EP0028227A1

EP0028227A1 - Electric storage batteries

Info

Publication number: EP0028227A1
Application number: EP80900779A
Authority: EP
Inventors: Keith Julian; Raymond Charles Irving
Original assignee: Chloride Group Ltd
Current assignee: Chloride Group Ltd
Priority date: 1979-05-09
Filing date: 1980-11-17
Publication date: 1981-05-13
Also published as: ES8104647A1; ES491332A0; WO1980002473A1; IN152648B

Abstract

Une batterie d'accumulateurs du type a recombinaison ou les gaz qui se degagent sont induits pour se recombiner comprend une pluralite de cadres en materiau plastique, chaque cadre comprenant un organe peripherique (12) et un ou plusieurs elements definissant des languettes (14) definissant ensemble deux ou plusieurs espaces de support de materiau actif. Les cadres sont empiles et l'organe peripherique et l'element definissant les languettes de chaque cadre sont relies a ceux des cadres adjacents. Chaque espace de support recoit un materiau actif portant une electrode, et chaque element de division alterne possede d'un cote deux evidements de liaison (16) s'etendant sur une petite partie de sa longueur, les evidements connecteurs de cadres adjacents d'un element definissant des languettes qui sont decalees les une par rapport aux autres. Les espaces de chaque cote de chaque element definissant les languettes recoivent des electrodes qui sont reliees les unes aux autres par des connecteurs (28) recus dans les evidements de connexion. Des electrodes opposees de cadres adjacents sont separees par un materiau separateur de fibres de verre microfines ayant un taux d'absorption de l'electrolyte de plus de 100%. La batterie ne contient sensiblement pas d'electrolytes non absorbe.A battery of the recombination type where the gases which evolve are induced to recombine comprises a plurality of frames of plastic material, each frame comprising a peripheral member (12) and one or more elements defining tabs (14) defining together two or more active material support spaces. The frames are stacked and the peripheral member and the element defining the tabs of each frame are connected to those of the adjacent frames. Each support space receives an active material carrying an electrode, and each alternate dividing element has on one side two connecting recesses (16) extending over a small part of its length, the connecting recesses of adjacent frames of one. element defining tabs which are offset relative to each other. The spaces on each side of each element defining the tabs receive electrodes which are connected to each other by connectors (28) received in the connection recesses. Opposed electrodes of adjacent frames are separated by a separating material of microfine glass fibers having an electrolyte absorption rate of more than 100%. The battery contains substantially no unabsorbed electrolytes.

Description

"ELECTRIC STORAGE BATTERIES"

TECHNICAL FIELD

The present invention relates to multicell electric storage batteries, in particular lead acid batteries, and is concerned with that type of battery known as "sealed" or "recombinant" in which the gases produced within the battery during operation and charging, at least at relatively low charging rates, are induced to recombine in the battery and are therefore not vented to atmosphere. BACKGROUND ART

Recombinant lead acid electric storage batteries are known, but it is an object of the present invention to provide a battery structure which may be easily assembled and is easily adapted to batteries of different voltages or capacities.

DISCLOSURE OF THE INVENTION

According to the present invention a multicell electric storage battery comprises two or more juxtaposed flat battery elements, each flat element comprising two or more cell elements disposed side by side and separated by cell element defining strips of electrolyte resistant polymer material, each cell element including a current conductor carrying active electrode material, the regions of active material being spaced from each other by the cell element defining strips, the polarity of the active material of each cell element being different to that of the or both adjacent cell elements of the same battery element, adjacent battery elements being disposed relative to each other such that the cell element defining strips of adjacent battery elements are in registry and the polarity of the active material of each cell element is different to that of the or both opposed cell elements of the or both adjacent battery elements, all the cell elements at one end of the battery elements of positive polarity having a terminal conductor disposed in the plane of the element and connected to the positive terminal of the battery and all the cell elements at the other end of the battery elements of negative polarity having a terminal conductor disposed in the plane of the element and connected to the negative terminal of the battery, all those cell elements which do not have a terminal conductor being connected to an adj acent cell element in the same battery element through the cell defining strip between them, electrolyte and gas permeable compressible fibrous separator material having an electrolyte absorption ratio of at least 100& being disposed between the juxtaposed regions of active material of adjacent battery elements, 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.

