Chemistry: electrical current producing apparatus – product – and – Cell support for removable cell – Support or holder per se
Reexamination Certificate
2000-11-28
2003-11-04
Ruthkosky, Mark (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Cell support for removable cell
Support or holder per se
C429S066000, C429S157000, C429S183000
Reexamination Certificate
active
06641951
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to battery cell tray assemblies for power applications involving deep discharge duty cycles. More particularly, the invention relates to a battery cell tray assembly having a battery housing sized for holding and compressing multiple stacks of valve-regulated lead-acid battery cells arranged in a horizontal position.
BACKGROUND OF THE INVENTION
For quite some time it has been known that lead-acid batteries are particularly suitable for applications involving “deep discharge” duty cycles. The term “deep discharge” refers to the extent to which a battery is discharged during service before being recharged. By way of counter example, a shallow discharge application is one such as starting an automobile engine wherein the extent of discharge for each use is relatively small compared to the total battery capacity. Moreover, the discharge is followed soon after by recharging. Over a large number of repeated cycles very little of the battery capacity is used prior to recharging.
Conversely, deep discharge duty cycles are characterized by drawing a substantial portion of the battery capacity before the battery is recharged. Typical applications that require deep cycle capability include Class 1 electric rider trucks, Class 2 electric narrow aisle trucks and Class 3 electric hand trucks. Desirably, batteries installed in these types of vehicles must deliver a number of discharges during a year that may number in the hundreds. The cycle life of batteries used in these applications typically can range from 500-2000 total cycles so that the battery lasts a number of years before it needs to be replaced.
Until recently, only lead-acid batteries of the flooded variety have been utilized for the aforementioned deep discharge applications. Flooded lead-acid batteries are designed to have an excess of electrolyte that floods the cell container, completely saturating the plate group and extending into the head space above the plate group to provide a reservoir. The electrolyte reservoir is necessary because as the battery is charged, water in the electrolyte is electrolyzed into oxygen and hydrogen gases, which escape from the cell and deplete the electrolyte volume. To make up for the loss of electrolyte, water must be periodically reintroduced into the cell, or the reservoir must be made large enough to compensate for the expected loss over the life of the battery.
More recently, valve-regulated lead-acid (VRLA) batteries have been introduced that are suitable for deep discharge applications. VRLA batteries rely upon internal gas recombination to minimize electrolyte loss over the life of the battery, thereby eliminating the need for re-watering. Internal gas recombination is achieved by allowing oxygen generated at the positive electrode to diffuse to the negative electrode, where it recombines to form water and also suppresses the evolution of hydrogen. The diffusion of oxygen is facilitated by providing a matrix that has electrolyte-free pathways. The recombination process is further enhanced by sealing the cell with a mechanical valve to keep the oxygen from escaping so it has greater opportunity for recombination. The valve is designed to regulate the pressure of the cell at a predetermined level, hence the term, “valve-regulated”.
There are two commercially available technologies for achieving the enhanced oxygen diffusion. One technology makes use of a gelled electrolyte. In gel technology, the electrolyte is immobilized by introducing a gelling agent such as fumed silica. Gas channels form in the gel matrix in the early stages of the cell's life as water is lost via electrolysis. Once the gas channels are formed, further water loss is minimized by the recombination process. Unlike a fibrous matrix, the gel matrix keeps the electrolyte immobilized and there is little bulk movement.
The other technology for enhancing oxygen diffusion makes use of a fibrous material separator between the electrodes. A widely used material for this purpose is an absorbed glass mat (AGM). The AGM is a nonwoven fabric comprised of glass micro-fibers that retain the electrolyte by capillary action, but also provide gas spaces as long as the matrix is not fully saturated with electrolyte. The electrolyte is still free to move within the matrix, but is more confined than in a flooded cell. Another fibrous material gaining acceptance is a non-woven mat constructed from a polymeric component such as polypropylene or polyethylene.
One important difference between the fibrous mat and gel technologies, stemming from the degree of electrolyte mobility, is the effect of cell orientation on cycle life. With fibrous mat technologies, particularly when dealing with cells over about 14 inches tall, it has been discovered that the cycle life in deep discharge applications can be significantly improved by arranging the cells so that the longitudinal axis of the cell lies in a horizontal plane rather than a vertical plane as is customary. With gel technology, there is little difference in deep cycle life when cells are arranged horizontally or vertically. Thus, to achieve maximum cycle life with fibrous mat constructions, it is desirable to orient the fibrous mat cells horizontally, but it is not necessary to orient gel cells horizontally. Presumably, this effect can be explained by stratification of the electrolyte in fibrous mat cells when subjected to deep discharge cycling due to the higher degree of mobility compared with gel technology. The stratification results in reduced discharge capacity and can only be reversed with great difficulty.
The benefits of valve-regulated, lead-acid cell batteries of the fibrous mat variety and cell arrangements for deep discharge applications are known in the art. Although there are applications that take advantage of horizontal cell orientation, they are not without their shortcomings. There is known a “monobloc” battery wherein individual cells are not individually formed and enclosed within separate containers. Rather, they are formed by installing plates in a housing having separate cell compartments, and filling each compartment with acid. Individual cell compartments are defined within the battery case between partitions that are sealed to the battery case walls. A significant disadvantage of this approach is the lack of flexibility to adapt the battery configuration to battery compartments of different sizes. That is, a “monobloc” battery constructed with 12 cells will not fit into a battery compartment sized to accept six cells. Another disadvantage of the “monobloc” approach is that, for applications requiring large capacity batteries, battery size may increase substantially. This large, heavy battery may be difficult to handle thus raising safety concerns for personnel and efficiency concerns for the powered equipment.
One of the solutions to these problems, as shown in the prior art, provides for the prefabrication of individual cells and the placement of individual cells in a preformed compartment in a steel tray assembly. Cell compartments are defined by cell-receiving members (partitions) attached to the tray. This approach is still somewhat limited in that each cell compartment is sized to accept only one or, at the most, two cells. Proper compression for the lead-acid cells is accomplished by limiting the cell compartments to one or two cells and dimensioning the cell compartments to be just slightly larger than the cell dimensions so that the cells may be moved into position without difficulty. Later, in use, the cell walls expand causing proper compression to be applied. Thus, for a tray to contain six cells, at least three, and possibly six, separately formed cell compartments are needed.
Recent research has demonstrated the necessity of initially applying and maintaining modest to high levels of compression in fibrous mat cells to keep the separators in close contact with the plates even before formation and use. Having multiple cell compartments, each dimensioned even slightly larger than the cells to be p
Cestone Christopher R.
Vutetakis David G.
Wilkie Stanley K.
Douglas Battery Manufacturing Company
Ruthkosky Mark
Womble Carlyle Sandrige & Rice PLLC
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