GB2051463A - Electric storage batteries - Google Patents

Abstract

A separator comprises a short fine diameter glass fibre mat with heat weldable fibres e.g. of polyester adhered to one face. It may be used in a recombinant lead acid battery.

Description

SPECIFICATION Electric storage batteries The present invention relates to lead acid electric storage batteries and to separators therefor.

Highly absorbent separators made of fibres of very small diameters e.g. less than 10 microns are known for use in lead acid batteries for example from G.B. 1364283, and are highly effective in absorbing the electrolyte in so-called sealed or recombinant lead acid batteries containing no free unabsorbed electrolyte.

However, these materials are very weak and present very substantial problems in the assembly of cells and batteries on a production scale especially the assembly of prismatic batteries.

Multilayer separator materials incorporating layers of fine fibre materials are known from U.S. patents 4137377 and 3753784 but both these utilize a dense interior layer of microporous polymer.

These multilayer separators are not susceptible to assembly by certain techniques which have advantages for continuous production.

Thus they are not heat weldable at convenient temperatures and thus cannot be used in assembly techniques in which plates are enclosed in an envelope of separator material by folding a sheet of separator around the plate and then welding the overlapping edges of the separator at least at the bottom or at the sides of the plates so that the separator is secured around the plate and desirably a seam is formed along the said edges so as to inhibit short circuits at the bottom or side edges of the plates either by detachment of active material, plate growth or treeing.

According to the present invention there is provided a lead acid electric storage battery or cell in which the plates of opposite polarity are separated by separator material having at least one layer of absorbent fine diameter fibre material having weldable fibres adhered to at least one face.

Preferably the plates of at least one polarity are enveloped in the separator material, which is preferably joined by welding overlapping portions thereof at or adjacent at least two edges of the said enveloped plates.

The layer of highly absorbent material preferably comprises a mat of fibres, e.g. staple fibres, of diameters less than 10 microns and these are preferably glass fibres especially staple fibres e.g. having diameters in the range 0.2 to 10 microns, e.g. 0.5 to 6 microns, the average of the diameters of the fibres being 0.5 to 3 microns e.g. 0.5 to 1.0 microns.

The weldable fibres are weldable to each other and to the fibres of the highly absorbent layer and are preferably adhered thereto by being welded thereto. The interlocking of these weldable fibres to the fibres of the absorbent layer may be chemical or mechanical or a combination of both. The fibres of the highly absorbent layer preferably intermingle with the weldable fibres so that at least some penetrate to the surface of the separator thus enhancing moisture pick up.

The absorbent material is preferably highly absorbent and compressible so that it can conform closely to the surface of the plates e.g. under compression so as to facilitate transfer of electrolyte from the pores in the separator to the pores in the active material and vice versa.

In the cell or battery the highly absorbent layer is preferably compressed to 85% to 95% of its original thickness.

At least the highly absorbent layer and preferably the whole separator has an electrolyte absorption ratio of at least 100% e.g. of 100 to 200% especially 110 to 170%. It is electrically non conducting and electrolyte resistant.

Electrolyte absorption ratio is the ratio, as a percentage, of the volume of electrolyte absorbed by the wetted portion of the material to the dry volume of that portion of the material which is wetted, when a strip of the dry material is suspended vertically above a body of aqueous sulphuric acid electrolyte of 1.270 SG containing 0.01 % by 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 20 C at a relative humidity of less than 50%.

We also prefer that the 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 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 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 at least 30 and especially at least 50.

The absorbent material preferably has a surface area in the range 0.1 to 20 square metres/gram especially in the range 1 to 10 or 2 to 8 square metres/gram.

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

This combination of properties is ideally suited to use in lead acid batteries in accordance with a preferred aspect of the present invention, in which the amount of electrolyte present is restricted so that there is no free unabsorbed electrolyte in the battery at least when it is in the fully charged condition.

Recombinant lead acid batteries in which gas recombination is used to eliminate maintenance during use, operate under superatmospheric pressure e.g. from 1 bar (atmospheric pressure) upwards and due to the restricted amount of electrolyte, the high electrolyte absorption ratio of the separator, and the higher electrochemical efficiency of the negative electrode, the battery operates under the so-called "oxygen cycle". Thus oxygen generated, 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 acic. Loss of water is thus avoided as is excess gas pressure inside the battery.

