CA1102409A - Battery plates covered with porous thermoplastic resin - Google Patents

Abstract

TITLE OF THE INVENTION:
BATTERY PLATES COVERED WITH POROUS
THERMOPLASTIC RESIN

ABSTRACT OF THE DISCLOSURE:
Softening and separation of an active material in flat-type pasted positive plates are prevented by a simple process with an inexpensive material so that the life may be comparable with that of clad type positive plates. Finely divided, corrosion-inhibitive thermoplastic resin powder is sprayed or otherwise applied over a pasted positive plate and heated and melted to form thin porous layers over the major surfaces of the positive plate. According to another aspect of the invention, the same resin powder is melted and bonded within the positive plate so that the porous layers may be formed within the plate in addition to the porous layers on the major surfaces.

Description

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ACKGROUND OF THE INVENTION
The present invention relates to lead acid batteries with pasted positive and/or ne~ative plates and a process for manufacture thereof.
The pasted positive plates in the lead acid batteries for stand-by service or in the portable lead acld bat-teries are divided based upon the construction into flat-plate type and clad type. In the clad -type, a cylinder o~ active material is surrounded by an envelope in the form of a slotted tube so that a long life of a lead acid ba-ttery with the clad type pasted positive plates is ensured. On the other hand, the flat- -plate type pasted positive plates are advantageous because of their simple production process and because of their unique discharge characteristics so that the lead acid batteries with -the flat-type positive plates have been widely used for start-ing the engines of automobiles. ~Since the heavy current is required to start an automobile engine, the batteries for auto-mobiles must have a high output characteristic and at the same time must be inexpensive. Thereore the batteries for auto~
mobiles are limited to the lead acid batteries with the flat-plate type pasted positive plates. Because of the low costs of the lead acid batteries with the flat-type pasted positive plates, they have been widely used for motive-power service, such as electric cars, golf carts, forklifts and so on.
The lead acid batteries with the flat-type pasted positive plates are also widely used as the power supplies for televisions and tape recorders. In order to factilita-te the handling of these batteries, the containers are totally closed to avoid the spill of an electrolyte or an electrolyte in the form of a gel is used. The flat-plate type lead acid batteries replace the clad-type lead acid ba~teries in some cases as

- 2 -2~9 stationary batteries. The lead acid batteries with flat-plate type pasted positive plates which have ~een widely used in various ~ields as described aboYe have their own problems de-pending upon their purposes. For instance, the lead acid bat-teries for starting automotive engines must have an excellent rapid discharge characteristic as well as a long life, and the batteries for driving electric cars must have a high energy density as well as a long :Life. Furthermore ~or any lead ac.id batteries whatever their uses may be, they must be fabricated by a simple process so that their cost may be low.
A most common process for preparation of flat-plate type pasted positive plates comprising a step of pasting a paste consisting of an active material and diluted sulfuric acid to pockets of a grid made of a lead alloy, a step of drying and a step of forming.
Therefore the improvements of flat-plate type pasted positive plates are dependent upon the composition of paste, materials and construction of grids, the improvement of the step for pasting the paste to the grids. Briefly stated, the more water and diluted sulfuric acid are added to an active material of lead powder, the higher the efficiency becomes, but the shorter the life becomes.
As the materials for grids, the quantity of antimony in a lead-antimony alloy is reduced in order to minimize the self-discharge and to facilitate the maintenance. Furthermore lead alloyed with calcium not antimony is used, and some lead acid batteries with the flat-plate type pasted positive plates made of a lead-calcium alloy have been already available in the market.
As to the bondiny of the paste to the grids made of such a lead-calcium alloy, it has been experimentally confirmed ~`` - 3 -24¢;~

