GB2127613A - Lead acid electric storage batteries - Google Patents

Abstract

A recombinant lead acid electric storage battery in which the electrodes in each cell have current conducting elements made from lead alloy, the positive and negative electrodes being arranged so that the relationship between the capacity of the negative electrodes and the capacity of the positive electrodes is such as to permit recombinant operation to occur (as taught herein), the positive and negative electrodes being separated by separators of electrolyte and gas permeable electrolyte absorbent compressible fibrous material, the said separators being compressed between the said electrodes and the battery containing aqueous sulphuric acid electrolyte absorbed in the electrodes and in the separator material, the battery at least when fully charged having substantially no free unabsorbed electrolyte, the arrangement being such that sufficient oxygen gas recombination occurs in the battery at charging rates not in excess of C/20 to achieve recombinant operation of the battery, characterised in that the negative electrodes in each cell have current conducting elements made throughout their thickness from an antimonial lead alloy containing from 0.5% to 0.6% antimony.

Description

SPECIFICATION Lead acid electric storage batteries The present invention relates to lead acid electric storage batteries, and is particularly concerned with such batteries of sealed or recombinant type in which the gas evolved during operation or charging is induced to recombine within the battery at the battery electrodes.

We have attempted to make recombinant lead acid batteries with flat plates such as miner's cap lamp batteries using pure lead electrode support or grids and we find that there are considerable problems in handling and pasting pure lead electrodes to produce a prismatic battery because of its softness. Because of the difficulty of casting pure lead one cannot overcome these strength problems by casting grids with reinforcing frames. In addition, pure lead suffers from unacceptable growth in use, and problems on charge/discharge cycling.

We have also tried using calcium containing lead alloys, since these alloys can be more readily cast and being substantially harder than pure lead, the grids can more readily be pasted and made into prismatic batteries.

Our French Patent Specification No. 2290048 discloses a sealed lead acid battery designed so that oxygen produced on overcharge recombines on the surface of the negative electrodes excess negative capacity being provided to ensure that this can occur.

Absorbent polyester separator material having fibre diameters of 5 to 50 microns, e.g. about 17 microns with gas channels impressed thereon is provided together with a restricted amount of electrolyte so that whilst the amount of electrolyte (E) is taught desirably to exceed the pore volume (X) of the separators plus the pore volume (Y) of the positive and negative electrodes it must not exceed 2X + Y.

However, as was the accepted attitude, our French patent taught that the electrodes should have metallic supports which minimize the evolution of hydrogen and specifically taught the use of lead calcium or lead-calcium-tin electrodes in preference to pure lead electrodes.

We found that batteries using alloys containing calcium are difficult to produce to a repeatable precise performance specification and in addition also suffer from problems on charge/discharge cycling.

In our published British Patent Specification No. 2048556 we revealed that we had found that, surprisingly, the inclusion of antimony in a recombinant battery does not have the disastrous effect on its performance, which would be expected in view of the widespread knowledge in the battery industry over many years and set out in text books on batteries that antimony is liable to cause unacceptable levels of gassing if it is present in the grids or the electrolyte of a lead acid battery, and which is corroborated by many separate studies.

High levels of gassing would result in unacceptably high losses of water in batteries described as maintenance free.

One attempt to deal with this problem of gassing in so-called maintenance free batteries which are designed to require no topping up during their life but which are not sealed and do not operate in a recombinant manner is shown in Japanese A 7512537 Chem. Abs. 1 34945j in Chem. Abs. Vol. 83 (1975) p.215.

This maintenance free battery has lead antimony alloy cathodes containing 1~3% antimony and 0.5 to 1% arsenic and lead-calcium anodes containing 0.06 to 0.1% calcium, and small pore diameter separator material of average pore diameter 1 to 8 microns.

