GB2129194A - Manufacturing electric storage cells - Google Patents

Abstract

A method of manufacturing a recombination electric storage cell comprises assembling a cell pack of alternating positive and negative electrodes 4 and 6 interleaved with separator material. The cell pack is inserted into a plastics bag, electrolyte is added to the cell pack, the plastics bag is surrounded by water in a water bath and an electric current is passed through the cell pack to form it. The cell pack is then removed from the water bath and inserted into its final outer container. <IMAGE>

Description

SPECIFICATION Manufacturing electric storage cells The present invention relates to the manufacture of electric storage cells, particularly those of recombination type, and batteries of such cells and relates in particular to the manufacture of such cells and batteries of lead acid type. The present invention is concerned with the formation of the electrodes for such cells and batteries, formation being the term applied to the initial conversion of the active electrode material into its electrochemically active form, e.g. the oxidization and reduction in a lead acid cell of the lead oxide electrode material on the positive and negative electrodes to lead dioxide and spongy lead respectively.

Recombination electric storage cells are those cells which contain substantially no free unabsorbed electrolyte and in which the gas evolved during operation or charging is normally induced to recombine within the cell and is thus not vented to the atmosphere.

When manufacturing lead acid cells and batteries other than of Plante type, e.g. automotive batteries, positive and negative electrode grids are pasted with damp positive and negative active electrode material respectively and the electrodes are then cured. Curing is the subjection of the electrodes to controlled conditions of heat and humidity so as to dry them and corrode and etch the grids thereby cementing the active material to the grids and to oxidise the free lead in the active material. Curing is conventionally effected by forming stacks of individual electrodes, e.g. on pallets, and leaving them for a protracted period of time in the controlled conditions.

Subsequent to curing, the electrodes are formed. The electrodes may either be formed individually or after connecting them together either into individual cell packs or into complete battery packs, each of which includes all the electrodes for a complete battery. In the latter case, the formation may either be effected prior to insertion of the cell or battery pack into the final container or after such insertion.

So called tank formation necessitates placing the cured electrodes into a tank of electrolyte and individually connecting them to a current supply and after formation is complete removing them from the tank and connecting them into cell or battery packs.

The insertion of the individual electrodes into the formation tank and their subsequent removal, cleaning and interconnection is a labour intensive task and for this reason so-called jar formation is the process usually used in which the cell or battery packs are formed whilst within their final container.

Whilst this eliminates one handling stage, it considerably slows down the formation process since the outer cell or battery container which is usually of hard rubber or polypropylene, is an effective thermal insulator which necessitates the formation process being conducted slowly since this process evolves a substantial amount of heat. In addition the outer container tends to be soiled by acid and must thus be cleaned and dried as a final step in the assembly process.

In conventional batteries of flooded electrolyte type it is essential that adjacent cells be electrolytically sealed from one another to avoid the occurrence of deleterious intercell ionic leakage currents and thus the containers for such batteries are provided with integral intercell partitions separating the container into sealed compartments. However, in batteries of recombination type it is found that such leakage currents do not constitute a major problem, the reason for which, it is believed, is that there is substantially no mobile electrolyte available for the conduction of such leakage currents.

British Patent Specification No. 2062945 of the present applicants discloses a recombination electric storage battery including a plurality of cell packs, each of which is accommodated in a respective open plastics bag, within a common uncompartmented outer container. Adjacent cells are thus separated only by two thicknesses of plastics film and communicate with the common headspace beneath the lid but nevertheless intercell ionic leakage currents are found to be a substantial problem.

It is an object of the present invention to provide a method of manufacturing an electric storage cell or battery, in particular of recombination type, which involves a reduced amount of handling of the electrodes at the formation stage. It is also an object of the invention to provide such a method which also permits the formation to be effected more rapidly than is usual.

According to the present invention a method of manufacturing an electric storage cell comprises assembling a cell pack of alternating positive and negative electrodes interleaved with separator material, inserting the cell pack into a plastics bag, adding an amount of electrolyte to the cell pack, dipping the plastics bag into a water bath and passing an electric current through the cell pack to form it, removing the cell pack from the water bath and inserting it into its final outer container. Thus in the method of the present invention the cell pack is formed whilst within a plastics bag which in turn is surrounded by water in a water bath.As a result of the fact that the walls of a plastics bag, which are preferably less than 0.25 mm thick and more preferably less than 0.1 mm thick, are relatively good thermal conductors, at least in comparison to the walls of a conventional battery container, the heat evolved in the cell pack during formation is rapidly conducted away thus enabling formation to be effected more rapidly than is usual. It will be appreciated that for any given throughput of cells, reduction in the time required for formation results in a reduced requirement for formation apparatus and thus reduced capital cost for such apparatus.

