This invention relates to battery plates and method for making same and, more particularly, to rechargeable battery plates in which the active material is impregnated in a sintered porous plaque.
In U.S. Pat. No. 3,790,408 assigned to the assignee of this invention, there is described a battery coil in which one of the longitudinal edges of the negative plate is extended transversely beyond one longitudinal edge of the positive plate at one end of the coil and the other longitudinal edge of the positive plate is extended beyond the other longitudinal edge of the negative plate at the opposite end of the coil. This construction permits the easy attachment of positive and negative plate current collectors to the extended edges of the respective plates by welding of the current collectors to the extended edges at a plurality of contact points. The plates are comprised of a thin metal substrate with a sintered porous plaque coating both major surfaces of the substrate. The active material of the battery is impregnated in the pores of the porous plaque. A small region adjacent one longitudinal edge of each plate which forms the extended edge is free of the porous plaque and active material for, among other reasons, assuring the formation of a good weld between the plate edge and the current collector. At the opposite longitudinal edge of each plate, the porous plaque extends to the substrate edge and terminates in a flat edge coplanar with the substrate edge (hereinafter referred to as "flat edge termination"). The principal reasons for flat edge termination is that it is important to maximize the volume of porous plaque per unit cell volume in order to maximize the energy density of the cell. This is particularly important in small rechargeable cells such as nickel-cadmium.
In the method for making such plates, an elongated metal substrate is used which has a width dimension such that a plurality of strips of plates can be formed by longitudinally severing the substrate into a plurality of strips. The substrate is coated on both major surfaces with a slurry comprised of a metal powder. The slurry coating is then divided into continuous strips by forming longitudinally slurry-free channels by using doctor blades to wipe the channels free of slurry. Also, a region adjacent each edge of the substrate is wiped free of slurry. The edge regions of the substrate eventually form the extended edge of each plate which is to be formed adjacent thereto.
Each continuous strip of slurry is dimensioned to be approximately equal either to the sume of the widths of two battery plates or to the width of a single battery plate (hereinafter referred to as "double width" and "single width" strips). For example, when using a substrate of 20.32 cm width, three double width strips of slurry are defined by two channels by running longitudinally of the substrate. The double width strips eventually form plates of 3.2 cm width used in a sub-C cell. A variation to accommodate plates of other widths is to form, on a substrate of 18.0 cm width, one double width strip and one songle width strip. The double width and single width strips are used to form plates of 4.7 cm used in D cell. Other variations are also possible. After the slurry strips are formed, the substrate is passed through a sintering furnace in which the slurry is transformed into a porous plaque. The porous plaque coated substrate is then severed into strips of plate of the finished width. The strips of plate cut from the double width strips have a slurry-free edge region along one longitudinal edge which is formed from the slurry-free channel or edge region of the substrate and the opposing longitudinal edge of the plate strip has a flat edge termination. Similarly, the single width strips are cut so that one longitudinal edge has a slurry-free edge region formed by the slurry-free edge of the substrate and the opposing longitudinal edge has a flat edge termination. The strips are then impregnated with active material and transversely severed into a plurality of plates.
While this construction has been found to be generally advantageous, it has been noted that there is a tendency for the cells to become shorted during cell assembly and testing. Cell shorting is also one of the major problems which will cut short the expected life of such cells.
Accordingly, it is an object of this invention to provide an improved porous plaque plate construction which is less subject to shorting during assembly into battery coils and during their subsequent use as cells.
Another object of the invention is to provide a porous plaque battery plate which is less subject to flaking and chipping of the porous plaque during assembly.