CA1041164A

CA1041164A - Maintenance-free battery and method for reducing the current draw of such batteries

Info

Publication number: CA1041164A
Application number: CA236,466A
Authority: CA
Inventors: George W. Mao
Original assignee: Gould Inc
Current assignee: Gould Inc
Priority date: 1974-10-11
Filing date: 1975-09-26
Publication date: 1978-10-24
Also published as: FR2287785B1; SE7511362L; BE834264A; JPS5163438A; FR2287785A1; AU8559775A; IT1058311B; NL7511961A; LU73549A1; GB1521975A; DE2544656A1; IN146036B

Abstract

ABSTRACT OF THE DISCLOSURE

A maintenance-free battery having improved current draw characteristics is provided by adding elemental cadmium, or a cadmium affording compound such as cadmium sulfate to the electrolyte in amounts sufficient to decrease the current draw that would otherwise occur during overcharge.

Description

` ``` ~04~ 4 RELATED APPLICATIONS
Millex, Serial No. 158,871, filed December 14, 1972, for: "Battery Contalner Cover".
Mao and Rao, Serial No. 209,677, filed September 20, 1974, for: "Lead Basa Calcium-Tin ~lloy and Use Thereof".
Rao and Mao, Serial No. 209,678, filed September 20, 1974, ~or: "Lead Base Cadmium Tin Alloy Useful for Forming Battery Components".
Mao ~nd Lannoye, Serial No. 214,431, filed November 22, ~0 1974, for: "Cadmium-Antimony-Lead-Alloy for Maintenance-free Lead-Acid Battery".
Mao, Rao and Trenter, Serial No. 236,233, filed on September 24, 1975, for: "Maintenance-free Battery".
This invention relates to lead-acid storage batteries and, more particularly, to maintenance-free batteries having improved current draw characteristics.
Lead base alloys have been used for storage battery plate grids for many years. The electrochemical characteristics of le~d as well as its low cost make it suitable as a primary materiaI, ~ut alloying ingredients must be included because of the ~nherent physical weakness of the lead. A large number of di~erent alloying materials in variou~ peraentages and com-binations ha~e been considered. Antimony-lead alloys containing anywhere from about 4.5 to 12~ by weight antimony have been used for the preparation of the grids for lead-acid batteries.
~he principal function of the antimony constituent is to impart adequate grid strength as well as permitting easy castins of the grid. Lithium and combinations of lithium and tin have likewise 29 been employed as shown in U. S. Patent No. 3,647,545. Still cb~ - 1 -1~411~
further, Canadian Patent ~20,393 describes a lead base alloy con-taining cadmium and antimony for use in forming battery grids.
As shown in that patent, alloying about 2.5 to 3% cadmium with

2.5% antimony in a lead alloy imparts a tensile strength con-siderably above that which would ordinarily be expected.
Recently, much interest has been placed upon providing automotive type, wet cell, lead-acid storage batteries in configur-ations which can be readily installed and which require,once in service, no further maintenance throughout the expected life of the battery. One aspect of this effort to provide such maintenance-free batteries is to utilize internal components that make it unnecessary to inspect and replenish electrolyte levels in the cells over the normal battery life.
To achieve this maintenance-free objective, substantial elimination of water losses must be achieved. This requires that the grids employed in the maintenance-free battery draw only a small current during constant voltage overcharge so that only minimum gas generation occurs with the accompanying water loss being concomitantly minimized. With conventional automotive batteries using antimony-lead grids typically containing about 4.5% by weight antimony, the current draw at the completion of charging is unacceptably high for maintenance-free battery applications. In addition, it is known that self-discharge of - a wet lead-acid battery employing an antimony alloy is caused primarily by the dissolution of antimony from the grids and its subsequent deposition on the negative plates, where it causes electrochemical reactions that discharge the lead to lead sulfate.
For these reasons, the development of suitable materials for grlds in maintenance-free batteries has primarily emphasized the use of lead base alloys not containing antimony.
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The copending Mao and Rao application, identified here-in, presents one type of alloy suitable for forming the grids of maintenance-free batteries. Thus, a lead base alloy containing, by weight, 0.06 to 0.10~ calcium and 0.10 to 0.40% tin is dis-closed. A further approach is described in the previously identi-fied Rao and Mao application. This discloses a cadmium-tin-lead alloy which is useful in forming components of the battery elements, including the battery grid in a maintenance-free battery.
A still further approach is described in the herein identified Mao and ~annoye application in which a lead based alloy containing, typically, from about 1.0 to 2.0~ antimony and fro~ a~out 1.2 to about 2.2% cadmium is disclosed. Lead-acid batter~ grids can be easily cast from such an alloy, and the grids may be advantageously employed in the preparation of ~aintenance~free batteries to provide superior characteristics.
While SUCll alloys do provide suitable materials for forming ~atter~ grids for use in maintenance-free battery applications, it would be highly desirable to be able to provide m~intenance-free batteries in which the current draw character-istics of the grids may be further decreased.
It is accordin~l~ an ob]ect of the present inventionto provide a maintenance-free battery having exceptional current draw characteristics~
Another object provides a method for decreasing the current draw characteristics of alloys used in such applications.
A still further and more specific object of this inven-tion lies in the provision of a maintenance-free battery employ-ing calcium-lead or calcium-tin~lead alloy grids which possess 29 improved current draw characteristics.
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Yet anotner object is to provide a method of lessen-ing the criticality of the impuritie~ typically observed for tne alloys used to form the battery grids in maintenance-free battery applications.
Other objects and advantages of the present invention will be apparent as the following description proceeds.
While the invention is susceptible of various modifi-cations and alternative forms, specific embodiments thereof will hereinafter be described in detail. It should be under-stood, however, that it is not intende~ to limit the invention to the particular forms disclosed, but, on the contrary, the ;
intention is to cover all modifications, equivalents and alter-natives falling wit~in the spirit and scope of the invention as expressed in the appended claims.
In general, the present invention is predicated on :
the discovery that the addition of elemental cadmium (e.g.--as a flne powder) or a cadmium compound to the electrolyte in certain levels in a maintenance-free battery application signifi-cantly diminishes the current draw, and thus the water consumption, so as to improve the performance of the battery.
The cadmium compound which is utilized in accordance with this invention may comprise cadmium sulfate or any other cadmium compound which is~ sufficiently soluble in aqueous sulfuric acid solutions to provide the requisite amount of the ~ ;
cadmium, (2~ not substantially harmful to either the battery com-ponents or to the performance of the battery in use and ~3) not susceptible to producing a lead salt that would likely precipitate in sufficient amounts which would significantly reduce the porosity 29 ~of the battery plates. For example, cadmium hydroxide could cbj` - 4 _ . . : ~ :. ::: - . . . :- : :...... : ,.. . .

