacturers stint on wire size. The normal pigtail between the tow vehicle and the camper is 10 awg and about a 10 foot run. Then inside the camper it sometimes get strange. The second biggest problem are the converters, they don't output high enough voltage "to protect the battery(s) from boiling".
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