GB2061604A - Electric storage batteries - Google Patents

Abstract

Each cell of an electric storage battery contains a pack comprising two or more electrode plates 2 and 4 separated by separators 6, of, e.g. microfine glass fibres, which extend above the plates and there form a continuous mat 8. The cells are spaced apart by partitions 12. The plate lugs 10 of one polarity of one cell are connected together and to those of the opposite polarity in an adjacent cell by a cast metallic bar within a mould 16. The bar extends through a hole 14 in the partition and rests on the fibre mat. In its preferred form the battery is of recombinant form and contains a reduced amount of electrolyte. <IMAGE>

Description

SPECIFICATION Electric storage batteries The present invention relates to electric storage batteries and in particular to intercell connectors for such batteries and to a method of forming such connectors.

According to one aspect of the present invention in a multicell electric storage battery each cell contains a pack comprising two or more electrode plates separated by compressible separators of heat resistant fibres which extend above the tops of the plates and there form a substantially continuous fibre mat, the cells being spaced apart by partitions of electrolyte impermeable material, the plate lugs of one polarity of one cell being connected together and to those of the plates of opposite polarity in an adjacent cell by a cast metallic bar which extends through a notch or hole in the intercell partition but not above its upper edge and which rests on the fibre mat.

A number of different methods of forming intercell connectors is known, both outside the battery such as by placing plates in a jig and pouring molten metal into a two part mould having a slotted bottom through which the plate lugs project, and inside the battery such as by locating a movable mould on the intercell partitions, sliding it around the plate lugs, pouring in molten metal and removing the mould.

The present invention is particularly concerned with electric batteries of the so-called "recombinant" type, in which substantially all of the gases evolved during operation or charging of the battery are induced to recombine within the battery.

Thus according to a further aspect of the present invention in a multicell electric storage battery each cell contains a pack comprising two or more electrode plates separated by compressible separators of heat resistant fibres which extend above the tops of the plates and there form a substantially continuous fibre mat, and contains substantially no free electrolyte, the cells being spaced apart by partitions of electrolyte impermeable material and the plate lugs of one polarity of one cell being connected together and to those of the plates of opposite polarity in an adjacent cell by a cast metallic bar which rests on the fibre mat.

In one embodiment of the invention the intercell partitions are substantially rigid and the metallic bar passes through a notch or hole formed in it but no above its upper edge.

In this case there is conveniently a mould integral with the intercell partitions in which the cast bar is moulded in situ.

In a further embodiment each cell pack is accommodated in a respective plastics bag, the material of which constitutes the intercell partitions. In this case the cast bars will pass over the intercell partitions.

The separator material preferably has an electrolyte absorption ratio of at least 100% Electrolyte absorption ratio is the ratio, as a percentage, of the volume of electrolyte ab sorbed by the wetted portion of the separator material to the dry volume of that portion of the separator material which is wetted, when a strip of the dry separator material is sus pended vertically above a body of aqueous sulphuric acid electrolyte of 1.270 SG con taining 0.01% by weight sodium lauryl sul phonate with 1 cm of the lower end of the strip immersed in the electrolyte, after a steady state wicking condition has been reached (that is to say electrolyte can rise no higher due to the capillary effect) at 20 C at a relative humidity of less than 50%.

The thickness measurement at least for the electrolyte absorption ratio measurement is carried out with a micrometer at a loading of 10 kilopascals (1.45 psi) and a foot area of 200 square millimetres (in accordance with the method of British Standard Specification No. 3983). Thus the dry volume of the test sample is measured by multiplying the width and length of the sample by its thickness measured as described.

We also prefer that the separator material should have a wicking height of at least 45 cms on the above test, namely that the elec trolyte should have risen to a height of at least 5 cms above the surface of the electrolyte into which the strip of separator material dips when the steady state condition has been reached.

We find that these two requirements are met by fibrous blotting paper like materials made from fibres, especially glass fibres, hav ing diameters in the range 0.01 microns or less up to 10 microns, the average of the diameters of the fibres being less than 10 microns and preferably less than 5 microns, the weight to fibre density ratio, namely the ratio of the weight of the fibrous material in grams/square metre to the density in grams/ cubic centimetre of the material from which the individual fibres are made preferably be ing at least 20 preferably 30 and especially at least 50.

Moreover the combination of properties gives a material which is highly resistant to "treeing through", namely growth of lead dendrites from the positive electrode of a lead acid battery to the negative electrode produc ing short circuits, whilst at the same time, even when containing large amounts of absor bent electrolyte, still providing a substantial degree of gas transmission capability.