This patent makes the same assumptions about the dangers of antimony on gassing and contends that water loss is reduced by the separator capturing antimony dissolved in the electrolyte and preventing it depositing on the anode. It quotes a water loss of 197 ml as against 275 ml for an identical battery with a separator having an average pore diameter of 20-30 microns used under identical conditions. Whilst this is a reduction in water loss it is still substantial and would not change the system with the larger pore diameter from being maintenance free to being recombinant. Such a method does not conceive of the anode containing antimony.

The text books and the patents referred to above teach that antimony and arsenic both depress the hydrogen over potential at the negative plate thus causing gassing. Patents heretofore on recombinant lead acid batteries have taught that impurities which depress the hydrogen overvoltage of lead should be substantially avoided (G.B. 1364283) and that antimony and arsenic specifically should be avoided (G.B. 1 032852), or that special, complicated and expensive steps should be taken.

Thus Australian Patent No. 407845 is alleged to be concerned with recombinant lead acid batteries and utilizes a thin microporous sheet separator in combination with a porous glass fibre retainer mat. It recognizes and endorses the conventionally held view that antimony must be avoided in a recombinant lead acid battery because of its effect on hydrogen over potential and the deleterious effect it is expected to have on recombinant operation. It recognizes the problems associated with the use of pure lead or lead calcium alloys and advocates the use of pure lead plating of conventional antimonal alloy grids.

This Australian patent contends that lead plating is simple but the process will require individual electrodes to be produced and then to be plated in a separate manufacturing stage which itself will involve a number of steps.

Gould in their British Patent Specification No. 2084791 advocate that in a recombinant battery antimony can be used in a cadmium containing positive electrode at an amount less than 2.0% by weight but that the negative electrode must be antimony free if antimony poisoning is to be avoided.

We have now discovered that unexpectedly certain advantages are obtained in an antimonial recombinant battery if contrary to Gould's teaching the negative contains antimony and the positive is antimony free.

As in our earlier G.B. Specification No. 2048556 the separator characteristics defined herein and shown in the photomicrographs accompanying this specification are most desirable in a recombinant battery since it is believed that they not only ensure good electrolytic conductivity but also permit rapid transport of oxygen from the positive to the negative electrodes through the open gas phase volume between the fibres on which the electrolyte is believed to cohere in view of the separator's observed high wicking or capillary activity.

Thus we have found that using the type of separator described herein we can use antimonial lead alloys as defined herein in recombinant batteries and avoid the problems associated with pure lead and lead alloys containing calcium whilst achieving excellent gas recombination, cycle life and operating capabilities with a range of battery types including automotive batteries, aircraft batteries and electric storage batteries suitable for providing, in body portable form, the power need of an individual, for example to power a miner's cap lamp.

The invention will be described with particular reference to automotive cells and batteries but is not limited in its applicability to such batteries.

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.

According to one aspect of the present invention there is provided a recombinant lead acid electric storage battery in which the electrodes in each cell have current conducting elements made from lead alloy, the positive and negative electrodes being arranged so that the relationship between the capacity of the negative electrodes and the capacity of the positive electrodes is such as to permit recombinant operation to occur (as taught herein below), the positive and negative electrodes being separated by separators of electrolyte and gas permeable electrolyte absorbent compressible fibrous material, the said separators being compressed between the said electrodes and the battery containing aqueous sulphuric acid electrolyte absorbed in the electrodes and in the separator material, the battery at least when fully charged having substantially no free unabsorbed electrolyte, the arrangement being such that sufficient oxygen gas recombination occurs in the battery at charging rates not in excess of C/20 to achieve recombinant operation of the battery, characterised in that the negative electrodes in each cell have current conducting elements made throughout their thickness from an antimonial lead alloy containing from 0.5% to 12% antimony.

The antimonial alloy desirably contains up to 2.29% by weight e.g. 0.8 to 2.29% and especially 1.01 to 2.29% by weight, since the latter range achieves gas recombination whilst providing for good charge acceptance on recharging.

