Forum Discussion

mikestock's avatar
mikestock
Explorer
Jan 27, 2017

Connecting the towed battery to the 12 v house system

I made a fused jumper to connect along side of the regular tail light system. I ran + and - to the starter battery to be sure I had a good constant ground. I ran the same type fused jumper from my house panel to the back of the rv and have a 5 foot jumper to connect the two when towing. I added this because I added a Patriot electric braking system and wanted to be sure I didn't rune the towed battery down while towing.

My question is: should i install some sort of diode in the system to isolate the towed battery so that the current only can flow from the motor home to the towed and not vice versa? I always heard it was a bad idea to parallel different systems.
Class A
  • wolfe10's avatar
    wolfe10
    Explorer
    Jan 27, 2017
    enblethen wrote:
    Roadmaster diodes are rated at 30 amperes.


    Amps is not the primary issue. Voltage drop, even if amps is rated way over actual could render the charge wire close to useless.

    If voltage on the coach side of an isolator (wire run from batteries, fuse, etc is, say, 13.5 VDC and you loose .7 VDC across a diode-based isolator, you are NOT going to charge a toad battery with 12.8 VDC.

    I agree, if you want/need it to be automatic, install a RELAY with an ignition-hot as the signal at the coach battery end. Fuse at coach chassis battery, fuse at toad battery and 8 gauge wire for hot and ground. DONE.
  • ets1996's avatar
    ets1996
    Explorer
    Jan 27, 2017
    I bought a kit I think Roadmaster Camping world or ebay can't remember which anyway it has a voltage reg and everthing needed to connect the toad to the motorhome. I wired it through the seven pin connector as my toad only uses 4 wires for the lights. It keeps the battery charged while using the supplemental brake.
  • hanko's avatar
    hanko
    Explorer
    Jan 27, 2017
    wolfe10 wrote:
    I see no reason for a diode-- that is unless you are dry camping and forget to unplug the charge wire.

    And if you decide to use a diode, make sure it is not a standard one that "charges" around .7 VDC. That much of a drop will render the charge wire pretty well useless.



    doent the 12 volt at the 7 wire plug run through and isolater. I know it does on my coach. the circuit goes dead when I shut the engine off on the motorhome
  • RoyB's avatar
    RoyB
    Explorer II
    Jan 27, 2017
    I would look into the smart charging relays over the diode based isolators...

    Todays charging schemes and diode based isolators doesn't mix well due to the built-in voltage drops of diodes...

    Roy Ken
  • wolfe10's avatar
    wolfe10
    Explorer
    Jan 27, 2017
    I see no reason for a diode-- that is unless you are dry camping and forget to unplug the charge wire.

    And if you decide to use a diode, make sure it is not a standard one that "charges" around .7 VDC. That much of a drop will render the charge wire pretty well useless.
  • mikestock's avatar
    mikestock
    Explorer
    Jan 27, 2017
    enblethen wrote:
    A standard diode like this would work.
    Diode


    That is actually the one I was looking at. My concern was that if, by some chance, there was a large differential between the towed voltage and the RV voltage if it would handle the current surge that could occur. Probably a foolish concern.

    There is a relatively inexpensive device on the market that will compare the voltage between the house and chassis batteries. It allows spill over to the chassis batteries if they drop too low while you are plugged in to a power source. I don't remember what it is called.
  • petrel's avatar
    petrel
    Explorer
    Jan 27, 2017
    some sort of battery isolater should do the trick. There is quite a selection of them on the marine market, I'm guessing the same goes for RVs.

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