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Battery disconnect

Discussion in 'The Hokey Ass Message Board' started by Hotrodmyk, Mar 27, 2013.

  1. BootleggerMatt
    Joined: Aug 17, 2011
    Posts: 258

    BootleggerMatt
    Member

    This is true, NHRA does require a push pull type cutoff on the positive side, and also requires you to wire the alternator in such a way that it stops powering the car when the cutoff is switched. Lookup "alternator interupt"... The logic behind this is if the car crashes down the track, the first repsonder can run up to your car, pull the battery cutoff and shut the car down. The reasoning behind the positve side cutoff is that if sheet metal happened to crush the battery and is hitting the negative post, a negative cutoff would be inefective.

    My opinion why this doesn't matter on a street car, not fact...
    #1. You probably aren't wiring in the alternator interupt, just cutting a cable and putting the switch between them. So Neg or Pos doesn't make a difference. Without the alternator interupt, the car will still run if you take off the Pos or Neg cable.

    #2. If you get in a crash no one knows to run up to your car and flip your switch, and chances are you didn't put it in an easily accessible spot to quickly flip it after a crash, or it's the last thing on your mind after a crash.
     
  2. tommy
    Joined: Mar 3, 2001
    Posts: 14,758

    tommy
    Member Emeritus

    You are right.
     
  3. Drive Em
    Joined: Aug 25, 2006
    Posts: 1,748

    Drive Em
    Member

    I install them on the negative side to completely kill any possibility of a live wire in an accident. If the cut off switch is installed on the positive side, you will still have a live cable going from the battery to the cut off switch. Everything after the switch will be dead. Depending on how far the switch is from the battery, the live wire could be long enough to cause trouble in an accident.
     
  4. Carter
    Joined: Mar 18, 2006
    Posts: 1,406

    Carter
    Member

    I believe, technically, within the conductor (wiring) the electrons move from negative to positive. I may be wrong, though, as it has been a long time since I studied physics, and even then it was not electricity-specific.

    Sent from my DROID device using the TJJ mobile app
     
  5. larry k
    Joined: Feb 23, 2009
    Posts: 317

    larry k
    Member

    PUT A DISCONNECT ON BOTH CABLES, NOW NOBODYS RIGHT OR WRONG ?????:confused:
     
  6. Carter
    Joined: Mar 18, 2006
    Posts: 1,406

    Carter
    Member

    I don't think it's a matter of right or wrong. Either will work in most cases. There are uncommon instances where both could be problematic, however, for the most part, it's just a matter of preference, or rules if you are going racing.

    Sent from my DROID device using the TJJ mobile app
     
  7. OMG, I'm putting mine on the positive side.
     
  8. LAROKE
    Joined: Sep 5, 2007
    Posts: 1,727

    LAROKE
    Member

    Thanx, G-son and 36roadster for the very good answers in layman's terms. It was a rhetorical question. I'd already learned that lesson in my youth in the school of hard sparks :D
     
  9. Guys, thanks so much for all the input.

    The reason I asked the question is because I am of the opinion it should be in the ground side. (Pos or Neg ground car doesn't matter). The instructions that came with it say "hot" side, conforming with NHRA.

    Bootlegger, you are right, it is a street car, not a race car.
     
  10. 1971BB427
    Joined: Mar 6, 2010
    Posts: 5,799

    1971BB427
    Member
    from Oregon

    I've always put them on the pos. side, but if it's used for the street it will not be a NHRA tech issue. I use them for safety and security.
     
  11. 40fordtudor
    Joined: Jan 3, 2010
    Posts: 2,503

    40fordtudor
    Member

    yep---have one just like it.
     
  12. Bluedot
    Joined: Oct 26, 2011
    Posts: 306

    Bluedot
    Member

    RE Bootlegger's comment about NHRA rules:"The reasoning behind the positive side cutoff is that if sheet metal happened to crush the battery and is hitting the negative post, a negative cutoff would be ineffective." I won't argue that it's their rule, but it seems to me if if that logic was reversed and the switch is on the positive side, and crushed metal contacts the positive post, you'd still have the very same problem. Nobody knows where that crushed sheetmetal is going in a wreck. A switch on either side will break the circuit between posts, and moving sheetmetal or other parts can certainly re-complete that circuit in a hurry. What have I missed here?

    I vote for the negative side to avoid the accidental wrench contact problem cited by others.

    Later...Oh, just thought of one more reason for negative. If NHRA is worrying about crushed sheetmetal making contact with the negative post in a wreck, consider this. Putting a cutoff on the positive side exposes two live positive posts: one on the battery, and one side of the switch, doubling the chance of a big problem: direct ground contact with positive. Switch on the negative side, any contact with the neg batt post or the batt side of the switch, worst case just regrounds the batt neg side to where it was in the first place. Not good when you want to disconnect the battery in a hurry, but far better than very quick battery meltdown or explosion.
     
    Last edited: Mar 28, 2013
  13. if you think about this statement, and the flow of electricity, you have a live cable/wires/system, from the battery + all the way to the disconnect on the negative cable. this reasoning is wrong.
    i don't know why, but i have always been told to "switch" hot side only. in cars, boats and houses.
     
