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path1's avatar
path1
Explorer
Jul 13, 2016

Doing stupid things to my elec system...question

I'm going bad to worse. Last week I posted that I forgot to disconnect my house battery on class c then we took off snowbirding for 7 months in our trailer. Came back and after charging only got 0.97 volt. Batteries are shot. Yesterday I took out the batteries, to go get new. First cable I undid had a red collar on the cable. Thinking to myself great, that's the pos side its marker red. Then after pulling all the way out, discover both cables have a red collar on them. I think red collars are what somebody used to make cables.
So I can't tell difference of pos and neg (they both have red collar)

So before I hook up new batteries and screw something else up...

Is this test right to determine pos and neg?

Neg side should be grounded to frame? So just follow cable back and look. Then to confirm, start motor (so alt puts out some juice), put one volt meter probe on cable that is not grounded (which should be pos) and the other probe, ground out to frame.

This would complete a connection and will show up on volt meter.

Do I have this right or going to mess something up?

Elec is my biggest short fall on RV's.

Thanks
  • path1's avatar
    path1
    Explorer
    Jul 14, 2016
    OP here...Thanks folks...After taking batteries out, I do see some corrosion on the metal shelf that they rest on, so I'll get on that.

    Thanks again
  • Almot's avatar
    Almot
    Explorer III
    Jul 14, 2016
    Don't overdo that grease part. Silicone grease is dielectric. It protects the surfaces but it also impedes the contact on terminals.

    Where Mex is living, things break, corrode and turn to dust ten times faster than anywhere else, and to get a replacement part you have to drive several hundred miles. Pretty much, - to get anything.
  • westend's avatar
    westend
    Explorer
    Jul 14, 2016
    If your meter has a "continuity" feature, check continuity to frame. The "negative" phase (-) cable will have continuity, the "positive" (+) cable will not.

    The resistance measure of ohms will also be identical, 0 ohms to frame with negative cable, infinite ohms to frame with positive cable.

    Or, power on converter, measure voltage by one probe to cable, the other to frame. Do not short the positive cable to ground, you will then be replacing fuses.

    Good luck with your battery replacement. FWIW, I use a protective spray from a spray can on my terminals. IIRC, it came from Napa Auto Parts.
  • rvten's avatar
    rvten
    Explorer
    Jul 13, 2016
    I suggest to anyone replacing batteries. Take a picture with your phone of the present batteries. If possible. So you can see how to put the new ones in.
  • MEXICOWANDERER's avatar
    MEXICOWANDERER
    Explorer
    Jul 13, 2016
    Please understand...

    Chassis grease contains sulfur

    Not the ideal mix-n-match with copper

    Dielectric silicone grease is light years better especially when it comes to very high temps. But even more important is sealing the junction between the lead battery post and plastic jar lid. The grease has to be applied meticulously to the underside of the battery lug as well as to the morphodite shaped "RV" lead post. Getting -under- the flat section is a chore. The terminals should be outright banned and a 3/8" + 5/16" stainless steel stud used. Anyway arguing will bring forth images of my 100 amp bussbar that was smeared with chassis grease then stored by a friend. "Chocolate" does not quite describe the patina. Sulfur and chlorine are both found in lubes and both are detrimental to shiny brass or copper. Pb does not seem to care.

    If thoughts of Kumbayah enter the picture sheep grease, anhydrous lanolin can be used. It works great but has a low melting thresh hold.
  • MEXICOWANDERER's avatar
    MEXICOWANDERER
    Explorer
    Jul 13, 2016
    When you have successfully finished, and cables assembled and tightened, SPRAY PAINT the positive terminal and post red, and the negative post and cable black. Then slime everything with protective coating.

    Works every time.

    With 3 cell batteries, I paint the series connections, green.
  • GordonThree's avatar
    GordonThree
    Explorer
    Jul 13, 2016
    path1 wrote:

    Neg side should be grounded to frame? So just follow cable back and look.


    Yes, whichever cables eventually connects to the frame is going to be negative. You can also use your volt meter in OHMS mode, one lead on a battery cable, one lead on a screw head or bolt on the frame. The negative side will have a very low / 0 reading, the positive cable will have a higher reading.

    I wouldn't start the engine to test, instead, plug into shore power.

    Your on board converter will put 12v on the battery leads and you can check the polarity with your volt meter.

    Wrap the negative lead with green or white tape... as black also can mean positive in the land of RV wiring (black = hot, white = neutral)

    edit: fixed a typo

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