5.3 Design (Part 1)
General Design Practices
Table 5-4 shows the factors which differentiate
the types of lead-acid batteries. Reliable lead-acid cell designs depend upon
the attention given to the design, the choice of materials used in the cell, and the care with
which the materials are processed.
TABLE 5-4. TYPES OF LEAD-ACID
II. Type of
IV. Cell Geometry
VI. State-of Charge as Shipped
1. Flooded, vented
b. Gelled Electrolyte
1. Pure Lead
2. Lead-Calcium (Pb-Ca)*
4. Hybrid (Pb-Ca negative
2. Prismatic, flat plate
1. Standby (float)
2. Engine starting
3. Cycle service
1. Wet and fully
2. Dry charged
3. Formed and damp-dry
4. Unformed and dry
|* These may
have additives such as tin, cadmium, slenium, or
Electrodes (or Plates)
The electrodes are composed of a structural member (the grid) and a lead
oxide which is converted to active material electrochemically.
The size of the positive electrode is determined by the capacity required.
The negative electrode has the same area, but its thickness is determined by
the amount of active material needed to balance electrochemically the amount
in the positive.
The grid is the support structure used in the electrodes. There are
definite design requirements to be considered:
Withstand all handling operations during production
- casting, trimming, pasting, and assembly
- Withstand transportation and operating shock and
- Contribute to the electrical performance
- Withstand corrosion to the extent that service life will be obtained
(Positive grid only).
The negative grid does not corrode and, therefore, is not required to have
members as heavy as the positive. It can have the minimum weight needed to
hold the required amount of active material.
Lead alloy is used to manufacture positive and
negative grids. Most stationary and valve-regulated batteries use either
lead-calcium (Pb-Ca) or a lead-antimony (Pb-Sb) alloy in their positive and
negative grids (See Table 5-5). Pure
lead is too soft for use as grids. Alloying ingredients improve the mechanical
strength of lead, increase its resistance to creep, and improve its
castability. The alloy is less corrosion resistant than pure lead; hence, it
requires additives that modify its grain structure to increase its corrosion
TABLE 5-5. POSITIVE GRID ALLOYS
|SLI and Aircraft
Good high Rate Performance
Good Recovery from Deep Cycle|
Good Cycle Life|
Good High Rate Performance
||Choice for "High Cycle" UPS|
|(1) These may have additives such as tin, cadmium, slenium, or others|
Traction or deep cycling batteries and some stationary batteries are
manufactured with Pb-Sb alloy grids with an antimony content between 2 and 11
SLI batteries, especially maintenance free batteries, require special grid
materials. Lead-calcium alloys and lead alloys with a low antimony content are
favored for this application.
Antimony improves castability, mechanical properties, and paste adherence,
but suffers from the drawback of reducing the grid's electrical conductivity.
Self-discharge and battery heat occur when antimony content is high. Antimony
can be completely eliminated from the battery by hardening lead with a small
percentage of calcium. Long shelf life associated with low water loss, and
good cold cranking characteristics are possible with these non-antimony
Addition of tin assists in casting. Tin-calcium grids reduce the tendency
of lead-calcium positive plates to "passivate" or resist recharge when deeply
discharged. A small amount of aluminium in this alloy causes the formation of
a protective skin over the surface of the molten lead pot. This reduces the
tendency of the calcium to dross and makes casting from that alloy more
uniform in composition and free of dross inclusions. Dross inclusions become
the focal points for corrosion.
Systems that use Pb-Ca positives usually also use the same alloys in the
negatives. The only usage of Pb-Sb alloys in both positive and negative grids
is in motive power cells. A small percentage of SLI batteries having Pb-Sb
alloy in the positive grid still use Pb-Sb in the negative grid.
There are two types of electrode construction:
flat-pasted and tubular. All negative plates are the flat-pasted type.
Positive plates can be either flat- pasted or tubular. "Planté" is another
type, but it is rarely used due to its high cost and low energy density.
The active material comes from the lead oxide
which is originally applied to the grid as a paste. This paste is later
electroformed into the active materials, lead dioxide (PbO2) in the positive
and sponge lead (Pb) in the negative. Most materials used in the paste of both
plates are the same - lead oxide (PbO), sulfuric acid, and water. Differences
result from the amount of acid and water used in the formulations and any
additives used, primarily in the negative mix. Some additional variation
occurs when the manufacturer uses oxides which have a more highly oxidized
To enhance mechanical strength, fiber
additives are commonly included in positive paste formulations for flat pasted
plates. Additives in the negative paste mix are included to keep the material
from densifying during the life of the cell. Material densification would
degrade electrical performance, both on charge and discharge. These additives
are called expanders.