The higher electrochemical efficiency of the negative active material enables the negative electrode to effect recombination of the oxygen produced by the positive electrode even at the beginning of the charge cycle. Thus it may not be necessary to have an excess weight of negative active material compared to the positive active material.

However recombinant operation of the battery may be facilitated by the use of a number of features in combination.

Thus firstly one desirably provides that, under the charge and discharge conditions, under which the battery is designed to operate, the capacity of the negative electrodes in each cell will normally and desirably always be in excess of that of the positive electrodes.

The electrochemical efficiency of the negative electrodes is in general greater than that of the positive electrodes but it must be born in mind that the efficiency of the negative electrodes drops more rapidly than that of the positive electrodes both as the cells undergo increasing numbers of cycles of charge and discharge and as the temperature of operation is reduced below ambient (i.e. 25 C). Excess negative capacity may thus conveniently be ensured by providing an excess of negative active material (calculated as lead) compared to the positive active material in each cell.

Secondly one provides a restricted amount of electrolyte as described above and thirdly one provides a separator, desirably having a high electrolyte absorption ratio as also described and defined above, which is compressible, so as to conform closely to the surfaces of the electrodes, and which has wicking or capillary activity, whereby transmission of electrolyte and electrolytic conduction between the electrodes is facilitated and preserved independent of the orientation of the cell, whilst gas trasnmission through the open spaces in the separator is maintained so that adequate and rapid gas transmission between the electrodes is also ensured.

Use of a fibrous separator having very small fibre diameters ensures that the open spaces in the separator are highly tortuous thus fulfilling the requirement that the separator resist "treeing through" as described above.

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 battery.

According to a preferred form of the present invention there is provided a lead acid electric storage battery in which the positive and negative plates are separated by separators of electrolyte and gas permeable compressible fibrous separator material in accordance with the invention preferably having an electrolyte absorption ratio of at least 100%, the volume E of electrolyte in the battery preferably being at least 0.8 (X + Y), where X is the total pore volume of the separators in the dry state and Y is the total pore volume of the positive and negative active materials in the dry fully charged state, the battery at least when fully charged having substantially no free unabsorbed electrolyte, whereby substantial oxygen gas recombination occurs in the battery at charging rates not in excess of C/20.

The ratio of X to Y is preferably in the range 6:1 to 1:1 e.g. 5.5:1 to 1.5:1 or more preferably 4:1 to 1.5:1.

We have found that surprisingly, oxygen gas recombination can still occur at the negative at these very high levels of saturation of the pores which are contrary to what is conventional in sealed lead acid cells. The charging rate is desirably kept at not greater than C/15 and preferably less than C/20 e.g.

C/20 to C/60. The volume of electrolyte is desirably in the range 0.8 (X + Y) to 0.99 (X + Y) and especially at least 0.9 (X + Y) or even at least 0.95 (X + Y). These values enable the active material to be utilized more efficiently than when lower amounts of electrolyte are used.

The electrolyte active material ratio is at least 0.05 e.g. at least 0.06 or at least 0.10 and is the ratio of H2 SO, in grams to the lead in the positive and negative active material calculated as grams of lead.

It is preferably in the range 0.10 to 0.60 especially 0.11 to 0.55 e.g. 0.20 to 0.50.

The container of the battery is provided at least with gas venting means.

The gas venting means preferably take 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.

The lid of the container may be formed with filling apertures to permit electrolyte to be introduced into each cell. The filling apertures may be closed after the electrolyte has been added but the closures should provide gas venting means or separate gas venting means should be provided.

The highly absorbent layer is preferably made of glass fibres and is preferably a waterleaf or airleaf such that the material can swell to absorb large quantities of liquids.

The material may contain binders but these will be present in such amounts and be of such nature as not to reduce the electrolyte absorption ratio below 80% and desirably not below 100%.

The weldable fibres are preferably present in an amount such as to form a self coherent layer capable of being delaminated from the highly absorbent layer. This provides the advantage of increasing the mechanical strength of the composite material and in particular reducing the tendency for pieces to break away from the layer during assembly operations.

Weldable fibres are preferably adhered to both faces of the highly absorbent layer. This has the advantage of reducing the tendency for individual fibres to separate from the surface of the highly absorbent layer.

The weldable fibres however desirably constitute a small proportion of the total weight and thickness of the separator material and preferably constitute merely a scrim through which the fibres of the absorbent layer are visible at least under a microscope rather than a layer such as to mask the fibres of the absorbent layer.

