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Understanding DC current in automobile electrical systems

  1. May 9, 2005 #1
    Ok so yesterday, I had to jump start my chevy blazer because I left the dome light on and the battery went dead. once it started, I let it run at high rpms for 8-10 minutes to semi charge the battery before I left. I know it was charging, because my alternator guage was reading 16, and a fully charged system runs at 14. However I got 50-75 feet down the road and everything went dead and the car just shutoff driving 35mph down the road. So i went to walmart got a new battery and everythings good... but it got me thinking.

    Even if the battery was dead, the alternator should have been enough juice to keep the car running, I know this because I can remove the black terminal off of the battery (I didnt actually do this, but I know it works, because years ago it was routinely performed by mechanics as a test to verify if the alternator is bad or not. ) while the car is running and the car will stay running. This implies to me that I had some sort of problem with the red side of the battery or the connection to it. Which makes sence because the red connector was heavily corroded. So I must have lost ground contact which is why everything just shut off dead.

    So here are my questions...
    1 Which side of the battery is the ground? + Red or - Black
    The negative side of the battery has all the electrons waiting to flow to the positive side of the battery(where the electron holes are)... why would you ground the negative side of the battery?
    2 Am I therefore correct in believing that the frame of the car is not truely ground in this circuit its just serving as a gigantic piece of wire. Surely the e- are not flowing from + to - on a car battery when its exactly the opposite on every other battery known to man.
    3 Why when jumping starting a car, do you connect good red to weak red, and strong black to engine block? Is this to provide a common ground for both batteries for the current to flow properly?
    4 When connecting up a suped up stereo... once again, why do you connect it to only the red side of the battery and ground the black to the frame of the car?

    It seems that electrons are flowing from the negative side of the battery, into the engine block, through the spark plug, back through the wire into the distributer, into the spark coil and from there by wire back into the positive side of the battery...
    Can someone confirm this?

    I read this on an auto site... and I believe it is definately incorrect...
    I was always taught, electrons do not flow from the positive red side of the battery, they flow to it.
  2. jcsd
  3. May 9, 2005 #2


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    A completely-drained car battery is not just "empty," it's often physically damaged. A damaged battery will soak up large amounts of current (converting the energy into heat), but will not charge. It sounds like your deep-discharge physically damaged your battery.
    The black terminal is connected to the chassis, and the chassis is considered ground.

    It doesn't make any difference to (most) electronics which direction the electrons are flowing. Despite the obvious "backwardness," conventional current is defined with the negative (electron-rich) terminal to be ground, and the positive (electron-poor) terminal to be at a higher potential. As far as the electronics are considered, it doesn't matter at all.
    Yes, the entire chassis of the car can be considered a "source" of electrons, which can flow into the positive (red) terminal of the battery.
    In essense, you're just connecting red to red and black to black -- you are wiring the new battery in parallel with the existing one. The only reason you don't connect both jumper wires directly to the battery is because the final connection will generate sparks, and you don't want sparks near the battery. The general protocol is then:

    1) Connect red to red, battery to battery.
    2) Connect black on the jumper to an large metal part on the jumpee.
    Because the black (negative) terminal of the battery is connected directly to the chassis. Look inside your engine compartment, and you'll probably see the thick wire that does it. The chassis is electrically the same node as the black terminal of the battery, but it's closer -- so you use it instead.
    That's correct.
    That's correct. I don't see how the previous paragraph is inconsistent with this statement. It talks about current, but never mentions electrons. Electrons leave the negative terminal and return to the positive terminal. You can just as easily imagine holes leaving the positive terminal and returning to the negative terminal, though, so use whichever convention you prefer.

    - Warren
  4. May 9, 2005 #3


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    Your voltmeter should not have read high if your battery was actually charging heavily. A battery that is passing a large current from the charging system will most likely keep the voltage down.

    Don't EVER disconnect the battery while the engine is running. You are doing what is called a 'load dump'. The voltage from the alternator can spike up towards 100 volts or so for a split second. Not good for todays vehicles. You cannot count on the alternator keeping the vehicle running in the even of a poor battery connection. I've been there, it can definitely cause everything to quit.

    The corrosion was likely the main problem. Aside from running your battery down in the first place it was most likely not defective. As chroot pointed out, black (negative) is ground. Why do you say you have lost ground when it is the + side that was corroded?

    Not quite. The ignition coil is like a large step up transformer. The main current from the battery is not going through the spark plug.
  5. May 11, 2005 #4
    if that's the case then you could burn out the PCM (power control module) computer on newer vehicles. Disconnecting the negative terminal on cars also used to be a quick shade tree mechanic way to clear error codes if you didn't have an actual OBD reader but on some vehicles it's been known to mess up the electronic ignition timing to the point where they won't run.

    Quick question. THe way a cars negative side of the electrical system is set up, isn't it called a floating ground, as it's not actual earth ground :confused: Jus twant to make sure my terminology is right.

    sao123...if you still have the old battery try charging it with a battery charger instead of in the car. That way you won't be asking a fully drained battery to charge and be loaded down at the same time.
  6. May 11, 2005 #5


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    Yes, the chassis ground is not necessarily at the same potential as Earth ground. Of course, the selection of "ground" is arbitrary, and even Earth ground is not the same as, say, Martian ground or Sun ground. In that sense, all grounds are "floating" grounds, so the distinction "floating" is redundant. Of course, you can use the term "floating" if you really want to ensure that your reader distinguishes it as being possibly different from Earth ground, but most engineers already understand the distinction.

    - Warren
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