A perimeter strip of electrolyte resistant polymer material may be provided extending around the edge of each battery element, the perimeter strips of adjacent battery elements being in registry.

In the preferred embodiment the battery includes a plurality of frames of plastics material, each frame comprising a perimeter member and one or more cell element defining strips, the spaces defined by the peri meter member and the or each cell element defining strip each receiving a current conductor carrying active material, every alternate cell element defining strip having on one side a connector recess extending over a minor proportion of its length, the connector recesses on adjacent frames being in cell element defining strips that are offset from one another, the current conductors in the spaces on each side of each cell element defining strip being connected together by a connector which is received in the recess in the cell element defining strip.

Preferably every alternate cell element defining strip has two connector recesses formed in it, and each connector recess is preferably formed adjacent the perimeter member.

Preferably that portion of each cell element defining strip of each frame which overlies a connector recess in the cell element defining strips of the adjacent frame has ah elongate recess formed in it which communicates with the exterior of the perimeter member, the connector recess and the longitudinal recess being filled with a sealing material which seals the connector in the connector recess. The sealing material, which may be epoxy resin or a hot melt adhesive thus seals the intercell connectors and substantially eliminates the possibility of intercell ionic leakage currents flowing around the connectors.

The ratio of X to Y may be 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 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 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 for automotive batteries and other batteries which are largely held in a condition close to fully charged, 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.

While the cell element defining strips can be sealed to each other e.g. by ultrasonic or other welding or adhesive means, such sealing may not be essential since due to the high affinity of the separator for electrolyte (as evinced by its electrolyte absorption ratio) and the restricted amount of electrolyte in the battery, electro lyte leakage between juxtaposed cell element defining strips is severely retarded and electrolyte conductivity paths by this route may not be present to such an extent as to cause significant self discharge problems in automotive batteries in use.

The electrolyte/active material ratio is desirably at least 0.05 e.g. at least 0.06 or at least 0.10 and is the ratio of H₂SO₄ 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 ratio of negative to positive active material on the basis of the weight of active material calculated as lead may be in the range 0.5:1 to 1.5:1 e.g. 0.6:1 to 1.4:1. The use of ratios below 1:1 is contrary to what is conventional in recombinant batteries but we find that recombinant operation can be achieved at these ratios and they have the advantage of providing more posi ti ve active material for the same cell volume. We thus prefer to use ratios in the range 0.6:1 to 0.99:1 e.g. 0.7:1 to 0.9:1.

As mentioned above the separator material is a compressible absorbent fibrous material having an electrolyte absorption ratio of at least 100Λ e.g. 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 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% 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 20ºC at a relative humidity of less than 50%.

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 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. It is also preferred 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, so that good electrolyte distribution is achieved in each cell.

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. This combination of properties gives a material which is highly resistant to "treeing through", namely growth of lead dendrites from the positive electrode in a cell to the negative electrode producing short circuits, whilst at the same time even when containing large amounts of absorbed electrolyte, still providing a substantial degree of gas transmission capability. 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 acid. 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 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. 25ºC). 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 defined 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 transmission 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. At charging rates not in excess of C/20 substantially all of the oxygen gas generated is recombined within the battery and is not vented. Thus typically water losses do not exceed 5 ml for a system in which on a Faradaic basis one would have expected a water loss of 125 ml. Thus the water loss is less than 5% of that expected on a Faradaic basis.

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 pack of battery elements is preferably located in a container with a lid sealed thereto and if desired end plates of electrolyte resistant polymer material may be located over the ends of the pack. The pack of battery elements, and the container if used, is desirably provided with gas venting means in the form of a pressure relief valve so that air cannot obtain access to the interior of the battery although excess gas generated therein can escape to atmosphere.

