WO1980002472A1

WO1980002472A1 - Electric storage batteries

Info

Publication number: WO1980002472A1
Application number: PCT/GB1980/000082
Authority: WO
Inventors: K Peters; B Culpin
Original assignee: Chloride Group Ltd; K Peters; B Culpin
Priority date: 1979-05-09
Filing date: 1980-05-08
Publication date: 1980-11-13
Also published as: EP0046749A1; ES491331A0; SG34384G; ES8103886A1; IN152628B

Abstract

A recombinant lead acid electric storage battery in which the separator is an electrolyte and gas permeable compressible fibrous material having an electrolyte absorption ratio of at least 100% and a volume E of electrolyte of 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 active electrode material in the dry fully charged state, the battery at least when fully charged having no free unabsorbed electrolyte. Gas recombination at least at charging rates not in excess of C/20 is achieved for miner's cap lamp, aircraft and automotive batteries which are described.

Description

ELECTRIC STORAGE BATTERIES TECHNICAL FIELD

The present invention relates to lead acid elec¬ tric storage batteries, and is particularly concerned

5. with such batteries of sealed or recombinant type in whic -the gas evolved during operation or charging is induced to recombine within the battery at the battery electrodes. BACKGROUND ART

10. Recombinant lead acid batteries are known and teach the use of reduced amounts of electrolyte.

The present invention is based on the discovery that surprisingly gas recombination can be achieved at defined high degrees of saturation and this produces

15. the advantage of more efficient utilization of the active material. DISCLOSURE- OF THE INVENTION.

The invention can be used with a range of battery types including automotive batteries, aircraf

20. batteries and electric storage batteries suitable for providing, in"body portable form, the power needs of an individual, for example to power a miner's cap lamp.

The invention will be described with particular reference to miner's cap lamp batteries.

25. Moreover, the invention, although described with reference to batteries, is not restricted to batteries but is also applicable to single cells e.g. spirally wound cells, and the claims to batteries thus include single cells within their scope. Thus according to one aspect of the present invention there is provided a recombinant lead acid electric storage battery in which the positive and negative electrodes are separated by separators of

5. electrolyte and gas permeable compressible fibrous material having an electrolyte absorption ratio of at least 100%, the volume E of electrolyte in the battery after formation being at least 0.8 (X+Y), where X is the total pore volume of the separators in the dry

10. state and Y is the total pore volume of the active electrode material in the dry fully charged state, the battery at least when fully charged having substantially no free unabsorbed electrolyte, whereby substantial oxy¬ gen gas recombination occurs in the battery at charging

15. rates not in excess of C/20.

The current conducting elements may be made of pure lead or lead calcium or lead-calcium-tin alloys or pre¬ ferably any other material which can provide a strong self-supporting readily handled and pasted electrode.

20. However, the current conducting elements of at least

. one of the electrode groups and preferably the positive electrodes preferably consist of anti onial lead alloy containing at least 1.0% by weight antimony.

The antimonial alloy may contain up to 12% by

25. weight antimony but desirably contains 1 to 6% by weight e.g. 1% to 4% by weight, since the latter range achieves gas recombination whilst economising on expen¬ sive antimony. In. addition in comparison with lower antimony content alloys the material can be more

OMPI_. readily cast and is more resistant to grid growth.

Whilst the antimony content can be as low as 1% it is preferably in excess of this in order to achieve good hardness and pastability within reasonable periods

5. after casting. In addition whilst the antimony content may be as high as 3% it is preferably less than 3% so as to keep the tendency of the plates to gas to a relatively low level as compared with the gassing tendency observed in flooded systems for such higher

10. antimony contents. Thus a preferred range is 1.01% to 2.99% e.g. 1.1 to 2.9 to 1.2 to 2.8% by weight antimony.

Preferably the current conducting elements of only the positive electrodes are made from a lead antimony

15. alloy and the negative is a lead-calcium-tin alloy or a lead-calcium alloy or other mechanically strong alloy or grid material adapted to produce a strong self supporting readily handled and pasted electrode or pure lead though this material being soft can introduce

20. problems in the assembly of the battery and high grid growth.

