AU5992280A

AU5992280A - Electric storage bateries

Info

Publication number: AU5992280A
Application number: AU59922/80A
Authority: AU
Inventors: Barry Culpin
Original assignee: Chloride Group Ltd
Current assignee: Chloride Group Ltd
Priority date: 1979-05-09
Filing date: 1980-05-08
Publication date: 1980-11-20
Also published as: ZA802797B; ES491333A0; WO1980002474A1; EP0028228A1; IN152679B; ES8103487A1

Description

*- . - . ϊ ^*

1.

ELECTRIC STORAGE BATTERIES TECHNICAL FIELD

The. present invention relates to lead acid electric storage batteries, and is particularly concerned with

5. 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 elec¬ trodes. BACKGROUND ART

10. Recombinant lead acid batteries are known. The cells of .such batteries usually have a highly absorbent separator material separating the electrodes and the amount of electrolyte added is such that the cells at least when fully charged contain substantially no free

15. unabsorbed electrolyte.

We have found that with such cells severe prob¬ lems of short circuiting occur with significant numbers of cells in any one batch if formation does not occur within a defined period after the acid has been added

20. to the cell. Thus if, as is conventional, the cell is left for 24 to 48 hours before formation, treeing through the separator by lead compounds occurs fre¬ quently, either during the electrolytic formation process or shortly thereafter.

21.. It is not known why this occurs but it is though : that it may be that with the reduced amount of acid present and with variations in density of the separator and compression of the separator from place to place there can be regions where the acid is neutralized by

OMPI the active material rather than vice versa and patches of alkalinity may occur which are conducive to the formation of soluble lead compounds which become precipitated in the separator during electrolytic 5. formation.

Whether this is the correct explanation or not we have observed that if the cells are formed as soon as they have cooled after addition of the acid, e.g. to 40°C, and provided formation commences within the 10. defined time interval the incidence of this problem is severely diminished if not eliminated.

The invention can be used with individual cells ' e.g. spirally wound cells or 'with batteries of cells. The invention has been developed with a par icu- 15. lar battery configuration in mind and is described with reference thereto but is not to be construed as being limited in its usefulness to such a battery. DISCLOSURE OF THE INVENTION

According to the present invention a method of 20. making a lead acid electric storage battery or cell comprises enclosing the cell group or groups in cell containers, evacuating the cells and introducing sulphuric acid electrolyte into the cells in an amount such that the cells are not flooded and within 2^*5. less than 6 hours preferably less than 4 hours and especially less than 2 hours of the first contact of the acid with the active material, commencing electro¬ lytic formation of the cells in the cell containers. The cells are preferably allowed to cool to a

OMP temperature not in excess of 40 C before electrolytic formation is commenced. Preferably the electrolytic formation is commenced within less than hour of the first contact of the acid with the active material

5. when the cells are at a temperature of less than 40 C. In a preferred form of the invention the method is applied to the production of a lead acid electric storage battery, and especially a battery adapted to provide one or more of the starting, lighting or

10. ignition battery functions for a vehicle, in which the positive and negative plates in each cell are separated by separators of electrolyte and gas permeable compressible fibrous .separator material having an electrolyte absorption ratio of at least

15. 100%, the volume E of electrolyte in the battery pre¬ ferably being at least 0.8 (X+Y), where X is the total pore volume of the separators in the dry state and Y is the total pore volume of the positive and negative active materials in the dry fully charged state, the

20. battery at least when fully charged having substant¬ ially no free unabsorbed electrolyte, whereby sub¬ stantial oxygen gas recombination occurs in the battery at charging rates not in excess of C/20. The charging rate is desirably kept at not

25. 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

- REAtT

OMPI_ 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

5. 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

10. 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 H„S0_u in grams to the lead in the active

15. 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 mater-

20. ial (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 conventional for recombinant batteries but we find that recombinant operation can be achieved at these ratios and they

25. 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

30. compressible absorbent fibrous material e.g. having

O PI 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

5. 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

10. 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 hμmidity of less

15. than 50%.

