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Put wire inside car battery cell to measure voltage?

Discussion in 'Electrical Engineering' started by JS, Nov 25, 2005.

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  1. JS

    JS Guest

    I recently read the following extract below which explains how to
    test a car battery by putting stiff wires right into the cells and
    measuring the voltage.

    I'm attracted by this method because I do not have a hydrometer to
    test the individual cells. Could someone answer these questions
    about this method.

    (1) Is the method described below a sensible approach?

    (2) Is the method a good indicator of the cell's health?

    (3) Is he advocating pushing the coat hanger wire right down
    between the plates of a cell or only pressing down on the top of
    the plates?

    (4) Does the method damage the cells either physically or


    -------------------- QUOTE ----------------------

    The Charging System. Part 1: The Battery

    The individual cells can also be tested with a voltmeter.

    Take a coat hanger and make two lead extensions about six inches
    long and attach them to the meters test leads.

    Touch the positive lead to the positive terminal and stick the
    negative lead inside the cell next to it.

    It should read about 2.1 to 2.3 volts.

    Now insert the positive lead in the first cell and the negative
    lead in the second cell.

    Proceed down the line until you get to the last cell. Here you
    will put the positive lead in the last cell and the negative lead
    on the negative terminal.

    All the cells should read the same, or within 0.2 volts.

    If one reads 4.0 or more, you have a shorted cell and the battery
    is no good.

    If you get a very low reading or a zero reading, the cell is open
    and again the battery is no good.

    -------------------- END QUOTE ----------------------
  2. Conor

    Conor Guest

    Nope. Dangerous as a spark can cause the battery to explode.
    Nope. You can only really get a good idea by putting a large load on
    the battery.
    Why not just take it to your local motorist shop who have the equipment
    and will gladly test it for free?
  3. SQLit

    SQLit Guest

    Static voltage is not a good indicator of the condition of the battery.
    Voltage under a measured load is better.

    As others have said. Putting wire in the cells is extremely BAD idea.
    I have seen a battery grenade, fortunately we were not close and all we had
    to do was hose the engine down so the acid did not eat everything.

    Get a hygrometer or load tester. Or visit the local auto parts store with
  4. Malc

    Malc Guest

    Just don't do it. OK. If you have to do a load test, turn your headlights on
    and use a DVM. As others have said, an off load test doesn't show very much
    at all.


    "The error of youth is to believe that intelligence is a substitute for
    experience, and the error of age is to believe that experience is a
    substitute for intelligence."
    - Slyman Bryson
  5. Stuart Gray

    Stuart Gray Guest

    Fuckin' hell, its cheaper just to buy a new battery, which means you won't
    have a pock marked face from the explosion from the old one.
  6. RT

    RT Guest

    <Original post snipped for safely reasons>

    "Dead man walking"
  7. The general consensus is that this is a bad idea. In addition to the
    problems mentioned, this is likely to damage the cell itself and shorten its
    life. You would be infinitely better off just measuring the voltage between
    the poles. Batteries which are less than about 12 volts are pretty much
    dead or on the way out. Even better is the suggestion of going to a battery
    shop and having them test it. If you think it is in need of a test or dead,
    it probably is ready for replacement anyway.

  8. Guy King

    Guy King Guest

    I do if he's doing it anywhere near me!
  9. Actually, we need a phrase for these sorts of situations:

    "Dead man posting"
  10. TimPerry

    TimPerry Guest

    from the same article:

    "A little bit about working with batteries. Whenever you disconnect a
    battery, ALWAYS disconnect the negative cable first. This will prevent
    sparks that may cause the gasses inside the battery from igniting."

    i consider lifting the neg first a good idea because afterward and
    accidental short from pos to almost any part of the cars body will have no
    adverse affect. this MAY be what the author was aiming to say but taken
    literally the statement is silly.
  11. Guy King

    Guy King Guest

    The message <>
    A lot of things that a lot of people who should know better say is silly.
  12. Guy King

    Guy King Guest

    You seen those teeshirts with "I am a bomb-disposal technician. If you
    see me running, try to keep up" written on the back?
  13. [Shudder]

    On batteries where this is possible you use cadmium probes.
  14. Chris Street

    Chris Street Guest

    I've spent a lot of time playing with batteries in a chemistry lab and this
    is about the best way to do yourself and the battery damage. Even with a
    "safe" CD or Pb probe you can still provoke a spark unless you know exactly
    what you are doing, and you are dipping into a hydrogen/oxygen gas pocket
    which is about the most explosive combination going.

    Try this

    1. Charge the battery fully and let it stand for at least 8 hours with no
    load (disconenct it)

    2. Get an accurate digital voltmeter and measure the off load
    (disconnected) battery voltage. If it's not over 12.4V at 68F (adjust down
    by 7mV for each degF) scrap it.

    3. Disconnect or isolate the sparks and crank the engine for 30 secs none

    4. Measure voltage again - if it fails to bounce back up to 12.2V within a
    minute, scrap it. (it's either screwed or too small for the car)

    Or just take it to the shop and get them to use the heavey discharge
  15. R. Murphy

    R. Murphy Guest

    If you want to measure the voltage, use a voltmeter on the regular
    terminals. If not up to par, then it needs charging. If it won't charge up,
    then it doesn't matter which cell is faulty, since you can't replace all of

    If you do want to check the electrolyte level/strength, buy a hydrometer.

    Or alternatively, stop winding us up! :)
  16. We were somewhere around Barstow, on the edge of the desert, when the
    <snip bollocks>

    The bit you quote is misleading and dangerous advice. I get the
    impression that guy's website is probably a collection of bad practices
    he's been doing for years and got away with.

    Regarding his battery testing advice...

    1. Car batteries are pretty much all in one case, so there's no point in
    identifying a dead cell. If the battery is a whole isn't up to snuff,
    whether it's caused by a dead cell or not, the battery's fit for

    2. The only type of battery where you'd need to do individual cell
    testing, is likely to have individual cells (fork truck or other
    heavies), or at the very least have cell interconnectors on top, where
    you can safely test cell voltages without risking your good looks by
    dipping probes into the electrolyte.

    Honestly, I've read some shit on the 'net, but that takes the individual
    baked sugary cake-like thing.
  17. John G

    John G Guest

    Whatever you might have done it does NOT WORK ALLRIGHT.

    The spark you make ignites the hydrogen and could easily kill you.

    As they say on TV "Do NOT try this at home".
  18. Colin Stamp

    Colin Stamp Guest

    I beg to differ. If generating a large volume of, probably toxic,
    smoke in a very short time is what you're after, then it works a
    Obviously. That's why I made sure I was a kid when I did it, so that
    personal safety wouldn't be a priority.
    Spotting tounge-in-cheek newsgroup posts isn't your strong point is


  19. As a young kid at school I decided that a bit of wire in the live and
    neutral points in a socket would be a good idea because the current would
    just flow along it. I even managed to convice a couple of mates, and tried
    it to prove the point.

    The resulting flash and bang scared the shit out of me, along with the rest
    of the *German* language class lol...
  20. PC Paul

    PC Paul Guest

    As apprentices we pratted about and wrapped solder around the live and
    neutral pins of a plug then plugged it back into a switched off socket.

    *Huge* flash when it was switched on. The solder vaporised almost instantly
    but the shock value was amazing.

    Would never do that sort of thing now though.

    A good way to demonstrate the high instantaneous current capacity of Nicads
    is to get an AA or even AAA Nicad and short it out with solder - again it
    vaporises immediately, but a bit less spectacularly.
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