104iltj4 suitably be used.
The amount of the cadruium compound which is used should be sufficient to decrease the current draw during constant voltage ovPrcharge. Typically, the ~attery at this stage will be in a fully charged condition. While the amount can vary so long as the amount of cadmium provided is sufficient to decrease the current draw to the extent required, it has been found suit-able to utilize amounts in the range of from about 0.1~ (or somewhat less) to about 0.3~, based upon the total weight of the electrolyte, and even up to about 0.5~ by weight when the cadmium compound employed is cadmium sulfate. All that is required is for the cadmium compound to be added to the elec-trolyte, suitably prior to the sealing of the cover to the battery container.
While it is certainly expeditious to add the cadmium affording compound directly to the electrolyte, it should be appreciated that the compound may be added to the battery in any manner so long as the resultant cadmium in the electrolyte is in the desired range. For example, the cadmium compound may be added to the electrode paste. When compounds other than cadmium sulfate are 2Q employed, the amount can desirably, of course, be varied to ~rovide the same amount of cadmium that would be provided by the amount of cadmium in the cadmium sulfate in the range set forth herein.
With respect to the materials used to form the electrodes or battery grids, any material can be used so long as the material does not contain any impurities in an amount that would adversely affect in a significant fa5hion the functioning of the cadmium.
For example, if an alloy containing antimony is used, the amount 22 of antimony present shouId be sufficiently low so as not to , 1~)4il~
completely eliminate the benefits achieved by the addition of the cadmium. Thus, it is preferred to utllize an alloy for the negative electrodes wnich is essentially free of antimony.
Typically, it is believed suitable to use an alloy for the positive electrodes where the antimony content is no more than about 2 to 3~ by weight. It is preferred to utilize calcium-lead alloys for the positive and negative grids, typically containing calcium in an amount from about 0.01 to about 0.15%
by weight of the alloy, and preferably about 0.06 to 0.10%.
The calcium-lead alloy may, if desired, include other alloying ingredients so long as the current draw characteristics are not significantly adversely affected. It is particularly pre-f~rred to employ a lead base alloy containing about 0.06~ to about 0.10% calcium, preferably 0.07% - 0.09%, and a tin ~ content of about 0.1% to about 0.4%, preferably 0.20~ - 0.30~.
Alloys of this latter type have a remarkakly improved resistance to drossing, thus permitting rapid casting and accurate calcium CDntent control. Batteries prepared with grids fabricated from this alloy also have unexpectedly improved capacity retention 2Q characteristics.
It should be appreciated that such alloys can contain negligible amounts of impurities such as are typically found in commercially available battery grade lead. While impurities may be contained in the calcium and tin components, the relatively small amounts of these components make the typical impurities unimportant. Thus, as has been set forth herein, it should be appreciated that the alloys of the present invention can include other ingredients so long as they do not adversely affect the 29 desirable features attributable to the present invention.