The thickness of the separator material is measured with a micrometer at a loading of 10 kilo- pascals C1.45 psi) and a foot area of 200 square millimetres (in accordance with the method of British

20. 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

25. 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

30. been reached, so that good electrolyte distribution is achieved in each cell.

We find that these two requirements are met by fibrous hotting paper-like materials made from fibres having diameters in the range 0.01 microns or less up

5. to lOmicrons, 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

10. centimetre of the material from which the individual fibres are made preferably being at least 20 prefer¬ ably at least 30 and especially at least 50.

This combination of properties gives a material which is highly resistant to "treeing through",

15. 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 con¬ taining large amounts of absorbed electrolyte, still providing a substantial degree of gas transmission

20. 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

25. the restricted amount of electrolyte, the high elec¬ trolyte 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

OMP charging or. overcharging, at the positive is trans¬ ported, 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

5. 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

10. negative active material enables the negative elec¬ trode 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

15. 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

20. 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 posi¬ tive electrodes.

25. The electrochemical efficiency of the negative electrodes is in general greater than that of the positive electrodes but it must be born in mind that the efficiency of the negative electrodes drops more rapidly than that of the positive electrodes both as the cells undergo increasing numbers of cycles of charge and discharge and as the temperature of operation is reduced below ambient (i.e. 25 C) . Excess negative capacity may thus conveniently be

5. 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

10. a separator, desirably having a high electrolyte absorp¬ tion ratio as also described and defined above, which is compressible, so as to conform closely to the sur- ^• faces of the electrodes , and -which has wicking or capillary activity, whereby transmission of electrolyte

15. and electrolytic conduction between the electrodes is facilitated and preserved independent of the orient¬ ation of the cell^', whilst gas transmission through the open spaces in the separator is maintained so that adequate and rapid gas transmission between the elec-

20. trodes 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 des-

25. cribed 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.

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

The gas venting means preferably take the form of a non-return valve so that air cannot obtain access 5. to the interior of the battery although gas generated therein can escape to atmosphere.

The lid of the container may be formed with filling apertures to permit electrolyte to be intro¬ duced into each cell. The- filling apertures may be 10. closed after the electrolyte has been added but the closures should provide gas venting means^• or separate gas venting means should be provided. BEST MODE OF CARRYING OUT THE INVENTION

The invention may be.put into practice in various 15. ways and one specific embodiment will be described by way of example to illustrate the invention with reference to the accompanying drawings, in which:-

Figure 1 is a partial cross-sectional side elevation of part of a starting, lighting and ignition 20. battery in accordance with the present invention;

Figure 2 is an end elevation on the line II-II of Figure 1;

Figure 3 is an electron scanning photomicrograph of a preferred separator material at 1000 fold agnifi- 25. cation; and

Figure 4 is a view similar to Figure 3 at 4000 fold magnification.

The battery has a capacity of 43 Ahr and has six cells accommodated in a container 2 made as a single 30. moulding of polypropylene plastics material and

_OMPI separated from each other by integral intercell partitions 4. The cells are sealed by a common lid 6 which is connected to the walls of the container 2 at 7 and the partitions 4 at 8 by the method known as "heat

5. sealing" in which the edges to be joined are placed in contact with opposite surfaces of a heated tool which is subsequently withdrawn and the partially melted edges are pressed together.

Each cell contains four positive plates 10 inter-

10. leaved with five negative plates 12 separated from one another by separators 14 of electrolyte and gas perm¬ eable compressible blotting paper-like glass fibre material whose composition and function will be des¬ cribed below. A sheet of separator 14 is also placed

15-. on both outside faces of each cell. The positive plates 10 and negative plates 12 are formed from a cast grid of lead alloy containing 0.07% calcium and 0.7% tin and carry positive and negative active elec¬ trode material respectively.

20. The positive plates are 2.0 mms thick and the negative plates are 1.8 mms thick and are held in inti¬ mate contact with the separators by solid polypropylene packing pieces 30. Both faces of all plates are covered by separator material which extends out above

25. 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 negative is 1.2 mms thick. The positive active material had the following composition before being electrolytically formed: Hardinge grey oxide 13640 parts, fibre 6 parts, water 1800 parts, 1.40 SG aqueous sulphuric acid 750 parts.