Another preferred antimonial alloy for use in the present invention contains 2.3 to 2.8% antimony, 0 to 0.5% by weight arsenic e.g. 0.2 to 0.49% or 0.25 to

25. 0.4% arsenic, 0 to 0.1%^' by weight copper e.g. 0.02 to 0.05% copper, 0 to 0.5% by weight tin, e.g. 0.02 to 0.4% tin and 0 to^" 0.5% by weight selenium e.g. 0.001% to 0.5% selenium and a particularly preferred alloy composition for the current conductors of the plates

OMPI is 2.3 to 2.8% by weight antimony, 0.25 to 0.35% by weight arsenic, 0.10 to.0.14% by weight tin, 0.02 to 0.05% by weight copper, 0.002% to 0.005% by weight selenium, balance substantially lead. 5. The charging rate is desirablykept 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 10. 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.

It has also been found that recombination can 15. still occur at the negative electrodes at these- very high levels of saturation of the pores which is con¬ trary to what is conventional in recombinant sealed lead acid cells.

The ratio of X to Y may be in the range 6:1 to 20. 1:1 e.g. 5.5:1 to 1.5:1 or more peferably 4:1 to 1.5:1. . 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 H-S0^ in grams to the lead in the active material on the positive and negative electrodes cal- 25. culated 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

5. these ratios and they have the advantage of providing more positive 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

10. compressible absorbent fibrous material having an electrolyte absorption ratio of at least 100% 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

15. 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

20. aqueous sulphuric acid electrolyte of 1.270 SG con¬ taining 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

2B. than 50%.

The thickness of the separator material is measured with a micrometer at a loading of 10 kilo- pascals (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.

5. 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

10. drips 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

15. 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 weightof the fibrous material

20. 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

2^*5. which is highly resistant to "treeing through", namely growth of lead dendrites from the positive electrode in a cell to the negative electrode pro¬ ducing short circuits, whilst at the same time even when containing large amounts of absorbed electrolyte,

- 3REΛ

OMPI still providing a substantial degree of gas transmission capability.

Recombinant lead acid batteries, in which gas re¬ combination is used to eliminate maintenance during use,

5. 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

10. 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

15. 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 in¬ side the battery.

The higher electrochemical efficiency of the

20. 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

2^*5. 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 desirablyprovides that, under

_O PI ^~~ 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

5. 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

10. 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

15. 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

20. 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 elec-

25. trolyte and electrolytic conduction between the electrodes is facilitated and preserved independent of the orientation of the cell, whilst gas trans¬ mission through the open spaces in the separator is maintained so that adequate and rapid gas transmission

OMP 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

5. 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

10. from the battery. The container of the battery is thus provided 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 excess gas generated therein can escape to

15. 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

20. provide gas venting means or separate gas venting means should be provided. BEST MODE OF CARRYING OUT THE INVENTION

Further features and details of the invention will be apparent from the following description of a number

25. of specific constructions of lead acid electric storage batteries embodying the present invention which are given by way of example only with reference to the accompanying drawings in which:-

Figure 1 is a partly cut-away perspective view of

___ OMPI a battery designed for use as a miner's cap lamp battery

Figure 2 is an electron scanning photomicrograph of a preferred separator material at 1000 fold magnifi¬ cation; and

5. Figure 3 is a view similar to Figure 4 at 4000 fold magnification. Example 1 (miner's cap lamp battery)

The battery shown in Figure 1 has a rated capacity of 16 Ahr and is designed for use as a miner's cap lamp

10. battery. It has two cells accommodated in a container 2 made as a single moulding of a polycarbonate plastics material and separated from one another by an integral inter-cell partition 4. The two cells are sealed by a common inner lid 6 which is connected to the walls of

15. the container 2 and the partition 4 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. The battery is

20. capped by a further outer lid 8 which is secured to the container and locked into position by means which form no part of the present invention.

Each cell contains three positive plates 10 inter¬ leaved with four negative plates 12 separated from one .

