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wet-cell battery gravity test

Discussion in 'Electronic Basics' started by Mike, Dec 17, 2004.

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  1. Mike

    Mike Guest

    A while back, I had a brand new battery that sat for a long time, eventually
    loosing its charge. It was slow-charged for several hours until the battery
    voltage read ~ 12/12.5 volts. I was actually having problems with the
    battery handling a load; cranking for 4-5 seconds revealed a slow-down on
    the starter. I proceeded to do a gravity test, which the battery failed. I
    brought the battery into Sears where they put the battery on some
    cutting-edge technology machine that didn't use gravity testing. I was
    actually arguing with the manager because he was telling me that gravity
    tests are very inaccurate. How can this be? Battery acid has greater gravity
    than plain water, which makes up most of the fluid in a dead battery, thus
    causing the "balls" to drop in the tube. This was a while ago, but the
    thought just popped in my head...

    Thanks
     
  2. Dbowey

    Dbowey Guest

    Mike posted:

    << A while back, I had a brand new battery that sat for a long time, eventually
    loosing its charge. It was slow-charged for several hours until the battery
    voltage read ~ 12/12.5 volts. I was actually having problems with the
    battery handling a load; cranking for 4-5 seconds revealed a slow-down on
    the starter. I proceeded to do a gravity test, which the battery failed. I
    brought the battery into Sears where they put the battery on some
    cutting-edge technology machine that didn't use gravity testing. I was
    actually arguing with the manager because he was telling me that gravity
    tests are very inaccurate. How can this be? Battery acid has greater gravity
    than plain water, which makes up most of the fluid in a dead battery, thus
    causing the "balls" to drop in the tube. This was a while ago, but the
    thought just popped in my head... >>

    I'm not certain what thought just popped into your head, but specific
    gravityreadings are a good indication of lead/acid cell condition.

    When you measured 12.5V you should have kept charging the battery. When a
    battery is badly discharged you will probably need to charge it at about 13.2
    Volts; that's 2.2 Volts per cell. During the heavy charge it is a good idea to
    monitor it's temperature, voltage, and cell specific gravity periodically.

    If your charger cannot reach 13.2 Volts you will need to charge it for more
    time. Or have a garage do a quick charge on it.

    Don
     
  3. Mike

    Mike Guest

    Hi Don,

    "~" meant approximate, so I don't remember exactly what it read. Could have
    been well over 13 volts, but the question in my post was whether that Sears
    manager was BS'in me.

    Thanks for your feedback.
    Mike
     
  4. Hi

    My recollection of the battery section that I once took and my work as a
    industrial controls mechanic, is that as a battery discharges its internal
    resistance increases. As the internal resistance increases the voltage will
    drop across a constant load.

    Further, allowing a battery to sit with the acid in it and not charged is
    one of the worst things that can happen to a battery aside from freezing.

    When a battery has been sitting for extended periods, the material on the
    plates turns into more or less insoluble lead sulfate crystals. This
    reduces the surface area in the active battery approximating a permanent
    discharged condition.

    There is a process that allows such batteries to sometimes be saved, but it
    has some dangers and you are well advised to get in the net and take a look
    at some of the interesting information that battery manufactures post.

    Al.
     
  5. Rich Grise

    Rich Grise Guest

    The manager was wrong about the "gravity test" being inaccurate. He might
    have been arguing with the term "gravity test" as opposed to "specific
    gravity test". But he has to justify the use of his expensive machinery.
    Other than that, you seem to be doing fine, except that there are people
    who will complain about your top-posting. :)

    Good Luck!
    Rich
     
  6. Mike

    Mike Guest

    Darn... have to remember about top posting, but then again you're the only
    complainer, yet ;)

    Thanks.
    Mike
     
  7. Mike

    Mike Guest

    Yeh, I've actually heard of that procedure. It's more or less "blasting" the
    battery with a load to dislodge the crystals....

    Thanks!
    Mike
     
  8. Gary Schafer

    Gary Schafer Guest

    The manager was not entirely wrong. Specific gravity tells you if the
    battery is charged and what percent of charge it is at. But the
    battery can have a high internal resistance as another poster said.
    What the guy at the store probably was talking about was a "load
    test". It is commonly done in the battery service shops. They put a
    load of so many amps and measure how much the voltage drops and how
    quickly. That is the real test to see if the battery is any good.

    By the way, when charging it needs to be charged at around 14 volts to
    14.6 volts. 13 volts will never charge it up fully. 13.6 volts is a
    good "float" voltage to maintain a battery at for long periods.

    Desulphating a battery to recover it requires a charge voltage of
    around 16 volts or so. The temperature must be monitored closely.
    Sometimes it helps but if the battery has been sitting for a long
    period with no charge it is probably history.

    If a battery is discharged to "flat" you must get it charged up as
    soon as possible. Letting it sit for more than a day in the discharged
    condition will pretty much ruin it.

    Regards
    Gary
     
  9. Rich Grise

    Rich Grise Guest

    Thanks for this. I knew it, but was negligent in not bothering to remember
    it before posting my response.

    Cheers!
    Rich
     
  10. Mike

    Mike Guest

    I performed a load test on the battery myself with a load tester, but I
    can't remember the exact results.

    Thanks for your words of wisdom.
    Mike
     
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