that the bonding st~ength is not sufficient and must be im-proved.
As to the energy storage density, the grid ser~es not only for holding the paste in its pockets but also for conducting a current. Although the grid is not directly in~
volved in the electrochemical reaction, it occupies a consider~
able part o~ the weight of the battery, which is one of the demerits of the grids~ From the standpoint of productivity, one grid is used for one positive plate so that the grid type pasted positive plates are not adapted ~or the continuous mass production. In order to attain the continuous mass production, the expanded metal type plates which are widely used in the alkaline storage batteries have been tried. The inexpensive lead acid batteries will be widely and mainly used for years for motive-power service such as electric cars, but they have their inherent problems that the energy shorage density is low and the life is short. In order to solve these problems, exten-sive studies and experiments have been conduc-ted in various fields.
In order to improve the energy storage density, first the efficiency of an active material must be improved and the voltage must be increased, but these impro~ements are limited.
~hen the efficiency is improved, the battery characteristics are improved in the initial service stage, but the life becomes shorter. Another countermeasure is to increase the ratio of the volume of an active material to the volume o~ a pasted positive plate; that is, to provide ~he grids light in weight and small in size.
To this endr expanded metal, metal meshes and perfor-ated metal sheets may be advantageously used, but the reduction in ratio of the volume of a support to the volume of a pasted 2~

positive plate is not preferred because the life is adversely affected. ~n other words, in order to improve the life the sUpport must contain the acti~e material in a three-dimensional manner. ~Iowever, grids capable of this are very complex in construction and the ratio of the volume of the grid to the overall volume of the positive plate is increased. ~s a result, such grids are not preferable from the standpoint of the im-provement of the energy storage density.
The flat-plate type pasted positive plates may be fahricated in a relatively simple process and have good dis-charge characteristics. However, even with the improved active materials and the improved materials and constructions of grids, there arises another serious problem of the increased separa-tion of the active material from the plate due the repetitive cycles of discharge and charging with an increased difference batween a discharged voltage and a charged voltage. As a consequence the life in terms of the number of cycles of dis-charge and charging is still short.
As is well known in the art, at a positive plate lead dioxide PbO2 is converted to lead sulfate while at a negative plate lead is converted into lead sulfate on discharge. Since diluted sulfuric acid is directly involved in the chemical reactions, it must be sufficiently diffused into the plates on discharge. In view of this fact, the idea of forming a thin resin layer on the plate has been rejected because the resin layer prevents the sufficient diffusion of diluted sulfuric acid into the plates so that the discharge reaction is~retarded even though the resin layer ~ill be effective for preventing the separation of the active material from the plates.

_ 5 -r ' :

SUMMARY OF l'HE IN~IENTION

Accordinyly~ one of the objects of the present inven-tion is to provide a lead acid battery wherein porous resin layers are formed on the pasted positive and/or negative plates so that the separation of the active material from the pla-tes may be prevented by the increase in bonding strength o~ the active material to the plates.
Another object of the present invention is to provide a lead acid battery wherein porous flat-plates are used as supports so that the discharge capacity may be considerably improved.
A further object of -the present invention is to pro-vide a cylindrical lead acid ~attery wherein porous thermoplastic layers are formed on the positive and negative plates to increase their mechanical stren~th to such an extent that even when they are coiled, their fractures and crackings may be prevented and consequently the separation of the active material from the plates may be prevented.
A further object of the present invention is to pro-vide a lead acid battery wherein thin porous thermoplastic resinlayers are formed on the surfaces of both positive and neyative plates and an electrolyte in the form of a yel is used so that the separator may be eliminated, short~ci`rcuits may be prevented and the life may be improved.
A still further object of the present invention is to provide a process for manufacture of lead acid batteries ~herein porous thin resin layers may be formed on the surfaces of the positive and/or negative plates in a simple step so that the separation of the actlve material from the plates may be pre-3G vented.

Accordinyly, there is provided a process for manu-facture of pasted lead acid batteries comprising; applying a paste~like active material to a support, irst applying over and bonding to the surfaces of said paste-like active material a finely di~ided corrosion-inhibitive, thermoplastic resin powder, heating and mel-ting said finely divided/ corrosion~
inhibitive, thermoplastic resin powder thereby forming a porous layer in the form oE a mesh-like .structure, impregnating a second finely divided, corrosion~inhibitive -thermoplastic resin powder having a particle size smaller than that of said first finely divided, corrosion-inhibitive, thermoplastic resin power through the pores of said porous layer in the form of a mesh-like structure into said paste-like active material, and heating and melting said impregnated second finely divided, corrosion-inhibitive, thermoplastic resin powder.

sRIEF ~ESCRIPTION OF THE DRAWINGS:

