GB2129198A - Expanded electrodes for electric storage batteries - Google Patents

Abstract

An electrode for an electric storage battery comprises a substantially rectangular expanded metal grid 2 carrying active electrode material and a current take-off member 8 connected to the grid and projecting therefrom. The grid comprises a plurality of meshes defining a plurality of substantially diamond-shaped apertures which have a minor axis and a major axis which is longer than the minor axis. The major axis extends substantially parallel to the direction in which the current take-off member extends thereby minimising the length of the current pathway within the grid. A bipolar electrode pair comprises two such electrodes of which the current take-off members are integral. <IMAGE>

Description

SPECIFICATION Expanded electrodes for electric storage batteries The present invention relates to expanded electrodes for use in electric storage batteries, particularly batteries of lead acid type. Electrodes for lead acid batteries of e.g. automotive type are conventionally of Faun6 type, that is to say they comprise a grid of lead or lead alloy pasted with damp active electrode material which is subsequently dried. The grid is conventionally of cast type but in recent years the battery industry has made increasing use of expanded grids, that is to say grids made by expanding a strip of lead or lead alloy, and the present invention is concerned with electrodes incorporating grids of this latter type.

One known method of manufacturing expanded electrode grids for a lead acid battery is illustrated in Figure 1 of the accompanying drawings. A strip of lead or lead alloy is progressively expanded inwardly from its outer edges to produce two elongate expanded meshes 2 interconnected by an unexpanded central land 4. Lozenges are successively punched from the central land to form regularly spaces apertures 6 thereby leaving the two meshes connected together by regularly spaced solid bridge pieces 8. Cuts 10 are then formed in each mesh at a regular spacing equal to the pitch of the bridge pieces, the cuts in the two meshes being offset by half a pitch to form a plurality of grid pairs, each comprising two grids offset by half a pitch and interconnected by a solid integral bridge piece.The grid pairs may then be separated into individual electrode grids by cutting them along the central line of the unexpanded land, as indicated by the chain-dotted line 12.

After making the cuts, or more preferably prior to making the cuts, the two grids are pasted with active electrode material. Expanded metal grids conventionally comprise a plurality of meshes, that is to say elongate metallic elements, which together define a plurality of elongate diamond shaped apertures each of which has a major axis and a minor axis shorter than the major axis. The method illustrated in Figure 1 coupled with the mechanism by which expansion is effected inherently results in the major axes of the diamond shaped apertures extending parallel to the length of the strip of lead or lead alloy. As a consequence, the major axes of the diamond shaped apertures in the finished electrodes extend perpendicular to the direction in which the bridge pieces 8 or the electrode lugs consituted by one half of the bridge pieces 8 extend.This means that the current path within each battery electrode from its solid lug to the edge of the electrode remote from the lug is inherently greater than the straight line distance between them since the curent is obliged to follow a tortuous path. The meshes of the grid have an appreciable resistance and the long current path within the electrodes thus results in each electrode having an appreciable resistance also.

It is an object of the present invention to provide an electrode for an electric storage battery which incorporates an expanded grid but which has a reduced resistance as compared to grids manufactured by the method illustrated in Figure 1.

According to the present invention an electrode for an electric storage battery comprises a substantially rectangular expanded metal grid carrying active electrode material and a current take-off member connected to the grid and projecting therefrom, the grid comprising a plurality of meshes defining a pluraltiy of substantially diamond-shaped apertures, the apertures having a minor axis and a major axis which is longer than the minor axis and extends substantially parallel to the direction in which the current take-off member extends.

Thus in the electrode in accordance with the present invention the major axis of the diamondshaped apertures does not extend perpendicular to the direction in which the current take-off member projects, as is conventional, but substantially parallel to this direction. This reduces the length of the current path within the electrode between the current take-off member and the edge of the electrode remote from this member and thus reduces the internal resistance, and consequently the maximum current output, of a battery incorporating a plurality of such electrodes.Typically, the diamond-shaped apertures of an expanded metal grid have two angles of about 450 and two angles of about 1350 and, depending on the precise geometry of the electrode, an electrode in accordance with the present invention may have an internal current path some 20 or 30% less than a conventional electrode and this is translated into a corresponding reduction in the resistance of the electrode.

The current take-off member may itself be of expanded metal and in this case it is preferably integrally formed with the grid. However, forming an electrical connection with such a current takeoff member may pose certain problems und thus, in an alternative embodiment, the current take-off member is of solid unexpanded metal and in this case it may be subsequently connected to the grid.

Alternatively, the expansion may be effected in a manner which leaves an integral unexpanded area which, or part of which, constitutes the current take-off member.