Recombinant batteries operate under super atmospheric pressure e.g. from 1 bar up wards, and due to the restricted amount of electrolyte, the high electrolyte absorption ra tio of the separator, and there being at least as asmuch negative active material capacity as positive active material capacity and the higher electrochemical efficiency of the negative electrode, the cell operates under the socalled "oxygen cycle" in which oxygen during charging or overcharging at the positive is transported, it is believed, through the gas phase in the separator to the surface of the negative which is damp with sulphuric acid and there recombines with the lead to form lead oxide which is converted to lead sulphate by the sulphuric acid. Loss of water is thus avoided as is excess gas pressure inside the cell.If the charging conditions generate oxygen at a faster rate than it can be transported to the negative and react thereat, the excess oxygen is vented from the cell by means of a vent which is provided merely as a safety vent.

For recombination of the evolved gases to occur satisfactorily the battery contains sub stantially no free electrolyte. In the most preferred condition of the battery the amount of electrolyte is not sufficient to saturate the pores in the electrode plates and in the separators. In addition it is necessary that the plates and separators be under a constant slight compressive stress so that the capillarity or wicking action of the separators operates efficiently to keep the entire surface area of the electrodes plates moistened with electrolyte.

However under production conditions the thickness of electrodes varies slightly, and this makes the formation of intercell connectors unpracticable by the conventional methods since such methods of necessity enforce a constant spacing between adjacent electrodes.

However, the fact that electrodes vary in thickness means that a constant spacing of the electrodes results in different pressures being exerted between adjacent pairs of electrodes and thus an inefficient utilisation of at least a proportion of the available active electrode material.

However in the present invention use is made of the fact that separator material of microfine glass material tends to fluff out and form a mat which is sufficiently dense to enable intercell connectors to be directly cast onto it using a mould.

Further features and details of the invention will be apparent from the following description of one specific embodiment, which is a 12 volt lead acid battery, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic perspective view of two adjacent plate packs during assembly into a battery; and Figure 2 is a diagrammatic plan view of the assembled battery showing the arrangement of the intercell connectors; The battery is assembled by making up six packs of alternate positive and negative-bat- tery electrodes 2 and 4 separated by separators 6 of microfine glass fibre material. The separators are of substantially the same width as the plates but are about 3mm higher and so project up above the plates.The separators are a fibrous blotting paper-like sheet of glass fibres having an average diameter of less than 5 microns, and are such that the edges which project above the plates fluff out and form a dense continuous mat 8. The plate packs are assembled so that the plate lugs 10 extend above the mat 8 and form two spaced lines, of positive and negative polarity respectively, extending perpendicular to the planes of the plates. Adjacent plate packs are separated by an intercell partition 12 of polypropylene sheet about 1 mm thick whose height is such that they extend about 1.5mum above the plate packs.

Fig. 1 shows two such plate packs separated by an intercell partition which has a rectangular hole 14 in its upper edge. Integral with the sides of the hole 14 is an elongate rectangular shaped open-bottomed mould or box 16 which is dimensioned and positioned such that it surrounds the line of positive plate lugs 10 of one plate pack and the negative lugs of the adjacent pack. The six plate packs and five intercell partitions are then inserted into a battery container 20 and the openbottomed boxes enclosing the lines of plate lugs are disposed as shown in Fig. 2.

As mentioned above it is necessary in a recombinant battery that the plates and separators be under a compressive stress. It will be appreciated that it is impoflant that the intercell connections be formed while the stress is present for if the plate packs are unstressed when the connectors are formed and the plate packs are subsequently compressed the intercell connectors or the plates may become deformed leading to premature failure or even internal short-circuiting of the battery. The compressive stress may be achieved by appropriately dimensioning the battery container, but this is unlikely to be satisfactory having regard to the inevitable slight variations in the plate thicknesses.

Thus in accordance with a preferred feature of the invention after insertion into the container the plate packs are placed under a predetermined compressive stress, for instance by the insertion of an appropriate packing piece (not shown) at one or both ends of the container. Molten lead is then poured into each of the boxes 14 to form the intercell connectors. By virtue of the poor conductivity and dense packing of the glass fibre mat 8 the lead solidifies in the boxes and does not trickle through the mat. One group bar at each end of the battery also has to be formed to serve not as an intercell connector but as a terminal bar.This may be achieved by any convenient means such as by a smaller openbottomed box on each of the end intercell partitions which does not extend through the partition and into which molten lead is also poured whereafter a preformed terminal post 22 is inserted into the molten lead and subsequently becomes integral with it.