Many of the heavy duty battery positive plates are made in porous tubular
sheaths. The grid is cast or injection-molded with long-finned lead alloy
spines attached to a header bar and a connection lug. The spines are inserted
in individual woven porous fiberglass sheaths assembled in a multitube
gauntlet. The tubes are filled with dry lead-oxide powder or with a slurry
paste. A plastic cap plugs the open sheath ends and becomes the bottom of the
The amount and density of the active material pasted on
the grid or filling the tubes determine the capacity and life of the positive
plate. In general, the higher the active material density, the lower the
diffusion rate of the electrolyte. Low electrolyte diffusion results in
capacity reduction. The lower the material density, the more susceptible the
plate is to shedding and washing the active material during cycling. To reduce
active material loss rate and to provide adequate life, the plates are usually
provided with some form of active material retention system (Table
TABLE 5-6. POSITIVE PLATE TYPES AND
ACTIVE MATERIAL RETENTION SYSTEMS
Good Mat on Separator
Fibers in Paste|
Glass Wrap Plus
Telephone - Glass Mat
Separators serve the purpose of preventing the plates from touching and
electrically shorting. They are usually made from a microporous rubber
material or a microporous polyethylene material. Other materials, such as
sintered polyvinyl chloride (PVC) or resin impregnated substrates, are also
used. The absorbed electrolyte type of valve-regulated stationary cell uses a
highly porous and absorbent glass mat and perforated plastic sheet as a
The electrical resistance of the separators is critical
to applications which require high discharge rates (such as UPS and SLI) and
is one of the properties for which a specified maximum is given. Other
important separator properties controlled by specification are:
- Burst strength
- Dimensional stability
- Oxidation resistance - critical for deep cycle,
long life cells
- Volume porosity
- Metallic impurities.
Other separator physical conditions which should be
controlled in order to avoid failures are:
Broken or chipped corners
Chipped, feathered, or wavy edges
Deformed, notched, or voided ribs
Pin-holes, cracks, or splits.
Sulfuric acid (H2SO4) solution is the electrolyte of
the lead-acid cell. This solution takes an active part in the chemical
reactions that produce energy from the cell. By changing the density or
specific gravity of the acid solution, performance factors such as capacity,
maintenance, and life can be affected. The relationship between acid specific
gravity and battery capacity, maintenance, and life are shown in Table
TABLE 5-7. ACID ELECTROLYTE EFFECT ON
In most cell designs, the acid is available as a free liquid
in which the plates are immersed. In valve-regulated cells, the acid is
immobilized and absorbed completely in the separator structure or converted to
a thixotropic gel by use of an additive.
Containers and Covers
The container design must bear the internal pressure
exerted by the elements, the electrolyte, and cell gases during service use or
in factory processing without distortion beyond specified dimensions. Wall
thickness can vary depending on the material selected for the container (Table
5-8). Polypropylene is selected for all types because of its excellent impact
properties, ease of heat sealing, and low cost.
TABLE 5-8. CONTAINER
SLI and Aircraft
Polypropylene (PP), High Impact Rubber,
Polypropylene (PP), High Impact Rubber,
Polypropylene (PP) Polysyrene (PS),
Polycarbonate (PC), Polyvinylchloride (PVC)
(SAN), Acrylonotrile Butadiene Styrene
In stationary cells,
most designs use SAN. Where flame retardancy is a requirement, PC, PVC, ABS,
or modified polypropylene are employed.
The specific cover design will depend on the seal designs used, both to the
jar and to the post. Material selected must be compatible with the material of
the container so pressure tight seals can be made.
Since operation of a lead-acid cell generates gases (hydrogen and oxygen),
these must be vented. Venting avoids pressure buildup which could result (at
very high pressures) in cracking of the container and spillage of electrolyte.
Even while standing on open-circuit, gases are released due to local
Venting is usually achieved through a baffled plug fitted into the cover.
In some designs, this plug is replaced by a flame arrester vent which does not
allow a flame to penetrate to the inside of the cell where there could be an
explosive concentration of hydrogen. Vents may be individually placed in each
cell or grouped into one piece for easy installation and removal.
Most cells are constantly vented to the atmosphere. However,
valve-regulated cell designs incorporate a pressure relief valve. These do not
vent unless internal pressure has exceeded the design pressure of the valve.
When the pressure bleeds, the valve reseals or closes.
Container/cover seals must withstand the particular application environment
(vibration, temperature extremes, or pressure) and remain intact to contain
the acid and gases within the cell. SLI, aircraft, and motive power cells
predominantly use heat sealing of container and cover materials. Some designs
still use adhesives or asphalt to effect a seal.
Container/cover seals vary widely. Joint designs
commonly used in these seals are:
Tongue and groove
All of these seals can be achieved using the various
materials or processes shown in Table 5-9, if the container, cover, and
adhesive materials are all mutually and electrochemically compatible.
The seal between the post and cover is usually made by one of two methods.
In the first method, the terminal post is welded (burned) to a lead component
in the cover. This is the most frequently used method for SLI, aircraft, and
motive power cells. This component may be:
- A lead bushing molded into the cover
- A lead ring cemented into the cover
- A lead ring gasketed at the cover.
In the second method, one of several variations using
combinations of gaskets, O-rings, and seal nuts is employed. The seal can be
made above the cover using an O-ring compressed against the cover by a seal
nut. The seal can also be made below the cover by compressing a seal gasket
against the post. Surface finish of lead parts and applied compression (to
gaskets/O-rings) are critical characteristics in this