An electrode in accordance with the invention may be simply made by continuously expanding a metallic strip, e.g. of lead or lead alloy, sequentially along its length such that the major axes of the diamond-shaped apertures extend transverse to the length of the strip rather than parallel to the length of the strip as is the case in the known method in which the strip is expanded sequentially inwardly from its edges. Thereafter, the method may proceed in a similar manner to the known method, that is to say portions are removed from the expanded strip thereby leaving the two halves of the strip connected by spaced portions and appropriate cuts are then made in the two halves of the strip and in the spaced connecting portions to form individual electrodes.

Each electrode grid may be pasted with active electrode material subsequent to being cut from the expanded strip but it is preferred that the two halves of the strip are pasted prior to being cut up, and in this connection it may be advantageous to paste the two halves of the strip with positive and negative active electrode material respectively.

The invention also embraces a bipolar electrode pair comprising two electrodes of the type referred to above of which the current take-off members are integral. Such a bipolar electrode pair may be made by a method very similar to that described above, but at the cutting stage cuts are formed only in the two halves of the expanded strip and not in the spaced connecting members which constitute the integral current take-off members in the finished bipolar electrode pairs. If the metallic strip is perforated over its entire area this will result in bipolar electrode pairs in which the integral current take-off members are expanded also.Alternatively, the method may be performed in a manner which leaves an unexpanded area which, or part of which, constitutes the integral current take-off members or in a still further possibility, two separate electrodes may be manufactured and subsequently connected by a bridge piece of solid metal which is burnt to the two expanded grids.

The two electrodes of the bipolar electrode pair may be offset from one another in the direction of the length of the strip from which they were cut in which case the integral current take-off members will extend only over a proportion of their length.

Alternatively, the two electrodes may be disposed side by side and in this event the integral current take-off members may extend over only a proportion of their length or alternatively over their entire length. Thus in its simplest form the bipolar electrode pair may constitute a single rectangular expanded metal grid of which one part carries positive active electrode material and the other part carries negative active electrode material, the two parts being separated by an unpasted area which constitutes the integral current take-off members.

Bipolar electrode pairs in accordance with the present invention may be used in a conventional battery of flooded electrolyte type in which the two electrodes are situated in adjacent cells and the integral current take-off members constitute the intercell connector between those two cells.

However, in such a battery it is necessary that adjacent cells are reliably electrolytically sealed from one another to prevent the occurrence of intercell ionic leakage currents and this may be a little problematic, particularly if the current takeoff members are of expanded metal. Thus the bipolar electrode pairs in accordance with the present invention find particular application in those batteries in which there is substantially no mobile electrolyte available for the conduction of intercell ionic leakage currents, e.g. recombination batteries which are batteries which utilise microfine glass fibre separator material and have substantially no free unabsorbed electroltye and gases evolved within the cells are induced to recombine and are not vented to the atmosphere.

There is believed to be a substantially diminished risk of intercell ionic leakage currents occurring in such batteries and thus the current take-off members which constitute intercell connectors may simply extend over the intercell partitions without being sealed thereto.

In a preferred embodiment of the invention, the two electrodes of a bipolar electrode pair are disposed side by side and the integral current take-off members extend from one end of the electrodes to substantially their centre point.

When incorporated into a battery, the integral current take-off member may remain planar or it may be bent through substantially 1800 to pass over or around an intercell partition. Such bipolar electrode pairs are particularly suitable for small capacity batteries having only one electrode of each polarity per cell since there will then be no need to from either plate straps or intercell connectors in a separate step. However, the capacity of the battery may be increased in the construction referred to above by making the electrodes approximately twice their normal size and forming a single fold in each of them.Thus one preferred embodiment of a bipolar electrode pair in accordance with the present invention comprises two electrodes disposed side by side integrally connected by expanded metal over half their length from one end to substantially their centre point, the two electrodes bearing positive and negative active electrode materials respectively. In use, a fold is formed in the integral current take-off members connecting the positive and negative areas and this folded portion extends over the full length of an intercell partition of a recombination battery. Each electrode has a single fold formed half way along its length transverse to the fold in the integral current take-off members and the folded areas of these areas are intercalated with those of adjacent similar electrodes so that the resulting battery effectively has four plates per cell.

Further features and details of the invention will be apparent from the following description of one specific method of making an electrode or a bipolar electrode pair for use in a lead acid electric storage battery which is given by way of example only with reference to Figure 2 of the accompanying drawings which is a plan view of an expanded strip of lead alloy subsequent to the removal of lozenges from it and the formation of spaced cuts in it.