Fig. 1 is a schematic side view of a pasted positive plate in accordance with the present invention;
Fig. 2 is a schematic sectional view thereof;
Fig. 3 shows the relationship between the polyethylene deposition rate (mg/cm2) and the discharge time in minutes of lead acid batteries of EXAMPLE l;
Fig. 4 shows the relationship between the polyethyl-ene deposition rate and the life in terms of the number of cycles of discharge and charging of the lead acid batteries of EXAMPL~ l;
Fig. 5 shows the relationship between the polyethyl~
ene deposition rate and the life of each of the lead acid bat-teries prepared in accordance with EXAM2LE 2;
Fig. 6 shows the relationship between the discharge

3~ in ampere-hours and the number of discharge-charging cycles of ~ 7 ~

lead acid batteries prepared in accordance with EXAMPLE 3;
Fig. 7 shows the relationship between the discharge time and the number of discharge-charging cycles of lead acid batteries prepared in accordance with EXAMPLE 4;
Figs 8A, B, C and D each show the relationship between the discharge capacity in % based on the reference discharge capacity and the nu~lber of discharge-charging cycles of each of the lead acid batteries prepared in accordance with EX~MPLE 5, the porosity of the support of the battery shown in Fig. 8A
being 75%, the porosity of the support of the battery shown in FigO 8B being 65%, the porosity of the supports of the battery shown in Fig. 8C being 50% and the porosity of the supports of the battery shown in Fig. 8D being 40%;
Fig. 9, which appears in the same sheet of drawings as Figs. 1 and 2, shows the relationship between the discharge in ampere-hours and the number of discharge-charging cycles of a lead acid battery prepared in accordance with EXAMPLE 6; and Figs. 10 and 11 are microphotos showing the surface conditions of the positive plates prepared in accordance with EXAMPLES 1 and 2, respectively.

_ SCRIPTION OF THE PRE~ERRED EMBODIMENTS:

EXAMPLE lo Referring to FigsO l and 2, a support cons~ists of a rectangular grid of lead alloyed with antimony (2.5% by weight~
and 75 mm in length, 50 mm in width and 1.5 mm in thickness, and a paste consisting of an active material of lead powder (1000 g), water (130 mQ) and sulfuric acid (70 m~ with the density of 1~35) is pasted into pockets of the grid, formed and dried, whereby a pasted positive plate is obtained which is substantially similar in composition to the conventional pasted positive plates for batteries for automobiles. This pastea ~"i` .
~ 8 positive plaie has the capacit~ o~ about 0.8 Ah ~or ~,en hours.
The pasted po~itive plate is supported ho~izontally, ana a solution conslsting of finely diviaed polyethylene tavera~e particle size: ~0 microns and the melting point 120C~ dis-persed in ethyl alcohol is sprayed uniformly over the sur~aces of the pasted positive plate and heat-treated at 130C Eor 0.5 hours so that polyethylene powders may be ri~idly honded to the surfaces of the positive plate.
The pasted positive plates with polyethylene powder deposited at the rates of 1, 2.5, 5, 7.5 and 10 mg per square centimeter (1 cm2) of the apparent surfaces of the positive plates are designated by A, ~, C, D and E, respectively, and the pasted positive plate provided with no porous layer of polyethylene powder is designated by F.
In Figs. 1 and 2 there is shown the pasted positive plate thus provided. 1 denotes the main body of the pasted positive plate; 2, the porous layer of polyethylene bonded to each of the major surfaces o the positive plate 1 in the man-ner described above; and 3, a lead. Fig. 10 shows a microscopic 2~ photo (x 10) of the pasted positive plate with the porous layers having the polyethylene powder deposition rate of 3 mg/cm2 .
Five pasted positive plates of each of A-F and six pasted negative plates with the discharge capacity substantially equal -to that of the positive plates are interleaved with separaters (made of cellulose and 1 mm in thickness~ interposed therebetween, whereby a single cell is constructed. An electro-lyte, that is, diluted sulfuric acid is adjusted to 1.28 in specific weight when the charging is completed.
Fig. 3 shows the continuous discharge hours o~ the cells constructed in the manner described above when discharged at the rate of 20 A at room temperature until the limiting voltage of 1.2 V is reached. As described above r the poly-ethylene deposition rates are l, 2,5, 5~ 7.5, 10 and 0 mg/cm .
The lives o~ these ceIls are subjected to repetitive discharge-charging tests in such a way that the discharge is continued for three hours at 0.8A and then the charging is continued for four hours at 0.4A. Fig. 4 shows the results (that is, the number of cycles un~il the terminal voltage be-comes 1.7 V after the discharge of three hours).
It is seen that in case of the disc~arge at such high a rate as 20 A, the higher the polyethylene powder deposition rate, the shorter the continuous discharge capacity becomes.
It is confirmed that there exists an optimum polyethylene deposition rate for a maximum life as shown in Fig. 4. An explanation for this phenonenon is that a small polyethylene deposition rate means a low resistance in case of the discharge, but does not contribute to the prevention of softening and sep-aration of the active material and that a high polyethylene powder deposition rate means a high resistance in case of the dlscharge and covers more active materials, thereby preventing them from undergoing the required chemical reaction.

