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POS or NEG Battery Disconnect switch

Discussion in 'The Hokey Ass Message Board' started by The37Kid, Mar 28, 2013.

  1. Positive Side

    131 vote(s)
  2. Negative Side

    118 vote(s)
  1. Clik
    Joined: Jul 1, 2009
    Posts: 1,864


    What was that practicle joke that was played with capacitors? I don't remember any battery needed, just an unsuspecting sucker to be the ground.
  2. TheEngineer
    Joined: Jul 17, 2012
    Posts: 239

    from New Mexico

    According to Electrical Engineering 101, it doesn't matter.

    In real life, the neutral bus can still kill you with all the breakers off in the house. (or so I was told when I was an electricians apprentice. I actually saw a few weird things happen between the neutral and ground in that job).

    In the car, if you switch the NEG and drop a wrench across the NEG terminal to the body, it will energize the system but nothing noticeable will happen and you won't know anything is wrong until you shock the shit out of yourself pulling the starter out. You can drop a wrench anywhere else on the car and it won't matter.

    If you switch the POS and drop a wrench across the POS terminal to the body, things will get pretty exciting and you will know something happened right away. It's not likely that you'll accidentally drop a wrench across the terminal and hit another exposed positive location to energize the whole system. This way you can avoid accidentally shocking yourself (unless you lean over the battery). Again, you can drop a wrench anywhere else on the car and it won't matter.

    Positive is the better way to go.
  3. Mike51Merc
    Joined: Dec 5, 2008
    Posts: 3,841


    I'll take that challenge. I bet you dollars to doughnuts that your coil cannot create a charge if your battery is disconnected on either side. You can turn your key all day long. It simply cannot get "powered" up if there is no CIRCUIT.

    A lot of guys here are confusing DC with AC. With AC there's a "hot leg" and a "neutral". You always switch the hot leg on a AC system.

    With AC, you can drive a spike into the earth and attach a wire to it and it will serve as a ground.

    With DC, that just doesn't work. The "ground" in an automotive system is just the metal frame and bodywork serving as a circuit system for the opposite polarity, it has nothing to do with the earth.

    Think about this: My car has positive ground. While parallel parking my bumper touches a car with negative ground. Do sparks fly?

    Everyone has touched a 9V battery to their tongue and got a shock. Try this: touch your tongue to only one side. Now switch to the other side. Any difference?
    Last edited: Mar 30, 2013
  4. Gus68
    Joined: Jan 29, 2007
    Posts: 402

    from Minnesota

    It's realy simple. Set a battery on your garage floor. Hook a set of jumper cables to the battery (Red to positive, black to neg). Now hook the other ends of the cables to a light bulb. What happens? The light lights, CORRECT??? Ok now, walk back over to the battery and unhook the RED clamp off of the POS side of the battery, what happens? The light turns off. Now do something CRAZY, take that red clamp that you just took off of the pos side and touch it to the neg side of the battery. What happens???? NOTHING!!!!! Now hook it back up to the POS side of the battery. Lights back on right??? Ok now unhook the BLACK clamp from the NEG side of the battery. HOLY CRAP!! the light STILL went off!!! Now take that same black clamp that's still in your hand and touch it to the POS side of the battery. STAND BACK cause NOTHING is gonna happen!! Now you can try this same experiment with 100 lightbulbs, or a radio all kinds of things. You will get the SAME results!
  5. boooooob
    Joined: Sep 14, 2012
    Posts: 38


    As said before do them both...........


  6. hotrodgypsy
    Joined: May 10, 2011
    Posts: 282

    from reading,pa

    you use the negitive side ....
  7. Wow. 4 pages? Haha, lookit me adding this anyway...

    Put the disconnect on whichever wire you would NOT touch to metal.

    If you're in a wreck and your fancy ammo box battery cover happens to hit the post or cut into the wire, a ground side switch isn't gonna shut everything off. And if you're drag racing, don't try to explain to the tech guy why you put it on the ground side...
  8. Carter
    Joined: Mar 18, 2006
    Posts: 1,406


    If you are in a wreck, it's just as likely a piece of metal could just jump your battery disconnect on either post. There are an unlimited number of possibilities in a wreck. There is no definite way to assure a battery disconnect is going to work in a crash.

    If you are going to go racing, make sure you put the battery disconnect where they require it. Also, try to take steps when placing your battery, wiring, and disconnect to minimize the possibility of metal coming in contact with any of it.

