From: REMOVE_THISdwilkins@means.net (Don Wilkins)
Newsgroups: rec.crafts.metalworking
Subject: Re: Lead Acid Batteries
Date: Sun, 24 Oct 1999 12:24:13 GMT

On 23 Oct 1999 17:40:43 -0400, dnichols@d-and-d.com (DoN. Nichols)
wrote:

>In article <RQmQ3.4516$pJ3.293470@news.flash.net>,
>H.G. <garciah@flash.net> wrote:
>>Brian:
>>
>>You could make them the same way that they were made in the early days of
>>electricity.
>>
>>Plates of PURE lead properly separated ( as close as possible), assign
>>polarity to the terminals then applied a constant current source with
>>voltage limiting of 2.37 Volts and let it run ( you may have to check the
>>H2SO4  for replenishing), get a battery electrolyte checker to determine
>>when the negative plate has reformed, check the current for minimum and
>>there you have a LEAD ACID battery.
>>I do not remember the electrolyte concentration level.
>
>	Well ... I remember a book that I read sometime around the mid
>1950s which was written either in the very early years of this century,
>or the late years of the previous century.  It was dealing with home
>experiments with electricity, and much of it was the construction and
>operation of various electrostatic generators.
>
>	However -- there was a section dealing with building your own
>lead-acid battery.  The case was wood -- heavily wax coated, of course.
>
>	The plates were made by taking strips of lead, and bending and
>soldering them into a grid.  The grid was then filled with a paste.  One
>I remember was "red lead", and I *think* the other was "white lead".of
>course, sulfuric acid was the electrolyte.

The grid usually is lead containing 7-12% antimony and small amounts
of tin, copper and arsenic. The Sb is added for structural strength.

The paste has been made from a variety of lead compounds but litharge
containing 20-30% metallic lead mixed with dilute sulfuric acid is a
generic one. Probably the "red lead" you refer to. Actually litharge
is usually yellow but can be reddish depending on prior treatment.

The oxide for the  negative plate contains about 1% of inert materials
called expanders. They are added to prevent deterioration of the plate
capacity and shrinking of the sponge lead.

After the plates are cured and assembled they are given their first
charge using a dilute sulfuric acid solution for the electrolyte.
Prior to the first charge there is no charge in the battery since the
chemical composition is essentially the same for both electrodes.
(Nitpickers will know better but true for all practical purposes.)

After the first charge the acid is replaced with stronger sulfuric
acid. Most lead acid batteries are made with acid between 1.250 and
1.275.

The initial charge oxidizes the lead oxide (litharge) on the positive
plate to PbO2 and reduces the lead oxide on the negative plate to lead
metal.

The chemical reaction during normal use is

Pb + PbO2  <-----> 2PbSO4  +  H2O  +2 electrons

Fortunately the reaction proceeds readily in either direction without
much heat. The chemistry usually is written as two half cell reactions
which makes it a little more clear just what is happening at each
electrode.

At the anode: PbO2  +  4H+   + 2 electrons---> PbSO4  +  2H2O

At the cathode Pb(metal)  ---> PbSO4  + 2 electrons

It is interesting that PbSO4 is the reaction product at both
electrodes and fortunate that the sulfuric acid electrolyte is already
saturated with PbSO4 so the products of the reaction remain on the
grids. (Equations above are not balanced)

As the battery discharges more water is produced which is why you find
the lower specific gravity in discharged batteries.

If you over charge you are forcing current through the battery and
there is no more Pb (actually the sulfate) left on the grids to be
oxidized and reduced. Whenever you force a current through a solution
there must be a reaction at both electrodes and since all of the stuff
on the plates is either oxidized or reduced something else happens and
it isn't good for the battery or perhaps for the person messing with
the battery.

The net reaction in the battery during overcharge is

2H2O + 4 electrons ----> 2H2  + O2

Note that the arrow points in one direction. The reaction is not
reversible so you would need to replace the water. Now if you happen
to provide a source of ignition (e.g. a spark) you have a mixture of
hydrogen and oxygen which can blow up the battery.

Those deposits which some have mentioned that fall to the bottom of
the battery and are sluiced out to rejuvenate the battery are PbSO4
and some of the inert ingredients in the grids. That PbSO4 is no
longer there to be returned to its original state on the grids and
therefore you have a battery which has less capacity. How much less
will  show up when you need the cranking power.