TECHNICAL FIELD

This invention relates to improvements to electrode plates and associated methods of manufacture. BACKGROUND ART

Certain types of batteries contain a number of electrode plates consisting of a frame to which is supplied active material. The frame is usually configured into a grid and its purpose is to conduct electricity and to hold the active material.

The active material for negative electrodes is a mixture of granules of lead, various oxides of lead and various fillers and/or expanders, and sulphuric acid which combine to form a (settable) paste with a consistency similar to cement plaster when wet. The positive electrode plates have an active material using similar constituents but in different proportions. After the plates are made arid set or "cured", they are assembled in an electrochemical cell, where both the positive and negative electrodes are separated, usually by an electrolyte permeable material known as a separator. The cell is then subject to an electrolytic forming process during which electric current is passed through the cell in the direction corresponding to recharging of the cell, and this process largely converts the active material in the positive plate to lead dioxide and that in the negative plate to metallic lead. There are 4 distinct areas in which there are notable problems with currently styled lead acid batteries. They are

(i) the conduction path through the active material (particularly the positive paste, with active ingredient PbO2) has excessive resistance due to the large mean distance from the grid structure, and (ii) the conductivity of the grid structure for the vertical current path is inadequate and uses the available lead inefficiently for carrying current, and

(hi) the access of the acid to the active materials is inadequate for medium and high current discharges, and

(iv) the balance of the active components is usually not achieved, with the amount of acid being inadequate compared to the other active materials. The problems associated with the above problems are examined in more detail below.

(a) Plate Problems in Conventional Starting, Lighting and Ignition (SLI) Battery Styles

1. The upper part of the grid (particularly in the vicinity of the tab) overheats due to the high current flow. As a result of this it corrodes and decays. 2. The imbalance between grid conductivity and current distribution in the grid makes for inefficient use of lead in most grid designs.

3. The voltage distribution over the grid during discharge causes the active matter near the top to discharge first and most, resulting in inefficient use of the active mass, and in excessive cycling in the top areas of the plate. 4. The poor conductivity of the positive active material means that in a conventional grid structure during discharge there is a voltage gradient across the area of active mass in each grid pocket. This causes excessive resistance in the discharge path, uneven and inefficient use of the active mass, distortion in the active mass pocket with increased shedding, and poorer contact of the active mass with the grid.

5. The corresponding potential gradients and the excessive discharge of certain areas of active mass adversely affect the recharging of the battery.

(b) Problems with the Active Pastes

1. For most if not all lead acid starting lighting and ignition (SLI) or starter SLI batteries for automobiles, acid starvation rapidly develops in the active mass areas of the plates during a high current discharge. This results in failure to use the accessible or available active mass efficiently in a high current discharge such as for engine starting.

2. There is usually a mismatch of the available plate active mass porosity and the amount of active mass in the plate. In addition, the paste pore sizes are inappropriate to the discharge process, so that it is common for the negative plate to suffer rapid loss of porosity during high current discharge from the formation of expanded insoluble PbSO4 from Pb, with the former occupying a larger volume than the latter. The porosity mismatch results in the ratio of available acid held within the plate pores to the active mass in the plate being inadequate to provide a reasonable capacity at high discharge currents, when diffusion replenishment is inadequate.

(c) Problems with the Balance of Active Materials in the Cell

1. Most SLI batteries have insufficient acid to allow full discharge, and this results in an inability of the battery to utilise all of its active mass, thus significantly limiting the available power to weight ratio for the battery. 2. Because of the preferential discharge of the active mass in the upper area, the acid in this region is depleted, reducing the battery performance compared to an optimum design with the same active mass.

There are several effects combining to limit the discharge current compared to the theoretical capabilities of the lead acid battery plates, namely the quantity and resistance of the depleted acid in the vicinity of the active material, the resistance from the centre to the edge (the area in contact with the grid wires) of the pocket of active mass, and the resistance of the grid structure itself from the active mass pockets to the connection tab. Thus there is a particularly high potential drop from the centre of a low-down active mass pocket to the plate tab when the battery is being cycled, which results in inefficient use of the battery active mass particularly at high discharge rates. This also inhibits efficient rapid charging of discharged cells.