EXAMPLE 2:

A support consists of a rectangular grid made of lead alloyed with antimony (2.5% by weight) and 75 mm in length, 50 mm in width and 3.0 mm in thickness. A paste consisting of finely divided lead active material (1000 g), water (140 mQ) and sulfuric acid (60 mQ and with the specific weight of 1.35) is pasted to pockets of the grid, formed and dried, whereby a pasted plate similar in composition to the positive plates for use in batteries intended for a long service life is obtained.

The positive plate has the discharge capacity of about 1.5 Ah 24C~

for 10 hours. Following the procedures of EX~MPLE 1, finely divided polyethylene pow~er (average particle size. 15 microns and the melting point: 115C) is sprayed over the major sur-faces of the plate and heat-treated for 20 minutes at 130C
so that polyethylene powder may be rigidly bonded to the plate.
The pasted positive plates thus prepared and with the poly~
ethylene powder deposition rates of 1, 2.5, 5, 7.5, 10 and 12.5 mg/cm are designated by A, B, C, D, E and F, respectively, while the pasted positive plate provided with no porous layer of polyethylene is denoted b~ G. Five pasted positive plates of each of A-G and six negative plates having the discharge capacity corresponding to that of the positive plates are inter-leaved with separators (made of cellulose and I.2 mm in thick-ness) interposed therebetween, whereby a single cell is obtain-ed. The specific weight of an electrol~te, that is, diluted sulfuric acid is adjusted to 1.28 when the charging is complet-ed. Fig. 11 is a ~icrophoto (X 18) of the pasted positive plate with the polyethylene powder deposition rate of 1.5 mg.
The cells thus constructed are subjected to repetitive discharge-charging tests for 12 hours in such a manner that the discharge is con-tinued discharge current of 1.5 A (that is, until the terminal voltage becomes 1.7 V) and then the charging is continued at the charging voltage of 2.5 V (the maximum current being 5 A). Fig. 5 shows the lives or cycles (until the dis-charge capacity becomes 60% of the discharge capacity of the positive plate).
In Fig. 5, the polyethylene powder deposition rates of 1, 2.5, 5, 7.5, 10, 12.5 and 0 mg/cm2 correspond to the pasted positive plates A, B, C, ~, E, F and G described else-where.
In EXAMæLE 1, the discharge is effected for a long ~ 2~

time at a low current as compared with EXAMPLE 1. It is seen that when the pol~eth~lene powder deposition rate is too high or too low, it does not contrl~ute to the improvement of service life. Thus it is very important to select-the polyethylene powder deposition rate depending upon the purposes of batteries.
So far polyethylene powder has been described as being spra~ed over the surfaces of the pasted positive plate after the paste has been dried and sufficiently hardened, but polyethylene powder may be sprayed over the surfaces of the pasted positive plate even before the paste has been hardened, and heat-treated at a temperature, for instance 130C, higher than the melting point of polyethylene after polyethylene powder is pressed lightly (this step being eliminated). Then when the paste active material is hardened, polyethylene powder is melted and then hardened on the surfaces of the active material in the form of nets. It is preferable to press lightly polyethylene powder against the paste as described above be-cause the bonding to the paste of polyethylene powder may be much facilitated.
In EXAMPLES l~and 2 polyethylene powder is used, but it is understood that any other suitable thermoplastic resins may be used. For instance, polypropylene may be used. In this case, the deposition rate is similar to that of polyethylene but the heat-treatment temperature is slightly increased to 160 to 180 because of the high softening point. The character-istics obtained are substantially similar to those attainable with polyethylene. ~hen polystyrene or polyvinylchloride is used, the deposition rates is about 2/3 of that of polyethylene, and the heat-treatment temperature is 110 to 130C because of the low melting point and high flowability. In like manner, the deposition rate and heat-treatment temperature may be suit-~ 2~

