Maintenance-free open industrial type alkaline electrolyte...

Chemistry: electrical current producing apparatus – product – and – Means externally releasing internal gas pressure from closed...

Reexamination Certificate

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C429S059000, C429S206000

Reexamination Certificate

active

06183899

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns an industrial aqueous alkaline electrolyte open secondary storage battery, i-e. one having a high capacity in the range 10 Ah to 200 Ah and “open” in the sense that it operates at a low pressure (less than 1 bar relative). Storage batteries of this kind are in particular of the nickel-cadmium (Ni—Cd) or nickel-metal hydride (Ni—MH) type.
2. Description of the Prior Art
An open industrial storage battery includes an electrode assembly comprising a plurality of electrode pairs consisting of a positive electrode, a negative electrode and a separator that is only slightly permeable to gases disposed between the positive and negative electrodes, together with an alkaline electrolyte in which the assembly is immersed and the level of which is above the top edge of the electrodes. On overcharge, aqueous electrolyte storage batteries generate oxygen at the positive electrode and hydrogen at the negative electrode. An open storage battery operates at a relative pressure (pressure difference relative to atmospheric pressure) less than 1 bar and the gases generated on overcharge escape, consuming water from the electrolyte. The storage battery therefore requires regular maintenance, i.e. water must be added periodically. The maintenance frequency depends on the operating conditions of the storage battery in the application concerned, in particular on the charged capacity.
To avoid topping up the electrolyte level after periods of operation sealed industrial type storage batteries have been derived from those previously described (U.S. Pat. No. 5,576,116). A sealed industrial (high-capacity) storage battery includes an electrode assembly comprising a plurality of electrode pairs consisting of a positive electrode, a negative electrode and a gas-permeable separator disposed between the negative and positive electrodes, a limited quantity of alkaline electrolyte and an oxygen recombination device. The oxygen formed at the positive electrode increases the pressure inside the storage battery which depends on the overcharge conditions employed. Permanent conditions are established thereafter in which the oxygen produced at the positive electrode is reduced, or recombined, at the negative electrode. A sealed industrial storage battery has a safety valve operating at a relative pressure higher than 1 bar. Although a sealed storage battery can solve the technical problem associated with maintenance, its energy per unit mass and per unit volume are lower than those of an open storage battery. On the one hand the sealed storage battery has a precharge and an excess of negative capacity designed to prevent the release of hydrogen at the end of charging. On the other hand such storage batteries contain a small quantity of electrolyte and consequently the yield of the active material is lower than in open storage batteries.
It has therefore appeared desirable to work towards reducing the maintenance of open storage batteries. U.S. Pat. No. 5,128,217 proposes self-limitation of the charge of an Ni—Cd storage battery based on the sharp increase in the voltage at the end of charging. This open industrial storage battery operates at a relative pressure less than 1 bar and contains excess electrolyte.
The aim of the present invention is to propose an open industrial storage battery requiring no maintenance with energy per unit volume and per unit mass higher than those of a sealed industrial storage battery.
SUMMARY OF THE INVENTION
The present invention consists in a maintenance-free open industrial storage battery including an electrode assembly comprising at least one positive electrode, one negative electrode, one separator disposed between the negative electrode and the positive electrode, an alkaline electrolyte covering the top end of the assembly before electrical cycling and a valve the relative operating pressure of which is less than 1 bar, wherein the total capacity of the negative electrodes is greater than the total capacity of the positive electrodes, the separator is permeable to oxygen and the storage battery contains an oxygen recombination device such that after at least one cycle of charging and discharging the storage battery operates without loss of electrolyte at a charging current at least equal to Ic/10 where Ic is the current discharging the capacity of the storage battery in one hour.
In the early cycles the storage battery of the invention operates like an open storage battery. The abundant quantity of electrolyte means that the high performance of open storage batteries can be achieved and conserved. As long as the electrolyte is in excess, the recombination device is relatively inaccessible and oxygen reduction is not encouraged. During charging the gas pressure rises and the valve allows the gas to escape, and water is therefore consumed. The water loss is estimated at approximately 0.3 cm
3
per ampere-hour of overcharge. As the excess electrolyte is consumed the reduction of the oxygen occurs with a higher yield. After a few cycles the storage battery has achieved equilibrium between the release and the recombination of oxygen. The pressure inside the storage battery remains below the operating pressure of the valve, generally in the range 0.5 bar to 1 bar. The storage battery of the invention then operates like a sealed storage battery and requires no further maintenance.
The negative electrodes have a low excess capacity relative to the capacity of the positive electrodes. At the end of charging the negative electrodes are completely charged. The total capacity of the negative electrodes is preferably in the range 100% to 150% of the total capacity of the positive electrodes.
The gas-permeable separator allows access from the negative electrode to the oxygen generated at the positive electrode so that recombination occurs. The presence of a recombination device significantly increases the rate of recombination and enables equilibrium to achieved, even in deep cycling duties. A recombination system of this kind is described in U.S. Pat. No. 5,576,116.
The distance between the negative electrodes and the positive electrodes is advantageously in the range 0.2 mm to 0.5 mm. The distance between the electrodes is made as small as possible whilst avoiding the risk of short-circuits. Depending on the application it can range from 0.2 mm for applications that are relatively undemanding, for example the aeronautical field, up to 0.5 mm if the cycling conditions lead to greater variations in the electrode dimensions, in particular in the case of use in an electric vehicle.
In one variant the positive electrodes are of the sintered type and the negative electrodes are of the paste type on a conductive support selected from a two-dimensional support such as a solid or perforated strip, expanded metal, a grid or woven material and a three-dimensional support such as foam or felt.
In another variant the positive electrodes are of the paste type on a three-dimensional conductive support and the negative electrodes are of the paste type on a two-dimensional or a three-dimensional conductive support.
The maintenance-free storage battery of the invention is particularly suitable for use in the aeronautical or railroad field and for electric vehicle propulsion.
The invention will be better understood and other advantages and features will appear from a reading of the following description of one embodiment given by way of non-limiting illustrative example and from the accompanying drawings.


REFERENCES:
patent: 4436795 (1984-03-01), Cooper et al.
patent: 4880435 (1989-11-01), Itou et al.
patent: 5128217 (1992-07-01), Liska et al.
patent: 5576116 (1996-11-01), Sanchez et al.
patent: 0354966A1 (1990-02-01), None
patent: 0401076A1 (1990-12-01), None
patent: 06666608 (1995-08-01), None
patent: 2628892A1 (1989-09-01), None
patent: 2203280 (1988-10-01), None
patent: WO9715959 (1997-05-01), None
Patent Abstracts of Japan, vol. 009, No. 321 (E-367), Dec. 17, 1985 corresponding to JP 60 154477 A (Shnikoube Denki K

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