Conventional Plate Construction

Conventionally, the paste is rolled or squeezed onto the electrode frames or grids which are then left to dry or cure for a period of time. The paste dries during curing, sets hard and sticks to the grid wires. This process is sometimes referred to as keying.

Unfortunately, the nature of conventional plate construction and the paste application process have meant that the active material often flakes off the grid when the battery is being cycled. This leads to an accumulation of active material in the bottom of battery cells increasing the risk of internal shorts occurring in the battery and lessening the battery life.

There are a considerable number of electrode plates which form part of the prior art and these are discussed below.

US Patent No. 388960 related to the manufacture of secondary battery plates. The essence of the invention was stamping out by cutting and pressing from a sheet of metal two similar plates with each plate having bevelled apertures. The plates were attached together so that a single plate with an oppositely bevelled aperture was formed. In addition to having a number of the aforementioned disadvantages, a plate of this

construction also utilised weighty horizontal wires which contributed little to the conduction of current to the tab attached to the electrode plate.

US Patent No. 1369353 related to a method of making electrode plates formed by punching a number of apertures into a metal alloy sheet. To provide the necessary structural strength, it was necessary to have a comparatively thick grid plate. For space reasons, this lead to a fewer number of electrode plates which can be fitted into a standard battery volume, and therefore a lower current carrying capacity for the total battery.

US Patent No. 2724733 disclosed a number of different types of electrode plates. The manufacture of each of the plates involved a multiple step process which is inherently expensive.

US Patent No. 3099899 relates to a battery grid plate comprised of expanded metal. The manufacture of these grid plates was a multiple step process comprising: a) stamping or cutting small slits in a suitable metal sheet or foil, and b) stretching the metal sheets to form small meshes, and c) stamping or cutting the resultant expanded metal sheets to provide slits which are larger than the slits for the preliminary stretching step, and d) stretching these metal sheets to obtain the final grid plate.

As is self evident, a disadvantage of this method is the multiple steps which contribute to an expensive and time consuming process.

It is also clear that unless the slits are made parallel to the intended direction of current flow (vertically in most cases), this method reduces the effective conductivity of the grid in the direction of current flow, with adverse effects on active mass usage throughout the plate.

US Patent No.4547939 related to a plate for use in a multi-cell secondary battery. This plate essentially comprised of a perimeter frame of thermoplastic material having a metal mesh spanning the area of the frame to provide support for the active battery paste. One of the disadvantages of this plate is the use of two different materials leading to an expensive multi-step manufacturing process.

US Patent No. 4805277 related to a process for producing a grid for use in lead acid batteries. The grid is formed by the following method: a) superposing a sheet or a foil of a lead alloy on a sheet bar of a lead calcium alloy wherein the sheet or foil has a composition different from that of the sheet bar and the thickness different from that of the sheet bar, and

b) subjecting the superposed materials to a cold rolling to integrate the two materials and to produce an elementary sheet having a thickness smaller than that of the sheet bar, and c) subjecting the elementary sheet thus obtained to an expanding process or a punching process.

Like much of the prior art, this multi-step process is expensive and time consuming.

US Patent No. 5093971 related to a method and apparatus forming an expanded mesh battery grid. The method of producing this grid plate expands mesh sheet from a pre- slit deformable strip having unslit portions along at least the lateral edges thereof. The steps of this method are comprised of:

a) positioning the lateral edges of the deformable strip within the same horizontal plane; and b) laterally expanding the pre-slit portion of the strip while maintaining the lateral edges within the horizontal plane, and c) vertically expanding the pre-slit portion of the strip concurrently with the lateral expansion thereof while maintaining the lateral edges of the horizontal plane.

Not only is the method complicated, but also the machinery required to perform the method is highly complex.

It is an object of the present invention to address the above problems or at least to provide the public with a choice.

Further objections and advantages of the present invention will become apparent from the following description which is given by way of example only.

DISCLOSURE OF INVENTION

According to one aspect of the present invention there is provided a frame for an electrode plate comprising a stmctural member or members and web material which provides additional surface area to the frame than that provided by the structural members.