5. The paste .had a density of 4.2 gr/cc.

•The negative active material had the following composition before being electrolytically formed: Hardinge oxide 13640 parts, fibre 3 parts, barium sulphate 68 parts, carbon black 23 parts, stearic

10. acid 7 parts, Vanisperse CB (a lignosulphonate) 41 parts, water 1525 parts, 1.40 SG aqueous sulphuric acid 875 parts. The paste had a density of 4.3. Vanisperse CB is described in British patent specifi¬ cation No. 1,396,308.

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

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

As the active material has sulphuric acid added

20. 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 separators 14 are highly absorbent blotting

25. paper-like short staple fibre glass matting about 1 mm thick, there being fibres 61 as thin as 0.2 microns and fibres 60 as thick as 2 microns in diameter, the average of the diameter of the fibres being about 0.5 microns. Figures 3.and 4 show this material at different magnifications, Figure 3 at 1000 fold and Figure 4 at 4000 fold.

It will be observed that the material whilst highly absorbent still has a very large amount of open

5. space between the individual fibres. The material when tested for its wicking and electrolyte absorption capabilities by being suspended vertically above a body of sulphuric acid of 1.270 SG- containing 0.01% by weight of sodium lauryl sulphonate with 1 cm of

10. its end dipping in the electrolyte in an atmosphere of 20°C and a relative humidity of less than 50% absorbs electrolyte so that the liquid has wicked up to a height of 20 cms after 2 hours and this is the steady state condition. This 20 cms of material

15. 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 by mercury intrusion penetrometry. The density of the glass from

20. 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 1 mms thick and weighs 200 grams/square metre. The total volume of separator for each cell before assembly is 218

25. cubic centimetres.

The separator in the cell is compressed by about 8% and thus the volume of separator in the cell is 200.6 cubic centimetres.

Since the porosity is 90-95% the separator void

"&0RE OMPI 13 .

volume is 180.5 to 190.6 ccs (this is the value of X). The weight of separator present in each cell is 39.7 grams.

The separators being compressible conform closely

5. to the surfaces of the plates thus facilitating electrolyte transfer and ionic conduction between the plates via the separator.

The total .thickness of separator should desirably be no thinner than about 0.6 mms since below this

10. value we have found that growth of dendrites through the separator is liable to occur with the material shown in Figures 3 and 4. It may be as high as 2 or more even 3 mms but a preferred range is 1 to 2 mms. The separator weight to fibre density ratio is prefer-

15. ably in the range 70 to 160 or 200.

The total geometric surface area of. the positive plates in each cell is 767 square centimetres and of the negative plates 959 square centimetres. The dry weight of the active material of the positive plates

20. is 4 x 109 x 1.07 i.e. 468 grams (as Pb0₂ i.e. 405 grams as lead) and that of the negatives is 5 x 105 x 0.93 i.e. 490 grams (as lead) an excess of 4.7% nega¬ tive active material based on the weight of the positive active material (21% as lead). The total

25. weight of the grids is 763 grams.

The true density of the positive active material (Pb0_) in the fully charged state is 9 gr/cc and the true density of the negative active material (sponge lead) in the fully charged state is 10.5 gr/cc.

/^< τ REΛ~Sr- Thus the true volume of the positive active material is 4 x 109 ÷ 9 i.e. 48.4 ccs and the true volume of the negative active material is 5 x 105 ÷ 10.5 i.e. 50 ccs.

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

10. apparent volume of the dry negative active material is 5 x 105 ÷ 4.4 i.e. 119.3 ccs.

Thus the pore volume of the positive active material is 55.4 ccs and of the negative active material is 69.3 ccs and the total pore volume of

15. the active material is 124.7 ccs, which is the value of Y. The ratio of X to Y is thus 1.45:1 to 1.53:1. (X+Y) is 305.2 to 315.3.

