ELECTRIC STORAGE BATTERIES

Abstract: A method of assembling a recombinant battery includes forming a cell pack (14) of positive and negative plates interleaved with microfine glass fibre separator material. The cell pack is placed in a plastics bag (18) and compressed so as to press the plates and separator material into intimate contact. The cell pack is then wetted with electrolyte and the compressive force removed. The cell pack maintains its compressed thickness and is inserted into a battery container (10).

Description: ELECTRIC STORAGE BATTERIES TECHNICAL FIELD The present invention relates to a method of assembling electric storage batteries, in particular, though not exclusively, lead acid storage batteries, and 5. is particularly 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 10. operation or charging are induced to recombine within the battery. BACKGROUND ART In conventional flooded lead acid batteries the cell packs are assembled and inserted into the battery 15. container, and the electrolyte is then of necessity added subsequently since the amount of elecτrolyte added is enough not only to saturate τhe electrodes and separators but also to fill the majority of the available space in the battery container. 20. Recombinant batteries are known, but it is an object of the invention to provide a simpler method of assembling such batteries. DISCLOSURE OF THE INVENTION According to τhe present invention a method of 25. assembling an electric storage battery includes forming a cell pack of alternate positive and negative electrodes interleaved with separators of compressible absorbent microfine fibre material, placing the cell pack into a plastics bag, compressing the cell pack so as to press 30. the electrodes and separators into intimate contact, wetting the cell pack with electrolyte and subsequently inserting the cell pack into a battery container. In the most preferred condition of the cells the amount of electrolyte added is not sufficient to saturate the pores 35. in the electrodes and in the separators Thus the amount of electrolyte added is not enough to saturate the cell components, and indeed after the electrolyte has been added the cell components may at first sight not even appear to be 5. wet. This therefore permits the electrolyte to be added before the cell packs are introduced into the battery container. Recombinant batteries utilise separators of compressible fibrous absorbent material and it is 0. believed to be important that in such batteries the separators be maintained in intimate contact with the electrodes so that the entire surface of the electrodes has adequate electrolyte for its electro¬ chemical requirements. The cell packs are therefore 15. compressed prior to insertion into the container to achieve the desired intimate contact between the separators and electrodes. The microfine separator material used, which is preferably microfine glass material, acts when dry 20. as a substantially elastic body. That is to say the width of the cell pack decreases when a compressive force is applied and then increases again substantially to its original value when the compressive force is removed. Thus it would, in theory, be possible to 25. apply the compressive force to the plate packs, insert them into the battery container whilst maintaining the compressive force and then to add the electrolyte. It will however be appreciated that this will present grave assembly problems since either all the cell 30. packs or at least the last cell pack will have to be fitted into a space whose width is considerably less than that of the cell pack. We have however discovered the surprising phenomenon that if such microfine separator material, e.g. 5. microfine glass separator material, is compressed and electrolyte then added,- the ceil pack retains its compressed size even when the compressive force is removed whilst nevertheless maintaining the desired intimate contact between the electrodes and separators. 10. Thus in accordance with a preferred feature of the invention the compressive force is removed from the cell pack prior to its insertion into the battery container. This considerably eases the assembly of the battery, and the method in accordance with the 15. invention therefore not only permits the electrolyte to be added to the cell packs prior to their insertion into the battery, which may under certain circumstances be desirable, but also permits the intimate contact between the electrodes and separators desired for 20. operation to be achieved without it being necessary to apply compressive force to the cell packs during their insertion into the container. The cell packs may be compressed so that the thickness of the separator material decreases by 2 to 30%, 25. and preferably 5 to 15%. The electrolyte absorption ratio of the separator material is preferably in excess of 100%, e.g. 100 to 200%, and the volume of electrolyte added is therefore between 100 and 200% of that of the uncompressed 30. volume of the separator material. 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 5. 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 SC- cont¬ aining 0.01% weight sodium lauryl sulphonate with 1 cm of the lower end of the strip immersed in the 10. electrolyte, after a steady state wicking condition has been reached at 20 C at -a relative humidity of less than 50%. The thickness measurement at least for the electrolyte absorption ratio measurement is carried 15. 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 20. 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 25. 