The thickness ratio of the layer of weldable fibres to the absorbent layer is preferably not more than 20% and preferably less than 10%.

The weldable fibres are themselves preferably fine e.g. having diameters less than 50 microns e.g. in the range 1 to 3 e.g. 5 to 20 microns.

The material is preferably 0.6 to 2.5 mms thick.

The thickness of the separator material is measured 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 3983). Thus the dry volume of the separator test sample or the delaminated highly absorbent layer is measured by multiplying the width and length of the sample by its thickness measured as described.

The invention also extends to a separator having at least one layer of highly absorbent fine diameter fibre material, which is preferably substantially free of binder, and has an electrolyte absorption ratio of at least 100%, having weldable fibres adhered to at least one and preferably both faces.

The invention further extends to an envelope of separator material having at least one layer of highly absorbent fine diameter fibre material, having weldable fibres adhered to at least one face and having edges of the envelope adhered together by means of the said weldable fibres. The envelope may be a single folded sheet with the weldable fibres on the inside face of the envelope welded face to face or may be two separate sheets overlapped and welded by the weldable fibres being welded face to face to each other or one surface of weldable fibres may be adhered to a surface of the sheet which is free of weldable fibres.

The weldable fibres weld at temperatures below the melting point of lead or lead battery alloys and preferably at below 300 C e.g. in the range 150 C to 250 C.

The welding can thus be safely carried out when the separator material is located around the plates.

The highly absorbent layer preferably has a porosity as determined by mercury intrusion porosimetry of at least 80% e.g. 85% to 95%.

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 drawings, in which: Figure 1 is a partial cross-sectional side elevation of part of a starting, lighting and ignition battery in accordance with the present invention; Figure 2 is an end elevation on the line Il-Il of Fig. 1; Figure 3 is an electron scanning photomicrograph of one face of a separator material in accordance with the invention at 1000 fold magnification; Figure 4 is a view similar to Fig. 3 at 4000 fold magnification; Figure 5 is a view similar to Fig. 3 of the other face of the material shown in Fig. 3; and Figures 6 and 7 are views simirar to Fig. 5 at 200 fold and 60 fold magnification.

The battery has a capacity of 43 Ahr and has six cells accommodated in a container 2 made as a single moulding of polypropylene plastics material and separated from each other by integral intercell partitions 4. The cells are sealed by a common lid 6 which is connected to the walls of the container 2 at 7 and the partitions 4 at 8 by the method known as heat sealing" in which the edges to be joined are placed in contact with opposite surfaces of a heated tool which is subsequently withdrawn and the partially melted edges are pressed together.

Each cell contains four positive plates 10 interleaved with five negative plates 12. Each negative plate is enclosed in an envelope of separator material 14, which has a thick absorbent layer 141 of fine glass fibre material and a thin heat weldable layer 142 of polyester fibres adhered to one face. The polyester layer 142 faces inwards to the negative and is heat welded up each side to form seams 143.

The positive plates 10 and negative plates 12 are formed from a cast grid of lead alloy containing 0.07% calcium and 0.7% tin and carry positive and negative active electrode material respectively.

The positive plates are 2.0 mms thick and the negative plates are 1.8 mms thick and are held in intimate contact with the separators which are thus under compression by solid polypropylene packing pieces 30. Both faces of all plates are in contact with separator material which extends out above and below and on each side of the plates.

More broadly the plates may be 1 to 2 mms thick e.g. 1.2 to 1.9 to 1.2 to 1.6 mms thick. In another alternative the positive is 1.4 mms thick and the negative is 1.2 mms thick.

The positive active material has the following composition before being electrolytically formed: Hardinge grey oxide 13640 parts, fibre 6 parts, water 1800 parts, 1.40 SG aqueous sulphuric acid 750 parts. The paste had a density of 4.2 gr/cc.

The negative active material has the following composition before being electrolytically formed: Hardinge oxide 13640 parts, fibre 3 parts, barium sulphate 68 parts, carbon black 23 parts, stearic acid 7 parts, Vanisperse CB (a lignosulphonate) 41 parts, water 1525 parts, 1.40 SG aqueous sulphuric acid 875 parts. The paste had a density of 4.3. Vanisperse CB is described in British patent specification No. 1,396,308.

Each positive plate carried 109 grams of positive active material on a dry weight basis.

Each negative plate carried 105 grams of negative active material on a dry weight basis.