The calculated true surface area for the positive active material is 1170 square metres and for the

20. negative is 220 square metres, using a factor of 0.45 square metre/gram for the negative active material and 2.5 square metres/gram for the positive active material.

Each dry electrolytically unformed cell was

25. evacuated to a high vacuum and had 325 ml i.e. 1.06 (X+Y) to 1.03 (X+Y) of 1.275 SG aqueous sulphuric acid (i.e. 153 grams of H^SO,,) added to the unformed cell. The cells were then allowed to cool to 40 C (1 to 2 hours) and electrolytically formed, formation

OMPI commencing within 2 hours of acid first being added, and 23 cubic centimetres of electrolyte was elec¬ trolysed off, the specific gravity of the electrolyte thus rising.

5. The electrolytic forming regime comprised 24 hours at 4.4 amps followed by 24 hours at 0.9 amps.

The amount of electrolyte remaining is thus 0.99 (X+Y) to 0.96 (X+Y).

The plates and separator are thus very nearly

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

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

20. acid per gram of positive and negative active material combined (as lead) .

There were thus 0.4 grams of H-SO^ per gram of lead in the positive active material and 0.35 grams of H„S0₄ per gram of lead in the negative active mater-

25. ial. The electrolyte active material ratio was thus 0.18.

The battery was then tested on the following test regime and completed this regime.

3 Cycles of C/20 discharge, namely discharge at 16 .

20°C at 2.2 amps to an end voltage of 10.5 volts followed by recharging at 14.1 volts for 24 hours.

1 Cycle of discharge at -18°C at 220 amps to an end voltage of 6 volts followed by recharging at 14.1 5. volts for 24 hours.

1 Cycle of discharge at 20°C at 25 amps to an end voltage of 10.5 volts followed by recharging at 14.1 volts for 24 hours.

1 Cycle of discharge at -18°C at 300 amps to an 10. end voltage of 10.5 volts followed by recharging at 14.1 volts for 24 hours.

1 Cycle of C/20 discharge.

Followed by 4 weeks of the SAE J240 test. This is a 2 minutes discharge at 25 amps at 20°Q followed 15. by a charge at 14.8 volts for 10 minutes repeated for 100 hours (i.e. 500 cycles will be carried out per week) followed by a discharge at 300 amps at 40 C to an end voltage of 7.2 volts; the discharge duration being at least 30 seconds. The battery had lost less 20. than 1 cubic centimetre of electrolyte after under¬ going this test regime.

As a further test of gas recombination batteries were charged at 2.35 volts (0.2 to 0.3 amps) for 6 months, a total current input of 1080 amphere hours, 25. and had lost only 3 ccs of electrolyte after this time.

On a Faradaic basis one would have expected a water loss of 1080 x 0.33 i.e. 356 ml.

This represents a 99.2% recombination efficiency.

The positive and negative plates are inter- 30. connected by a respective positive and negative group

O bar 16 and 18. Integral with the negative group bar 18 in the right hand cell as shown in Figure 1 is a later¬ ally^'pro ecting portion which terminates in a "flag" or upstanding portion 20 which is adjacent to the

5. intercell partition 4 and overlies a hole 22 in the partition. The positive flag in the left hand of the two cells shown in Figure 1 is connected to the similar negative flag in the right hand cell through- the hole 22 so as to form an intercell connection by a method

10. known as "extrusion fusion". This method comprises placing welding jaws against the two opposed flags before the lid 6 is placed in position, applying pressure so that the flags distort and meet in the hole 22 and then passing an electric current between

15. the two welding jaws so that the material of the- two flags is melted together and seals the hole 22.