25. another by double layer separators 14 of electrolyte and gas permeable compressible blotting paper-like glass fiber materialwhose composition will be described below. Each negative plate is wrapped in a U-shaped sheet with the separator enclosing the bottom of the

OM

/*, IP plate. A sheet of separator 14 is also wrapped right around the sides of the electrode pack. There are thus eight thicknesses of the double layer separator and one double layer wrapping in each cell. The posi-

5. tive plates 10 are 2.1 mms thick, 5.5 cms wide and 13 cms high and are formed from a cast grid of lead alloy and carry positive and negative active electrode material respectively. Each positive grid weighs 38 grams and each negative grid weighs 33 grams.

10. The grid alloy composition in % by weight is 2.43% antimony, 0.22% arsenic, 0.04% tin, 0.006%- copper, 0.004% selenium, balance lead.

The positive active material had the following composition before being electrolytically formed:

15. grey oxide 1080 parts, fibre 0.45 parts, water 142 parts, 1.40 SG aqueous sulphuric acid 60 parts. The paste had a density of 4.2 jξr/cc and a lead content of 79%.

The negative active material had the following

20. composition before being electrolytically formed: grey oxide 1080 parts, fibre 0.225 parts, barium sulphate 5.4 parts, carbon black 1.8 parts, stearic acid 0.54 parts, Vanisperse CB (a lignosulphonate) 3.27 parts, water 120 parts, 1.40 SG aqueous sulphuric

25. acid 70 parts. The paste had a density of 4.3 gr/cc and contained 79% lead.

Vanisperse CB is described in British patent specification No. 1,396,308.

Each positive grid carries 50 grams of active

30. material on a dry weight basis. Each negative grid carries 45 grams of active material on a dry weight basis.

The separators 14 are of highly absorbent blotting paper-like short staple fibre glass matting about 1.2 mm

5. 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. Figures 2 and 3 show this material at different magnifi¬ cations, Figure 2 at 1000 fold and Figure 3 at 4000 fold.

10. It will be observed that the material whilst highly absorbent still has a very large aπtount of open space between the individual fibres. When tested for its wicking and electrolyte absorption capabilities as described above it was found that the liquid had wicked

15. 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.

A double layer of separator is used between the

20. plates i.e. providing a total separator thickness between adjacent plates of 2.4 mms before assembly.

Each 1.2 mm sheet of separator 14 weighs 200 grams/square metre and has a porosity of 90 to 95% as measured by mercury intrusion penetrometry. The den-

25. sity 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.

Each sheet of separator material is 0.12 cms thick, 6.1 cms wide and 14.1 cms high. Each sheet of

OMPI ^" the outer wrapping is 32 cms by 6.4 cms by 0.12 cms thick.

The total volume of separator for each cell before assembly into the cell is thus 173.1 cubic

5. centimetres. The separator in the cell is compressed by about 10% and thus the volume of the separator in the cell is 155.8 cubic centimetres. Since the .poro¬ sity is 90-95% the void volume is 140 to 148 ccs (this is the value of X).

10. The weight of separator present in each cell is 31.6 grams.

The separators being compressible conform closely to the surfaces of the plates thus facilitating elec¬ trolyte transfer and ionic conduction between the

15. plates via the separator.

The total thickness of each separator should desirably be. no 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

20. material shown in Figures 2 and 3. It may be as high as' 3 or more even 4 mms but a. preferred range is 1.5 to 2.5 mms. The separator weight to fiber density^' ratio is preferably 70 to 160 or 200.

The total geometric surface area of the positive

25. plates in each cell is 429 square centimetres and of the negative plates 572 square centimetres. The dry weight of active material of the positive plates is 150 x 1.07 i.e. 160.5 grams (as Pb0₂ i.e. 139 grams as lead) and that of the negatives is 180 x 0.93 i.e..

30. 167.4 grams (as lead) an excess of 4.3% negative active

- JRE^ material based on the weight of the positive active material (20% as lead) or a ratio of negative to positive active material (on a lead weight basis) of 1.20:1. The total weight of the grids is 246 grams.

