GB2080606A - Lead acid electric storage batteries - Google Patents

Abstract

A method of making a recombinant electric storage battery includes providing electrodes carrying an active electrode material and converting half of them to the positive formed condition and the other half to the negative formed condition. The plates are placed in a container under compression with interleaved microfine glass fibre separator material. The container is then sealed and evacuated and electrolyte is introduced into the container in an amount such that substantially all of it is absorbed in the electrodes and the separator material.

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.

The invention will be described with particular reference to aircraft 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 a method of making a recombinant lead acid electric storage battery comprises providing current conducting electrodes carrying electrolytically unformed material convertable to positive active material and to negative active material, converting substantially half the electrodes to positive electroly ticallyformed condition and the remainder to negative electrolytically formed condition, locating the thus charged positive and negative electrodes in a sealable container, adjacent positive and negative electrodes being separated by a fibrous absorbent separator material, which is compressed between the said adjacent plates, sealing the container, evacuating the container and introducing electrolyte into the evacuated container in an amount such that substantially all of the electrolyte is absorbed in the electrodes and the separator material.

Prior to sealing the container the positive and negative groups will be connected together, any necessary intercell connections made and the positive and negative groups connected to positive and negative terminals outside the battery respectively.

This method of making a recombinant battery alleviates the severe problems which are met in electrolytically forming the electrodes in situ in the battery after assembly.

It is preferred that the current conducting elements of at least one of the electrode groups, and preferably the positive electrodes are made of antimonial lead alloy containing at least 1.0% by weight antimony.

The antimonial alloy may contain up to 12% by 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 expensive antimony. In addition in comparison with lower antimony content alloys the material can more readily be 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 pastabilitywithin reasonable periods 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 antimony contents. Thus a preferred range is 1.01% to 2.99% e.g. 1.1 to 2.9 or 1.2 to 2.8% antimony.

According to a preferred aspect of the present invention, the separators are of electrolyte and gas permeable compressible fibrous material having an electrolyte absorption ratio of at least 100%. In addition, the volume E of electrolyte in the battery is preferably 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.

Preferably the other electrodes e.g. the negatives are 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. Pure lead can be used but this material being soft can introduce problems in the assembly of the battery and high grid growth.

Preferably the current conducting elements of the positive elctrodes are made from the lead antimony alloy, 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 0.4% arsenic, 0 to 0.1% by weight copper e.g. 0.02% to 0.05% copper, 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 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 0.05% by weight selenium, balance substantially lead.

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 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. to 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 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.

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 20"C 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 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 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 b;utte closures should provide gas venting rnaans -or separate gas venting means should be provided.

Further features and details of the invention will be apparent from the following description of a specific construction of lead acid electric storage cell embodying the present invention which is given by way of example only with reference to the accompanying drawings in which: Figure 1 is a scrap cross sectional view of a gas vent for an experimental recombinant lead acid electric storage cell for an aircraft battery embodying the invention, Figure 2 is an electron scanning photomicrograph of a preferred separator material at 100 fold magnification and Figure 3 is a view similar to Figure 2 at 4000 fold magnification.

Example 1 The cell in this example has a rated capacity of 25 Ahr at 25 amps i.e. the 1 hr rate and is designed for use in an aircraft battery. The cell is accommodated 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 container by cementing with an appropriate conventional cement. The cell is provided with a vent described in more detail below with reference to Figure 1 and the terminal posts are sealed into the end walls of the container as is conventional for flooded aircraft batteries. The cell contains six positive plates interleaved with seven negative plates separated from one another by separators of electrolyte and gas permeable blotting paper like glass fibre material whose composition will be described below.

Each negative is wrapped in a single U shaped layer of the 1.2 mm thick material (which weighs 200 grisquare metre) with the separator enclosing the bottom of the plate. Apart from the separator and the vent, the cell is of conventional aircraft battery structure and will not be further described.

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 over-charged, so that oxygen gas is evolved at a faster rate than it can be combined a relief valve is provided to exhaust the excess gas. As can be seen in Figure 1 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.

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 positive and negative active electrode material respectively. Each positive grid weighs 43 grams and each negative grid weighs 37 grams.