In preferred embodiments of the present invention, the stmctural members will be the conductive wires typically used in the grid or frame of electrode plates. Also in preferred embodiments the web material is a thin extension or "flashing" of the wire. However, it should be appreciated that the present invention can be applied to other situations.

As an example, the web material may be a conductive sheet into (and onto) which the active material can be keyed. A stmctural member may in this embodiment be non- conductive filaments or otherwise which serve only to provide structural strength and perhaps support the web material. In alternative embodiments, the web material may be a non-conductive substrate for the active material to attach to and the stmctural members may be electrically conductive. Reference throughout this specification will now be made to the preferred embodiment as described above.

Throughout this specification the terms web material, flashing and sheet material will be used interchangeably.

The preferred embodiment consists of replacing the conventional lead grid system by a highly "flashed" grid stmcture, or a continuous lead sheet stmcture.

One advantage of this method of manufacture is that the frame is made in a single step process. This is less time consuming and less expensive than the previous more complicated methods of manufacturing frames for electrode plates. The present invention also fits in with conventional grid manufacture. A number of conventional frames are made by casting the lead material. With the present invention the frame may also be cast, but in such a manner that flashing or the creation of the web material is encouraged. The use of horizontal (at right angles to the direction of main current flow) leaden bars or wires may also be minimised.

Appropriate ribs may be used in the vertical direction, and stiffening may be achieved by the use of corrugations or indentations in the sheet. Adhesion of the pastes to the sheet may be assisted by many small perforations and other techniques. This format enables the same amount of lead to be used to give a grid stmcture with a significantly reduced resistance in the direction of the current flow (e.g. by a factor of 5), while making the plate more rigid. It also results in reducing the average current path in the active pastes by a factor of at least 15, reducing the resistance and voltage drop incurred in the positive paste pockets by a factor of around 15. This provides a vastly improved contact area between the active pastes and the current collecting grid, and it allows much more even use of active mass to be achieved in the discharge of the battery, particularly at high currents.

By reducing the plate resistance dramatically, localised plate heating and the associated corrosion and short life of the positive plates is virtually eliminated, extending the life of

the plate. By providing a more uniform use of active mass over the plate surface, the cycling ability of the cell is also likely to be significantly improved.

Because the resistivity of the positive active mass is typically around three orders of magnitude higher than that of lead, the standard lead grid, comprising pockets filled with active mass surrounded by cast lead "wires" which comprise the grid can be replaced by a reinforced or appropriately shaped sheet of lead upon or through which is deposited active mass.

In preferred embodiments this may alternatively take the form of a thin layer of flashing from the grid "wires" extending throughout the pocket areas, with suitable perforations to key the active mass to the plate. A basic grid structure may nevertheless be left in place around the sheeting, to give the sheet plate strength and to help cany the current to the tab.

With a plate of typical thickness of 1 millimetre, where typical grid packets are 10- 15mm squares or larger, the average current path in the highly resistive active material is reduced from around 5-8mm to less than 0.3mm. This gives a reduction in average path resistance to the lead stmcture for current generated in the pocket of active mass by a factor typically 15 or more.

The effect of the grid resistance between a pocket of active mass in the grid and the current collecting tab at the top of the grid is fourfold. First, the grid "wires" in the area near the tab carry a large proportion of the total current drawn out of the plate. This causes overheating of the grid "wires" in this region, increased conosion, distortion, and degradation of the plate. This is the area where most batteries fail, and the most common reason they fail.

Second, the resistance of the grid "wires", especially near the top of the plate, cause a significant voltage gradient to develop over and across the upper part the plate. This inhibits cunent supply from other regions of the plate, resulting in inefficient use of active mass and over-discharge of the active mass in the top region of the plate, enhancing premature aging (loss of capacity, increase in paste resistance and poor paste-to-grid connection) of the plate in the region and leading to earlier than necessary failure of the plate.

Thirdly, battery current capacity sometimes referred to as cold cranking amps (CCA) is reduced by the inefficient use of active mass in the lower regions of the plate due to the voltage gradient between it and the tab.