Whilst the antimony content can be as low as 0.5% it is preferably in excess of this in order to achieve good hardness and pastability within reasonable periods after casting. In addition whilst the antimony content may be as high as 2% it is preferably less than 2% 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 antimony contents. Thus a preferred range is 1.3% to 1.8% e.g. 1.4 to 1.75 or 1.5 to 1.6% antimony.

The current conducting elements of only the negative electrodes are made from a lead antimony alloy and the positive is antimony free e.g. made a lead-calcium-tin alloy or a lead-calcium alloy, or pure lead though this material being soft can introduce problems in the assembly of the battery and high grid growth. Lead-calcium-tin alloy is preferred for the positive.

The top straps, bars and terminals are conventionally cast of high antimony alloy and we have found that when these are used with calcium-tin-iead negatives corrosion problems can arise at the juncture of the two different alloys.

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.

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

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, 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 H2SO4 in grams to the lead in the active material on the positive and negative electrodes 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 a weight of lead basis) 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 taught in G.B.

1364283 but we find that recombinant operation can be achieved at 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 compressible absorbent fibrous material e.g.

having an electrolyte absorption ratio of at least 100% e.g. 1 00 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 200C 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.

We also prefer that the separator material should have a wicking height of at least 5 cms on the above test, namely that the electrolyte should have risen to a height of at least 5 cms above the surface of the electrolyte into which the strip of separator material dips when the steady state condition has been reached, 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 socalled "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 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. 250C). 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 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. 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 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.

Further features and details of the invention will be apparent from the following description of various specific constructions of lead acid electric storage cells and 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 scrap perspective cut-away view of an experimental recombinant lead acid electric storage cell embodying the invention; Figure 2 is a scrap cross-sectional view on the line Il-Il of Figure 1 showing the gas vent; Figure 3 is a scrap cross-sectional view on the line Ill-Ill of Figure 1 showing the way in which the group bar and terminal post are sealed into the lid; Figure 4 is an electron scanning photomicrograph of a preferred separator material at 1000 fold magnification; and Figure 5 is a view similar to Figure 4 at 4000 fold magnification EXAMPLES The cell shown in Figure 1 has a capacity of 21 Ahr and is accommodated in a container 42 made as a single moulding of a polypropylene plastics material. The cell is sealed by a lid 46 which is connected to the walls of the container 42 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 cell contains 3 positive plates 50 (which are not shown since they are hidden by the separator) interleaved with 4 negative plates 52 (which are normally also hidden by the separator). The plates are separated from one another by separators 14A of electrolyte and gas permeable compressible blotting paper-like glass fibre material wrapped around the positive plates. The composition and function of the separator will be described below. An end separator 14B half the thickness is also placed in both outside faces of the cell. There are thus 8 sheets of separator 14A and two sheets of 14B in each cell. The positive plates 50 and negative plates 52 are both 1.6 mm thick, 15 cms wide and 10.7 cms high and are each formed from a cast grid of lead alloy and carry positive and negative active electrode material respectively.

The grid alloy composition for the positive and the negative is given in Table 1 below. The top lead, 48 and 49, was 4% antimony balance lead.

The positive and negative plates are interconnected by a respective positive and negative group bars 46 and 48. Integral with the negative group bar 48 is an upwardly projecting post 49 which is sealed into the lid 46 as shown in Figure 3. A tin plated brass terminal 47 is soldered to the top of post 49. The post passes up through a collar 70 in the lid the inside edge of which is dished so that a rubber '0' ring 71 can nest tightly in it, whereby as shown the post 49 compresses the ring 71 in use. A ring of hot melt adhesive 72 seals around the '0' ring and between the inside face of the lid and the top of the group bar 48. A further ring of hot melt adhesive 73 seals around the post 49 at the top of the collar outside the lid.

Polypropylene packing sheets 75 are positioned between the ends of the electrode pack and the container to assist compression of the pack.