5. The true volume of the positive active material is 160.5 divided by 9 i.e. 17.8 ccs and its apparent volume is 160.5 divided by 4.2 i.e. 38.2 ccs; the void volume of the' positive active material is thus 20.4 ccs. The true volume of the negative active

10. material is 167.4 divided by 10.5 i.e. 15.9 ccs and its apparent volume is 167.4 divided by 4.4. i.e. 38.0 ccs. The pore volume of the negative active material is thus 22.1 ccs. The total pore volume of - the active material is 42.5 ccs, which is the value of

15. Y. The ratio of X to Y is thus 3.3:1 to 3.5:1. (X+Y) is 182.5 to 190.5 ccs.

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

20. charged condition is about the same as it is in the unformed state before addition of electrolyte.

The calculated true surface area for the positive active material is 160.5 x 2.5 i.e. 401.3 square metres and for the negative is 167.4 x 0.45 i.e. 75.3 square

25. 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 is evac¬ uated to ahigh vacuum and has 200 ml i.e. 1.10 (X+Y) to

_Q 1.05 (X+Y) of 1.27 SG aqueous sulphuric acid i.e. 91 grams of H.SO added to the unformed cell.

The cell is then allowed to cool to 40°C (about 1 to 2 hours) and then electrolytically formed and about 5. 30 cc of water is electrolysed off, the specific gravity of the electrolyte thus rising.

The electrolytic forming regime comprises 48 hours at 2.0 -amps, i.e. 5 C/20 Ahrs.

The amo^'ύnt of electrolyte remaining is thus 0.93 10. (X+Y) to 0.89 (X+Y).

The battery in the fully charged condition con¬ tains 0.61 grams of H»S0^ per gram of lead in the posi¬ tive active material and 0.55 grams of H-SO^ per gram of lead in the negative active material. The electro- 15. lyte active material ratio was thus 0.29.

The battery had a capacity at 1.0 amps of 15 Ahr.

The positive and negative plates are inter¬ connected by respective positive and negative group bars 16 and 18. Integral with the negative group bar 20. in the left hand cell as shown in Figure 1 is a laterally projecting portion which terminates in a "flag" or upstanding portion 20 which is adjacent to the inter-cell partition 4 and overlies a hole^' 22 in the partition. The positive flag in the^' left hand 25. cell is connected to the similar negative flag in the right hand cell through the hole 22 so as to form an intercell connection.

The negative group bar in the left hand cell and the positive group bar in the right hand cell are also 30. each provided with a flag 24 overlying a hole in the

_OMPI container 2. Each of the flags 24 is connected to a lug 26 outside the wall of the container but within a space defined by the outer lid 8. The lugs 26 are connected to a respective connecting wire of a

5. connecting lead 28 for connecting the battery to a miner's cap lamp.

The cell or each cell of a battery is normally sealed,- that is to say-that during normal operation a cell does not communicate with the atmosphere.

10. Hoever in case a substantial over-pressure should build up in a 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

15. exhaust the excess gas-.

Each cell is provided with a Bunsen type vent valve. Each valve comprises a passage 36 communi¬ cating with the interior of the cell and leading to the space between the internal and external lids 6

20. and 8. Each passage 36 is within a boss in a respect¬ ive rescess in the internal lid, and the boss is seal- ingly covered by a resilient cap 40 having a depending skirt around the boss. A downwardly extending pro¬ jection 42 on the outer lid 8 engages each cap 40

25. thus ensuring that it is not blown off by the gas pressure.

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

OMPI . v form e.g. perforated or punched sheet.

Alternative antimonial alleys include those dis¬ closed in the United States patents Nos. 3,879,217 and 3,912,537.

5. Example 2

The battery in this example has the same structure as that described in Example 1 except that it has -four positive plates and three negative plates. It has a rated capacity of 20 Ahr. Each positive

10. plate is wrapped in a U-shaped sheet of separator with^' the separator enclosing the bottom of the-plate. A sheet of separator is also wrapped right around the sides of the electrode pack as in Example 1.

The plate dimensions and grid weight, active

15. material composition and density and weight per plate for the positive and negative plates are the same as in Example 1.

