Environmental Engineering Reference
In-Depth Information
conditions (temperature, humidity, and time) affect the development of
crystallinity and pore structure. The cured plate consists of lead sulfate,
lead oxide, and some residual lead (<5%). The positive active material
formed electrochemically from the cured plate is a major factor influenc-
ing the performance and life of a lead-acid battery. In general, the negative
or lead electrode controls cold-temperature performance (such as engine
starting).
Pure lead is generally too soft to serve as a grid material and was hardened
traditionally by the addition of antimony metal. The amount of antimony
varied between 5 and 12% by weight, generally dependent on the avail-
ability and cost of antimony. Typical modern alloys, especially for deep-
cycling applications, contain 4 to 6% antimony. The trend in grid alloys
is to use even lower antimony content, in the range of 1.5 to 2%, to reduce
the required maintenance (water addition) of the battery. As the antimony
content goes below 4%, the addition of small amounts of other elements is
necessary to prevent grid fabrication defects and grid brittleness. These
elements, such as sulfur, copper, arsenic, selenium, tellurium, and various
combinations of these elements act as grain refiners to decrease the lead
grain size.
Lead oxide is converted to a plastic dough-like material so that it can be
affixed to the grids. Lead oxide is combined with water and sulfuric acid
in a mechanical mixer. Sulfuric acid acts as a bulking agent—the more acid
used, the lower the plate density will be. The total amount of liquid and the
type of mixer used will affect final paste consistency (viscosity). The pro-
cess is a form of extrusion by which the paste is integrated with the grid to
produce a battery plate; it is known as pasting. The paste is pressed by hand
trowel or by machine into the grid interstices. A curing process is used to
form the paste into a cohesive, porous mass and to help produce a bond
between the paste and the grid. The simplest cell consists of one negative
electrode, one positive electrode, and one separator between them. Most
practical cells contain 3 to 30 plates with the required number of separa-
tors. Individual or leaf separators are generally used. The use of ''envelope''
separators surrounding the positive or negative plate or both is becoming
more popular in small, sealed cells, motive power, and standby batteries to
facilitate production and control lead contamination during manufacture.
Separators are used to electrically insulate each plate from its nearest coun-
ter-electrode neighbors but must be porous enough to allow acid transport
into or from the plates. As a cell discharges, both electrodes are converted
to lead sulfate. The process reverses on charge. The half cell reactions in
discharge are as follows:
Positive:
PbO
2
+ 3H
+
+ HSO
4
-
+ 2e
-
→ PbSO
4
+ 2H
2
O
Negative:
Pb + HSO
4
-
→ PbSO
4
+ H
+
+ 2e
-