Electrolyte is now added to the battery in a reduced amount, typically 7 to 12 mis of sulphuric acid of 1.27 specific gravity per Ampere hour of capcity of the battery. However it has been found that the amount of electrolyte added is not critical since if the amount is excess to requirements the excess is electrolysed off during charging and not subsequently recombined and is therefore vented to the atmosphere. A lid is then secured to the battery, for instance by hot plate welding such that a seal is formed with the upper surface of the container, the intercell partitions 12 and the boxes 14. The lid is provided with a safety vent for each cell adapted to vent the cell at a pressure in excess of 1 bar.

It will be appreciated that a great many modifications may be made to the embodiment described. For instance the moulds need not be integral with the intercell partitions but may be constituted by a loose open-bottomed mould of plastics material or metal which is simply placed around the plate lugs through a hole in the top of the intercell partition.

In a further modification that portion of the intercell connectors which extend through the intercell partitions may be constituted by a preformed metallic button or insert set in the intercell partitions.

In a still further modification in the intercell connectors are formed before the plate packs are inserted into the container. This may be achieved by inserting them into a jig, applying the required compressive force, forming the intercell connectors as before and inserting the complete assembly into the container.

In a modified construction, which is not illustrated, each cell pack is accommodated in a respective plastics bag and the walls of the plastics bags constitute the intercell partitions.

In this case the cast bars will pass over the intercell partitions and are moulded in a rectangular mould which may either be left around the cast bars or may be removed after the metal has solidified.

Claims (19)

1. A multicell electric storage battery in which each cell contains a pack comprising two or more electrode plates separated by compressible separators of heat resistant fibres which extend above the tops of the plates and there form a substantially continuous fibre mat, the cells being spaced apart by partitions of electrolyte impermeable material, the plate lugs of one polarity of one cell being connected together and to those of the plates of opposite polarity in an adjacent cell by a cast metallic bar which extends through a notch or hole in the intercell partition but not above its upper edge and which rests on the fibre mat.

2. A multicell electric storage battery in which each cell contains a pack comprising two or more electrodes separated by compressible separators of heat resistant fibres which extend above the tops of the plates and there form a substantially continuous fibre mat, and contains substantially no free unabsorbed electrolyte, the cells being spaced apart by partitions of electrolyte impermeable material and the plate lugs of one polarity of one cell being connected together and to those of the plates of opposite polarity in an adjacent cell by a cast metallic bar which rests on the fibre mat.

3. A battery as claimed in Claim 2 in which the cast bar passes through a hole or notch formed in an intercell partition.

4. A battery as claimed in Claim 3 in which that part of the cast bar which extends through the partition is constituted by a preformed insert set into the partition.

5. A battery as claimed in Claim 2 in which each cell pack is accommodated in a respective plastics bag the material of which constitutes the intercell partitions.

6. A battery as claimed in any one of the preceding claims in which the sides and ends of the cast bar are surrounded by a mould.

7. A battery as claimed in Claim 6 and any one of Claims 1 to 4 in which the mould is of plastics material and is integral with the intercell partition.

8. A battery as claimed in any one of the preceding claims in which the separator material has an electrolyte absorption ratio of at least 100%.

9. A battery as claimed in any one of the preceding claims in which the separators comprise microfine glass fibre material.

10. An electric storage battery substantially as specifically described with reference to the accompanying drawings.

11. A method of forming an electric storage battery including the steps of assembling two or more packs of electrode plates separated by separators of heat resistant fibre material which extend above the tops of the plates and there form a substantially continuous mat, spacing adjacent pairs of packs apart by means of an intercell partition of electrolyte impervious material and connecting together the plate lugs of one polarity of one pack to those of the plates of opposite polarity in an adjacent pack by introducing molten materal into a mould, whose bottom is constituted by the glass fibre mat, surrounding the plate lugs of the said packs.

12. A method as claimed in Claim 11 in which the intercell partition has a notch or hole formed in it through which the connection between the two sets of pate lugs extends.

13. A method as claimed in Claim 12 in which the notch hole in the intercell partition is occupied by a preformed metallic insert.

14. A method as claimed in Claim 12 or Claim 13 in which the mould is integral with the intercell partition.

15. A method as claimed in Claim 11 which includes placing each pack in a respective plastics bag, the material of which constitutes the intercell partitions.

16. A method as claimed in any one of Claims 11 to 13 or Claim 15 in which the mould is removed after the metal has solidified.

17. A method as claimed in any one of Claims 11 to 16 in which the plate packs are placed under a compressive stress before and during the introduction of the molten metal into the mould and are so held until the metal has solidified.

18. A method as claimed in any one of Claims 11 to 17 in which the plate packs are introduced into the battery container prior to the introduction of molten metal into the mould.

19. A method of forming an electric storage battery substantially as specifically described with reference to the accompanying drawings.