As the active material has sulphuric acid added to it, its porosity decreases. When the active material is charged its porosity increases and in the fully charged condition is about the same as it is in the unformed state before addition of electrolyte.

The layer 141 of the separators 14 is composed of highly absorbent blotting paperlike 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 diameter of the fibres being about 0.5 microns. Figs. 3 and 4 show the free face of this material at different magnifications, Fig. 3 at 1000 fold and Fig. 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 delaminated from the polyester scrim 142 and tested for its wicking and electrolyte absorption capabilities by being suspended vertically above a body of sulphuric acid of 1.270 SG 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 layer 141 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 gr/cc; the weight to fibre density ratio is thus 74.

The total volume of the layer 141 of separator for each cell before assembly is 218 cubic centimetres. The separator in the cell is compressed by about 8% and thus the volume of separator in the cell is 200.6 cubic centimetres.

Since the porosity is 90-95% the separator void volume is 180.5-190.6 cubic centimetres (this is the value of X).

The separators being compressible conform closely to the surfaces of the plates thus facilitating electrolyte transfer and ionic conduction between the plates via the separator.

The total thickness of separator should desirably be not thinner than about 0.6 mms since below this value we have found that growth of dendrites through the separator is liable to occur with the material shown in Figs. 3 and 4. It may be as high as 2 or more even 3 mms but a preferred range is 1 to 2 mms. The separator weight to fibre density ratio is preferably in the range 70 to 160 or 200.

The layer or scrim 142 is made of polyester fibres of density 1.3 and weighs 50 grams/ square metre. The fibres 144 are much thicker e.g. 5 microns, fibre 145, to 20 microns, fibre 146, than the fibres 60 or 61 and thus the material is much more open than the layer 141.

Referring to Figs. 5, 6 and 7 the glass fibres 60 and 61 can be seen between the polyester fibres.

As can be seen from Figs. 6 and 7 the polyester fibres in the scrim 142 are fused and welded to each other at many of their intersections, e.g. 147. Even though the scrim has a high open area it holds together as a continuous sheet which can be peeled away from the underlying glass fibre layer 141 without itself disintegrating. It is apparent that it is the structure of the layer 141 which breaks down when this delamination is carried out because the underside of the scrim 142 can be seen to be carrying glass fibres adhered to the polyester fibres.

The delamination requires the scrim to be pulled away from the material and thus it clearly has a reinforcing effect of the glass fibre layer.

The scrim is located only on the free major face of the glass fibre layer, the edges being free. This means that the glass fibres are not constrained from movement relative to each other in the thickness of the layer and the glass fibre layer can swell to absorb electrolyte.

The scrim is about 0.1 to 0. 15 mms thick or less and appears to be up to about 5 fibres thick and certainly not more than 10 fibres thick.

Referring to Fig. 7 the polyester fibres form an interconnected open-mesh network. The fibres themselves having gaps about 50 to 100 or 200 microns across between them through which the glass fibres of the layer 141 are visible. A cluster 148 of such fibres are particularly apparent in the top right hand corner of Fig. 7.

The delaminated scrim can be stretched in its major plane e.g. up to 15 to 20% and then recovers largely to its original dimensions.

Pulling the scrim beyond its yield point enabled one to tease long fibres up to 3.5 cms long from the scrim. Pulling the glass fibre layer resulted in small portions breaking off very readily, no long fibres could be teased out, the largest seemed to be no more than 2 or 3 mms long and most seemed of the order to 1 mm or less.

Referring to Fig. 6 thin glass fibres 61 can be seen through the mesh of polyester fibres and extending up between some of the polyester fibres.

The total geometric surface area of the positive plates in each cell is 767 square centimetres and of the negative plates 959 square centimetres. The dry weight of active material of the positive plates is 4 x 109 x 1.07 i.e. 468 grams (as PbO2 i.e.

405 grams as lead) and that of the negative is 5 x 105 x 0.93 i.e. 490 grams (as lead) an excess of 4.7% negative active material based on the weight of the positive active material (21% as lead). The total weight of the grids is 763 grams.

The true density of the positive active material (PbO2) in the fully charged state is 9 gr/cc and the true density of the negative active material (sponge lead) in the fully charged state is 10.5 gr/cc.

Thus the true volume of the positive active material is 4 x 109 i 9 i.e. 48.4 ccs and the true volume of the negative active material is 5 x 105 . 10.5 i.e. 50 ccs.