The separator composition, volume and weight are also the same as in Example 1.

20. The total geometric surface area of the positive plates in each cell is 572 square centimetres and of the negative plates is 429 square centimetres. The dry weight of active material of the positive plates is 200 x 1-07 i.e. 214 grams (as PbO i.e. 185 grams

25. as lead) and that of the negatives is 135 x 0.93 i.e. 126 grams (as lead) an excess of 70% positive active material based on the weight of negative active material (47% as lead) or a ratio of negative to posi¬ tive active material (on a lead weight basis) of 0.68:1. The total weight of the grids is 251 grams. The true volume of the positive active material is 214 T 9 i.e. 23.8 ccs and its apparent volume is 214 4.2 i.e. 50.9 ccs; the pore volume of the posi- 5. tive active material is thus 27.1 ccs.

The true volume of the negative active material is 126 x 9 i.e. 14 ccs and its apparent volume is 126 T 4.4 i.e. 28.6 ccs; the pore volume of the nega¬ tive active material is thus 14.6 ccs. The total pore 10. volume of the active material is 43.7 ccs, which is the value of Y. The ratio of X to Y is thus 3.2:1 to 3.4:1. <X+Y) is 183.7 to 191.7.

The calculated true surface area for the positive active material is 214 x 2.5 i.e. 535 square metres and 15. for the negative is 126 x 0.45 i.e. 56.7 square metres. Each dry electrolytically unformed cell was evacuated to a high vacuum and had 200 ml i.e. 1.09 (X+Y) to 1.04 (X+Y) of 1.27 SG aqueous sulphuric acid i.e. 91 g^&rams of H„2S04., added to the unformed cell.

20. The cells were then formed as described for Example 1. The battery in the fully charged condition con¬ tains 0.46 grams of H„S0^ per gram of lead in the positive active material and 0.72 grams of H„S0 per gram of lead in the negative active material. The

25. electrolyte active material ratio was thus 0.28 to 1. Examples 3 and 4

These have the same structures and compositions as the batteries of Examples 1 and 2 except that the negative grids consisted of 0.08% by weight calcium and 0.8% by weight tin, balance substantially lead. Example 5 (aircraft battery)

The cell in this example has a rated capacity of 25 Ahr at 25 amps i.e. the 1 hr rate and is designed

5. for use in an aircraft battery. The cell is accommo¬ dated in a container made as a single moulding of a polystyrene plastics material. The cell is sealed by a. lid which is connected to the walls of the con¬ tainer by cementing with a^'n appropriate conventional

10. cement. The cell is provided with a vent of the type described for Example 1 and the terminal posts are sealed into the end walls of the container as is con¬ ventional for flooded aircraft batteries. The cell contains six positive plates interleaved with seven

15. negative plates separated from one another by separators of the same material used in Example 1. Each negative is wrapped in a single U shaped layer of the 1.2 mm thick material (which weighs 200 gr/: : square metre) with the separator enclosing the bottom

20. of the plate.

The positive plates are 0.15 cms thick, 8.25 cms wide and 14.5 cms high, the negative plates are 0.13 cms thick, 8.25 cms wide and 14.8 cms wide. They are formed from a cast grid of lead alloy and carry posi-

25. tive and negative active electrode material respect¬ ively. Each positive grid weighs 43 grams and each negative grid weighs 37 grams.

The grid alloy composition is 6% by weight anti- . mony, balance substantially lead.

OMPI The positive active material had the following composition, before being electrolytically formed: grey oxide 1080 parts, fibre 0.45 parts, water 140 parts, 1.400 SG aqueous sulphuric acid 76 parts. The

5. paste had a density of 4.1 gr/cc.

The negative active material had the following composition, before being electrolytically formed: grey ox de .1080 parts, fibre 0.255 parts, barium sulphate 5.4 parts, carbon ^'black 1.8 parts, stearic

10. acid 0.56 parts, Vanisperse CB (a lignosulphonate)

2.25 parts, water 126 parts, 1.40 SG aqueous sulphuric acid 66 parts.. The paste had a density of 4.35 gr/cc.