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. However 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 overcharged, 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. As can be seen in Figure 2 the valve is of the Bunsen type and comprises a passage 76 communicating with the interior of the cell and leading from inside the cell to atmosphere. The passage 76 is within a boss 77 in a collar 78 in the lid, and the boss is sealingly covered by a resilient cap 80 having a depending skirt around the boss.The cap 80 normally seals the passage 76, 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 strip of adhesive tape (not shown) is secured over the cap 80 and the collar 78 thus ensuring that cap 80 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 form e.g. perforated or punched sheet.

The electrolyte as described below is added to each cell 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 cell at least in the fully charged condition.

Thus, in use, the cell or each cell in a 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 cell generally operates at superatmospheric pressure at least on charge, and the relief valves are arranged to open only if the pressure becomes excessive, say when it reaches 1.1 bar i.e. 0.1 bar above atmosphere.

The positive active material had the following composition before being electrolytically formed: 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 gr/cc and a lead content of 79%. Each positive plate carried 55 grams of active material on a dry weight basis.

The negative active material had the following 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 acid 70 parts. The paste had a density of 4.3 gr/cc and contained 79% lead. Each negative plate carried 55 grams of active material on a dry weight basis.

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

The separators 14A are of highly absorbent blotting paper-like short staple fibre glass matting about 2 mm thick, and weighing 200 grams/square metre, 14B weighing 100 grams/square metre and being 1 mm thick. The separator contains 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 4 and 5 show the material at different magnifications, Figure 4 at 1000 fold and Figure 5 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. When tested for its wicking and electrolyte absorption capabilities as described above it was found that the liquid had 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 14 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 100.

Each sheet of separator material 14A and 14B is 16 cms wide and 11 cms high. The total volume of separator for each cell before assembly in the cell is 1 70 cubic centimetres. The separator in the cell is compressed by about 10% and thus the volume of separator in the cell is 140 cubic centimetres.

Since the porosity is 90~95% the separator void volume is 125-133 ccs (this is the value of X). The weight of separator present in the cell is 280 grams.

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 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 material shown in Figures 4 and 5. 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 fibre density ratio is preferably 90 to 120.

The total geometric surface area of the positive plates in each cell is 960 square centimetres and of the negative plates 1250 square centimetres. The dry weight of active material of the positive plates is 165 grams (as PbO2) and that of the negatives is 220 grams (as lead) an excess of negative active material based on the weight of the positive active material or a ratio of negative to positive active material (on a lead weight basis) of 1.33:1. The total weight of the grids is 455 grams.

The true density of the positive active material (PbO2) in the fully charged state is 9 gr/cc and the true dry density of the negative active material (sponge lead) in the fully charged state is 1 0.5 gr/cc.

Thus the true volume of the positive active material is 165 t 9 i.e. 1 8.3 ccs and the true volume of the negative active material is 220 . 10.5 i.e. 20.9 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 1 65 s 4.2 i.e. 39.3 cc. 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 220s 4.4 i.e. 50 ccs. Thus the pore volume of the positive active material is 21 ccs and of the negative active material is 29.1 ccs and the total pore volume of the active material is 50.1 ccs, which is the value of Y.

The ratio of X to Y is thus 2.5:1 to2.7:1.(X+Y)is175to183.

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 calculated true surface area for the positive active material is 165 x 2.5 i.e. 412 square metres and for the negative is 220 x 0.45 i.e. 99 square metres using a factor of 0.45 square metre/gram for the dry weight of negative active material and 2.5 square metres/gram for the dry weight of positive active material.

The dry electrolytically unformed cell was evacuated to a high vacuum and had 240 ml i.e. 1.4 (X + Y) to 1.5 (X + Y) of 1.27 SG aqueous sulphuric acid (chilled to 50C) added to the unformed cell.

The cell was then electrolytically formed for 72 hours in a water bath at 1 50C at 1.66 amps, i.e. C/5 Ahrs.