  14. J'st Wandering
    Joined: Jan 28, 2004
    Posts: 1,674

    J'st Wandering
    Member

    The house example doesn't apply because house wiring has an earth ground. Car does not.

    I weigh in on switching ground side.

    Neal
     
  15. boooooob
    Joined: Sep 14, 2012
    Posts: 38

    boooooob
    Member

    Something to think about, maybe way off topic, why do you think they ever tied + or - to a frame, firewall, or sheet metal............because it was a better conductor then copper,,,,,,,,,,,,,NO,,,, it was to save lots of money on wires.............look at the electric grid it is tied to earth ground, the earth is not a good conductor but it is big, and hopefully wet enough,.......and saved much money...........


    Everything would have been better with a positive and negative wire to all device's on a vehicle, then you wouldn't have to worry about all the frame ground BS............


    Get you two high current contactors and a big red mechanical kill switch, kill both sides with one stone...


    boooooob
     
  16. G-son
    Joined: Dec 19, 2012
    Posts: 658

    G-son
    Member
    from Sweden

    You are correct, as long as we're talking physics. But they had no idea about that back in the day when they first begun to figure out how electricity worked, so they decided to call one side positive, and that electricity flowed from there to negative, just as positive pressure causes a flow of gas/fluid towards negative pressure.

    They got it wrong and the electrons do move the opposite direction, but since it doesn't really matter for any normal use you still say that electricity/current moves from positive to negative. In science you learn that the electrons moves from negative to positive.
     
  17. Carter
    Joined: Mar 18, 2006
    Posts: 1,406

    Carter
    Member

    Model A's and other early cars used a positive ground electrical system because they did know about the movement of the electrons from negative to positive.
    In the end it matters not in the context of this discussion.

    Sent from my DROID device using the TJJ mobile app
     
  18. Alfster
    Joined: Jan 15, 2002
    Posts: 1,174

    Alfster
    Member

    Most times a cut off is put onto a car to 'save' the battery when it is in storage. Let's face it not many of us are really building competition cars.

    One thing I do with a cut off switch is fit a small inline fuse accross the two terminals. This will keep things like your clock and stereo memory alive but should a short appear while the vehicle is parked then the fuse will blow.
     
  19. it applies, you could [it's wrong] put a switch on the neutral. again, to be clear, it is wrong.
     
  20. big bad john
    Joined: Aug 11, 2010
    Posts: 4,727

    big bad john
    Member

    ...Ditto......it is wrong
     
  21. I always put mine on the pos. and close to the battery. Right or wrong?? been doing it that way for 49 years.:eek: Im going to bed now:cool:
     
  22. tommyd
    Joined: Dec 10, 2010
    Posts: 11,330

    tommyd
    Member
    from South Indy

    Good advice Don. We killed a diode in a expensive alternator once doing that.
     
  23. GearheadsQCE
    Joined: Mar 23, 2011
    Posts: 2,514

    GearheadsQCE
    Member

    @ 8:17? You must be really old! :)
     
  24. Bad Eye Bill
    Joined: Sep 1, 2010
    Posts: 841

    Bad Eye Bill
    Member
    from NB Canada


    You are deadass wrong my friend. If the negative side of the battery is disconnected you can feed any positive wire to any part of the car without any fireworks at all.
     
  25. 5window
    Joined: Jan 29, 2005
    Posts: 7,531

    5window
    Member

    Right. Finally. Friends, you leave the grounded wire alone with the best contact you can get. If you switch the grounded wire (in most cases the negative pole from your battery) you leave everything powered with the possibility of inadvertently creating a completed circuit with anything that complwtes the circuit. No electrical profession would every interrupt the grounding.
     
  26. tjet
    Joined: Mar 16, 2009
    Posts: 1,272

    tjet
    Member
    1. Early Hemi Tech

    FWIW, all new BMW's are shipped from Germany with a 6' disconnect on the neg side. The dealer removes these in the new car prep procedures.
     
  27. Bad Eye Bill
    Joined: Sep 1, 2010
    Posts: 841

    Bad Eye Bill
    Member
    from NB Canada

    This is true, good points.
     

  28. This is the reason I mount the fire extinguisher and the battery kill in a position that can be reached from the drivers seat while wearing a seat belt .
    In a nasty crash , if you can reach them both you may stand a better chance of avoiding Frying!


    .
     
  29. tjet
    Joined: Mar 16, 2009
    Posts: 1,272

    tjet
    Member
    1. Early Hemi Tech

    Household vs automotive - 2 different animals.

    Your logic on the neg is not correct. There's a 1000 x's more negative circuit "area" than there's positive. There's more of a chance that a battey could ground the pos post (even if disconnected) to a body panel in a crash.
     
  30. Bad Eye Bill
    Joined: Sep 1, 2010
    Posts: 841

    Bad Eye Bill
    Member
    from NB Canada


    True, but, disconnect the positive and you still have the same situation all the way to ground.

    It doesn't matter which side of the circuit you disconnect there will be no flow of electricity.
     

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