This invention is directed to nickel-hydrogen battery structure, and particularly the structure of both the positive and negative electrodes, the electrodes having an optimum current path thereon.
In any battery the electric current produced by a change of valence state must be electrically conducted to the battery terminals. Electrical resistance loss in this conduction both adds heat to the battery and reduces the cell voltage to reduce cell efficiency. Therefore conductivity paths must be maximized to minimize electrical resistance. However, ion transfer through the cell requires liquid continuity through the cell stack so that maximized open area is required to maintain adequate ion transfer for optimized cell conduction. In order to achieve these two objectives, open areas are provided in the electrode substrate. Expanded mesh screen has previously been used for hydrogen electrode substrates and woven wire mesh has been used for negative, nickel hydroxide electrode substrates in nickel-hydrogen batteries. One manufacturer makes such electrode substrates from punched metal foil to provide a uniform geometry of equal diameter punched holes.
Sometimes the punched metal foil may be nickel plated iron which will corrode and may degrade the battery performance. The punched holes are circular, and thus provide no optimum relationship between the generally radially directed conduction and the open area for ion passage in a direction perpendicular to the electrode.
Both the expanded mesh electrode substrate and the woven wire screen electrode substrates can cause cell short circuits because the wire-like substrate strands at the edges of the substrate can extend and make contact where such contact is undesirable. This is one disadvantage of the prior art construction. The pressure vessel which contains the electrodes and the electrolyte is cylindrical through the central portion thereof and has hempispherical domes on the ends to contain the internal pressure. When there is a change in temperature of the system the clearance between the electrodes and the outer wall changes. When the electrodes have wire screen substrates, then the radial clearance is a problem. However, the solid outer rim construction of the electrode substrate, in accordance with this invention, allows precise minimum clearance between the electrodes and the cylindrical center section of the pressure vessel, which was not possible with the prior art woven wire substrate.
Furthermore, the woven screen wire construction of the prior art has wires in two directions which are substantially normal to each other. This construction provides difficulty in the current path, to result in nonuniform current density through the electrode substrate to result in higher electrical losses and less electrode utilization. With properly designed etched foil electrode substrates in accordance with this invention, there is lower electrical losses as compared to the same weight electrodes of other construction. Thus, the battery designer can achieve a lower weight battery with the same electrical performance, or a battery of the same weight with higher efficiency, by use of the concepts of this invention. Additionally, the incorporation of a shape substrate which is self-edged with both interior and exterior rims can reduce manufacturing operations and thus reduce cost.