The grid alloy composition is 6% by weight antimony, balance substantially lead.

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 paste had a density of 4.1 gr/cc.

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 0.56 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.

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

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

Each negative grid carrid 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 alternating with pairs of negatives, the pairs being spaced 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.

The separators 14 are of highly absorbent blotting paper-like short staple fibre glass matting about 1.2 mms 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 magnifications, Figure 2 at 1000 fold and Figure 3 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 weighs 200 grams/square metre and has a porosity of 90-95% as measured bv mercury intrusion penetrometry. The densitv 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.

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 mums since below this value we have found that growth of dendrites through the separator is liable to occur with the 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 fibre density ratio is preferably 90 to 120.

Each sheet of separator material is 0.12 cms thick 8.8 cms wide and 31.3 cms long. The total 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 separator 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 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 the negative plates is 1709.4 square centimetres. The dry weight of the active material of the positive plates if 55 x 6 x 0.83 i.e. 274 grams (as PbO2 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% 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.

The true volume of the positive active material is 274 divided by 9 i.e. 30.4 ccs and its apparent volume is 274 divided by 4.2 i.e. 65.2 ccs; the pore volume of the positive active material is thus 34.8 ccs.

The true volume of the negative active material is 346.5 divided by 10.5 i.e. 82.4 ccs and its apparent volume is 346.5 divided by 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 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 mi i.e. 1.08 (X+Y) to 1.09 (X+Y) of 1.30 SG aqueous sulphuric acid i.e. 156 grams of H2SO4 added to it.

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. 23 Ahr or 125% of the cells capacity) and then discharged at 1.5 amps to 1.667 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 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 discharge) 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 hr rate) to 1.667 volts (i.e. 100% depth of discharge). The discharge duration was 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 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.

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.

Alternative antimonial alloys include those disclosed in the United States patents No.3,879,217 and 3,912,537.

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 state.

Thus, in use, the cell or each cell in a battery is normally sealed and is arranged so that essentially oxygen recombines with a negative plate. The cell generally operates at superatmospheric pressure at least on chanrge, 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.

Example 2 Example 1 qas repeated except that the grids were made from an alloy having a composition in % by weight of 2.43% antimony, 0.22% arsenic,0.04% tin, 0.006 copper, 0.004% selenium, the balance lead.

The cell was subjected to the same test regime as the cell of Example 1 and the water loss after 28 cycles was 1.7 ccs, after 54 cycles was 1.4ccs, 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 1 and 2 on constant potential recharging at 2.37 volts also indicated good cycle lives with even lower water losses than those quoted for the constant current recharging regime described above.

Example 3 Example 1 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 Example 1 was obtained.

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

Claims (8)

1. A method of making a recombinant lead acid electric storage battery comprising providing current conducting electrodes carrying electrolytically unformed material convertable to positive active material and to negative active material, converting substantially half the electrodes to positive electrolyticallyformed condition and the remainder to negative electrolyticallyformed condition, locating the thus charged positive and negative electrodes in a sealable container, adjacent positive and negative electrodes being separated by a fibrous absorbent separator material, which is corn;> ti between the said adjacent plates, sealing the container, evacuating the container and introducing electrolyte into the evacuated container in an amount such that substantially all of the electrolyte is absorbed in the electrodes and the separator material.

2. A method as claimed in Claim 1 in which the current conducting elements of the positive electrodes are made of antimonial lead alloy containing between 1 and 4% by weight antimony.

3. A method as claimed in Claim 1 or Claim 2 in which the current conducting elements of the negative electrodes are made of a lead alloy containing calcium.

4. A method as claimed in any one of the preceding claims in which the separators are of electrolyte and gas permeable compressible fibrous material having an electrolyte absorption ratio of at least 100%.

5. A method as claimed in any one of the preceding claims in which the volume E of electrolyte in the battery 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, 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.

6. A method as claimed in Claim Sin which the ratio of X to Y is in the range 4:1 to 1.5:1.

7. A method as claimed in any one of the preceding claims in which the ratio of negative to positive active material (on the weight of lead basis) is less than 1:1.

8. A method of making a recombinant electric storage battery substantially as specifically herein described with reference to the accompanying drawings.