GB2062944A - Electric storage batteries - Google Patents

Abstract

A recombinant multicell electric storage battery has two or more cells each containing a cell pack 14 comprising electrodes interleaved with separators of microfine glass fibre material and substantially no free unabsorbed electrolyte. Each cell pack is accommodated in a respective plastics bag 18, the material of which constitutes the intercell partitions. The plates of one polarity in each cell are connected to those of opposite polarity in an adjacent cell by a combined plate strap and intercell connector 22. The battery lid 24 is provided with depending wedge- shaped projections 28 which extend into the gaps between the ends of adjacent plate straps 22 thereby preventing internal short-circuits and restraining the cell packs from vibration. <IMAGE>

Description

SPECIFICATION Electric storage batteries The present invention relates to electric storage batteries, and is particularly though not exclusively concerned with lead acid storage batteries.

According to the present invention there is provided an electric storage battery having two or more cells in which the electrodes in each cell are separated by separators of absorbent fibrous material and the cells contain substantially no free unabsorbed electrolyte and are spaced from one another by intercell partitions which do not extend up as far as the top of the plate straps connecting together the plates of the same polarity in each cell, and in which the space between adjacent ends of the plate straps in adjacent cells is at least partially occupied by insulating material preventing electrical contact between the said plate straps.

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

In the preferred embodiment, the electrodes and separators of each cell are contained within a respective plastics bag, the material of the plastics bags constituting the intercell partitions.

The plates of the same polarity in each cell are connected together by respective plate straps, and each plate strap is connected to the plate strap of opposite polarity in one of the adjacent cells to form intercell connectors in the conventional manner.

One end of each plate strap is spaced from the end of the plate strap in the adjacent cell by a small distance. In a conventional flooded lead acid storage battery the intercell partitions are integral with the battery container and sealed to the lid, and thus there is no possibility of electric contact occurring between the spaced ends of adjacent plate straps.

However, in the type of battery with which the present invention is concerned in which the partitions terminate short of the top of the plate straps, electrical contact between adjacent plate straps would be possible. If the intercell partitions are very thin, perhaps plastics film having a thickness of only a small fraction of a millimetre, the gap between the ends of adjacent plate straps may be of the order of only a few millimetres and thus electrical contact may be a real possibility if the battery should be subjected to vibration, with the concomitant deleterious effects on the battery's performance and service life.

However, the provision in accordance with the invention of insulating material in the gap between the ends of adjacent plate straps effectively ensures that such short-circuits between adjacent cells can not occur.

The insulating material may take many forms, such as a film of organic material around the ends of at least one of each pair of plate straps which may be applied by dipping the plate straps into an organic liquid, such as a laquer, which subsequently hardens. Preferably, however, such a film is applied by inverting the battery and dipping the plate straps whilst hot, e.g. immediately after their formation, into a fluidised bed of epoxy resin incorporating a curing agent. The epoxy dust sticks to the hot surface of the plate straps and cures to form an insulating layer but does not stick to the other components of the battery which are cold. Alernatively or in addition the insulating material comprises one or more projections depending from the lid of the battery.This enables the insulating material to be simply positioned as required merely by appropriately positioning projections on the underside of the lid.

In order to ensure that the projections will pass into the gaps between the ends of adjacent plate straps they are preferably of tapered shape converging away from the lid, and in the preferred embodiment they are so dimensioned that when the lid is in position the opposed surfaces of the projections engage the opposed ends of the plate straps. This assists in restraining the plate straps, and thus the plates, against vibration and adds rigidity to the battery components. This restraining function is of considerable value even when an insulating film of plastics material is provided on the plate straps.

As mentioned above the cells contain essentially no 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 at least 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 fibres having 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 gramsisquare 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 at least 50.

Moreover 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.

This combination of properties is ideally suited to use in reduced electrolyte recombinant lead acid batteries in accordance with the invention. 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.

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 absorption ratio of the separator, and there being at least as much negative active material capacity as positive active material capacity 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 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 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 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 used is established.

The electrodes may be prismatic or may be spirally wound. Prismatic electrodes may be separate rectilinear plates e.g. cast grids, or cast or rolled sheets, slit and expanded to make expanded to make expanded mesh grids or cast or rolled sheets punched to produce perforated grids. The prismatic electrodes may be 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 from expanded mesh grids or perforated grids and these are preferably provided with unexpanded selvedges from which the current take offs are made orto which they are connected.

Conventional grid alloys may be used to make the current conducting supports for the electrodes but for the folded or wound embodiments, 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.

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 escapt to atmosphere.

The material of the intercell partitions must be electrolyte impermeable and resist degradation by the electrolyte. Thus it may be polyethylene or polypropylene or polyvinyl chloride film for a lead acid battery. The partitions may be very thin e.g. of film thickness such as less than 0.010 inches (0.25 mms) e.g. 0.001 to 0.005 inches (0.025-0.125 mms) thick.

The invention may be put into practice in various ways and a specific embodiment will be described by way of example to illustrate the invention with reference to the accompanying diagrammatic drawings in which: Figure 1 is a perspective view of a battery in accordance with the present invention in which one corner of the battery container has been cut away.

