GB2070844A

GB2070844A - Electric storage batteries

Info

Publication number: GB2070844A
Application number: GB8005971A
Authority: GB
Original assignee: Chloride Group Ltd
Current assignee: Chloride Group Ltd
Priority date: 1980-02-22
Filing date: 1980-02-22
Publication date: 1981-09-09
Also published as: ES8206099A1; GB2070844B; AU6745781A; DE3106203A1; AU536606B2; ES499644A0

Abstract

A multicell electric storage battery includes a compartmented container covered by a lid and has two or more cells, each containing alternate positive and negative electrodes interleaved with compressible fibrous absorbent separator material and substantially no free unabsorbed electrolyte. Every alternate electrode in the two end cells is a unipolar plate 12 and the remaining electrodes are one half of a bipolar plate 14 connected to its other half in an adjacent cell by a bridge piece 16. The bridge pieces 16 constitute the intercell connectors and pass around at least one of the sides of the intercell partitions 10. <IMAGE>

Description

SPECIFICATION Electric storage batteries The present invention relates to electric storage batteries and in particular though not exclusively to lead acid storage batteries.

The invention is concerned with batteries of so called "sealed" or "recombinant" type. These are batteries in which the amount of electrolyte present is restricted so that there is no free unabsorbed electrolyte in the cells, and the gases evolved during operation or charging are induced to recombine within the battery.

We have found that, contrary to conventional teaching, in recombinant batteries containing substantially no free unabsorbed electrolyte the cells do not necessarily need to be sealed from each other in order to prevent the battery failing prematurely due to ionic intercell leakage.

It is an object of the present invention to provide an electric storage battery which may be simply and economically assembled, and which is economical of materials and has good high rate discharge characteristics.

According to the present invention a multicell electric storage battery includes a compartmented container covered by a lid and having two or more cells each containing alternate positive and negative electrodes interleaved with separator material of compressible fibrous absorbent material and substantially no free unabsorbed electrolyte, every alternate electrode in each of the two end cells being a unipolar plate and the remaining electrodes being one half of a bipolar plate connected to its other half in an adjacent cell by a bridge piece, the bridge pieces constituting the intercell connectors and passing round at least one of the sides of the intercell partitions.

The invention also embraces a method of assembling such a battery which includes providing a battery container having one or more intercell partitions dividing the container into compartments, inserting into the container a layer of electrodes and compressible fibrous absorbent separator material, with one electrode in each compartment, and then inserting further such layers until the container is substantially full, whereby every alternate electrode in each of the end compartments is a unipolar plate and all the remaining electrodes constitute one half of bipolar plates connected to the other half in an adjacent compartment by a bridge piece which extends around one edge of an intercell partition, the intercell connectors in the finished battery being constituted by the bridge pieces, and inserting an amount of electrolyte into each compartment that is substantially all absorbed by the electrodes and separator material. The method preferably includes the step, subsequent to inserting the electrodes and separator material, of inserting a cover plate having slots corresponding to the intercell partitions, moving the cover plate into a position in which the electrodes and separator material are under compression and securing it in that position.

As mentioned above the cells contain essenfiallyno free unabsorbed electrolyte, and in the most preferred condition of the cells the amount of electrolyte is not sufficient to saturate the pores in the electrodes and in the separators. The electrolyte absorption ratio of the separator material is preferably greater than 100%.

Electrolyte absorption ratio is the ratio, as a percentage, of the volume of electrolyte absorbed 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 suspended vertically above a body of aqueous sulphuric acid electrolyte of 1.270 SG containing 0.01% weight sodium lauryl sulphonate with 1 cm of the lower end of the strip immersed in the electrolyte, after a steady state wicking condition has been reached at 200C 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 5 cms on the above test, namely that the electrolyte 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 fibrous having diameters in the range of 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 meter to the density in grams/cubic centimetre of the material from which the individual fibres are made preferably being at least 20 preferably 30 and especially 50.

This 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 producing short circuits, whilst at the same time, even when containing large amounts of absorbent electrolyte, still providing a substantial degree of gas transmission capability.

Recombinant lead acid batteries on charge may operate undersuperatmospheric pressure e.g. from 1.1 bars upwards and due to the restricted amount of electrolyte, the high electrolyte absorption ratio of the separator, the fact that the capacity of the negative active material is generally at least as great as that of the positive active material and the higher electrochemical efficiency of the negative electrode, the cell operates under the so-called "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 that it can be transported to the negative and react thereat, then the excess oxygen is vented from the cell.