Each positive grid carried 55 grams of active material on a dry weight basis (65 grams of wet paste).

15. Each negative grid carried 49.5 grams of active material on a dry weight basis (62 grams of wet paste).

The plates were dry charged before assembly into the cell by immersion as pairs of positives altern¬ ating with pairs of negatives, the pairs being spaced

20. apart by 1 cm, in a tank of aqueous sulphuric acid having an SG of 1.010 (measured at 15°C) for 20 hours at 1.39 amps per plate.

Each sheet of separator material is 0.12 cms thick 8.8 cms wide and 31.3 cms long. The total

25. volume of separator in the cell before assembly is thus 7 x 0.12 x 8.8 x 31.3 i.e. 231 ccs. The separa¬ tor in the cell is compressed by 5% and thus the volume of the separator in the cell is 219.4 ccs. Since the porosity of the separator is 90-95% the

^<g^"0 EA OMPI separator pore volume is 197.5 to 208 ccs (this is the value of X) .

The total geometric surface area of the positive plates in the cell is 1435.5 square centimetres and of

5. the negative plates is 1709.4 square centimetres. The dry weight of the active material of the positive plates is 55 x 6 x 0.83 i.e. 274 grams (as PbO. i.e. 237 grams .as lead) and that of the negatives is 49.5 x 7 x 1.00 i.e^'.^' 346.5 grams (as lead) an excess of 26%

10. of negative active material based on the weight of positive active material (46% as lead) or -a ratio of negative to positive active material (on a weight of lead basis) of 1.46:1. The total weight of the grids is 517 grams.

15. The true volume of the positive active material is 274 T 9 i.e. 30.4 ccs and its apparent volume is 274 f 4.2 i.e. 65.2 ccs; the- pore volume of the posi¬ tive active material is thus 34.8 ccs.

The true volume of the negative active material

20. is 346.5 -r 10.5 i.e. 32.4 ccs and its apparent volume is_.346.5 f 4.4 i.e. 78.8 ccs; the pore volume of the negative active material is thus 46.4 ccs. The total pore volume of the active material is thus 81.2 ccs, which is the value of Y. The ratio of X to Y is thus

25. 2.9:1 to 2.6:1. (X+Y) is 278.7 to 289.2.

The calculated true surface area for the positive active material is 274 x 2.5 i.e. 685 square metres and for the negatives is 346.5 x 0.45 i.e. 156 square metres. Each dry charged cell was evacuated to a high vacuum and had 300 ml i.e. 1.08 (X+Y) to 1.09 (X+Y) of 1.30 SG aqueous sulphuric acid i.e. 156 grams of H^O^ added to it.

5. THe cell was then discharged to 1..667 volts and if the discharge time was in excess of 55 minutes it was used. If it was less the cell was recharged at 2 amps. for 16 hours (i.e. 25 Ahr or 125% of the cells capacity) and then discharged at 1.5 amps to 1.667

10. volts once or twice after which time the discharge time had risen to 60 minutes in almost all cases. Despite the amount of electrolyte seeming by calculation to exceed the pore volume of the system in the fully charged state recombination still ^'

15. occurred as can be seen from the performance in the following tests.

The cell was recharged at 2 amps for 16 hours and then subjected to 25 cycles of discharge at 6 amps across a 0.33 ohm resistance (80% depth of dis-

20. charge) and then recharged at 4 amps for 8 hours. After this the cell had lost 2.9 cc of water... The cell was then taken at top of charge and discharged at 25 amps (1 hour rate) to 1.667 volts (i.e. 100% depth of discharge) . The discharge duration was

25. in excess of 48 minutes.

The cell was then given 30 more cycles as in the first 25 cycles after which it had lost a further 1.7 ccs of water. On a Faradaic basis one would have expected the^' cell to have lost 110 ml over this

30. period . The cell survived this regime, with discharge at 79 and 104 cycles at which the further water losses were 1.5 and 1.1 ccs respectively, to in excess of 125 cycles.

5. Example 6

Example 5 was repeated except that the grids were made from the alloy described for Example 1.

