In article <1993Apr25.013013.13717@cronkite.ocis.temple.edu>, camter28@astro.ocis.temple.edu (Carter Ames) writes: >  >    I thought the first thread was hilarious, so here goes another post. >  > Some more background information on what has happened to my poor batteries. >  >   One year, I left the batteries in the garage.  The garage is Unheated.   > They were left in their places that they needed to be.  One in the riding > lawnmower (what a lux) and one in each motorcycle.  The battery in the  > riding lawnmower was about 3-4 years old, and one of the cycle batteries  > was new, and one was around 5 years old.  Upon spring time, I discovered > that all of the batteries were dead, and needed to be recharged.  They all > worked properly, except for the older ones, and they had to be replaced  > about halfway through the summer.  (they had originally been stored > in the garage, and I think I remember my dad charging them in spring) >   Last year, I decided to bring them in to the basement, which has two parts. > one is dirt and brick, and the other is concrete.  I brought in all three, and > discovered in the spring, (about 15 days ago) that not only were the batteries > dead, but when I put the charger on them, the charger said "HMMMMMMMM" and  > the amp-meter read around 1,000,000,000  (In other words, it was pinned) > the internal circut tripped, and I went on to the other batteries.  They > were the same.   >    Realizing that we still had 2 GIGANTIC sailboat batteries, also lead > acid, I decided to put the charger on them.  (these had been sitting for around 1 and a half years, on the brick and dirt part) and noticed that they only > needed a small charge, around 2 hours or so on trickle.  I tested them > by using them at the local gas station for giving jump starts, and they > worked fine for around 25 of 'em.  >   This leads me to believe one of several things. >      1) I bought really #$%tty batteries last year. (sorta true) >      2) the concrete has something to do with the discharge of the >         batteries. >      3) There was enough moisture to short out the terminals in the  >           room where the concrete is,  >      4) The dirt room was able to absorb the moisture in the air >          better than concrete. >  > (BTW, I can almost waterski in the dirt room)   >  >    Any other ideas? >     *could be the aliens, anything is possible.* >     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