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ThomasM's avatar
ThomasM
Explorer
Dec 09, 2014

Additional Battery

I have a spare car battery that I would like to hook up to the 2 deep cycle batteries of my RV.
I want to use it as an emergency battery, so I would not attach it permanently. I'd connect it for charging and then when I dry camp and the other batteries get empty I would use the car battery.

I tried to put the + of the car battery to the + of the rv and the - to the -, but I get strange readings on my multimeter.

From what I read the + cable of the rv goes in one battery and the - cable leaves from the other battery. I can't do that with the spare battery because I don't want to disconnect the battery pack when I attach the spare battery.
What can I do?

16 Replies

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  • midnightsadie's avatar
    midnightsadie
    Explorer II
    Dec 09, 2014
    12.4 is not a charged battt. I,d take all three to a auto shop and load tested.
  • ThomasM's avatar
    ThomasM
    Explorer
    Dec 09, 2014
    Yes I know the wires a too thin, I will buy #8. Thanks for the advise cleaning the connectors, will do...

    Both battery are somewhat loaded (12.4 volt)
    When I connect the green (car) battery to the left of the RV batteries the meter jumps from 3-9 volts. I didn't had it connected long, because i am scared to do some damage.

    If I connect the green battery the reading on the meter should still be 12 something, right?

    Again this is only for emergency purposes. Or maybe, since I have the battery I could hook it up permanently. Or does it hurt when you mix battery types?
  • Dakota98's avatar
    Dakota98
    Explorer
    Dec 09, 2014
    If you can change out the wire used on those two battery clamps, I would. Use a min. 8 gauge wire. If not, maybe get THESE CLAMPS & 8 guage wire.



  • MrWizard's avatar
    MrWizard
    Moderator
    Dec 09, 2014
    picture finally showed

    i agree..clean the terminals ..and try again with larger wire

    but your spare on a battery charger
    charge it and test it
    maybe it is Not so good
  • time2roll's avatar
    time2roll
    Nomad
    Dec 09, 2014
    Wire looks thin. Several terminals could be cleaned.
    Best might be a battery selector switch to choose the spare, existing or both.
  • CA_Traveler's avatar
    CA_Traveler
    Explorer III
    Dec 09, 2014
    The two batteries both appear to have corrosion on the terminals which should be cleaned.

    What are the strange readings? The wires on the green battery are much to small to do anything useful - maybe trickle charge over a long period of time maybe days.

    Have the green battery (or even all three) load checked at a auto store to make sure it's OK.

    + to + and - to - is correct. Connect the green battery to the upper left terminal and the lower right.

    It appears that the batteries and the two are connected in a balanced manner which is good. But the cables are much longer than needed.

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