Fourthly, because battery grids are normally made with a more or less uniform cross section from the bottom to the top, whereas the current flow in the (vertical) "wires" in an efficient battery would increase linearly with distance from the bottom, there is too much lead at the bottom and, for the potential CCA capacity of the battery, too little lead at the top. Thus the same amount of lead and active mass more wisely used is capable of giving a significantly higher CCA and a longer life battery. These effects are particularly important because the active mass usage, and therefore current generation by the battery, decreases exponentially with the overpotential that is generated by the ohmic current flow in the grid. The alleviation of this problem is greatly assisted by the use of sheet or sheet flashing in accordance with the present invention. This can be arranged to increase the conductivity of the grid in the vertical direction by providing a greater cross sectional area of lead for vertical current flow.

The conventional grid form is not adequate to carry the maximum expected current in the vicinity of the tab. If the tab is centralised, it can collect an important portion of the current that would otherwise flow in the grid, reducing the heating and voltage gradient significantly.

The stiffness of the plate can also be improved by structured sheet flashing.

The keying of the paste to the plate may be better with a perforated flashing as in some embodiments rather than plane sheet or sheet flashing.

The stiffness of the plates can also be improved by structured sheet flashing. Thus, either in a mould for casting the frame, or in a separate operation later, the sheet or flashing could be perforated and ridged or corrugated or suitably distorted to improve the keying of the paste to the plate, and especially to increase the rigidity and stiffness. Ridging or dishing of the sheet flashing would greatly increase the stiffness and rigidity of the plate, and this could allow a reduction in the amount of lead needed in the lower region of the plate, where its main purpose is to provide strength to allow the plate to be self supporting, and to allow the plate to be pasted without undue care and support being given to it. In addition to the stiffening by cormgating or dishing of the sheet form, as the paste is now keyed to the grid by slots, holes, buns etc. in the sheet flashing, it is no longer necessary to have the grid "wires" or ribs the common diamond shape, which helps to key the paste to the grid apertures. This shape is poor for providing stiffness, since most of the material is close to the neutral axis, giving no stmctural benefit. When ribs

are added, (vertically for conductivity improvement, and horizontally for stiffness enhancement if required) they can be shaped to provide much improved structural stiffness for the same amount of material. Both the conductivity and stiffness improvements of these grids allow improved properties for the same amount of lead, or similar properties for considerably less lead in the grid.

Efficient use of lead in a rib-reinforced sheet-style grid can improve the grid conductivity by a factor of 5, compared to current conventional grids. The improvement in plate performance from this method is of course substantially greater due to the non¬ linear effect of the overpotential developed by the current flow in the grid. To improve the usage of the lead in the grid, the size of conductors (wires in particular, but also taking note of the conductivity of the thin sheet flashing) can be matched to the current flow. For example, in a conventional battery plate, the horizontal wires carry little current, and represent inefficient use of lead, as they are there for ease of casting (as vertical runners in the standard book mould) and strength only. However, in a design with one or more extended tabs, the horizontal wires can be used to feed current sideways to the tab (central) and borders. The thickness and spacing of the grid "wires" in a sheet flashed plate with dishing, cormgating or ridging of the sheet providing the necessary strength can then be matched to the lateral current requirements, without other constraints. It can be seen that the present invention can provide a frame for an electrode plate which does not require discrete horizontal grid wires or bars and which derives its strength from using webs or sheets which are largely or wholly continuous in the vertical direction, this being the intended overall direction of the cunent flow. Preferably the stmctural function of the horizontal grid wires or bars is largely or wholly replaced by a thin web or sheet having a vertical continuity to enable it to carry current effectively in the vertical direction.

A frame for an electrode plate in which stiffness and rigidity is provided by or enhanced by a suitably deformed web or sheet substantially continuous in the vertical direction so as to both being an effective cunent carrying system in the vertical direction and a stiffening method obviates the need for horizontal wires or bars.

Ideally the frame in which the pockets or mechanism used for containing the active material has such dimensions and shapes that the average cunent path in the active material to the nearest conducting frame component is not more than 3 mm.

The use of a frame in accordance with the present invention has other advantages.