The amount of electrolyte added is not highly critical since it is observed that if a slight excess of electrolyte is added above that required to saturate the porosity of the cell components the recombination mechanism is suppressed and electrolyte is lose by electrolysis until the electrolyte volume has reached the correct amount for the cell in question, i.e. the cell porosity has reached the correct degree of unsaturation, when the recombination mechanism comes into operation again and a steady state recombination occurs.

Gas venting means are preferably provided in the form of a non-return valve so that air cannot obtain access to the interior of the battery although gas generated therein can escape to atmosphere.

In recombinant reduced electrolyte batteries it is believed to be important that the plates and separators are maintained in intimate contact so that the entire surface of the plates has adequate electrolyte available for its electrochemical requirements.

However, under manufacturing conditions the thickness of the plates tends to vary slightly, and if the plates were merely inserted into a compartment the intimate contact might be lacking, or the plates might be too tight due to the variations in plate thickness. One way of overcoming this problem is to assemble the elements of each cell outside the container and then to secure them together with bands of plastics material such that a predetermined compressive face is exerted. This is however inconvenient in production and uneconomical of time.

However, the use of plates that can slide along the intercell partitions, and subsequent compression of them and maintaining them in that position by means of the cover plate largely overcomes these problems in an economical and simple manner.

Further features and details of the invention will be apparent from the following description of two specific embodiments which is given by way of example with reference to the accompanying diagrammatic drawings in which: Figure 1 is a perspective view of a battery container with one layer of electrodes; Figure 2 is a plan view of a closure plate on a reduced scale; Figures Sa and 3b are views of a unipolar and bipolar electrode respectively; Figure 3c shows how the separator material is arranged to overlie the two halves of a bipolar plate; Figure 3d is a perspective view of a modification in which one half of the bipolar plates is sheathed with separator material.

Figures 4, 5 6a and 6b are views similar to Figures 1,2, 3a and 3b of a modified embodiment.

Figure 7 is an electron scanning photomicrograph of a preferred separator material at 1000 fold magnification; and Figure 8 is a view similar to Figure 7 at 4000 fold separation.

Figure 1 shows a battery container of polypropylene having a bottom 2, two end walls 4 and 6 and a side wall 8. The top and other side wall of the container are missing, and the container is divided into six equally sized compartments by five intercell partitions 10 integral with the bottom 2 and side wall 8. The end walls and intercell partitions all extend up to a point about 5 mm short of the top of the side wall 8.

To assemble the battery the container is, as shown in Figure 1, laid with its side wall downwards. A unipolar plate 12, as shown in Figure 3a, is placed in each of the end compartments and two bipolar plates 14 each made in two halves connected at one end by a bridge piece 16, and which can be thought of as generally z shaped, or of flat bottomed U shape, as shown in Figure 3b, are then inserted into the remaining four compartments, so that each of the four compartments contains one half of one plate. The bridge piece 16 (at the upper end of the plate in Figure 3b) passes over the top of the intercell partitions 10. The bridge pieces are about 3 mm wide and are arranged to be such that they do not extend above the top of the side wall 8.An upper strip, and the bridge piece of each plate is of solid metal, that is to say that it is not expanded. A rectangular strip of separator material 17, which will be described in more detail below, is then inserted into each compartment so that is completely overlies the plate beneath it and extends beyond the edges thereof at the top and bottom and sides (see Figure 3c). A further layer of plates is then inserted into the container comprising three bipolar plates 14. It will be appreciated that the bridge pieces 16 of these three bipolar plates will be offset from those of the plates beneath them. Further strips of separator material are then inserted followed by a third layer of plates similar to the first, and the process is repeated until the container is full.

In recombinant reduced electrolyte batteries it is believed to be important that the plates and separators are maintained in intimate contact so that the entire surface of the plates has adequate electrolyte available for its electrochemical requirements. For this reason a cover plate 20, as shown in Figure 2, having slots 22 corresponding to the partitions 10 is then placed on top of the stack of plates and separators, and then pushed down to place the plates and separators under compression. The cover plate is then secured in position to the bottom, side walls and partitions of the container by welding or adhesive and a side wall equal in size to that of the wall 8 is then secured to the battery, again by welding or adhesive to give the battery a neat flush finish.