ably selected depending upon the melting point, flowability and ability of diffusing into the pastea positive plates of a thermoplastic resin used.
In both EXA~PLES 1 and 2 polyethylene powder is sprayed over the surfaces of the positive plate and heated and melted to form the porous layer. Furthermore, a resin layer may be formed within the pasted positive plate in order to improve the life as will be described below.

EXAMPLE 3:

A pasted positive plate is 80 mm in length, 50 mm in width and 1.5 mm in thickness and has the discharge capacity of 1.5 Ah for about ten hours.
Polyethylene powder (particle size: about 30 microns and the melting point: about 120C) is dispersed in methyl alcohol and sprayed over the major surface of the positive plate at the deposition rate of about 4 mg per cm2 of the ap-parent surface. Thereafter the positive plate is dried to remove alcohol, and then heated at 140C for 20 minutes to melt poly~thylene powder, thereby forming a thin resin layer of very fine pores. Next the positive plate is immersed into a dis-persion consisting of finely divided polyethylene powder (aver-age particle size: 15 microns and melting point: about 120C) dispersed at the ratio of 3% by weight into water, and withdrawn and dried at 140C for 0.5 hours. The pasted positive plates thus prepared are referred to as "A". Three pasitive plates A
and four conventional pasted negative plates are interleaved with separator (made of cellulose and 1 mm in thickness) inter-posed therebetween, whereby a cell is constructed. An electro-lyte is injected into a contàiner, and the cell is charged and formed. The specific weight of the electrolyte which is diluted sulfuric acid is adjusted to 1.28 at the end of charging.

For the sake of comparison single cells are construct-~
ed with the pa~-ted positive plates B not subjected to the above polyethylene deposition and dispersion, the pasted positive plates C subjected to the polyethylene deposition and the pasted positive plates D subjectea only to the polyethylene dispersion.
The single cells A-D are subjected to the repetitive discharge-charging tests in such a wa~ that each cell is dis-charged at a rate of 1~ until the terminal voltage drops to 1.8 V and then is continuously charged for 10 hours with the maximum charging current of 5 A until the terminal voltage rises to 2.5 V. The results are shown in Fig. 6.
It is seen that as compa~red with other cells the cell A of EXAMPLE 3 shows a low decrease in discharge capacity. An explanation to this effect is that because of the fine porous layer of resin distrlbuted at a high density over the major surfaces of the positive plate the separation of active materials which are finely divided within the positive plate may be pre-vented from being separated from the positive plate; because of the mesh-like structure of finely divided resin powder pene-~
trated relatively deeply into the positive plate, softening of the active material within the positive plate is favourably suppressed; and the synergistic effect of -these two phenomena may be attained.
So far the present invention has been described in conjunction with the preparation of the pasted positive plates, but it will be understood that the present invention may be equally applied to the pasted negative plates in order to avoid the softening and separation of the active material and to im-prove the service life.
Instead of the steps of EXA~PLE 3 for first forming a fine porous layer and then immersing the positive plate into ~he dispersion containing finely divlded ~esin powder, the plate may be first immersed in the dispersion, and then the fine porous la~er of the same resin may be formed~ Howeyer finely divided resin powder diffused into the plate by the dispersion step is heated and meltea to form a relatively smooth thin layer over the relatively rough major surfaces of the plate. ~s a result finely divided resin powder cannot be attached sufficiently so that the fine porous layer sufficient for preventing the separation of the active material cannot be formed.
The average particle size of finely divided resin powder difused in the dispersion or emulsion is selected small-er than the average particle size of finely divided resin spray-ed over the major surfaces of the positive or negative plate because the former resin powder may penetrate into the plate through the finely porous resin layers formed over the major surfaces of the plate. When the average particle size of the resin in the dispersion or emulsion is greater, the resin powder cannot penetrate through the porous resin layers into the plate.
Instead of the steps of EX~MPL~ 3, finely divided resin powder may be mixed with the paste, and after the paste has been pasted the finely divided resin powder may be sprayed over the major surfaces of the plate and heat-treated to form the porous resin layers. These steps may avoid the problems caused by the steps of impregnating the plate into the disper-sion or emulsion and then forming the porous resin layers.
In order to attain nonspillability and to facilitate handlin~ an electrolyte in the form of a gel has been used in small-sized batteries. The present invention may be equally applied to these bat~eries as will be described below.