Battery performance is significantly limited by the poor conductivity of the positive paste, which is a semiconductor. Consequently additives are used to improve its conductivity.

By using the new sheet flashing-style plates, the mean effective resistance from the positive paste to the grid can be reduced by a factor of about 10. Therefore efforts to artificially increase the conductivity of the positive paste with additives are no longer important. This allows much more freedom to concentrate on improving the porosity of the paste.

A form of active paste in which there is incorporated a high degree of porosity so that the density of the active material when incorporated in the frame is less than 4 gm/per cm ³ is prefened. This allows the acid electrolyte to intermingle in good proportions with the active mass to alleviate the problem acid starvation during a high cunent discharge.

Taking advantage of the present invention to extend the porosity of the pastes will have several effects, including: a) It will increase the CCA capacity of the plate by making more acid immediately available for the discharge to continue, and by reducing the development and effects of an insulating surface layer of PbSO4 as the cell discharges, since the exposed surface area will be very much greater; b) It will allow a trade-off in the battery of reducing the amount of paste (which is very dense) and increasing the amount of acid, so that there is a better balance in the three required constituents, namely positive active mass, negative active mass, and acid. The power-to- weight ratio of the battery is significantly improved (e.g. by more than 25%) by this approach. c) . It will alleviate the effects of pore blockage at or near the surface by PbSO4 early in the discharge, especially at high currents. d) . However, increased porosity increases the mean resistivity of the positive plate paste, as poorly conducting PbO2 will be replaced in part by much poorer conducting acid. This is only acceptable if a sheet-flashing-style of plate is used to reduce the cunent path in the paste to the grid to counter the effect of increased positive paste resistance.

The present invention can be manufactured by a variety of methods.

At present the majority of battery grids are cast. This method is easily modified to allow sheet flashing to fill the median plane of the grid, with or without keying holes for the active matter. Flaws in the casting of both the wires and the flashing are of little matter because of the added strength and conductivity given to the grid by this method. It is also possible to modify the moulds to include small but significant amounts of dishing or ridging in the flashing to stiffen the grid. However, such modifications could also be done cheaply and easily (as well as perforating the flashing) in a press in a subsequent operation.

In a preferred method of manufacture, the frames are cast in a mould so that the vertical runners for the casting process within the frame region become the vertical cunent carrying bars for the frame. This makes efficient use of the lead in the larger size runner channels needed in the vertical pour of the casting.

Alternatively, the techniques for plate improvement discussed above allow for the manufacture of battery grids by pressing or stamping techniques. For example, it would be possible to use rolled lead sheeting with formed wires or ribs in the longitudinal direction, and with a central wider tab extending the length of the grid. The next operation is then to deform the rolled sheet to provide ridging for lateral and longitudinal stiffness. The thin sheet between the ribs would provide adequate lateral conductivity, and the result is a better and cheaper and stiffer battery plate with all of the above advantages except that the conductivity in the vertical direction will not change with height. This is of course no worse than conventional battery grids.

An improved manufacturing step is to use an extruding roller that concentrates the lead towards the top of the plate by incoφorating a rolling pattern that provides tapered rib borders, a central tapered rib developing into the tab, further tapered ribs, and also, or alternatively, allowing the thickness of the thin sheeting between the ribs to increase towards the top of the grid. Such a roller extrusion system could also put in the dishings or corrugations to provide the stiffened plate.

Because of the ductility of lead and the alloys used in battery grids, stamping processes could be used to completely form a grid in one operation, incorporating all of the improvements considered above. Obviously the initial set-up cost would be higher, and it is a departure from cunent methods. However, it eliminates the considerable difficulties with casting methods in present use, and should be adaptable without modification to a wide range of alloys for the grids.

According to one aspect of the present invention there is provided a method for manufacturing an electrode plate including applying active material to electrode plates, by spraying the active material onto said electrode plates.

It should be appreciated that although in many embodiments, the active material will be comprised of lead and lead oxide as described previously, the present invention will also apply to active material having different compositions.

