Newsgroups: sci.electronics
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From: vanderby@mprgate.mpr.ca (David Vanderbyl)
Subject: Re: Lead ACid Batteries Part 2!!!
Message-ID: <1993Apr26.221422.17208@mprgate.mpr.ca>
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Reply-To: vanderby@mprgate.mpr.ca (David Vanderbyl)
Organization: MPR Teltech Ltd.
References:  <1993Apr26.050848.1@ulkyvx.louisville.edu>
Date: Mon, 26 Apr 1993 22:14:22 GMT
Lines: 80

>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
[heats of formation deleted]
>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.)

The major problem with this is that the reaction takes place in an ACID solution.
PbSO4 is soluble in an acid solution and will not precipitate out.  Also, H2SO4
is in a water solution as 2H30+ and SO4--.  Thus the heats of formation of
PbSO4 and H2SO4 are for the most part irrelevant.

>What is important to notice here is that the reaction, as you knew it would be,
>is exothermic, or energy discharging.

As it turns out the reaction is indeed exothermic (heat producing).
(More about this later.)

What actually happens to make the battery completely useless is this:
(we're talking lead-acid batteries of course)
The battery slowly self discharges.  As this discharge takes place two things
happen. -The level of Pb++ ions in the acid solution increases (i.e. the lead
           and lead oxide plates are dissolved).
        -The level of H30+ ions in the acid solution decreases (i.e. the solution
           becomes less acidic, or more like water if you like).
Now, as the post to which I am responding correctly stated, PbSO4 will precipitate
in a WATER (non-acid) solution.  When the battery dies (i.e. is fully discharged)
we end up with a high concentration of Pb++ and SO4-- in water.  So PbSO4
forms in the solution and FALLS TO THE BOTTOM OF THE BATTERY (of course this
happens in varying degrees, the more discharged, the more precipitate forms).

The precipitate forms a conductive layer on the bottom of the battery.  If
there is enough of the lead and lead oxide plates left to touch the precipitate
(more common in a newer battery) a dead short results.

I have seen products in automotive shops to correct this condition, but they
are for the most part useless.  They can dissolve the PbSO4 but cannot restore
the lead and lead oxide plates properly.  You may have some success with
these products for a newer battery.

[stuff deleted]
>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:
[stuff deleted]
>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.

This stuff is just made up by the author and is completely invalid.
In fact the discharge reaction takes place at a higher rate at higher
temperatures.  A logical consequence of the above argument is this:
"If you really want your car to start, lower the battery temperature to
 -50 to 'pull' the reaction along."  We all know from experience (at
 least those of us in Canada do (it gets cold up here)) that this is
not true.  If we want to start our car on a really cold day we warm
the battery.

(Besides which, there is not enough energy released through self discharge
 to appreciably raise the temperature.  The air would amply dissipate any
 such heat, whether the bottom of the battery was insulated or not.  This
 is of course irrelevant, since you would WANT the battery to be cool
 during storage.)

Just keep the battery in a cool dry place and keep it charged!