,. .

EXAMPLE 4:

Con~entional pasted positiVe plates ~ 80 mm in length, 30 mm in width and 2.5 mm in thickness and conventional pasted negative plates s 80 mm in len~th, 30 mm in width and 2.0 mm in thickness are used. Both the positive and negative plates A and s are immersed in water so as to ba moiste~edO Next polyethylene powder (the melting point: 120C and particle size:
smaller than 100 mesh) is sprayed over the major surfaces of the positive and negative plates A and B. The plates A and B
are suspended vertically and heat-treated at 1~0C for 20 min-utes to ~use and attach finely divided polyethylene powder to the positive and negative plates A and s at the rate of about 3 mg/cm2 of the apparent surfaces thereof.
Two positive plates A and three negative plates thus prepared are interleaved with spacers so that the positive and negative plates may be spaced apart from each other by approxi~
mately 1.5 mm. The plate assembly is then placed in a contain-er, and a mixture consisting of sulfuric acid (40% by weight) and silicic acid (15~ by weight) is mixed well and poured into the container above the upper edges of the positive and negative plates A and B, whereby the mixture is gelated. Thus con-structed cell is referred to as "a".
For the sake of comparison with two positive plates and three negative plates not subjected to the polyethylene treatment, a cell b is constructed. With three positive plates subjected to the polyethylene treatment and t~ree negative plates not subjected to the polyethylene treatment, a cell c is constructed. With two positive plates not subjected to the polyethylene treatment and three negative plates subjected to the above polyethylene treatment, a cell d is constructed.
Three of these cells (of the same type) are combined to produce a battery. I'hese batteries are subjected to the repetitive discharge~charging tests in such a way that each battery is discharged through a fixed resistor load of 7 ohms until the terminal voltage drops to 1.7 V and then charged ~or ten hours with the maximum charging current of lA until the terminal volt-age rises to 7.5 V. The results are shown in E'ig. 7.
It is seen that the lives of the batteries _ and d are longer than that of the battery b and that the li~e of the battery a is by far longer. The discharge capacity of the battery a is lower than those of the batteries b d when the number of cycles of discharge and charging is less, but the decrease in discharge capacity is lower as compared to the batteries b-d and the life in terms of cycles of discharge and charging is by ~ar longer. An explanation is that the thin porous poly-ethylene layers on the positive and negative plates supress the softening and separation of the active materials within the plates so that the decrease in discharge capacity due to cyclic discharge and charging is less. This is apparent from the comparison with the batteries c and d wherein only positive or negative plates are subjected to the polyethylene treatment and with the battery b where neither of the positive and nega-tive plates were not subjected to the polyethylene treatment.
The existence of gels between the positive and nega-tive plates is ~ery effective for firmly holding the positive and negative plates and for preventing the short circuit. That is, the gels between the positive and negative plates serve as the separators so that no separator is required.
As described above the lead acid battery of EXAMPLE