A strip of lead or lead alloy is continuously expanded over its entire area to form an expanded grid or mesh 2. This may be effected by any suitable expander machine known per se but is preferably effected by a reciprocating machine which successively forms a single line of diamond shaped apertures across the full width of the strip with the major axis of the apertures extending transverse to the length of the strip. It will be appreciated that, if desired, a thin unexpanded selvedge may be left along each edge of the strip to provide a pathway of increased electrical conductivity and to ensure the mechanical integrity of the edges of the strip.The expanded strip is then passed through a reciprocating lozenge punch which punches out apertues 6 in the expanded mesh which are situated along its longitudinal centre line and whose length is substantially equal to the spacing between them thereby effectively dividing the strip into two halves integrally interconnected by regularly spaced bridge pieces 8 of expanded metal. The expanded strip is then passed through a conventional pasting machine and positive active electrode paste is applied to one half of it and negative active electrode paste to the other half.

The areas of active electrode material are dimensioned to terminate at or adjacent to the side edges of the aperture 6.

Either before or after drying of the active electrode pastes, spaced cuts 10 are formed in the two halves of the expanded and pasted strip which extend perpendicular to its length and are spaced apart by a distance equal to the pitch of the aperture 6. The cuts are arranged to communicate with one or other end of the aperture 6 and in this embodiment the cuts in the two halves of the strip are offset by half a pitch. The resultant bipolar electrode pairs (from which the active electrode material has been omitted for the sake of clarity) then have the form shown in Figure 2 and may be used as such or alternatively each bridge piece 8 may be severed at its centre point along the chaindotted line designated 12 to cut them into individual electrodes.In either event, each electrode comprises a rectangular expanded grid of which the major axis of the diamond-shaped apertures extends parallel to the current take-off member 8.

In an alternative embodiment, which is not illustrated, the cuts 10 in the two halves of the expanded strip are aligned with each other and this will result in bipolar electrode pairs of which the two halves are not offset as illustrated in Figure 2, though it will be appreciated that if these bipolar electrode pairs are separated into individual electrodes the resulting electrodes will be identical to those produced in the method illustrated in Figure 2.

The resulting electrodes or bipolar electrode pairs are then used in the manufacture of an electric storage cell or battery. If a battery is to be manufactured, each cell may contain only one electrode of each polarity and each electrode, with the exception of every alternate electrode in the two end cells, may constitute one half of a bipolar electrode pair manufactured by the method described above and the remaining electrodes in the end cells may constitute a single electrode manufactured in the method described above. In the preferred embodiment, the battery is of recombination type containing substantially no free unabsorbed electrolyte and the integral bridge pieces simply extend over or round the intercell partitions and are not sealed thereto. In such a battery there is no need to form plate straps or intercell connectors but it will be appreciated that terminal connections must be made to the current take-off members of the two single electrodes in the end cells and this may conveniently be done by burning a solid member of lead or lead alloy to it. If a battery of somewhat greater capacity is required, each half of each electrode may be folded once and its folds intercalated with those of an electrode of adjacent polarity which effectively results in a battery having two plates of each polarity in each cell.

Claims (12)

1. An electrode for an electric storage battery comprising a substantially rectangular expanded metal grid carrying active electrode material and a current take-off member connected to the grid and projecting therefrom, the grid comprising a plurality of meshes defining a plurality of substantially diamond-shaped apertures, the apertures having a minor axis and a major axis which is longer than the minor axis and extends substantially parallel to the direction in which the current take-off member extends.

2. An electrode as claimed in Claim 1 in which the take-off member is of expanded metal and is integrally formed with the grid.

3. An electrode as claimed in Claim 1 in which the current take-off member is of solid unexpanded metal subsequently connected to the grid.

4. A bipolar electrode pair comprising two electrodes as claimed in any one of the preceding claims of which the current take-off members are integral.

5. An electrode pair as claimed in Claim 4 in which the two electrodes are disposed side by side and the integral current take-off members extend from one end of the electrodes to substantially their centre point.

6. An electrode for an electric storage battery substantially as specifically herein described with reference to Figure 2 of the accompanying drawings.

7. A bipolar electrode pair for an electric storage battery substantially as specifically herein described with reference to Figure 2 of the accompanying drawings.

8. An electric storage battery including a plurality of electrodes as claimed in any one of Claims 1 to 3 and 6.

9. An electric storage battery including a plurality of bipolar electrode pairs as claimed in any one of Claims 4, 5 and 7.

10. A battery as claimed in Claim 9 which is of recombination type and in which each cell contains substantially no free unabsorbed electrolyte.

1 A battery as claimed in Claim 10 in which the integral current take-off members extend over an intercell partition of the battery and are not sealed thereto.

12. A battery as claimed in Claim 1 1 in which each electrode has a single fold in it substantially half way along its length and the integral current take-off members extend over substantially the full length of the associated intercell partition.