From: elmore@virga.ucar.edu (Kim Elmore)
Subject: Re: batteries on concrete
Organization: Research Applications Program/NCAR, Boulder, CO
Date: Wed, 9 Sep 1992 16:58:01 GMT

In article <6+sn+=n@dixie.com> jgd@dixie.com (John De Armond) writes:

>The issue is not concrete per se.  The issue is placing the battery in a 
>cold, damp environment, such as the envrionment that exists right over
>most concrete floors in basements and so on.  This cold, damp enviroment
>will supply moisture to the hydroscopic deposits that frequently
>collect on the outside of a battery.  THIS process will discharge the 
>battery.
>
>Discovering this is as simple as placing a well used battery in such an
>environment and probing around the case with a DVM.  The voltage you
>observe is the tramp current finding its way across the case.
>
>When a battery suffering this kind of discharge is picked up off the floor,
>it is likely to be placed in a warmer and drier location.  This will obviously
>stop the discharge.  Since most people are not engineers and since there 
>is seldom asphalt in basements, the concrete gets the blame.  Regardless
>of the cause, the "cure" works.

	I considered this when dueling with HR over concrete and
batteries and don't buy it as sufficient to discharge the battery.  If
this were the case, batteries wouldn't last long at all on John's neck
of the woods because there is generally so much moisture in the air.
Consider that dewpoints generally run in the 70's during summer
yielding mixing ratios of about 20 g/kg and vapor pressures of about
31.5 mb.  Thus, there is far more water available for hygroscopic
nuclei in the realtively moist environment there.  Things are usually
so hazy because hygroscopic aerosols are "activated" by the high
humidity and grow to be effective scatterers (usually scattering in
the Mie regime, if anyone cares).

	H2SO4 is *extremely* hygroscopic as are most sulfate salts.
Expose a quantity of CuSO4 or PbSO4 to the air, especially in the
humid South, and watch it turn to jelly.  If reagent 18M sulfuric acid
is exposed to air, it's volume will slowly increase while its molarity
will decrease because water is being absorbed from the air.  Any salt
solution is indeed a good conductor, but I submit that the discharge
rate is insufficient to kill the battery much faster than the
self-discharge rate.  While we can, no doubt, measure voltages across
the surface of the battery, the fact that they can be measured
indicates a relatively high impedance and so a rather low discharge
rate.  If the impedance were, say, 1000 ohms, we would see a 12 mA
discharge rate.  Since batteries are 60 to 100 AH affairs, it would
require 5000 hours (125 days) to completely discharge a 60 AH battery
and 8333 hours (347 days) to discharge a 100 AH battery. This is
probably around the self discharge rate of automotive-formulated
batteries.  If we use a 100 ohms, then we begin to get discharge rates
that become suspicious: 12.5 days to kill a 60 AH battery and 15 days
to kill a 100 AH battery.  I submit that we just don't see this in
reality because if we did, aircraft batteries (formulated more like
marine batteries) wouldn't last long enough between uses to be of any
benefit.  I choose aircraft batteries because they typically have
capacities of 25 to 35 AH and are typically lightly coated with acid
because they reside in small compartments vented to the outside.  Even
though they are vented, the vapor produced by charging is deposited on
the battery and the compartment; just try handling one with a tiny cut
on a finger and you'll soon come to respect this fact.  Besides, if
the path were low impedance, any water would rapidly boil away and the
impedance would rise again.

	Face it, friends: the battery-killing concrete secnario is a
myth even more tenacious than the Craig Shergold story!


					Kim Elmore
					N5OP, PP ASEL/Glider 2232456
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