The cell was subjected to the same test regime as the cell of Example 5^'and the water loss after 28

10. cycles was 1.7 ccs, after 54 cycles was 1.4 ccs after

84 cycles was 1.0 ccs and after 112 cycles was 0.9 ccs, the cell surviving in excess of 125 cycles.

Testing of the cells of Examples 5 and 6 on constant potential recharging at 2.37 volts also

15. indicated good cycle li.ves with even lower water losses than those quoted for the constant current recharging regime described above. Example 7 (automotive battery)

The battery has a capacity of 24 Ahr and has six

20. cells accommodated in a container made as a single moulding of polypropylene plastics material and separ¬ ated from each other by integral intercell partitions. The cells are sealed by a common lid. which is connected to the walls of the container and partitions by the

25. 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. The lid is provided with a vent of the type described

30. for Example 1. Intercell connectors and terminal posts are provided in conventional manner as for a flooded automotive battery.

Each cell contains three positive plates inter¬ leaved with four negative plates separated from one

5. another by separators of electrolyte and gas permeable compressible blotting paper-like glass fibre material whose composition and function will be described below. A sheet of separator is also placed on both outside faces of each cell. Each positive is wrapped in a

10. sheet 23 cms by 16 cms by 1.2 mms thick weighing 200 grams/square metre with the separator enclosing the bottom of the plate. A single sheet of separator 0.6 mms thick by 11.5 cms high by 16 cms wide weighing 100 grams/square metre is placed against the outside

15. face of the two end negative plates. The positive plates are formed from a cast grid of the antimonial alloy of Example 1 and the negative plates are formed from a cast grid of lead alloy containing 0.07% cal¬ cium and 0.7% tin and carry positive and negative

20. active electrode material respectively.

The positive and negative plates are 1.27 mms thick 15.0 cms wide and 10.7 cms high and are-held in intimate contact with the separators by solid poly¬ propylene packing pieces. Both faces of all plates

25. are covered by 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 or 1.2 to 1.6 mms thick. In another alternative the positive is 1.4 mms thick and the

OMPI negative is 1.2 mms thick.

The positive and negative active material compo¬ sitions, densities and lead contents are as described in Example 1.

5. Each positive grid carried 75 grams of active material and each negative 74 grams, both on a dry weight basis.

_. ^'The separators are as described in connection with Example^"1.

10. The interplate separators weighed 200 and the end separators 100 grams/square metre respectively and have 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

15. 2.69 gr/cc; the weight to fibre density ratio is ^"thus 74.

Each sheet of the three sheets of interplate separator material is 1.2 mms thick by 23 cms by 16 cms. Each of the two sheets of end separator is 0.6

20. mms thick by 11.5 cms by 16.0 cms. The total volume of. separator for each cell before assembly iεr 154.6 cubic centimetres. The separator in the cell is compressed by 10% and thus the volume of the separ¬ ator in the cell is 139.1 cubic centimetres. Since

25. the porosity is 90-95% the separator void volume is 125 to 132 ccs (this is the value of X). The weight of the separator present in each cell is 25.8 grams.

The total geometric surface area of the positive plates in each cell is 6 x 15 x 10.7 i.e. 963 square

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OMPI centimetres and of the negative plates 8 x 15 x 10.7 i.e. 1284 square centimetres. The dry weight of active material of the positive plates is 75 x 3 x 1.07 i.e. 240.8 grams (as Pb0₂ i.e. 208 grams as lead) and

5. that of the negatives is 74 x 4 x 0.93 i.e. 275.3 grams (as lead) an excess of 14% negative active material based on the weight of the positive active material (32% as lead) or a ratio of negative to positive active material (on^' a lead weight- basis) of 1.32:1.

10. The true volume of the positive active material is 240.8 •_^■ 9 i.e. 26.8 ccs and its apparent volume is 240.8 r 4.2 i.e. 57.3 ccs the void volume of the positive active material is thus 30.5 ccs.