The apparent density of the dry positive active material is 4.2 gr/cc and thus the apparent volume of the dry positive active material is 4 x 109:4.2 i.e. 103.8 ccs. The apparent density of the dry negative active material is 4.4 gr/cc and thus the apparent volume of the dry negative active material is 5 X 105:4.4 i.e. 119.3 ccs.

Thus the pore volume of the positive active material is 55.4 ccs and of the negative active material is 69.3 ccs and the total pore volume of the active material is 124.7 ccs, which is the value of Y. The ratio of X to Y is thus 1.45:1 to 1.53:1 (X + Y) is 305.2 to 315.3.

The calculated true surface area for the positive active material is 1170 square metres and for the negative is 220 square metres, using a factor of 0.45 square metre/gram for the negative active material and 2.5 square metres/gram for the positive active material.

Each dry electrolytically unformed cell was evacuated to a high vacuum and had 325 ml i.e. 1.06 (X + Y) to 1.03 (X + Y) of 1.275 SG aqueous sulphuric acid (i.e. 153 grams of H2SO4) added to the unformed cell. The cells were then allowed to cool to 40 C (about 1 to 2 hours) and electrolytically formed water being electrolysed off, the specific gravity of the electrolyte thus rising.

The plates and the separator are very nearly saturated but since no venting occurs there must be sufficient gas space in the separators and plates at least at conditions approaching full charge for gas phase (oxygen) transport to be occurring at this rate of charging.

The battery contained 0.7 ml of 1.275 SG aqueous sulphuric acid per gram of positive active material (as lead) and 0.66 ml of 1.275 SG aqueous sulphuric acid per gram of negative active material as lead. The battery contained 0.34 ml of 1.275 SG aqueous sulphuric acid per gram of positive and negative active material combined (as lead).

There were thus 0.4 grams of H2SO4 per gram of lead in the positive active material and 0.35 grams of H2SO4 per gram of lead in the negative active material. The electrolyte active material ratio was thus 0.18.

The positive and negative plates are interconnected by a respective positive and negative group bar 16 and 18. Integral with the negative group bar 18 in the right hand cell as shown in Fig. 1 is a laterally projecting portion which terminates in a "flag" or upstanding portion 20 which is adjacent to the intercell partition 4 and overlies a hole 22 in the partition. The positive flag in the left hand of the two cells shown in Fig. 1 is connected to the similar negative flag in the right hand cell through the hole 22 so as to form an intercell connection by a method known as "extrusion fusion".This method comprises placing welding jaws against the two opposed flags before the lid 6 is placed in position, applying pressure so that the flags distort and meet in the hole 22 and then passing an electric current between the two welding jaws so that the material of the two flags is melted together and seals the hole 22.

The positive group bar in the right hand cell is provided with a flag 24. The flag 24 is connected to a terminal 26 in the lid of the container.

Each cell of the battery is normally sealed, that is to say that during normal operation of the battery the cells do not communicate with the atmosphere. However in case a substantial over-pressure should build up in the cell, for instance because the cell is exposed to a very high temperature or over-charged, so that oxygen gas is evolved at a faster rate than it can be combined, a relief valve is provided to exhaust the excess gas and is arranged to operate at a pressure of only 2 to 3 psi. Each valve is of the Bunsen type and comprises a passage 36 communicating with the interior of a cell and leading to the exterior of the lid.

Each passage 36 is within a boss in a respective recess 38 in the lid, and the boss is sealingly covered by a resilient cap 40 having a depending skirt around the boss. The cap 40 normally seals the passage 36, but if an excessive pressure should occur in the battery the skirt of the cap lifts away from the boss to vent the cell. A disc 42 provided with a vent hole or clearance and keyed into the undercut top edge of the recess 38 engages each cap 40, thus ensuring that it is not blown off by the gas pressure, whilst allowing venting to atmosphere.

Reference has been made above to cast lead alloy grids. Whilst this is preferred the electrodes could be made from slit expanded sheet or be of wrought form e.g. perforated or punched sheet or from fibrous supports provided with electrically conductive coatings or deposited conductors such as are disclosed in the present applicants British applications Nos. 9876/76 and 15664/76 Serial No.

1,577,223. The grids are preferably 0.1 to 3.0 mms thick especially 1.5 to 2.5 mms thick. The preferred alloy is a lead calcium tin alloy preferably containing 0.06 to 0.13% e.g. 0.07 to 0.09% calcium and 0.3 to 0.99% tin e.g. 0.4 to 0.8% tin e.g. of 0.07% calcium and 0.7% tin.