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 30. having diameters in the range 0.01 microns, or less, WIPO * 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 5. in grams/square metre 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. Moreover this combination of properties gives a 10. 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 15- absorbent electrolyte, still providing a substantial degree of gas transmission capability. This combination of properties is ideally suited to use in reombinant lead acid batteries in which the amount of electrolyte present is restricted so 20. that there is no free unabsorbed electrolyte in the cell, 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 discharge state of the cell, per amphere hour of capacity of the cell. 25- Recombinant lead acid batteries operate under superatmospheric pressure e.g. from 1.1 bars upwards and due to the restricted amount of electrolyte, the high electrolyte abosorption ratio of the separator, and there being at least as much negative active 30. material capacity as positive active material capacity and the higher electrochemical efficiency cf the negative electrode, the cell operates under the so- called "oxygen cycle" . in which oxygen during charging cr overcharging at the positive is transported, it is 5. 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 10. excess gas pressure inside the ceil. If the charging conditions generate oxygen at a faster rate than 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 15. 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 lost by electrolysis until the electrolyte volume has reached 20. 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 condition related to the rate of charging which is 25. used is established. The plastics bags not only assist in sliding the cell packs into the container, but also serve to retain the electrolyte and to hold the electrodes and separators of each cell pack together both before and after applicaτion of the compressive force. It has been found that, contrary τo convenτional teaching, in recombinanτ batteries containing 5. substantially no free unabsorbed electrolyte the intercell connectors do not need to be sealed to the intercell partitions , and indeed the intercell partitions do not need to be'sealed to the battery lid in order to prevent the battery failing 10. prematurely due to ionic intercell leakage. Thus although the invention embraces a method in which two or more cell packs are formed, each of which is inserted into a respective compartment in the baττery container, in the preferred embodiment each 15. cell pack within its respective plastics bag is inserted into the same compartment in the battery container, and the material of the plastics bags then serves as the intercell partitions. The electrodes may be prismatic or may be 20. spirally wound. Prismatic electrodes may be separate rectilinear plates e.g. cast grids, or cast or rolled sheets, slit and expanded to make expanded mesh grids or cast or rolled sheets punched to produce perforated grids. The prismatic electrodes may be 25. folded and interleaved or arranged in interleaved zig zag relationship, the longitudinal axes of the plates being parallel to each other or at right angles to each other. Spirally wound electrodes are preferably made 30. from expanded mesh grids or perforated grids and these are preferably provided with unεxpanded selvedges from which the current take offs are made or to which they are connected. Conventional grid alloys may be used to make the 5. current conducting supports for the electrodes but for τhe folded or wound embodimenτs , softer materials such as pure lead or lead/calcium alloys e.g. with up to 0.1% calcium or lead/calcium/tin alloys e.g. with up to 0.1% calcium and up to 1.0% tin are preferred. 10. The material of the plastics bags must resist degradation by the electrolyte. Thus it may be poly¬ ethylene or polypropylene or polyvinyl chloride film for a lead acid battery, and may have a thickness of less than 0.010 inches (.0.25 mms) e.g. 0.001 to 0.005 15. inches (.0.025-0.125 nuns). The invention may be put into practice in various ways and one specific embodiment will be described by way of example to illustrate the invention with reference to the accompanying diagrammatic drawings. 20. BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a perspective view of a cell pack within a plastics bag; Figure 2 is a perspective view of an assembled 12 volt recombinant lead acid multicell battery in 25. which the lid is not shown and one corner of the battery container is partially cut away; Figure 3 is a electron scanning photomicrograph of a preferred separator material at 1000 fold magnification; and 30. Figure 4 is a view similar to Figure 3 at 4000 fold magnification. V. ' IP BEST MODE OF CARRYING OUT THE INVENTION Referring first to Figure 2, the battery has a container 10 of plastics material such as polypropylene containing six cell packs 14. A cell pack ' - , seen in 5. Figure 1, is made up ' by assembling a stack of positive and negative plates, each of which has a plate lug 16, interleaved with compressible absorbent microfine glass material which will be described in more detail below. The stack is then inserted into a plastics 10. bag 18, of for instance polypropylene which is seamed at 20. The plastics bag extends up above the plates and separators, but not as far as the tops of the plate lugs. The plate pack is then inserted into a suitable 15- jig and compressed transverse to the plane of the plates until the thickness of the separators