The perimeter strips of the battery elements which will be juxtaposed to the lid or form the upper surface of the battery in use may be formed with filling apertures to permit electrolyte to be introduced into each cell. The electrolyte may be added by immersing the battery in a bath of electrolyte, evacuating it and optionally then transferring the saturated cells to the container. Alternatively the cells may be located in the container in the dry state, the lid sealed on, the cells evacuated and the electrolyte injected into the cells preferably in an amount of (X+Y) or 1.1 (X+Y) to 1.2 (X+Y) or 1.5 (X+Y). The 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.

Conventional grid alloys may be used to make the current conducting supports for the electrodes but 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 Further features and details of the invention will be apparent from the following description of a 12 volt lead acid storage battery which is given by way of example only with reference to the accompanying drawings BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is a plan view of two frames for use in the battery;

Figure 2 is an underneath plan view of the two frames shown in Figure 1, showing an end grid element and a bipolar grid element in position;

Figure 3 is an electron scanning photomicrograph of a preferred separator material at 1000 fold magnification; Figure 4 is a view similar to Figure 7 at 4000 fold magnification; and

Figure 5 is a transverse section through a battery in accordance with the invention showing all the frames, electrodes and separators of one cell. BEST MODE OF CARRYING OUT THE INVENTION

The battery is made from a number of frames, which in accordance with a preferred aspect of the present invention are formed in pairs as a one piece injection moulding of polypropylene connected by an integral hinge 10. Each frame is of rectangular shape having an outer perimeter member 12 defining a space which is spanned by five equispaced cell element defining strips 14 parallel to two of the sides of the perimeter member. The perimeter member and cell element defining strips together define six equally sized rectangular cell elements or active material support spaces. Each frame has external dimensions of about 250 mm by 200 and is 5 mm thick, whilst the perimeter member is about 7 mm wide and the cell element defining strips are 6 mm wide. As may be seen in Figure 1 the lower frame of the pair has in each of the two outer and the central cell element defining strips 14 a pair of connector recesses 16, each adjacent the perimeter member extending across the full width of the strips 14 and having an overall length of about 13 mm. Each connector recess comprises a relatively deep central groove 18 on either side of which is a somewhat shallower recess 20 whose length is about 10 mm. The upper frame, as seen in Figure 1, has elongate recesses 22 communicating with the exterior of the perimeter member formed in its cell element defining strips in positions such that a recess 22 overlies each of the connector recesses 16, when the surfaces of the two frames seen in Figure 1 are placed in contact. Figure 2 shows the rear surface of the two frames of Figure 1, and as may be seen, that frame which has on one surface connector recesses 16 has on its other surface elongate recesses 22 and vice versa. In any one frame the connector recesses and the elongate recesses are formed in strips 14 that are laterally offset from one another.

When making up a battery from such frames, current conductors comprising electrode grids 23 carrying active material are inserted and retained in the frames. For this purpose expanded lead grids are used, either in the form of single unipolar grids 24 (of which one is shown in Figure 2) whose shape corresponds to that of an active material support space carrying either positive or negative active material as required, or in the form of double bipolar grids 26 (of which one is shown in

Figure 2), whose shape corresponds to that of two active material support spaces, whose two halves are connected by two connector portions 28 whose size and position corresponds to that of the connector recesses, the two halves of the bipolar grids carrying positive and negative active material respectively.

A battery is made up as follows: three bipolar electrode grids are placed on the lower frame seen in Figure 1, such that their connector portions are received in the connector recesses 16, and the polarity of the grids alternate across the width of the frame. The depth of the connector recess and the thickness of the electrode grids are such that the grids lie wholly within the space defined by the frame. A separate strip 27 of microfine glass fibre separator material of the type described above is then laid over each active material support space such that the surface of the perimeter member and the division elements are at the most only partially covered by the edges of the strips of separator material. The visible surfaces of the two frames shown in Figure 1 are then connected together, for instance by hotplate welding, retaining the electrode grids and separators between the two frames. If one imagines that the upper frame in Figure 1 is placed on the lower frame, the upper surface of the two frame stack will look like the upper frame shown in Figure 2.