The initial electrolyte volume in each cell after formation was approximately 220 ml of 1.310 SG sulphuric acid, 20 ml of water being gassed off during formation.

Despite the amount of electrolyte seembing by calculation to exceed the pore volume of the system in the fully charged state recombination still occurred as can be seen from the performance in the following tests.

Groups of cells having the alloy compositions given in Table 1 were made up as described above.

TABLE 1

Example Positive grid alloy Negative grid alloy 1 Pb/Ca/Sn11 Pb/Ca/Sn 2 Pb/Ca/Sn GSM 3 Pb/Ca/Sn 6% Sub(3 4 6% Sb Pb/Ca/Sn 5 GSM Pb/Ca/Sn Notes on Table 1 (1) PB/Ca/Sn means an alloy of % calcium, % tin, balance lead.

(2) GSM means an alloy of 2.43% antimony, 0.22% arsenic, 0.04% tin, 0.006 copper, 0.004% selenium, balance lead.

(3) 6% Pb means an alloy of 6% antimony balance lead.

The cell potentials were measured during formation and the mean value of twelve cells is quoted for each example in Table 2 below.

TABLE 2

Mean Cell Potential/Volt Example Time 5 minutes 1 hour 24 hours 72 hours 1 2.11 2.02 2.28 2.77 2 2.09 2.03 2.27 2.63 3 2.08 2.00 2.36 2.67 4 2.24 2.09 2.29 2.67 5 2.18 2.07 2.26 2.68 Three cells of each example were overcharged at 2.4 V for an extended period. The cell potential was selected as representative of the alternator setting in cars, a battery potential of 14.4 V. The cells were weighed at intervals and the current through each cell was monitored.

Table 3 gives results for means cell current during float overcharge.

TABLE 3

Mean Cell Current/mA Example Time 13 days 30 days 50 days 80 days 114 days 136 days 165 days 1 40 65 90 100 95 85 60 2 160 250 240 185 120 80 25 3 150 180 250 235 120 90 35 4 140 300 370 460 515 610 695 5 115 235 270 320 240 270 265 From the information in Table 3 it is possible to estimate the mean ampere-hours of overcharge which have been passed through each cell type during the 165 days on float.This is shown in Table 4, along with the estimated Faradaic water loss and the measured recombinant water losses, the ratio of these is the recombination efficiency.

TABLE 4

Faradaic Overall Re Cumulative Mean Cell Ah weight combination Example weight lossig Input loss/g Efficiency 60 days 110 days 165 days After 165 days 1 13 14 14 310 105 87% 2 21 29 30 592 200 85% 3 19 23 26 611 205 87% 4 57 68 71 1720 580 87% 5 42 55 57 964 320 83% From Tables 3 and 4 it will be observed that the recombination efficiencies of the five examples are not measurably different.

Examples 2 to 5 containing antimony have a lower negative electrode potential and thus an increased overcharge current is passed through the cell if a previously set potential continues to be applied.

This effect is however surprisingly much more marked in Examples 4 and 5 (see Table 4) and the mean cell current on float overcharge for Examples 2 and 3 most surprisingly falls progressively after a peak between 30 and 50 days and is the same or lower than that of Example 1 after 136 days whereas that for Example 4 rises continuously and that for Example 5 whilst peaking at 80 days ends at level at least three times higher than that of Examples 1, 2 and 3 after 136 days.

At the end of the float overcharge test all cells were discharged at 25A to 1.75 V. Before the test began the cells gave durations on this test of 31-36 minutes.

Table 5 gives the results for this mean reserve capacity after float overcharge.

TABLE 5

Example Mean duration/minutes 1 41 2 40 3 38 4 3 5 23 The cells were then discharged at 150A at -1 80C to 1 V, and the discharge durations recorded and are given in Table 6 for this cold rate performance test.