The invention is applicable to both conventional cells of flooded electrolyte type and also to recombination cells in which the separator material is compressible fibrous absorbent material, e.g.

microfine glass fibre material. In either case the cell pack and its associated bag may be placed in the outer container or the cell pack may be removed from its plastics bag prior to inserting it into the container in which case the bag may form part of the formation installation.

If the cell pack includes a plurality of separate electrodes at each polarity, these must be connected together prior to formation, but in the preferred embodiment the cell pack contains a single electrode member of each polarity, each electrode member having one or more folds in it to define two or more electrodes integral with one another. Thus in this embodiment the electrodes are electrically connected together automatically by virtue of their construction and thus a separate step to interconnect them is not required.

The invention embraces also a method of manufacturing an electric storage battery including two or more cells and is particularly applicable to a recombination battery of the type referred to above in which the cell packs are placed in an uncompartmented container whilst within their plastics bags, the material of which serves as the intercell partitions in the finished battery. At some stage in the manufacture of such a battery the cell packs must be interconnected and this may be effected either before or after their formation.Preferably, however, each cell pack contains a single electrode member of each polarity, each electrode member having one or more folds in it to define two or more electrodes integral with one another and each electrode member of the battery with the exception of one electrode member in each end cell is integrally connected to an electrode member of opposite polarity in an adjacent cell. In this embodiment no positive step is required to interconnect the electrodes of the same polarity in each cell or to interconnect adjacent cells since all the necessary connections are automatically present by virtue of the inherent constructional features of the battery. In this embodiment the whole battery is of necessity formed as a single unit rather than its constituent cell packs being formed individually.

As referred to above, recombination batteries contain a reduced amount of electrolyte and the amount of electrolyte is preferably insufficient to saturate all the pores in the electrodes and separator material. It is however desirable that during formation an amount of electrolyte in excess of this reduced amount should be present and in practice at least a part of this excess is gassed off during the formation process. Thus the finished battery may contain an amount of electrolyte in excess of that at which efficient recombination of the gas evolved in the battery occurs and thus initially the battery may produce both hydrogen and oxygen which is not recombined but is vented to the atmosphere.This will result in a progressive decrease in the amount of electrolyte present in the battery and as this amount approaches the critical value the amount of oxygen produced on gassing will increase relative to the amount of hydrogen and the recombination efficiency will improve. Consequently the battery may not perform efficiently as a recombination battery for some time after it has been put in service.

Further features and details of the invention will be apparent from the following description of one specific embodiment which relates to the assembly of a three cell six volt lead acid standby battery and is given by way of example only with reference to the accompanying drawings, in which Figure 1 is a diagrammatic plan view of the two different types of electrode member used in the assembly of the battery; Figure 2 is a diagrammatic perspective view of the electrode assembly for the battery; and Figure 3 is a graph showing the variation of the temperature of the interior of the battery with time whilst the battery is being formed.

The battery incorporates two intermediate electrodes, generally designated 2 in Figure 1, and 2 end electrodes designated 4 in Figure 1. Each intermediate electrode comprises two elongate rectangular grids 6 of lead or lead alloy along one edge of which is a solid selvedge 8. The selvedges of the two grids are connected by an integral bridge piece 1 0 which is slightly offset from the central portion of the two grids. The end electrodes 4 can be considered to be one half of an intermediate electrode, that is to say when cut across the centre of the bridge piece. The end electrodes can be manufactured in any convenient manner, for instance by casting.It is however preferred that they are produced from a strip of lead or lead alloy which is expanded, and in one embodiment such a strip is expanded inwardly from its two edges to leave a central unexpanded land from which portions are subsequently removed to leave two expanded meshes with a solid selvedge connected at intervals by bridge pieces. The two expanded meshes are then pasted with positive and negative active electrode material respectively and the strip is then cut to form intermediate electrodes as shown in Figure 1. As an alternative to using positive and negative active electrode materials on the two halves of the intermediate electrodes, both halves could bear the same universal active electrode material. In an alternative embodiment, which is not illustrated, a strip of lead or lead alloy is expanded over substantially its entire area and a central unexpanded land is not left. The method then proceeds as before and it will be appreciated that this will result in intermediate electrodes whose two halves are connected by a bridge piece of expanded metal rather than of solid metal. This results in a reduced wastage of metal but due to the fact that expanded metal has a lower conductivity per unit area than solid metal the bridge pieces in this construction preferably have a length greater than that shown in Figure 1.