The active material may be sprayed onto the electrode plates by a variety of methods. In one embodiment, a spraying apparatus similar to that used for spraying cement may be used. For example, there may be provided a hopper positioned above a spray nozzle. Compressed air may be injected into the nozzle apparatus propelling or drawing the active material through the spray nozzle. Other spraying apparatus may of course be used.

In preferred embodiments of the present invention, the electrode plate may be sprayed with active material simultaneously from spray nozzles at opposing sides of the electrode plate. This method of spraying has a number of advantages.

Firstly, the force of the opposing sprays with respect to each other will limit undue spattering of the active material beyond the electrode plates.

Further, the interaction of the two sprays at the electrode plate assists to key the active material strongly to the electrode plate. The applicant has found that the above process enables thinner grid wires to be used which reduces the cost and weight of a battery incorporating electrode plates made as described.

The present invention lends itself readily to automation and in preferred embodiments, the electrode plates will be conveyed past the spray nozzles. In one embodiment, the electrode plates may be gripped in a clamp situated either above or below the spray nozzles leaving the majority of the surface area of the electrode plate exposed to the spray from the nozzles.

In some embodiments, the electrode tab, which is not required to be coated with active material, may be the handle by which the electrode plate is moved through the coating process.

Preferably the whole of the process is contained within an enclosure such as a tunnel due to the toxic nature of the active material. There may be provided drainage and other

means for removing excess amounts of active material that do not adhere to the electrode plates.

The present invention lends itself to be adapted to apply various materials to the electrode plates rather than just the active material. For example, prior to the application of active material through spray nozzles, another set of spray nozzles may apply a conditioning treatment such as cleansing fluid to the electrode plates. Conditioning chemicals which may also form part of a conditioning treatment can be applied to the plates at the start of the process making them more receptive to the later application of active material. With the present invention, a number of coats of active material may be apphed. For example, there may be multiple pairs of spray nozzles set up so that the electrode plate can be conveyed by the conveying system from one pair of spray nozzles to the other for subsequent coats of active material.

Multiple spray coats may ensure that the active material adheres more strongly to the electrode plate.

In some embodiments, the coats applied to the electrode plates may have differing densities, consistencies and compositions from each other.

Following the application of active material, there may be provided a further coat or coats to the electrode plates. For example, a porous material may be sprayed onto of the active material. This material can act as a separator holding the electrode plates apart. Being porous, the material can still allow electrolyte to pass through the separator and reach the active material on the electrode plate. In one embodiment, the separator material may be a foam.

There may be provided further conditioning means such as a diyer. It should be appreciated that the present invention provides a means by which a greater amount of active material can be applied to an electrode plate than previously and be adhered more strongly. This leads to greater battery efficiency. Further, the present invention provides an automated way by which electrode plates can be made which leads to lower manufacturing costs. In addition, thinner grids can be used. And, in some embodiments, there is no need to supply a discrete separator material as an application of a foam or some other material makes the separator integral with the electrode plate.

The applicant believes incorporating inert fibres into that active mass of the battery improves the behaviour of the plate under rapid discharge conditions. This is probably

because it improves the porosity of the material to the electrolyte, and in addition, allows capillary action to pull electrolyte into the paste, improving the effective conductivity of the active matter.

The fibres may be made of polymers, glass, metal, be conducting, hollow or otherwise. The fibres can also be crimped or distorted to increase the reinforcing effect of their present and to increase the porosity of the paste by making voids which become open during the curing process, and/or to increase the conductivity of the paste.

In the case of sheet flashing style grids, with such methods, it can be relatively easy to incorporate fibres. They also have the additional advantage in this case that they provide reinforcing for the active matter, similar to the use of chopped strands in cement fibre boards (such as fibrohte).

An additional application of such fibres is to pierce the sheet flashing and insert a tuft of fibre in each pierced hole. Then the fibre can not only fulfil the above effects of

(a) improving the electrical performance of the plate by the actions described above, and

(b) reinforcing the paste to make it stronger and to hold it together, but

(c) it can also cause the paste to be restrained very strongly to the grid. Even if a microscopic gap occurs between the reinforced paste and the sheet flashing lead grid at the interface, the paste is still unable to be shed, and the porosity will ensure that a thin layer of electrolyte still maintains electrical connection over a wide surface area between the body of the active matter paste and the lead grid.

