 
The lead-acid secondary cell releases energy (electricity) with the following
chemical reaction:
 
Pb + PbO2 + 2H2SO4 --> 2PbSO4 + 2H20
 
Lead and Lead (IV) Oxide and Sulfuric Acid produce Lead Sulfate and Water
 
The standard heats of formation at 25 C (77 F) of the various compounds are:
 
              Heat of formation
Compound      at 25 C, kcal/mole
--------      ------------------
   Pb               0.00
  PbO2            -65.0
 H2SO4           -193.69
 PbSO4           -218.5
  H20             -68.3174
 
The heat of reaction at 25 C is therefore -60.6 kcal per mole PbSO4 produced.
Note that lead sulfate is not very soluble (0.0048 grams per 100 grams water
at 25 C), and it will thus precipitate out of solution where the reaction is
occurring, or the cathode (positive terminal) of the battery.  (I am almost
sure it is the positive terminal where the precipitate forms, but I may be
wrong.  Oh well, I don't have a corroded battery to corrobate, and I don't feel
like thinking through it right now.)
 
What is important to notice here is that the reaction, as you knew it would be,
is exothermic, or energy discharging.  If moisture with dissolved electrolytes
(acid rain) condenses on the battery, a conductive path between the terminals
may form.  This will discharge the battery as the chemical reaction proceeds.
The reaction is reversible, if electricity (from an alternator or battery
charger) is put back into the battery.
 
Lead-acid batteries must change in chemical composition to discharge.  Each
cell has all the reactants necessary for the reaction.  The non-reacting posts
(terminals) of the lead-acid battery are there to remove (or add) electric
energy during a discharge (or charge).  Because the reaction is exothermic, it
has a tendency to happen, though quite slowly under normal circumstances.  So
while keeping the battery dry is a good idea, it is not a total solution.
 
To understand why lead-acid batteries DO INDEED discharge faster when stored on
concrete as opposed to wood or earth (dirt), one should recall LeChatelier's
Principle, which can be paraphrased as:  anything subjected to some stress will
act to move to a more comfortable position.  Here are the thermal conductivities
of a some selected materials:
 
                                              thermal conductivity
       material           Temperature, C (F)  BTU/(hr-ft-ft)(F/ft)
       --------           ------------------  --------------------
diatomaceous earth,            204                   0.051
 natural, across strata        871                   0.077
diatomaceous, natural,         204                   0.081
 parallel to strata            871                   0.106
diatomaceous earth powder     (100)                  0.039
  (density, 18 lb/cu.ft)      (200)                  0.042
                              (300)                  0.044
                              (400)                  0.048
                              (500)                  0.051
concrete (cinder)              ---                   0.20
         (stone)               ---                   0.54
         (1:4 dry)             ---                   0.44
pine (across grain)             15                   0.087
   (parallel to grain)          21                   0.20
oak (across grain)              15                   0.12
maple (across grain)            50                   0.11
 
Note that the thermal conductivity for "dirt" (diatomaceous earth) goes down
with temperature.  Also, 204 C is about 400 K, and the values for earth powder
are on the same order of magnitude as natural earth.  The thermal conductivity
of concrete is around a factor of 10 greater than that of dirt, and is 2-4
times greater than wood.
 
It is these differences in thermal conductivity that cause the battery on
concrete to discharge faster than that on dirt or wood.  At any instant the
discharge reaction is occurring, energy is being released, either as electricity
or heat.  (Thermal conductivity of air is about 0.015 BTU/(hr-ft-ft)(F/ft).)
During storage, except for the trickle that passes through any condensate on
the battery, this energy is mostly released as heat.  The higher conductivity
surface will remove the heat proportionately faster than the lower conductivity
surface.  Thus, if you perfectly insulated a dry battery on all but the bottom
surface, at any given battery temperature above the storage surface temperature,
the battery on a concrete surface will be discharging about 10 times faster
than that on dirt.
 
This is where LeChatelier's principle comes into play.  Removing energy from
the exothermic reaction will drive the reaction further to completion.  If the
reaction normally occurs at room temperature, keeping the battery at that
temperature requires the removal of any heat produced.  A concrete surface is
a better heat sink than a dirt or wood surface.  Store a battery in the corner
of a poured concrete basement, and you have 3 surfaces removing energy, which
"pulls" the reaction along.
 
Also, if water evaporates from the battery, that elevates the ratio of sulfuric
acid to water.  This will tend to drive the discharge reaction.
 
Jeff White      jhwhit01@ulkyvx.louisville.edu