TABLE 6

1 50A discharge @ -1 80C Estimated Electrolyte Volume Example to IV Mean duration/seconds cm3 1 120 206 2 118 190 3 122 194 4 O 149 40 163 Tables 5 and 6 show that the performance of Examples 2 and 3 are substantially the same as that of Example 1 whilst that of Examples 4 and 5 is substantially less good.

EXAMPLE 6 Example 2 was repeated using a modified GSM alloy in which the antimony content was 1.7% and the amount of lead making up the balance of the alloy correspondingly increased.

Claims (8)

1. A recombinant lead acid electric storage battery in which the electrodes in each cell have current conducting elements made from lead alloy, the positive and negative electrodes being arranged so that the relationship between the capacity of the negative electrodes and the capacity of the positive electrodes is such as to permit recombinant operation to occur (as taught herein), the positive and negative electrodes being separated by separators of electrolyte and gas permeable electrolyte absorbent compressible fibrous material, the said separators being compressed between the said electrodes and the battery containing aqueous sulphuric acid electrolyte absorbed in the electrodes and in the separator material, the battery at least when fully charged having substantially no free unabsorbed electrolyte, the arrangement being such that sufficient oxygen gas recombination occurs in the battery at charging rates not in excess of C/20 to achieve recombinant operation of the battery, characterised in that the negative electrodes in each cell have current conducting elements made throughout their thickness from an antimonial lead alloy containing from 0.5% to 12% antimony.

2. A recombinant lead acid electric storage battery in which the electrodes in each cell have current conducting elements made from lead alloy, the positive and negative electrodes being arranged so that the relationship between the capacity of the negative electrodes and the capacity of the positive electrodes is such as to permit recombinant operation to occur (as taught herein), the positive and negative electrodes being separated by separators of electrolyte and gas permeable electrolyte absorbent compressible fibrous material, the said separators being compressed between the said electrodes and the battery containing aqueous sulphuric acid electrolyte absorbed in the electrodes and in the separator material, the battery at least when fully charged having substantially no free unabsorbed electrolyte, the arrangement being such that sufficient oxygen gas recombination occurs in the battery at charging rates not in excess of C/20 to achieve recombinant operation of the battery, characterised in that the negative electrodes in each cell have current conducting elements made throughout their thickness from an antimonial lead alloy containing from 0.5% to 12% antimony and the positive electrode is a lead-calcium-tin alloy, the compressible fibrous separator material comprises fibres having diameters ranging from 0.01 #m or less up to 10 #m with an average diameter of less than 5 Hm, the compressible fibrous separator material has a weight to fibre density ratio of at least 20 and the separator material has an electrolyte absorption ratio of at least 100%, when separately tested, the electrolyte absorption ratio being 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 20 C at a relative humidity of less than 50%.

3. A battery as claimed in Claim 1 or Claim 2 in which the antimonial alloy electrodes contain from 1.01% to 2.29% of antimony.

4. A battery as claimed in Claim 1, 2, or 3 in which each negative electrode has current conducting elements made from antimonial alloy and each positive electrode has current conducting elements made from lead, a lead-calcium alloy containing up to 0.13% by weight calcium or a lead-tin-calcium alloy containing up to 0.13% by weight calcium and up to 1.0% by weight tin.

5. A battery as claimed in Claim 1,2,3 or 4 in which each antimonial electrode has current conducting elements made from an antimonial lead alloy consisting of 1.3 to 1.8% 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 0.05% selenium, balance substantially lead.

6. A battery as claimed in any one of Claims 1 to 5 in which each positive electrode has current conducting elements made from a lead-calcium alloy containing 0.03 to 0.10% by weight calcium, balance lead.

7. A battery as claimed in any one of Claims 1 to 5 in which each positive electrode has current conducting elements made from a lead-calcium-tin alloy containing 0.03 to 0.1% by weight calcium and 0.4 to 0.99% by weight tin, balance lead.

8. A battery as claimed in any one of Claims 1 to 7 in which the volume (E) of electrolyte in the battery after formation is 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.