    Sent from my DROID device using the TJJ mobile app
  9. The37Kid
    Joined: Apr 30, 2004
    Posts: 27,267


    If NHRA wants it on the positive side were a safety crew member can shut off the power, wouldn't it be wise to have one on the negative side within reach of the driver?
  10. Weasel
    Joined: Dec 30, 2007
    Posts: 6,694


    I am posting this here as a 'Public Service Announcement', which I posted on the original thread on this topic regarding an incident that happened to me and resulted in losing a high dollar engine. In my case, 'none of the above' worked....:rolleyes:

    Okay, Weasel chiming in: a little known and somewhat obscure issue with GM alternators is that they do, under certain conditions, backfeed. In this instance the alternator was backfeeding to the electric fuel pump, even though the ignition was off, key pulled from ignition and HD master cutout switch turned off. The plug wires were pulled off the engine and the HT lead disconnected as well, but the engine was hot enough to be glowing or dieseling. As long as the alternator continued to supply electricity to the fuel pump and the engine would ignite the mixture, it would continue to run - as it did for ten minutes or more after everything was shut off. The only solution was to starve the carburetor of air by pulling off the blow through supercharger hat and stuffing rags down it. By this time the engine was literally toast and it was a very dangerous situation as it could have seized and let go at any moment. Painless Wiring recognizes this alternator backfeeding issue and has a kit to prevent backfeeding. I would strongly urge that this is a must have safety device.

    Painless has this to say:

    This kit, complete with 250 amp solenoid, will effectively handle alternator feedback of the highest amperage alternators. Eliminates engine run-on when the master disconnect is turned off. Alternator feedback can keep your engine running even with the master disconnect turned off - this kit eliminates engine run-on. Highly recommended by most racing sanctioning bodies. Easy to install with all hardware and instructions.

  11. Dick Dake
    Joined: Sep 14, 2006
    Posts: 788

    Dick Dake

    I believe I can hear Mr. Wells, my Electrical Core instructor from the Navy, rolling his eyes and wanting to slap some people after reading this.
  12. Buzznut
    Joined: May 9, 2008
    Posts: 2,349


    I think this is a case where even those who are wrong, or lets just call them "less right", will never admit that the OPTIMAL situation is cutting the FEED side as opposed to the GROUND side. If your system was ground on the positive side, then you would put a cut-off on the negative or FEED side.

    I think it's pretty absurd that rules set forth by the NHRA, SCTA, NASCAR, SCCA, FORMULA Racing, etc... are all being called incorrect so that a few can justify their stance.

    Let's see if we can get ten more pages of reasons why all these race sanctioning bodies are wrong....
  13. Carter
    Joined: Mar 18, 2006
    Posts: 1,406


    Buzznut, I really don't want to argue this anymore, and the only reason I have so far is that people come on the HAMB to learn about things. There are a lot of people who don't understand wiring and electrical systems. The more misinformation there is, the worse it gets.

    By the NHRA rule book, disconnect must be wired on positive side. So if I showed up with a positive ground car, it would still be required to be on the positive side, which should tell you that the rule isn't the most well thought out regulation out there.

    As far the feed side being switched, you say that the positive side is the feed in a negative ground system, and the negative is feed in a positive ground system. This is incorrect. They don't sell positive ground and negative ground batteries. The movement of the electrons from the negative towards the positive occurs in both systems. By this alone, the negative side should be considered the 'feed' side. But I won't claim that because by creating an open at any point stops all flow.

    The only real difference between the hot side and the ground side in most electrical systems is the conductor. The frame of a vehicle is used as the conductor on the ground side of most electrical systems to save cost and weight.
    If you ran hot and ground wires to and from the battery and each electrical component, the body, chassis and engine would no longer be a part of the electrical system.

    Also, alternator backfeed can occur with either side switched.

    In summation, I will state again that it does not make any difference to the electrical system which side is switched. Everything other than that is a matter of personal preference or opinion.

    As I stated in an earlier post, if you are going to race with a sanctioning body, please follow the rules that they enforce.

    Sent from my DROID device using the TJJ mobile app
  14. Mike51Merc
    Joined: Dec 5, 2008
    Posts: 3,841


    Agreed! Although I spoke for the "equivalence" of switching the ground side, it was only from an electrical theory viewpoint.
  15. Drive Em
    Joined: Aug 25, 2006
    Posts: 1,748

    Drive Em

    Poll: This is a stupid poll. I cannot believe that this is still wasting space.
  16. blowby
    Joined: Dec 27, 2012
    Posts: 6,805

    from Nicasio Ca

    Can someone please explain how this 'backfeeding' from the alternator can make a fuel pump or dizzy that has had it's input wire switched off still operate?
  17. BootleggerMatt
    Joined: Aug 17, 2011
    Posts: 258