BRIEF DESCRIPTION OF DRAWINGS

Aspects of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 illustrates an electrode plate in accordance with one embodiment of the present invention; and

Figure 2 illustrates an enlarged cross-section of the plate illustrated in Figure 1 , and

Figure 3 illustrates part of a die which could be used for casting the electrode plate, and

Figure 4 is a diagrammatic view of an electrode plate being sprayed with active material, and

Figure 5 is a diagrammatic plan view of one possible process in accordance with the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

With respect to Figure 1, there is illustrated an electrode plate 19 which has been manufactured in accordance with the present invention. The paste and active material is indicated by numeral 19A.

Wires 19B form a rectangular grid across the electrode plate 19.

A moulding flash containing perforations is indicated by an anow 19C. A closer view of these perforations is indicated by anow 19D.

Figure 2 illustrates in an enlarged cross-section of at least two wires 19B of the grid 19. The active material 19A has been applied to the grid 19 and is shown to cover both sides of the mould flash 19C. In addition to this, the active material also keys in between the perforations 19D of the mould flash.

Figure 3 illustrates part of a die which can be used to cast the electrode grids. In particular, two parts of a section of the die closed together is illustrated. A wire cavity is indicated by 19E. The large perforation at the centre of the grids is created by the die being closed tightly at the points approximately centre of the wire cavity in the mould flash. Smaller perforations are indicated by anow 19G.

Using a die as illustrated above, will create an electrode plate which is moulded to allow a flash of conductive material to fill part of the space between each grid wire. The mould flash contains a major perforation created by the die as well as a mould lining material apphed to the moulding.

The active material can be applied to the grid by conventional methods as described earlier.

A further preferred method however which is believed to be inventive in itself is described below and illustrated in Figures 4 and 5.

With respect to Figure 4, there is illustrated an electrode plate 1 which has been sprayed with active material 2.

The electrode plate 1 is being held into a substantially vertical position by a clamp 3 which is situated on a conveying belt 4. The belt 4 has a wide mesh configuration to allow for drainage of any excess active material 2 through the apertures 5 in the belt 4.

Two spraying devices 6 and 7 apply the active material 2 to the electrode plate 1. The spraying devices 6 and 7 each comprise of a hopper 8 (not fully shown), nozzle housing 9, a nozzle 10 and an injection port 11. The configuration and operation of the spraying devices 6 and 7 are similar to that used for spraying plaster. Compressed air enters through the injection port 11 and draws active material from the hopper 8 into the nozzle housing 9 and out through the nozzle 10. The nozzle 10 has been adjusted to ensure that the spray pattern covers the whole of the surface area of the electrode plate 1. By having competing sprays of active material 2, undue spatter of active material is reduced. Further, if the electrode plate has increased flashings as prefened, there are fewer and smaller apertures through which the active material can pass. Figure 5 illustrates one possible process in accordance with the present invention. For the purposes of illustration, the conveyor belt 4 and the clamps 5 which support the electrode plates 1 are not shown in Figure 5.

A conveying system (not shown) passes electrode plates 1 through a tunnel 12 in the direction of the anows. The first pair of spraying devices 13 that the electrode plates counter spray the electrode plates 1 with a conditioning chemical 14. In some embodiments, the conditioning chemical 14 may include cleansers or possibly abrasives or some other material which could cause a slight pitting of the electrode plates to allow the active material to more readily key into same.

The second pair of spraying devices 15 spray active material 2a having a high density onto the electrode plates.

Spraying devices 16 spray active material 2b having a lower density than active material 2a.

Finally, spraying devices 17 spray a foam 18 onto the electrode plates 1. The foam 18 hardens to become a separator for the electrode plates yet allowing electrolyte to reach the active material 2 on the electrode plates 1.

After the application of the foam, the electrode plates 1 proceed past hot air dryers 20.

Preferably the spacing of the spraying devices 17 with respect to each other and the speed of the conveyor are matched to ensure that each application of material to the

electrode plates has reached a condition that makes it receptive to the next part of the process.

Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof as defined in the appended claims.