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Battery discharge when sitting on the floor

  1. Jun 20, 2010 #1
    This is an old saying that always annoys me! People swear by it but I just don't see how it can be unless the theory has been distorted over the years. Could it be that just the coldness of the floor weakens the battery? As it goes if you put a car battery on a cement floor it will discharge so if it has to sit on the garage floor you want to set it on a board. I ask where is the conductor that provides a short between the poles? An electrician once told me it was just absolutely true although he didn't know how. Sounds like a wives tale or superstition to me.
     
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  3. Jun 20, 2010 #2

    mheslep

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    Maybe the saying derives from the observation that batteries http://en.wikipedia.org/wiki/Self-discharge" [Broken] some types/chemistries faster than others, though they do so no matter where they rest without some kind of periodic recharge.
     
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  4. Jun 20, 2010 #3
    Batteries discharge in two ways. There is an internal discharge rate which varies with temperature and an external dischare that is due to conductive contaminates on the exterior of the battery. Placing a battery on a concrete floor provides a heat sink which tends to cool the battery. This slows the chemical processes with in the battery to a small degree depending on how cold the concrete is. The fact is that there is negligeable difference in the discharge rate of a battery placed on a concrete floor, a metal rack or an insulating surface.
     
  5. Jun 21, 2010 #4
    One theory on why lead acid batteries discharge when setting on concrete floor is that there is a temperature difference between the top of the battery and the bottom of the battery. This temperature difference causes a difference in voltage between the top of the battery and the bottom of the battery. This difference in voltage causes the battery to discharge.
    I don't know if this theory is correct or not, however a lot of people believe that lead acid batteries should not be left on concrete floors.
     
  6. Jun 21, 2010 #5

    mheslep

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    I don't know either but I agree that's a clever, plausible explanation.
     
  7. Jun 21, 2010 #6

    sophiecentaur

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    How does it 'know' it's on the floor?
    Perhaps it feels rejected and lacking in status?

    It could be that old batteries get taken out a the car and put on the garage floor, preparatory to being dumped.
     
  8. Jun 21, 2010 #7

    mheslep

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    The theory (proposed here) is the floor is a cold plate.
     
  9. Jun 21, 2010 #8

    sophiecentaur

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    The same effect should be seen when the top of the battery is warmer than the bottom - say when it is close to the hot bonnet (=hood) in the engine compartment or the side nearer the engine could also be hotter.
    How 'well known' is this effect?
     
  10. Jun 21, 2010 #9

    mheslep

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    Yep.
    The battery is nearly constantly charged when in a vehicle.
     
  11. Jun 21, 2010 #10
    Car batteries are relatively heavy, and may be leaking some acid. As a result the most likely place people would store them is on the floor of garages or basements. I honestly don't know anyone who would put one on the top shelf.

    And so there is a lack of control to compare against.
     
  12. Jun 21, 2010 #11

    turbo

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    My father would always place a battery on a piece of plywood or a plank instead of directly on the concrete floor of the garage. There may be nothing to it - just common misperception. For instance, why is a lead-acid battery removed from a car, truck, tractor, etc? Because of a weak cell, poor cold-weather starting, or some other problem? In such cases, it would be no surprise that the battery would go dead flat in storage. After all, even a weak battery can remain usable for quite a while when coupled with a robust charging system.

    My grandfather was a heavy-equipment mechanic and I think my father got the plank/plywood idea from him. If the effect is real, we should consider that in older non-sealed batteries (especially in rough use) acid would seep out and lightly coat the battery casing. In such a case, setting the battery on bare concrete might allow the leaching of electrolytes that could allow the discharge of the battery in a relatively short time.
     
  13. Jun 21, 2010 #12
    If anything a concrete floor should slow the discharge rate of a lead acid battery. Lower temperatures generally result in slower chemical kinetics and increased impedance.

    However, generating a temperature gradient may have an effect. In a lead acid battery you do have two dissimilar solid metals in direct contact with each other so the seeback effect may come into play.
     
  14. Jun 22, 2010 #13

    sophiecentaur

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    But not after you turn off the engine! LOL

    I reckon this is bordering on an urban myth.
     
  15. Jun 22, 2010 #14
  16. Jun 22, 2010 #15

    sophiecentaur

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    You mean the LEAKED all over the floor?
    And that is the 'accumulated' wisdom of Google.
     
  17. Jun 22, 2010 #16

    mheslep

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    In which case the posited under-the-bonnet temperature gradient vanishes in the steady state.
     
  18. Jun 22, 2010 #17

    sophiecentaur

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    It takes an hour or so for an engine to cool down and the not only is the temperature difference high but the actual temperature is also high - causing a speeding up of chemical reactions. But I suppose that effect would not be noticed in the comparatively short times involved, compared with months on a cold garage floor. But what would actually be the temperature difference over 20cm of a good thermal conductor like lead, whatever the situation?
     
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