    I totally agree Alden. It is very embarrasing to see that so many car guys do not understand the way a battery works. It's so simple. I have tried to convince them on the original thread that started this poll that the battery is sending no power through the car when the negative side is disconnected, but have come to the conclusion that they will never understand. Instead I have decided to take a battery, hook a peice of wire on the positive terminal, and carry it around scaring the shit out of these folks by telling them I'm gonna shock em!
    Last edited: Apr 1, 2013
  18. J'st Wandering
    Joined: Jan 28, 2004
    Posts: 1,674

    J'st Wandering

    Buzznuts, I agree with you. Only problem is that I feel that you are the one "less right". ;) I don't see this as a big deal but the thread just goes on and on. Must be the weather.

  19. Abomb
    Joined: Oct 14, 2006
    Posts: 1,659


    I have a loop of rope, and I need a straight piece of rope with two ends....which side of the circle should I cut ???

    Same question, same doesn't matter
    Joined: Jun 3, 2005
    Posts: 8,056


    If you're racing and need an emergency cut off it should be on the pos side and use an alternator diode to break current (or relay since some love them and the word so much). If it's simply to cut power for storage/service/whatever, breaking the main ground does the same job.
  21. Mr48chev
    Joined: Dec 28, 2007
    Posts: 27,368


    That is also the reason that Cat has the shut off on the negative side on their equipment. Taking the ground out of the system means that everything electrical is shut off. A large number of cars have a secondary power feed line tied into the battery terminal that powers the computer or other things like the clock in the radio and you still have 12 volts running through the car where it may cause a spark if the wire is compromised.
  22. Morrisman
    Joined: Dec 9, 2003
    Posts: 1,600

    from England

    Back-feeding. I think it may be referring to the fact that even with the battery disconnected the alternator is still feeding power to the system when the motor is running, by whatever point its output wire is connected to. In most cars it is the big starter motor power connection. As long as that point is where your power circuit is getting its power from, for the pumps and stuff, then your motor will happily run, even with the battery ++ wire physically removed from the battery terminal.

    Some battery isolators switches have a specific connection that you wire the alternator output to, that is also disconnected from all circuits when you flip the switch.

    At least I think that is what they are referring to. :D

    My new industry is going to produce dual pole isolation switches, that disconnect both negative and positive wires with one key. ;)
  23. Buzznut
    Joined: May 9, 2008
    Posts: 2,349


    Not to belabor it, but I don't think race sanctioning bodies even allow a positive grounded system.
  24. blowby
    Joined: Dec 27, 2012
    Posts: 6,805

    from Nicasio Ca

    That must be what they are referring to, and I have seen vehicles wired that way (switch between battery and alternator, not ignition), however the proper way, and the way in which most vehicles are wired, is to have the switch between ignition (or fuel pump, etc.) and the battery/alternator. Yes, it will still run with battery removed but not with switch turned off.
  25. ShoeStringFiftyThree
    Joined: Mar 5, 2013
    Posts: 36


    ^^makes sense.

    Think of the alternator as a second battery (when its spinning).

    If you want to disconnect power from a running engine,
    you need to disconnect the battery (pos, or neg terminal)

    AND the alternator (positive terminal on a neg grounded system).

    Since the alternator case is what connects the neg terminal to the chassis/ground, you can't disconnect the alt's negative from ground with just a switch.

    Makes sense that the NHRA would spec disconnecting the positive side, for safety
    so you can disconnect the alternator at the same time (kill all power even if the engine is running)

    Anyone wiring a disconnect to isolate the battery from draining,
    would also want to disconnect the battery from the alternator somehow.
  26. child9
    Joined: Mar 25, 2013
    Posts: 8

    from Austin

    "if the electrons are flowing from the positive to the negative"

    They don't. Ben Franklin messed this up for everyone. Still causes a LOT of confusion. Positive negative are just arbitrary designations. They could be red charge blue charge. All that matters is strength of charge and that like charges repel, and unlike charges attract.

    Example of this confusion:
    "will never admit that the OPTIMAL situation is cutting the FEED side as opposed to the GROUND side. If your system was ground on the positive side, then you would put a cut-off on the negative or FEED side."

    Again, technically the negative side is the "feed" side as you call it. It where the electrons are coming from.

    "If it's on the negative side, EVERYTHING has power to it"

    Power=Current x Resistance. No current, no power.

    "what if the car has positive ground?"

    Dunno what that means, but I hope that's a troll.

