Metal working – Battery-grid making
Reexamination Certificate
1999-04-03
2002-03-05
Kalafut, Stephen (Department: 1745)
Metal working
Battery-grid making
C029S623100, C429S233000
Reexamination Certificate
active
06351878
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to lead-acid cells and batteries, and, more particularly, to a method for making positive grids using calcium-tin-silver lead-based alloys.
BACKGROUND OF THE INVENTION
Over the last 20 or so years, there has been substantial interest in automotive-type, lead-acid batteries which require, once in service, little, or more desirably, no further maintenance throughout the expected life of the battery. This type of battery is usually termed a “low maintenance” or “maintenance-free battery.” The terminology maintenance-free battery will be used herein to include low maintenance batteries as well. This type of battery was first commercially introduced in about 1972 and is currently in widespread use.
It has been well recognized over the years that lead-acid batteries are perishable products. Eventually, such batteries in service will fail through one or more of several failure modes. Among these failure modes are failure due to positive grid corrosion and excessive water loss. The thrust of maintenance-free batteries has been to provide a battery that would forestall the failure during service for a period of time considered commensurate with the expected service life of the battery, e.g., three to five years or so.
To achieve this objective, the positive grids used initially for maintenance-free batteries typically had thicknesses of about 60 to about 70 mils or so. The batteries were likewise configured to provide an excess of the electrolyte over that needed to provide the rated capacity of the battery. In that fashion, by filling the electrolyte to a level above that of the top of the battery plates, maintenance-free batteries contained, in effect, a reservoir of electrolyte available to compensate for the water loss occurring during the service life of the battery. In other words, while the use of appropriate grid alloys will reduce water loss during the service life of the battery, there will always be some water loss in service.
The principal criteria for providing satisfactory positive grids for starting, lighting, and ignition (“SLI”) automotive lead-acid batteries are stringent and are varied. 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 battery in the intended application. Satisfactory alloys thus must impart the desired corrosion resistance, and avoid positive active material softening that will result in a loss of capacity.
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 and to a variety of processes using alloy strips from which the grids are made, e.g., by stamping or the like.
The resulting cast grids need to be strong enough to endure processing into plates and assembly into batteries 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 battery performance as will be more fully discussed hereinafter.
Considering now the electrochemical performance required, the grid alloy for the positive plates must yield a battery having adequate corrosion resistance. Yet, the use of a continuous direct casting process, or other processes using grid alloy strips, desirable from the standpoint of economics, ostensibly can compromise corrosion resistance. 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. Casting a thick strip and then cold rolling or the like to the grid thickness desired even further exacerbates the problem.
Positive grid corrosion thus can be a primary mode of failure of SLI lead-acid batteries, particularly at higher ambient temperatures. When positive grid corrosion occurs, this lowers the electrical conductivity of the battery itself. Battery 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 positive grid corrosion, involves failure due to “grid growth.” During the service life of a lead-acid battery, 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 positive grid 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 and thus occupy a larger volume. 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 non-uniform. 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.
As is now appreciated, what has occurred in the last several years is the substantial increase in the under-the-hood temperature to which the battery is exposed in automobile service. Obviously, the under-the-hood temperature is particularly high in the warmer climates. One automobile manufacturer has perceived that the temperature to which an SLI battery is exposed under-the-hood in such warmer climates has risen from about 125° F. to about 165° F.-190° F. in new automobiles.
The specific temperature increase which is involved is not particularly important. What is important is that such under-the-hood temperatures have in fact increased. The impact of the under-the-hood vehicle service temperature increases on the failure modes has been to substantially increase the occurrence of premature battery failures. The incidence of premature battery failures due to excessive positive grid corrosion has been significant.
A breakthrough was achieved in utilizing the positive grid alloys disclosed in U.S. Pat. No. 5,298,350 to Rao. Utilizing such positive grid alloys provided batteries that exhibited substantial improvements in service life and have effectively eliminated premature positive grid corrosion at elevated temperatures as being the primary mode of failure.
The subject Rao patent has spurred considerable interest in the type of positive grid alloys utilized, i.e., calcium-tin-silver lead-based alloys. Thus, substantial effort has been made to investigate this type of alloy through testing of various properties with varying levels of the alloying constituents.
The interest has also extended to utilizing this family of alloys in sealed lead-acid cells and batteries (often termed “VRLA,” viz., valve-regulated lead-acid). Sealed lead-acid cells and batteries are widely used in commerce today for variou
Alejandro Raymond
GNB Technologies, Inc.
Kalafut Stephen
Leydig , Voit & Mayer, Ltd.
LandOfFree
Method for making positive grids and lead-acid cells and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for making positive grids and lead-acid cells and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for making positive grids and lead-acid cells and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2880649