has reduced by say 10%. Whilst the compressive force is still applied the appropriate volume of electrolyte, equal to, say, 110% of the dry volume of the separator 20. material, is introduced into the plastics bag. The compressive force is then removed, and due to the fact that electrolyte has been added, the plate pack does not expand back to its original thickness but the intimate contact between the plates and separators 25. is maintained. The cell pack is then inserted into the battery container ? and the process repeated five times so that the container has six cell- packs within it. It will be appreciated that the final cell pack may be a relatively tight fit within the container, but 30. the provision of the plastics bags which have a relatively low coefficient of friction both with each other and the battery container enables the final cell pack to be slid in such that all the cell packs are a relatively tigh ' fit within the container. 5. In an alternative method, the six cell packs are made up and then compressed simultaneously in a larger jig and subsequently inserted simultaneousl into the container. The container has no fixed intercell partitions 10. and the plastics bags 18 serve the function of the partitions. As mentioned above the plastics bags extend above the tops of the separators and plates and this is important since it ensures that the separator material in adjacent cells cannot come 15. into contact. It is not critical if the bags should extend slightly above the plate lugs, since the flexible plastics material v/ill simply be depressed by the mould in which the plate straps are formed, or by the plate straps themselves. 20. The negative and positive plates within each cell pack are then connected together by respective plate straps ' 22 (seen in Figure 2) which are fused to the plate lugs in any conventional manner. Every alternate plate strap passes over an intercell 25. partition constituted by the walls of two plastics bags to form an intercell connector in the usual manner. ■ Terminal connectors are then formed on or secured to the plate straps in the usual manner, and a 30. lid sealed to the container, e.g. by hot plate welding. O \ n . V/ The electrode supports are cast prismatic grids made from a lead, 0:07% calcium, 0.7% tin alloy. The grids are 1.2 nuns thick and are rigid and self supporting and resist deformation even under load. They have good 5. creep resistance. The separators are highly absorbent blotting paper¬ like short staple fibre glass matting about 1mm thick, there being fibres 61 as thin as 0.2 microns and fibres 60 as thick as 2 microns in diameter, the average of the 10. diameters of the fibres being 0.5 microns. Figures 3 and 4 show this material at different magnifications , Figure 3 at 1000 fold and Figure 4 at 4000 fold. It will be observed that the material whilst highly absorbent still has a very large amount of 15. open space between the individual fibres. The material when tested for its wicking and electrolyte absorption capabilities as described above absorbs electrolyte so that the liquid has wicked up to a height of 20 cms after 2 hours and this is the steady state condition. This 20. 20 cms of material absorbs 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 pqrosity of 90 - 95% as measured by 25. mercury intrusion penetrometry. The density of the glass from which the fibres of the separator are made is 1.69. g/cc; the weight to fibre density is thus 74. After electrolytic forming the cells may then be brought to a gas recombination steady state (Lf they 30. are not already in that state) by appropriate charging to electrolyse off any excess electrolyte. OMPI /,, WIPO ..

Claims: CLAIMS 1. A method of assembling an electric storage battery including forming a ceil pack of alternate positive and negative electrodes interleaved with separaτors of compressible absorbent microfine fibre material characterised by the steps of placing the cell pack into a plastics bag, compressing the cell pack so as to press the electrodes and separators into intimate contact, wetting the cell pack with electrolyte and then subsequently inserting the cell pack into a battery container. 2. A method as claimed in Claim 1 in which the amount of electrolyte added is insufficient to satisfy the pores in the electrodes and in the separators. 3. A method as claimed in Claim 1 or Claim 2 in which the separator material is glass fibre material having an average diameter of less than 10 microns. 4. A method as claimed in Claim 1 or Claim 2 in which the compressive force is removed from τhe cell pack prior to its insertion into the battery container. 5. A method as claimed in Claim 1 or Claim 2 in which the cell pack is compressed so that the thickness of the separator material decreases by 2 to 30%. 6. A method as claimed in Claim 5 in which the cell pack is compressed so that the thickness of the separator material decreases by 5 to 15%. O WIP 7. A method as claimed in Claim 1 or Claim 2 in which the volume of electrolyte added is equal to 100 to 200% of that of the uncompressed volume of the separator maτerial. 3. A method as claimed in Claim 1 or Claim 2 which includes forming two or more cell packs , each of which is inserted into a respective compartment in the battery container. 9. A method as claimed in Claim 1 or Claim 2 which includes forming two or more cell packs each of which is inserted into the same compartment in the battery container.

Inventors: PEARSON E
Applicants: CHLORIDE GROUP LTD, PEARSON E
Application year: 1980
Main classification: H01M10
IPC codes: H01M10