As will be seen connector recesses are formed in the cell element defining strips of this surface of this frame in the other surface of which elongate recesses are not formed. Electrode grids are now placed in the active material support areas of this frame, but in this case a unipolar grid is placed in each of the two outer support areas, and two bipolar grids in the remaining four central areas. The polarity of these grids is again arranged to alternate across the width of the frame, and also to be of reverse polarity to the electrode grids of the adjacent frame to which they are opposed. Six strips of separator material are then placed over the electrode grids and a further frame is then secured to the stack. It will be appreciated that the lower surface of the further frame will look like the visible surface of the lower frame in Figure 2, and that its upper surface will be that of the lower frame in Figure 1.

The process is now continued until a battery of the desired capacity is formed. The finished stack of frames has a top, bottom and two end walls, and two side walls may be provided by securing, e.g. welding, a plastics sheet to each of the sides of the stack. It will be appreciated that an outer container for the battery is not strictly necessary but is desirable to ensure that the plate assemblies are held pressed tightly together since this facilitates operation in a gas recombinant mode. Such a container 29 is shown in Figure 5, sealed by a lid 31. As the assembly is proceeding, or when it is complete, a sealant such as epoxy resin is injected into each recess 22. This passes down around the connectors 28 and fills the grooves 18, thus completely sealing adjacent cells from each other. Terminal connections are of course required for the battery, and for this purpose every alternate frame is provided with a connector recess 30 in each limb of the perimeter member parallel to the cell element defining strips in that surface in which the connector recesses are formed. Those electrode grids which are placed in the support areas adjacent the connector recesses are provided with a terminal projection 32 extending in the plane of the grids which is received in the recess 30 and passes out of the battery. The terminal projections may be sealed in the terminal recesses, for instance by means of epoxy resin, and are connected together by an external terminal strap to form a battery terminal.

The separator material is highly absorbent blotting paper-like short staple fibre glass matting about 1 mm thick. As seen in. Figures 7 and 8 there are fibres 61 as thin as 0.2 microns and fibres 60 as thick as 2 microns in diameter, the average diameter 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. 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 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 gr/cc the weight to fibre density ratio is thus 74.

As mentioned above the use of a reduced amount of electrolyte and microfine glass separator permits the battery to be of "recombinant" type. For effective recombination of evolved gases to occur the separators should be under a certain compressive pressure to assure that their capillarity or wicking action is brought into play whereby adequate supply of electrolyte is provided to the entire surface of the active material carried by the electrode grids. In addition the battery should be designed to operate at superatmospheric pressure. The battery is therefore preferably filled with the appropriate amount of electrolyte at the time of manufacture, through a hole 33 in each cell which may either be formed in one of the frames, or which may be subsequently formed, for instance by drilling. Each cell is then fitted with a safety vent 35, designed to vent the cell at a pressure in excess of 1 bar, either in the filling hole, or in a separate hole. The safety vent is intended during normal operation of the battery not to function, but merely to act as a relief valve if the rate of evolution of gas should be greater than the maximum possible rate of recombination. The fact that the intercell connectors, constituted by the connector portions 28, extend over a minor proportion of the length of the cell element defining strips means that the maximum area available for intercell leakage currents is relatively small. The fact that the battery is of reduced electrolyte, "recombinant" type means that the electrolyte rarely if ever need be topped up, and that there is a reduced (or even zero) amount of free electrolyte available for the conduction of intercell leakage currents. However a great many modifications may be made to the specific construction of battery described. Thus although the frames have been shown as formed in integral pairs, which construction slightly reduces the amount of welding required and means that only one mould is required instead of two moulds of different construction the frames may be formed as quite separate units. Instead of being connected by a heat-welded butt joint the frames may be connected by any suitable means, such as adhesive. Even a mechanical interconnection such as a snap-fit tongue and groove connection may be adequate since intercell leakage is not a grave problem in such reduced electrolyte batteries. Instead of using six strips of separator material, a single sheet may be used for each frame. By using adhesive or welding for a sufficiently long time the seal between adjacent frames through the separator material may be made adequate.

Instead of being pasted with positive and negative active electrode material respectively, it may be simpler if adjacent electrode grids carry a common universal active electrode material capable of acting as either polarity,

One suitable universal paste composition comprises:

60 lbs of Hardinge grey oxide

12 grams of fibre

82 grams of Vanisperse CB (a lignosulphonate material)

3.47 litres of water

1.93 litres of 1.400.sp. gravity sulphuric acid.