    In example, when we tig weld on DC negative, the electrons are flowing from the negatively charged tungsten to the positively charged work piece via what we call a "ground clamp". The clamp is in fact not a ground. When we reverse the electron flow by setting the machine to DC positive, the now reversed flow (from the negatively charged work piece to the positively charged tungsten) literally focuses the heat and power up into the tungsten, causing the tungsten atoms to melt and be pushed up and in...we see this evidenced when the tip balls up.
    The flow of electricity is similar to air flow, in that just as air naturally requires a pressure differential to cause movement (weather systems), so electrical flow requires a potential difference in order to flow (voltage). Pressure flows from high to low in air, and electricity does a similar thing...again pointing at Ben Franklin for the electrical mix-up.

    Enough with basic the context of our vehicles it doesn't matter. Ideally you want the disconnect as close to the source (battery) as possible.

    The only time I personally ever consider an order of operations is when disconnecting a battery by hand. I always disconnect the negative side first, since if I happen to turn a metal wrench and make contact with the vehicle, nothing will happen since there is no potential difference between the negative terminal and the chassis due to them being in contact already and thus sharing the same charge. If the negative terminal is connected I will have an electrical potential difference (12 volts with a potential of the CCA rating of the battery) between the wrench and the chassis of the vehicle the instant I put the metal tool on the positive terminal. In the process of turning the nut, if I inadvertently touch the chassis, I'll get some pretty good sparks...though it could possibly short well enough to weld the tip of something to whatever was touched. The latter case is roughly equivalent to connecting the two battery terminals with a screwdriver. All the electrons, as fast as the battery can, directly through the conducting tool.

    As far as an alternator continuing to power the vehicle after the battery has been disconnected, that scenario is only possible if the alternator or generator is wired in parallel with the battery instead of in series. Wire it in series and you will never have to worry about it.

    Backfeeding, as I understand it, is caused by an effect that happens according to Lenz's Law, which basically says that electromagnetically induced current tends to flow in such a direction that it opposes the very cause producing it. Look it up on wiki or better yet you tube videos explaining it. It's actually pretty cool.

    Just my 2 cents.
    Last edited: Apr 1, 2013
  27. 5CHERO8
    Joined: Feb 22, 2013
    Posts: 71


    I would use the negative terminal for the following reason: anything metallic attached to the positive terminal increases the area that could be contacted by a grounded metal object, e.g., a wrench lying on the fender, exhaust, frame, etc. If the kill switch is COMPLETELY shielded, then it can go on either terminal.
  28. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,284


    Hahaha! :D

    All this bullshit about the flow of electrons blah blah blah!

    It has nothing to do with this, but more importantly how the electrics are wired. I would only run an isolator switch on the negative post if the vehicle has positive grounding.

    Most vehicles are negative ground so it is better to isolate the positive side. There are TWO sources of electricity in a vehicle that both use a [-] ground but only one can be isolated.
    The two sources are the Battery and the Charging system. I haven’t figured out how to isolate the ground on the charging system yet!

    In "Road Racing" the sanctioning body requires the isolator switch to cut the ignition supply for safety reasons.
    To stop the charging system looping back to the ignition we run the alternator wire back to the battery side of the isolator switch. Unfortunately this wire to the alternator is live while the ground strap is still connected , so to remedy this we add a Diode at the battery end of this alternator wire [so the battery can charge but not discharge through this wire ]

    The added side benefit is the battery will still hold charge even with a “buggered” regulator.

    Ten years ago I owned a Lotus Cortina that was involved in a serious accident on the track. The car was nearly cut in half at the rear quarter.
    We had the battery in the trunk and the isolator switch on the front fender [ the rules mandated this ] ,The main power feed to the isolator switch was severed and dead shorted against the body shell.
    It got quite exciting for a while

    Now my present race car ALSO with the battery in the trunk so I have gotten a little more creative.

    I bridged the wire from the starter motor to the solenoid [so it is going all the time] then I mounted a Ford style solenoid at the battery that is triggered by the starter switch.
    This main power feed is dead all the time except when I start the engine.
    Also from the battery I ran the ignition/ accessory main feed up to the mandated isolator switch in the front fender [ there is a fusible link at the battery also ]and every thing was switched from this wire.
    The alternator wire went back to the battery side of the isolator switch via a diode

    Apart from running a starter switch wire back to the rear solenoid it is quite simple to do and a lot safer.
  29. child9
    Joined: Mar 25, 2013
    Posts: 8

    from Austin

    Lulz. Science is bullshit. Well said. Still killing cats to make it rain. Wait a sec...
  30. Don's Hot Rods
    Joined: Oct 7, 2005
    Posts: 8,319

    Don's Hot Rods
    from florida

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