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The particular configuration for the battery can vary wlthin wide limits, and the specific construction is not critical insofar as this invention is concerned. Suitable embodiments are shown in the copending Miller and Mao and ~-Lannoye applications identified herein.
It is theorized that the cadmium that is added to the electrolyte preferentially migrates to the negative electrode and is deposited thereon during charging.
The following examples are illustrative, but not in limitation of tne present invention. Unless otherwise specified, all percentages are by weight.
Example 1 A single 70 Ampere Elour capacity cell having 6 positive and 7 negative electrodes formed of a lead based alloy having 0.08~ by weight calcium and about 0.25 - 0.30~ tin was exposed to varying cell voltages at ambient condition and at elevated temperatures. The current draw was compared for the cell having no additive ~control cell~ to the same cell containing 0.5%
by weight cadmium in the electrolyte. An electrolyte compris-2~ ing sulfuric acid having a specific gravity of 1.265 was u~ed.
The current draw charact~ristics at the various equili-brium voltages ~i.e.--after the cell has been at the voltage charge condition for about one week~ are set forth in Table 1 below:

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1~4il64 Example 2 Two Group 24 maintenance-free batteries having capacities of 81 Ampere Hours were tested to show the effects of the inclusion of a cadmium compound in accordance with the present invention on the current draw characteristics of the batteries. Two different charge voltages were used, and the grids were formed of the lead base alloys described in Example l. The gassing current draw was then measured after the batteries had been on charge conditions overnight. The 10 results ~re shown in Table 2: ~

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ffl-' - 10 -~ 411fà4 Example 3 A number o~ tl~e Group 24 batteries described in the previous example were subjected to tests at higher temperatures with varying amounts of additives and were tested as described in the previous example. The results are shown in Table 3.

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Thus, as has been seen, the present invention provides a maintenance-free battery which is characterized by improved -gassing current characteristics. As is also illustrated in Example 1, the inclusion of suitable amounts of the cadmium in accordance with the present invention results in a signifi-cantly more electronegative electrodQ. It should also be noted that the current draw values taken at equilibrium conditions in Example 1 may be more representative of the improvement that can be achieved pursuant to this invention rather than the values taken prior to reaching equilibrium, as in Examples 2 and 3.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a maintenance-free lead-acid battery comprising a battery container having a plurality of cells, a cover sealed to the container, venting means providing passages for the escape of evolved gas and an electrolyte contained in the cells, each cell including a plurality of positive and negative electrodes dis-posed therein comprising a grid supporting structure having a layer of active material attached thereto, the improvement where-in said electrolyte contains cadmium in an amount sufficient to decrease the gassing current during constant voltage overcharge of said maintenance-free battery, the cadmium being provided by a cadmium compound which is (1) sufficiently soluble in aqueous sulfuric acid solutions to provide the requisite amount of the cadmium, (2) not substantially harmful to either the battery components or to the performance of the battery in use and (3) not susceptible to producing a lead salt that would likely pre-cipitate in sufficient amounts which would significantly reduce the porosity of the electrodes.

2. The maintenance-free battery of Claim 1 wherein the catmium is provided by the addition of cadmium sulfate to the electrolyte.

3. The maintenance-free battery of Claim 2 wherein the cadmium sulfate is present in an amount of from about 0.1 to about 0.5%, based upon the weight of the electrolyte.

4. The maintenance-free battery of Claim 1 wherein at least the grid supporting structure of the negative electrodes consists essentially of a lead base alloy containing calcium in an amount of from about 0.01 to about-0.15%, based upon the weight of the alloy.

5. The maintenance-free battery of Claim 4 wherein at least the grid supporting structure of the negative electrodes consists essentially of a lead base alloy containing from about 0.01 to about 0.15% calcium and from about 0.1 to about 0.4%
tin, based upon the weight of the alloy.

6. The maintenance-free battery of Claim 5 wherein said lead base alloy contains calcium in an amount of 0.07 to 0.09% and tin in an amount of 0.20 to 0.30%.

7. A method of reducing the gassing current during con-stant voltage overcharge in a maintenance-free battery com-prising a battery container having a plurality of cells each including a plurality of positive and negative electrodes dis-posed therein comprising a grid supporting structure having a layer of active material attached thereto and an electrolyte contained in the cells which comprises adding cadmium to said electrolyte in an amount sufficient to decrease the gassing current during constant voltage overcharge, the cadmium being provided by a cadmium compound which is (1) sufficiently soluble in aqueous sulfuric acid solutions to provide the requisite amount of the cadmium, (2) not substantially harmful to either the battery components or to the performance of the battery in use and (3) not susceptible to producing a lead salt that would likely precipitate in sufficient amounts which would significantly reduce the porosity of the electrodes.

8. The method of Claim 7 wherein said cadmium is pro-vided by the addition of cadmium sulfate.

9. The method of Claim 7 wherein at least the grid sup-porting structure of the negative electrodes consists essentially of a lead base alloy containing calcium in an amount of from about 0.01 to about 0.15%, based upon the weight of the alloy.

10. The method of Claim 9 wherein at least the grid supporting structure of the negative electrodes consists essentially of a lead base alloy containing calcium is an amount of from about 0.01 to about 0.15% and tin in an amount of from about 0.10 to about 0.40%, based on the weight of the alloy.