Each of the unipolar plates 12 in the two end compartments is provided with a laterally extending terminal lug 24, and the battery therefore has a row of such lugs projecting laterally from each end of the battery extending up to a level slightly below the top of the side walls. A line of sealant material, such as epoxy resin, hot melt adhesive or the like is then laid along each of the rows of lugs up to the level of the side walls, and a lid is then sealed to the container by welding or adhesive. The lugs of each row are then connected, for instance by burning or pouring molten lead into a mould around the lugs to form terminal connectors. The electrolyte may be added to the cells either before the lid is seated to the container, or afterwards, for instance by injection through holes in the lid.The amount of electrolyte added is typically in the range 7 to 12 mis of sulphuric acid of 1.270 SG per cell in the discharged state of the cell, per Amphere hour of capacity of the cell.

It will be appreciated that the bridge pieces 16 will constitute the intercell connectors, and that it is therefore not necessary to form intercell connectors in a separate step. This saves a large proportion of the lead usually used in the relatively massive plate straps and intercell connectors and results in a shortening of the current paths within the battery and thus a reduction in its internal resistance and an increase in its high rate discharge characteristics.

The bridge pieces are not sealed to the intercell partitions or to the battery but the battery's performance is not significantly impaired because intercell ionic leakage is not a serious problem in recombinant reduced electrolyte batteries. However, it may be desirable to provide means to prevent intercell ionic leakage currents along the bridge pieces.

Figures 4, 5, 6a and 6b show a modified embodiment in which the same reference numerals are used to designate similar items. In this embodiment the partitions 10 are integral with only one wall of the container, in this case the side wall 8. The two halves of each bipolar plate are connected to each other by two bridge pieces one at each end of the plate and the bipolar plates can thus be thought of as of generally 0 shape having a central slot closed at each end by a bridge piece 16. These bipolar plates are therefore structurally more rigid than the U shaped plates, and the provision of two bridge pieces per bipolar plate increases the area of the conductive pathways between adjacent cells and thus leads to a still further reduction of the internal resistance of the battery. The method of assembly is identical to that described above.

The electrode supports are cast or expanded prismatic grids made of a lead, 0.07% calcium, 0.7% tin alloy. The grids are 1.2 mms thick and are rigid and self supporting and resist deformation even under load.

The separators are shown in Figures 7 and 8 and are highly absorbent blotting paper-like short staple fibre glass matting about 1 mm thick, there being fibres 61 as thin 0.2 microns and fibres 60 as thick as 2 microns in diameter, the average of the diameters of the fibres being about 0.5 microns.

It will be observed that the material whilst highly absorbent still has a very large amount of open space between the individual fibres. When tested, the material absorbed 113% of its own dry volume of electrolyte, and this is its electrolyte absorption ratio.

The separator material weighs 200 grams/square metre and has a porosity of 90-95% as measured by mercury intrusion penetrometry. The density of the glass from which the fibres of the separator are made is 2.69 g/cc; the weight to fibre density is thus 74.

In both embodiments the two halves of each bipolar plate act as plates of opposite polarity.

Although it would be possible to coat the two halves with positive active material and negative active material respectively, this would add complication to the assembly procedure and it is thus preferred that the bipolar plates are pasted with a universal active electrode material, that is to say one which after electrolytic formation will act as either positive or negative active material depending on the formation. Similarly the unipolar plates are conviently pasted with universal active material to avoid having to keep a stock of two types of unipolar plate.

In a modification of the methods described above the separator material 17 is inserted simultaneously with at least some of the plates. Conveniently this may be achieved by sleeving or wrapping one half of each bipolar plate (see Figure 3d). This will ensure that separator material is present between each pair of adjacent plates and results in a considerable simplication of the assembly procedure. If the separator material is wrapped around the bipolar plates the fold may occur down an edge of the plate, or along the bottom. If desired the whole of each bipolar plate may be so wrapped or sleeved, in which case there will be two layers of separator material between adjacent bipolar plates. The separator material typically has a weight of 100 gm per square metre.

In a further modification the bridge pieces 16 are sealed, e.g. with epoxy resin, to the partitions and container to eliminate the possibility of intercell ionic leakage.

In a still further modification the terminal lugs 24 extend upwardly from the unipolar plates, and therefore pass through the lid of the finished battery where they may be burnt together in the conventional manner. It will be appreciated that in this case the end walls 4 and 6 will extend up as far as the side wall 8.