The battery is assembled by forming a single fold half way along the length of one end electrode and half way along the length of one half of one intermediate electrode and then intercalating the layers of the two folded electrodes with microfine glass fibre separator material between them. The bridge piece 10 of the intermediate electrode is then bent through 1800 and the other half of this electrode is then folded and intercalated with one half of the second intermediate electrode in a similar manner. The bridge piece of the second intermediate electrode is then bent through 1800 and the other half of this second intermediate electrode is then folded and similarly intercalated with the other end electrode.The resultant structure then has the form shown in Figure 2 which comprises three cells, each of which has two electrodes of each polarity integrally connected by a fold. Adjacent pairs of cells are connected by a folded bridge piece 10 which constitutes an intercell connector and the two end cells have a projecting terminal connector.

Each cell is placed within a respective open-topped plastics bag either as the assembly of the battery progresses or when all the folding and intercalation is complete and the battery has reached the stage illustrated in Figure 2. The electrodes are then cured within the bags or alternatively they may already have been cured prior to assembly of the battery. Electrolyte is then added to each cell in an amount in excess of that required to saturate the pores and electrodes and the excess electrolyte is retained in position by the plastics bags.

The battery unit is suspended in a water bath to a depth such that the electrodes and separators are substantially below the water level but the tops of the plastics bags are not. An electric current is then passed through the battery element to form it during which process a proportion of the excess electrolyte is electrolysed and thus gassed off.

The cell packs are then inserted into an uncompartmented rectangular section container of e.g.

polypropylene and a lid with a single vent is then sealed to the container. The two terminal connectors are connected to battery terminals in any conventional manner.

In this specific embodiment the grids and active paste of the various electrodes had the weight in grammes shown in the table below:-

Positive Negative end Intermediate Intermediate end Electrode Electrode Electrode Electrode Total Grid Weight 86.6 192.3 178.9 87.5 545.3 neg. pos. neg. pos. Paste Weight 129.0 115.5 129.0 117.2 126.8 115.6 738.1

The battery was formed at a current of 10 amps for 56 hours and the formation current was then continued overnight for 1 3 hours at a reduced current of 0.5 amps. During formation the temperature in the interior of one cell was measured and this is shown in Figure 3. As may be seen, the temperature was initially about 480C and this was due to the usual evolution of heat which occurs when adding sulphuric acid electrolyte to a dry cell pack. The temperature then fell relatively rapidly to about 340C as a result of the cooling effect of the water bath. The temperature then rose slightly due to the resistive heating effect which results from the fact that at an early stage in formation there are not enough active formation sites for the electric current to be efficiently converted and the excess is converted into thermal energy.After a short time, however, the formation proceeds increasingly rapidly and after about 1 hour the peak of resistive heating was finished and formation proceeds efficiently. From this point the temperature fell to about 250C at which point nearly all the active material was converted to its active form which occurred after about three hours. After this, a decreasing proportion of the applied electrical energy is stored in chemical form and an increasing amount produces heat and the temperature thus increased again to about 350C. After about four hours the heating effect ceased to increase and the temperature remained constant. When the formation current was reduced to 0.5 amps the temperature fell rapidly to about 240C by reason of the reduced resistive heating effect and then remained constant.

Throughout the formation the temperature of the water bath was maintained at 1 80C.

Claims (7)

1. A method of manufacturing an electric storage cell comprising assembling a cell pack of alternating positive and negative electrodes interleaved with separator material, inserting the cell pack into a plastics bag, adding an amount of electrolyte to the cell pack, dipping the plastics bag into a water bath and passing an electric current through the cell pack to form it, removing the cell pack from the water bath and inserting it into its final outer container.

2. A method as claimed in Claim 1 in which the cell is of recombination type and the separator material is compressible fibrous absorbent material.

3. A method as claimed in Claim 1 or Claim 2 in which the cell pack contains a single electrode member of each polarity, each electrode member having one or more folds in it to define two or more electrodes integral with one another.

4. A method of manufacturing an electric storage battery in which each cell is manufactured by a method as claimed in any one of the preceding claims.

5. A method as claimed in Claim 4 when dependent on Claim 2 or Claims 2 and 3 in which the cell packs are placed in a common uncompartmented outer container and the material of the plastics bags serves as the intercell partitions in the battery.

6. A method as claimed in Claims 3 and 5 in which each electrode member of the battery with the exception of one electrode member in each end cell is integrally connected to an electrode member of opposite polarity in an adjacent cell.

7. A method of assembling an electric storage battery of recombination type substantially as specifically herein described with reference to Figures 1 and 2 of the accompanying drawings.