On Mon, 30 Oct 1995, Denise Thorpe wrote:
> (You can't imagine how many jokes I've heard based on the words: big, red,
> and box.)
No comment.
> One thing that was said in all this winter storage debate that's a hit
> and not a myth is that batteries shouldn't be set on concrete. Concrete
> is a heat sink; it continuously removes heat from the battery thereby
> driving the chemical reaction that discharges it. This is helped along
> by the battery's natural tendency to want to return to an equilibrium
> state, which is half-charged. Which explains why a completely discharged
> battery will recharge itself slightly when left sitting. Any discussion?
>
> As Popeye muttered, "I ain't no physikist, but I know what matter's."
Well, Denise. I respect the heck out of your apparent mechanical
expertise. But the best I can say about your battery statements is that
following your advice won't hurt.
I've heard that the concrete business comes from days of hard rubber
battery cases, and doesn't apply to batteries with plastic cases. Dunno
why, I think it was that the rubber was slightly permeable to salts in
the concrete.
If you sit a warm battery on cold concrete, then yes, the concrete will
serve as a heat sink until the battery temperature equals the concrete
temperature. But I know of no reason why this should discharge the
battery. It will reduce the capacity, which is decreased by cold because
cold slows the chemical reaction that produces current, but that would be
true whether the battery was on wood, concrete, or suspended in air. I
can't think of any reason why withdrawing heat from the battery should
drive a reaction that discharges it. If anything, cold preserves the
state of charge. I keep spare drycell batteries in the refrigerator
because the cold slows their discharge.
Finally, a wet cell battery's equilibrium state isn't half charged. If
that were the case, you could not discharge a battery by running the
starter. Instead, to get the battery below half charged, you would have
to add energy to it. If you run a battery completely down quickly, by
drawing a large current from it with the starter, you can sometimes get a
grunt or two from it by waiting a bit. It does rebound very slightly,
perhaps because the chemicals in it redistribute to allow a small amount
of new discharge. Once you really and truly run the battery down,
however, it is dead and stays that way. Leave your parking lights on for
a day, to discharge the battery slowly, and you will find there is no
rebound no matter how long you wait.
Now you're going to hate me.
Ray
P.S. Science aside, do I store batteries on concrete? No, of course
not. Why tempt the gods?
Ray Gibbons Dept. of Molecular Physiology & Biophysics
Univ. of Vermont College of Medicine, Burlington, VT
gibbons@northpole.med.uvm.edu (802) 656-8910
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