Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-05-04
2002-07-23
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S225000, C029S002000
Reexamination Certificate
active
06423451
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to lead-acid cells, and, more particularly, to calcium-tin-silver lead-based alloys used for the positive grid alloys in such cells.
BACKGROUND OF THE INVENTION
Sealed lead-acid cells (often termed “VRLA” cells, viz., valve-regulated lead-acid) are widely used in commerce today. As is known, sealed lead-acid cells utilize highly absorbent separators, and the necessary electrolyte is absorbed in the separators and plates. Accordingly, such cells may be used in any attitude without electrolyte spillage as would occur with a flooded electrolyte lead-acid battery. Such cells are normally sealed from the atmosphere by a valve designed to regulate the internal pressure within the cell so as to provide what is termed an effective “oxygen recombination cycle” (hence the use of the terms “sealed” and “valve-regulated”).
The advantages that are provided by sealed lead-acid cells in comparison to conventional, flooded lead-acid batteries are substantial and varied. Sealed lead-acid technology thus offers substantial benefits by eliminating maintenance (e.g., cell watering), expense (e.g., acid purchases), environmental (e.g., expensive waste treatment systems and air-borne acid mist) and safety (e.g., acid burns) concerns.
It is thus not surprising that sealed lead-acid cells are widely used in commerce today for various applications that have widely differing requirements. In one type of application, generally termed as stationary applications, lead-acid cells are used, for example, for load leveling, emergency lighting in commercial buildings, as standby power for cable television systems, and in uninterruptible power supplies. The uninterruptible power supply may be used to back up electronic equipment, such as, for example, telecommunication and computer systems, and even as a backup energy source for entire manufacturing plants. When the principal power supply to the electronic equipment has been cut off, such as during a power outage, the sealed cells (typically many electronically connected together) provide a source of reserve power to allow the telecommunication or computer system to remain operational until the principal power supply can be restored. The uninterruptible power supply also will accommodate short, or intermittent, losses in power, so that the function of the electronic equipment will not be impaired during a brief power outage.
In addition, there are many applications where sealed lead-acid cells are used in what are termed as motive power application. Sealed lead-acid cells are thus used as the power source for electric vehicles, fork-lift trucks, and the like.
The performance requirements for these two basic types of applications vary significantly. On the one hand, stationary applications are generally float applications, i.e., the cells are generally on float (i.e., an external voltage supply connected to the cells is held slightly above the cell potential to maintain charge), with an occasional need for a deep discharge when the main power source fails or is otherwise interrupted.
On the other hand, motive power applications require repetitive deep discharges, down to a 80% depth of discharge or even somewhat greater. Suitable cells must thus be capable of enduring repetitive charge-deep discharge-charge cycling regimes for up to 500 cycles or even more. Indeed, it would be desirable to provide cells capable of enduring from 1,000 to 2,000 cycles or so.
Developing grid alloys that adequately satisfy the diverse criteria for both stand-by and motive power applications has been largely unsuccessful. This lack of success has resulted even though substantial attention has been given to this issue by those working in this field.
This relative lack of success can perhaps best be appreciated when the principal criteria are considered because such criteria are stringent and are varied. These criteria must be satisfied, regardless of the type of application. In general, and by way of a summary, suitable alloys must be capable of being cast into satisfactory grids and must impart adequate mechanical properties to the grid. Still further, the alloys must impart satisfactory electrical performance to the VRLA cell in the intended application. Satisfactory alloys thus must impart the desired corrosion resistance, not result in thermal runaway (i.e., must not raise the tendency for the cell to lose water via gassing) and avoid premature capacity loss (sometimes referred to as “PCL”).
More particularly, and considering each of the criteria previously summarized, suitable alloys in the first instance must be capable of being cast into grids by the desired technique, i.e., the cast grids must be low in defects as is known (e.g., relative freedom from voids, tears, microcracks and the like). Such casting techniques range from conventional gravity casting (“book molds” or the like) to continuous processes using expanded metal techniques.
The resulting cast grids need to be strong enough to endure processing into plates and assembly into cells in conventionally used equipment. Even further, suitable grids must maintain satisfactory mechanical properties throughout the expected service life. Any substantial loss in the desired mechanical properties during service life can adversely impact upon the cell performance as will be more fully discussed hereinafter.
Considering now the electrochemical performance required, the grid alloy for the positive plates must yield a cell having adequate corrosion resistance. Yet, the use of a continuous direct casting process, desirable from the standpoint of economics, ostensibly can compromise corrosion resistance. Such continuous processes thus orient the grains in the grids, thereby making the intergranular path shorter and more susceptible to corrosion attack and to early failures.
Positive grid corrosion thus is a primary mode of failure of VRLA lead-acid cells. When positive grid corrosion occurs, this lowers the electrical conductivity of the cell itself. Cell failure occurs when the corrosion-induced decrease in the conductivity of the grid causes the discharge voltage to drop below a value acceptable for a particular application.
A second failure mechanism, also associated with grid corrosion, involves failure due to “grid growth.” During the service life of a lead-acid cell, the positive grid corrodes; and the corrosion products form on the surface of the grid. In most cases, the corrosion products form at the grain boundaries and grid surface of the lead-acid where the corrosion process has penetrated the interior of the “wires” of the grid. These corrosion products are generally much harder than the lead alloy forming the grid and are less dense. Due to the stresses created by these conditions, the grid alloy moves or grows to accommodate the bulky corrosion products. This physical displacement of the grid causes an increase in the length and/or width of the grid. The increase in size of the grid may be nonuniform. A corrosion-induced change in the dimension of the grid is generally called “grid growth” (or sometimes “creep”).
When grid growth occurs, the movement and expansion of the grid begins to break the electrical contact between the positive active material and the grid itself. This movement and expansion prevents the passage of electricity from some reaction sites to the grid and thereby lowers the electrical discharge capacity of the cell. As this grid growth continues, more of the positive active material becomes electrically isolated from the grid and the discharge capacity of the cell decays below that required for the particular application. The mechanical properties of the alloy thus are important to avoid undue creep during service life.
Still further, and importantly, the use of the alloys must not result in thermal runaway. VRLA cells must avoid conditions in service in which the temperature within the cell increases uncontrollably and irreversibly.
It has been hypothesized that excessive water loss resulting in cell dry-out is the driving mechanism for
Chaney Carol
GNB Technologies, Inc.
Morrison & Foerster / LLP
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