Claims

1. A multicell electric storage battery including a compartmented container covered by a lid and having two or more cells each containing alternative positive and negative electrodes interleaved with separator material of compressible fibrous absorbent material and substantially no free unabsorbed electrolyte, every alternate electrode in each of the two end cells being a unipolar plate and the remaining electrodes being one half of a bipolar plate connected to its other half in an adjacent cell by a bridge piece, the bridge pieces constituting the intercell connectors and passing round at least one of the sides of the intercell partitions.

2. A battery as claimed in Claim 1 in which the two halves of each bipolar plate are connected by a single bridge piece which passes around one edge of the associated intercell partition.

3. A battery as claimed in Claim 1 in which the two halves of each bipolar plate are connected by two bridge pieces which pass around two opposed edges of the associated intercell partition.

4. A battery as claimed in any one of Claims 1 to 3 in which each bridge piece is not sealed to the container or to the intercell partition so that the cells are in communication with each other.

5. A battery as claimed in any one of the preceding claims in which the end walls parallel to the intercell partitions are shorter than the side walls, and each unipolar plate has a terminal lug extending laterally over an end wall, the terminal lugs at each end being connected together to form a terminal connector.

6. A battery as claimed in Claim 5 in which the gap between the lid and the end walls through which the terminal lugs extend is filled by sealant material and the lid is sealed to the side walls and the sealant material.

7. A battery as claimed in any one of Claims 1 to 4 in which the end walls parallel to the intercell partitions are of the same height as the side walls, and each unipolar plate has a terminal lug extending up through the battery lid, the terminal lugs at each end being connected together to form a terminal connector.

8. A battery as claimed in any one of the preceding claims which includes a cover piece extending transverse to the intercell partitions and affording slots through which the intercell partitions pass, the cover piece being secured in position so as to maintain the electrodes material in intimate contact.

9. A battery as claimed in any one of the preceding claims in which one half of each bipolar plate is sleeved with separator material.

10. A battery as claimed in any one of the preceding claims in which the separator material is microfine diameter glass fibre material.

11. A battery as claimed in any one of the preceding claims in which the amount of electrolyte present is not sufficient to completely saturate the pores of the separator material and the electrodes.

12. An electric storage battery substantially as specifically herein described with reference to Figures 1,2, 3a and 3b or Figures 4, 5, 6a and 6b of the accompanying drawings.

13. A method of assembling a multicell electric storage battery which includes providing a battery container having one or more intercell partitions dividing the container into compartments, inserting into the container a layer of electrodes and compressible fibrous absorbent separator material, with one electrode in each compartment, and then inserting further such layers until the container is substantially full, whereby every alternate electrode in each of the end compartments is a unipolar plate and all the remaining electrode constitute one half of bipolar plates connected to the other half in an adjacent compartment by a bridge piece which extends around one edge of an intercell partition, the intercell connectors in the finished battery being constituted by the bridge pieces, and inserting an amount of electrolyte into each compartment that is substantially all absorbed by the electrodes and separator material.

14. A method as claimed in Claim 13 in which two adjacent walls of the container perpendicular to the partitions are absent, the layers of electrodes and separator material are slid into the container substantially parallel to the intercell partitions so that, in position, they extend perpendicular to the partitions and the two said walls are subsequently connected to the container.

15. A method as claimed in Claim 13 or Claim 14 in which each bipolar plate has two halves connected by a single bridge piece at or adjacent one end.

16. A method as claimed in Claim 13 or Claim 14 in which the intercell partitions are integral with or sealed to one wall of the container only, and the two halves of each bipolar plate are connected to each other by two bridge pieces one at or adjacent each end of the plate.

17. A method as claimed in any one of Claims 13 to 16 in which the electrodes and separator material of each layer are inserted sequentially.

18. A method as claimed in any one of Claims 13 to 16 which includes the step of sheathing or wrapping one half of each bipolar plate with separator material, whereby the bipolar plates are inserted simultaneously with their associated separator material.

19. A method as claimed in any one of Claims 13 to 18 which includes the step, subsequent to inserting the electrodes and separator material, of inserting a cover plate having slots corresponding to the intercell partitions, moving the cover plate into a position in which the electrodes and separator material are under compression and securing it in that position.

20. A method of assembling an electric storage battery substantially as specifically herein described with reference to Figures 1,2, 3a and 3b or Figures 4, 5 6a and 6b of the accompanying drawings.