Figure 2 is a perspective view of the underside of the battery lid; Figure 3 is a electron scanning photomicrograph of a preferred separator material at 1000 fold magnification; and Figure4 is a view similarto Figure 3 at 4000 fold magnification.

Referring first to Figure 1, the battery has a container 10 of plastics material such as polypropylene containing six cell packs 14. In use, the battery is sealed by a lid 24 of polypropylene welded to the container 10. The lid is shown separately in Figure 2 and is described in more detail below.

A cell pack is made up by assembling a stack of positive and negative plates, each of which has a plate lug, interleaved with compressible absorbent microfine glass material which will be described in more detail below. The stack is then inserted into a plastics 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. Six such cell packs are slid into the container, and the relatively low coefficient of friction between the bags and the container enables the cell packs to be a relatively tight fit in the container, thus ensuring intimate contact between the plates and the separators. The material of the plastics bags 18 serves the function of the intercell 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 is adjacent cells cannot come into contact. It is not critical if the bags should extend slightly above the plate lugs, since the flexible plastics material will simply be depressed by the mould in which the plate straps are formed, or by the plate straps themselves.

The negative and positive plates within each cell pack are connected together by respective plate straps 22 which are fused to the plate lugs in any conventional manner. Every alternate plate strap passes over an intercell partition consituted by the wall of two plastics bags to form an intercell connector in the usual manner.

At one end each plate strap defines, together with the opposed end of the adjacent plate strap, a narrow gap 26 whose width may be as small as 3 mm. In use the plates and plate straps may be subject to vibration with the risk that the opposed ends of the plate straps may contact across the gap, thus partially internally short-circuiting the battery.

To prevent this occurring the lid 24 (seen in Figure 2) is, in accordance with the present invention, provided on its underside with five integral wedgeshaped projections 28, whose positions correspond with those of the gaps 26 between the plate straps.

When the lid is sealed to the battery the tapered ends of the projections 28 serve as a lead-in and guide the projections into the gaps 26. When the lid is in position the opposed surface of the projections engage the opposed end surfaces of the plate straps and thus not only restrain the plate straps against vibration but also ensure that short-circuiting between adjacent plate straps can not occur.

The electrode supports are cast prismatic grids made from 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.

They have good creep resistance.

The separators are highly absorbent blotting paper-like short staple fibre glass matting about 1 mm 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 diameters of the fibres being about 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 open space between the individual fibres. The material when tested for its wicking and electrolyte absorption capabilities by being suspended vertically above a body of sulphuric acid of 1.270 S.G containing 0.01 % by weight of sodium lauryl sulphonate with 1 cm of its end dipping in the electrolyte in an atmosphere of 20 C and a relative humidity of less than 50% 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 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 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 ratio is thus 74.

The arrangement shown in Figure 1 may be assembled by stacking the cell packs in a jig and then inserting the complete assembly into the container, or the cell packs may be inserted sequentially into the container. The electrolyte is added to the cells and the plate straps/intercell connectors are formed after the various components have been inserted into the container. The battery terminals are then connected to or formed on the plate straps and the lid sealed to the container.

After electrolytic forming the cell may then be brought to a gas recombination steady state (if it is not already in that state) by appropriate charging to electrolyse off any excess electrolyte.

In a modified embodiment, which is not illustrated, the wedges 28 are dispensed with and the insulating function is performed by a film of organic material adhering to the entire surface area of the plate straps. This is applied immediately after formation of the latter by dipping them whilst still hot into a fluidised bed of epoxy resin particles incorporating a curing agent. This sticks to the plate straps and cures, but not to the other battery components which are cold. Alternatively, it may be desirable to provide the wedges 28 in addition to the plastics film so as to restrain the plate straps against movement and thus protect the cell elements from damage caused by vibration.

Claims (11)

1. An electric storage battery having two or more cells in which the electrodes in each cell are separated by separators of absorbent fibrous material and the cells contain substantially no free unabsorbed electrolyte and are spaced from one another by intercell partitions which do not extend up as far as the top of the plate straps connecting together the plates of the same polarity in each cell, and in which the space between adjacent ends of the plate straps in adjacent cells is at least partially occupied by insulating material preventing electrical contact between the said plate straps.

2. A battery as claimed in Claim 1 in which the insulating material comprises a film of organic material covering at least the opposed surfaces of the said plate straps.

3. A battery as claimed in Claim 2 in which the film of organic material covers substantially the entire surface of the said plate straps.

4. A battery as claimed in Claim 3 in which the film of organic material is applied to the plate straps whilst they are hot by dipping them into a fluidised bed of particles of the organic material.

5. A battery as claimed in any one of the preceding claims in which the insulating material comprises one or more projections depending from the lid of the battery.

6. A battery as claimed in Claim 5 in which the projections are wedge-shaped tapering away from the lid, their opposed surfaces engaging the opposed end surfaces of the said plate straps and thereby restraining the plate straps from movement within the battery container.

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

8. A battery as claimed in any one of the preceding claims in which the electrolyte absorption ratio of the separators is 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. A battery as claimed in any one of the preceding claims in which the electrodes and separators of each cell are accommodated in a respective plastics bag, the material of the bags constituting the intercell partitions.

11. A multicell electric storage battery substantially as specifically herein described with reference to Figures 1 and 2 of the accompanying drawings.