The true volume of the negative active material

15. is 275.3 10.5 i.e. 26.2 ccs and its apparent volume is 275.3 4.4. i.e. 62.6 ccs; the void volume of the negative active material is thus 36.4 ccs. The total void volume of the active material is 66.9 ccs which is the value of Y. The ratio of X to Y is thus

20. 1.9:1 to 2.0:1. (X+Y) is 191.9 to 198.9 ccs.

The calculated true surface area for the- posi¬ tive active material is 240.8 x 2.5 i.e. 602 square metres and for the negative is 275.3 x 0.45 i.e. 124 square metres, using a factor of 0.45 square metre/

25. 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 2-40 ml of cold (5°C) 1.270 SG aqueous sulphuric acid i.e. 1.21 (X+Y⁾

_/^J l

OMP to 1.25 (X+Y) i.e. 109 grams of H S0₄ added to the unformed cell. The cells were electrolytically formed within two hours of filling and 20 cc of water was electrolysed off, the specific gravity of the 5. electrolyte rising to 1.310.

The electrolytic forming regime comprised 72 hours at 1.67 amps i.e. 5 C__Q Ahrs; the C__Q capacity being 24 Ahr.

The amount of electrolyte remaining is thus 1.15 10. (X+Y) to 1.11 (X+Y) and would thus appear by calcu¬ lation to exceed the pore volume of the system in the fully charged state.

However the battery characteristics indicate that at a C/15 charging rate substantial oxygen gas recom- 15. bination occurs and recombination still occurs at C/10.

Example^' 8

Example 7 was repeated except that the negative grids were made of the same antimonial alloy as the positive grid. 20. ^' Example 9

Example 7 was repeated except that the positive grid was made of an alloy of 6% by weigh antimony, balance substantially lead.

^■Example^' 10 25. Example 9 was repeated except that the negative grids were made of the same 6% antimonial alloy as the positive grids.

Continuous overcharge tests at -2 amps for 193 hours indicated that the batteries of Examples 7, 8, 30. 9 and 10 are all achieving a high level of gas recombination in the light of the water loss incurred, which was less than 5 ml per cell.

On a Faradaic basis one would have expected the water loss to be 386 x 0.33 i.e. 127 ml per cell. 5. Example 11

Example 5 was repeated using a lead calcium, tin alloy of 0.07% calcium, 0.7% tin, balance substantially lead for the positive and negative grids.

Gas recombination as good as for the cell of 10. Example 5 was obtained. Example 12

Example 2 was repeated using the same lead, calcium tin alloy as Example 11 for both positive and negative grids. 15. Gas recombination as good as for the battery of Example 2 was obtained. INDUSTRIAL APPLICABILITY

The invention is applicable to recombinant lead acid electric storage batteries and cells.

O

Claims

1. A recombinant lead acid electric storage battery in which the positive and negative electrodes are separated by separators of electrolyte and gas permeable compressible fibrous material having an elec¬ trolyte absorption ratio of at least 100%, the volume E of electrolyte in the battery after formation 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 active electrode material 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.

2. A battery as claimed- in Claim 1 in which the current conducting elements of at least one of the electrode groups consist of antimonial lead alloy con¬ taining 1% to 4% by weight antimony.

3. A battery as claimed in Claim 2 in which the antimonial lead alloy contains 1.01% to 2.9-9% by weight antimony.

4. A battery as claimed in Claim 1, 2 or 3 in which the volume of electrolyte is in the range 0.8 (X+Y) to 0.99 (X+Y).

5. A battery as claimed in Claim 1, 2, 3 or 4 in which the volume of electrolyte is at least 0.95 (X+Y).

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6. A battery as claimed in any one of Claims 1 to 5 in which the ratio of X to Y is in the range 6:1 to 1:1.

7. A battery as claimed in Claim 6 in which the ratio of X to Y is in the range 4.0:1 to 1.5:1.

8.' A battery as claimed in any one of Claims 1 to 7 in whicft the electrolyte active material ratio is in the range 0.20 to 0.50.

9. A battery as claimed in any one of Claims 1 to 8 in which the ratio of negative active material to positive active material is in the range 0.6:1 to 0.99:1.

OM