Alternative alloys include 99.9% lead and antimonial alloys such as those disclosed in United States patents No. 3,879,217 and 3,912,537.

The electrolyte is added to the battery in a very limited quantity, that is to say much less is added than in the case of a conventional fully flooded cell. The electrolyte that is added is substantially all absorbed and retained by the separators and the active material and there is substantially no free electrolyte in the cells at least in the fully charged condition.

This reduced electrolyte condition is important not only because it is desirable to have as little electrolyte as possible in a battery having regard to safety and spillability but also because this condition is conducive to recombination of the gases within the cell.

Thus, in use, the battery is normally sealed and is arranged so that essentially only oxygen is evolved on over-charge. Any such oxygen recombines with a negative plate. The battery generally operates at superatmospheric pressure, and the relief valves are arranged to open only if the pressure becomes excessive, say 1 to 3 pounds per square inch.

Due to the fact that the cells rarely if ever communicate with the atmosphere, the battery will not need topping up with electrolyte and is therefore maintenance free. Furthermore the battery is unspillable firstly because it is sealed and secondly because there is substantially no free electrolyte in the cells, the electrolyte being retained within the microfine glass separators and the active material. The fact that the cells are sealed also means that no spark or explosion can propagate from the atmosphere into the battery or vice versa.

Claims (15)

1. A battery separator having at least one layer of highly absorbent fine diameter fibre material, which has an electrolyte absorption ratio of at least 100%, and which has weldable fibres adhered to at least one face.

2. A separator as claimed in Claim 1 in which the weldable fibres weld at temperatures below the melting point of lead or lead battery alloys.

3. A separator as claimed in Claim 1 or Claim 2 in which the layer of highly absorbent material comprises a mat of staple fibres of diameters less than 10 microns.

4. A separator as claimed in Claim 3 in which the fibres are glass fibres having diameters in the range 0.2 to 10 microns, the average of the diameters of the fibres being 0.5 to 3 microns.

5. A separator as claimed in Claim 1, 2, 3 or 4 in which the weldable fibres are weldable to each other and to the fibres of the highly absorbent layer and are adhered thereto by being weldable thereto, and the fibres of the highly absorbent layer intermingle with the weldable fibres so that at least some penetrate to the surface of the separator thus enhancing moisture pick up.

6. A separator as claimed in any one of Claims 1 to 5 in which the highly absorbent material has a wicking height of at least 5 cms on the test defined herein 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 material dips when the steady state condition has been reached.

7. A separator as claimed in any one of Claims 1 to 6 in which the weldable fibres are present in an amount such as to form a self coherent layer capable of being delaminated from the highly absorbent layer.

8. A separator as claimed in any one of Claims 1 to 7 in which the thickness ratio of the layer of weldable fibres to the absorbent layer is less than 10%, and the weldable fibres have diameters of less than 50 microns.

9. A separator as claimed in Claim 1 substantially as specifically described herein with reference to Figs 3, 4, 5 and 6 of the accompanying drawings.

10. An envelope of separator material having at least one layer of highly absorbent fine diameter fibre material, having weldable fibres adhered to at least one face and having edges of the envelope adhered together by means of the said weldable fibres.

11. An envelope as claimed in Claim 10 when made from a separator material as claimed in any one of Claims 1 to 9.

12. A lead acid electric storage battery or cell in which the plates of opposite polarity are separated by separator material having at least one layer of absorbent fine diameter fibre material having weldable fibres adhered to at least one face.

13. A battery or cell as claimed in Claim 12 in which the plates of at least one polarity are enveloped in the separator material, which is joined by welding overlapping portions thereof at or adjacent at least two edges of the said enveloped plates.

14. A lead acid electric storage battery or cell as claimed in Claim 12 or Claim 13 in which the separator material is as claimed in any one of Claims 1 to 9.

15. A recombinant lead acid electric storage battery or cell in which the positive and negative plates are separated by separators of electrolyte and gas permeable compressible fibrous separator material as claimed in any one of Claims 1 to 9, the volume E of electrolyte in the battery being at least 0.8 (X + Y), where X is the total pore volume of the separators in the dry state and Y is the total pore volume of the positive and negative active materials in the dry fully charged state, the battery at least when fully charged having substantially no free unabsorbed electrolyte, whereby substantial oxygen gas recombination occurs in the battery at charging rates not in excess of C/20.