Electricity: battery or capacitor charging or discharging – Battery or cell charging – Gas controlled
Reexamination Certificate
1999-07-13
2001-09-04
Wong, Peter S. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell charging
Gas controlled
C429S058000
Reexamination Certificate
active
06285167
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to improving the service life of valve regulated lead acid (VRLA) cells in standby service.
The construction of a VRLA cell is shown schematically in FIG.
1
. Like the traditional flooded cell, it has at least two electrodes or plates: a positive plate and a negative plate. Each of these plates is made of a current-collecting grid and an energy-storing active material. The VRLA cell differs, however, from the flooded cell in two ways.
First, the plates, instead of being immersed in a bath of electrolyte, are sandwiched between sponge-like separators which are made usually from absorbent glass fiber. Most of the electrolyte is absorbed in these separators. This type of VRLA cell is called the “absorptive glass mat” type, or AGM cell. Another exemplary type of VRLA cell is the “gel cell” in which liquid electrolyte of the type used in a conventional flooded cell is replaced by a gelled electrolyte. The present invention applies also to this type of VRLA cell. However, for the sake of clarity, the following description will be in terms of the AGM cell only.
A second difference between the VRLA cell and the flooded cell is that the flooded cell is vented to the atmosphere through a simple orifice, whereas the VRLA cell is vented through a one-way valve. The purpose of the one-way valve is to allow gas to escape from the cell to prevent over-pressurizing of the cell and prevent ingress from the air of oxygen that would oxidize and, therefore, discharge the negative plate. (Note that the negative plate of a flooded cell is protected by submersion in the acidic electrolyte, but the negative plate of the VRLA cell is exposed and very vulnerable to free oxygen in the cell).
As in any lead-acid cell on charge, oxygen is produced on the positive plate; some of this oxygen corrodes the positive grid. This is a fundamental characteristic of the lead acid cell and cannot be avoided.
The rate of corrosion of the positive grid is one of the two critical reactions that, in a VRLA cell, must be balanced or compensated for to avoid problems of short service life. This rate depends on the cell design. For example, two thin grids will corrode faster than a single thick grid of the same capacity due to their larger surface area. Different alloys also have differing rates of corrosion.
The negative grid is protected cathodically and does not normally corrode. However, the material comprising the negative electrode plays a major role in the design of the cell because it has an inherent tendency to self-discharge if the cell is left on open circuit. Such discharge is accompanied by the formation of hydrogen. The rate of the self-discharge reaction represents the second critical reaction in a VRLA cell that must be balanced to avoid service problems.
To the battery user, the VRLA cell has important advantages over a conventional flooded cell. One advantage is that the electrolyte, which is immobilized by the glass mat separators, cannot leak out of the cell even if the case or housing is punctured or inverted. Another advantage is that the cell has a reduced water consumption and, therefore, lower associated maintenance costs.
VRLA cells have been very successful in replacing conventional “flooded” cells in many standby applications such as, for example, a source of uninterruptable power supplies in telephone and computer systems. Much of this success is due to the claims by the manufacturers that the VRLA cells will provide a full 20-years of service without requiring water addition of any kind.
There is, however, evidence which has been collected from extensive laboratory testing over a two-year period that indicates that such claims may be overly optimistic. This is especially true at higher operating temperatures such as, for example, 90° F. (32° C.) at which many of the VRLA cells tend to fail in much shorter time periods. This problem is described in more detail below.
First, and by way of background, it is noted that the VRLA cell operates on a well-known principle called the “oxygen cycle” which gives the cell its ability to operate at reduced levels of water consumption.
FIG. 1
shows schematically a VRLA cell on charge. The oxygen gas produced by the positive plate, instead of bubbling to the surface of the electrolyte and leaving the cell as it would do in a flooded cell, penetrates the glass mat separator and comes into direct contact with the negative plate. (For example, a major portion of the oxygen gas so produced can migrate from the positive to the negative plate.) The result is the immediate “depolarization” of the negative plate, that is, a reduction in the voltage of the negative plate to approximately its open-circuit value.
This lower voltage causes the negative plate to produce less hydrogen so, in effect, the oxygen cycle suppresses the quantity of hydrogen produced. However, it does not eliminate the production of hydrogen (as may be erroneously believed), but reduces it to the minimum value possible, namely, the open-circuit value, for example, about 20 to about 80 cc/day/100 ampere hours (at 30° C.).
On the negative plate surface, the oxygen recombines with hydrogen ions from the electrolyte (plus the necessary electrons which are not shown for the sake of clarity) to reform water. Thus, the cell has a much reduced level of water consumption.
On the basis of this model, the industry has produced millions of VRLA cell for a multitude of applications. In many of these applications, the cells are operating successfully and are well accepted by their users. Surprisingly, however, in some commercial applications, high quality, heavy duty cells are demonstrating a serious problem of reduced capacity and short life. Such cells include those that have been designed for long-life by equipping them with highly corrosion-resistant positive grids. The extent of the problem is that cells designed for 20 years of service life may fail (defined as 80% or less capacity) in as little as 5 years or even less.
Various battery manufacturers have tended to assign the blame for such failures, including failures which have been observed in the field, to manufacturing defects or customer abuse. Research has shown that there are other reasons for the failures, as the failures have tended to continue.
The present invention relates to improvements in the design of and operation of VRLA cells.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a method for charging a valve-regulated, lead-acid (VRLA) cell at a charge voltage which has a value that is slightly in excess of the value of the open-circuit voltage of the cell, said cell including, in spaced relationship, a positive electrode and a negative electrode, and sandwiched therebetween electrolyte-containing separator means in which electrolyte is contained, wherein, during charging of the cell, there is produced at the positive and negative electrodes respectively oxygen gas and hydrogen gas in a predetermined amount, a portion of the oxygen gas tending to migrate through the electrolyte-containing separator means to the negative electrode and cause depolarization thereof, and wherein there is also formed at the positive electrode hydrogen ions which migrate to the negative electrode to form hydrogen gas in an amount less than said predetermined amount, the negative electrode tending to discharge over a prolonged period of time during charging, the improvement comprising inhibiting the tendency of the negative electrode to discharge during charging by controlling the amount of oxygen gas in the cell by catalytically converting a portion of the oxygen gas and a portion of the predetermined amount of hydrogen gas to water.
In accordance with another aspect of the present invention, there is provided an electrical cell comprising:
(A) a sealed housing;
(B) a positive electrode positioned in the housing;
(C) a negative electrode positioned in the housing in spaced relationship from the positive electrode;
(D) electrolyte-containing separator means positioned betw
Luk Lawrence
Synnestvedt & Lechner LLP
Wong Peter S.
LandOfFree
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