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

20. Each cell of the battery is normally sealed, that is to say that during normal operation of the battery the cells do not communicate with the atmos¬ phere. However in case a substantial over-pressure should build up in the cell, for instance because

25. 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 non-return relief valve is provided to exhaust the excess gas and is arranged to operate at a pressure of only 2

30. to 3 psi. Each valve is of the Bunsen type and comprises a passage 36 communicaxing with the interior of a cell and leading to the exterior of the lid. Each passage 36 is within a boss in a respective recess 38 in the lid, and the boss is sεalingly covered by a

5. resilient cap 40 having a depending skirt around the boss. The cap 40 normally seals the passage 36, but if an excessive pressure should occur in the battery the skirt of the cap lifts away from the boss to vent the cell. A disc 42 provided with a vent hole or

10. clearance and keyed into the undercut top edge of the recess 38 engages each cap 40, thus ensuring that it is not blown off by the gas pressure, whilst allowing venting to atmosphere.

Reference has been made above to cast lead alloy

15. grids. Whilst this is preferred the electrodes could be made from, slit expanded sheet or be of wrought form e.g. perforated or punched sheet or from fibrous supports provided with electrically conductive coat¬ ings or deposited conductors such as are disclosed in 0. the present applicants British applications Nos.

9876/76 and 15664/76. The grids are preferably 0.1 to 3.0 mms thick especially 1.5 to 2.5 mms thick. The preferred alloy is a lead calcium tin alloy prefer¬ ably containing 0.06 to 0.13% e.g. 0.07 to 0.09%

25. calcium and 0.3 to 0.99% tin e.g. 0.4 to 0.8% tin e.g. 0.07% calcium and 0.7% tin.

Alternative alloys include 99.9% lead and anti- monial alloys such as those disclosed in United States patents Nos. 3879217 and 3912537.

OMPI In an alternative arrangement a thinner sheet of separator 0.5 mms thick but otherwise identical is used as a continuous strip and is fed up between each pair of plates as a double sheet. It thus starts on

5. an outside face of the cell passes down and round the bottom edge of the first plate, thus insulating it and avoiding short circuits, up between the plates to above their top edge then is folded back on itself -and passes down again and round the bottom of the second

10. plate and continues in this manner to wrap the bottom edge of all the plates in the cell and then passes up the outside face of the last plate at the opposite end ^• of the cell.

Due to the fact that the cells rarely if ever

15. communicate with the atmosphere, the battery will not need topping up with electrolyte and is therefore maintenance free. Furthermore the battery is unspill- able firstly because it is sealed and secondly because there is substantially no free electrolyte in the

20. cells, the electrolyte being retained within the-micro- fine glass separators and the active material. The fact that the cells are sealed also means that no spark or explosion can propogate from the atmosphere into the battery or vice versa.

25. INDUSTRIAL APPLICABILITY

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

OMPI

Claims

1. A method of making a recombinant lead acid electric storage battery or cell which comprises enclosing the cell group or groups in cell containers, evacuating the cells and introducing sulphuric acid electrolyte into the cells in an amount such that the cells are not flooded and within less than 6 hours of the first contact of the acid with the active material, commencing electrolytic formation of the cells in the cell containers.

2. A method as claimed in Claim 1 in which ^'the cells are allowed to cool to a temperature not in excess of 40 C before electrolytic formation is commenced.

3. A method as claimed in Claim 2 in which electrolytic formation is commenced within less than hour of the first contact of the^' acid with the active material.

4. A method as claimed in any one of Claims 1 to 3 in which the cells have a highly absorbent separator material separating the electrodes and the amount of electrolyte added is such that the cells at least when fully charged contain substantially no free unabsorbed electrolyte.

OM

5. A method as claimed in any one of Claims 1 to 4 in which the method is applied to the production of a lead acid electric storage battery, adapted to provide one or more of the starting, lighting or ignition battery functions for a vehicle, in which the positive and negative plates in each cell are separated by separators of electrolyte and gas perm¬ eable compressible fibrous separator material having an electrolyte absorption ratio of at least 100%, the volume E of electrolyte in the battery being at least 0.8 (X+Y), where X is the total pore' volume of the separators in the dry state and Y is the total pore volume of the positive and negative active materials in the dry fully charged state, the battery at least when fully charged having substantially no free unabsorbed electrolyte, whereby substantial oxygen gas recombination occurs in the battery at charging rates not in excess of C/20.