This is readily converted eiectrochemically in the cell either to positive or negative active form.

Details of Vanisperse CB are given in British patent specification No. 1396308. INDUSTRIAL APPLICABILITY

Batteries in. accordance with the invention find application in all those areas. where batteries are conventionally used. They may however be particuarly advantageous in those applications where their facility to be substantially maintenance-free and spill-resistant is required.

Claims

1. A multicell electric storage battery comprising two or more juxtaposed flat battery elements, each battery element comprising two or more cell elements disposed side by side and separated by cell element defining strips of electrolyte resistant polymer material, each cell element including a current conductor carrying active electrode material, the regions of active material being spaced from each other by the cell element defining strips, the polarity of the active material of each cell element being different to that of the or both adjacent cell elements of the same battery element, adjacent battery elements being disposed relative to each other such that the cell element defining strips of adjacent battery elements are in registry and the polarity of the active material of each cell element is different to that of the or both opposed cell elements of the or both adjacent battery elements, all the cell elements at one end of the battery elements of positive polarity having a terminal conductor disposed in the plane of the element and connected to the positive terminal of the battery and all the cell elements at the other end of the battery elements of negative polarity having a terminal conductor disposed in the plane of the element and connected to the negative terminal of the battery, all those cell elements which do not have a terminal conductor being connected to an adjacent cell element in the same battery element through the cell defining strip between them, electrolyte and gas permeable compressible fibrous separator material having an electrolyte absorption ratio of at least 100% being disposed between the juxtaposed regions of active material of adjacent, battery elements, 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.

2. A battery as claimed in Claim 1 in which a perimeter strip of electrolyte resistant polymer material is provided extending around the edge of each battery element, the perimeter strips of adjacent battery elements being in registry.

3. A battery as claimed in Claim 1 including a plurality of frames of plastics material, each frame, comprising a perimeter member and one or more cell element defining strips, the spaces defined by the perimeter member and the or each cell element defining strip each receiving a current conductor carrying active material, every alternate cell element defining strip, having on one side a connector recess extending over a minor proportion of its length, the connector recesses on adjacent frames being in cell element defining strips that are offset from one another, the current conductors in the spaces on each side of each cell element defining strip being connected together by a connector which is received in the recess in the cell element defining strip.

4. A battery as claimed in Claim 3 in which that portion of each cell element defining strip of each frame which overlies a connector recess in the cell element defining strips of the adjacent frame has an elongate recess formed in it which communicates with the exterior of the perimeter member, the connector recess and the longitudinal recess being filled with a sealing material which seals the connector in the connector recess.

5. A battery as claimed in Claim 3 or Claim 4 in which at least every alternate frame has a terminal recess formed in the perimeter member on that side of the frame in which the connector recesses are formed, the terminal recess accommodating a terminal conductor connected to a current conductor carrying active material, which terminal conductor extends from the inside to the outside of the battery.

6. A battery as claimed in Claim 3 or Claim 4 in which each connector recess includes a groove extending along the length of the cell element defining strip and communicating with its associated elongate recess, on either side of which groove is a shallow recess extending to the edges of the division element.

7. A battery as claimed in any one of Claims 1 to 3 in which the volume of electrolyte is in the range 0.8 (X+Y) to 0.99 (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.

8. A battery as claimed in any one of Claims 1 to 3 in which the separator material is a fibrous blotting paper like material made from fibres having diameters in the range 0.01 microns or less up to 10 microns, the average of the diameter of the fibres being less than 10 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 being at least 20.

9. A battery as claimed in .any one of Claims 1 to 3 in which the ratio of X to Y is the range 6:1 to 1:1.

10. A battery as claimed in any one of Claims 1 to 3 in which the pack of battery elements is located in a container with a lid sealed thereto and provided with gas venting means in the form of a pressure relief valve so that air cannot obtain access to the interior of the battery although excess gas generated therein can escape to atmosphere.