4 in accordance with the presen-t invention comprises pasted positive and negati~e plates each having the major surfaces coated with the thin porous layers of a thermoplastic resin and a gelated electrolyte which also serves as separators. There-fore the life in terms of cycles of discharge and charging may be considerably i~proved as descri~ed abo~e.
When the so~called porous plates such as expanded sheets, perforated sheets and screens all made of lead alloyed with a small quantity of antimony are used instead of support-ing grids, the ratio of the volume of the supporting member to the volume of the positive or negative plate may ~e reduced so that the discharge capacity may be improved and that the step for applying the paste to the plates is adapted for the mass production of batteries.
EXAMPLE 5:
Rectangular pasted positive plates 50 mm in width, 75 mm in height and 1.0 mm in thickness and so formed as to have the discharge capacity of approximately 1 Ah for ten hours are prepared. Supports are expanded lead metal sheets 0.5 mm in thickness with the porosity of 75, 65, 50 and 40~. The plates are moistened with waterr and polyethylene powder (average particle size: about 30 microns and the melting point: about 128C) is sprayed uniformly over the major surfaces of the plates at a rate of 0.3g per plate. Thereafter the plates are heated at 145C for a quarter hour and then cooled to room temperature.
With three pasted positive plates thus prepared, four pasted negative plates 50 mm in width, 75 mm in height and 1.0 mm in thickness and with the discharge capacity of approximate-ly 1.0 Ah for ten hours, positive plate separators approximately 1 mm in thickness and made of glass mat and negative plate separators approximately 0~6 mm in thickness and made of cellu-lose, a single battery is constructed. The specific weight of , an electrolyte; that is, diluted sulfuric acid is adjusted to 1.28 at the end of the charging.
These batteries are subjected to the repetitive is discharge-charying tests in such a way that each bat-tery is discharged continuously at a rate o~ 0.6A until the terminal voltage drops to 1.7V and then ~ conventional paste consisting of finely divided lead compound and diluted sul~uric acid is applied to the supports and is followed by drying, forming, washing with water and drying. Thus prepared pasted positive plates are 40 mm in width, 120 mm in length and 1.2 mm in thick-ness and have the discharge capacity of about 2 Ah for 10 hours.
Polyethylene powder (average particle size: approximately 10 microns and the melting point: approximately 126C) is uniform-ly diffused into methyl alcohol and sprayed over the major sur-faces of the positive plates. Thereafter the pasted positive plates are heated at 145C for 25 minutes. The polyethylene deposition rate is approximately 2 mg/cm of the apparent sur-faces of the plates.
Next the preparation and treatment of the negative plates will be described. Following the procedures of the pre-paration of positive plates, the supports are prepared, and a conventional paste consisting of finely divided lead compound and diluted sulfuric acid is applied, driea, formed, washed with water and dried. Thus prepared negative plates are 120 mm in length, 40 mm in width and l.l mm in thickness and have the discharge capacity of approximately 3 Ah for 10 hours. Finely polyethylene powder (averaye particle size: approximately 30 microns and the melting point: approximately 126C) is sprayed over the major surfaces of the pasted negative plates and heat-3Q treated at 145C for 0.5 hours. Prior to spraying polyethylene powder, the pasted negative plates are moistened with water, and the deposition rate is 4 mg/cm2 of the apparent surfaces of the plates.
The positive~and negative plates thus prepared are overlayed one upon another with a separator interposed there-between, the separator being 0.2 mm in thickness and made of nonwoven cloth of polypropylene, and they are coiled into the form of a cylinder approxi~lately 25 mm in outer diameter and 40 mm in height. The coil thus prepared is inserted into a container substantially similar in outer dimension to a single-cell dry-cell/ and the specific weight of an electrolyte which is diluted sulfuric acid is adjusted to 1.2~ at the end of charging.
Thus constructed battery is subjected to the repeti-tive discharge-charging test in such a way that the battery is continuously discharged at a rate of ~on mA until the terminal voltage drops to 1.7 V and then continuously charged at 2.5 (with the maximl~ charging current less than 0.7 V). The re-sult is shown in Fig. 9. It is seen that according to the pre-sent invention the improved characteristics of batteries may be maintained even when the pasted positive and negative plates are coiled with the separator interposed therebetween.
For the sake of comparison, the pasted positive and negative pla-tes not subjected to the polyethylene treatment described above are coiled, but fractures, separation and cra~kings of active materials occur even beore the pasted positive and negative plates are completely coiled. The dis-charge capacity in the initial stage is 1.2 Ah. As a con-~
sequence, the repetitive discharge-charging ~est was not con-ducted~
As described above, the pasted positive and negative plates may be coiled simultaneously, but it will be understood ~,~;

" ~IL~IIL~2~9 that the positive and negative plates may be coiled independ-ently of each other and then assembled. Thus according to the present invention there may be pro~ided batteries with coiled positive and negative plates which have been so far unattain-able bv any prior art methods.

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for manufacture of pasted lead acid batteries comprising:
applying a paste-like active material to a support, first applying over and bonding to the surfaces of said paste-like active material a finely divided corrosion-inhibitive, thermoplastic resin powder, heating and melting said finely divided, corrosion-inhibitive, thermoplastic resin powder thereby forming a porous layer in the form of a mesh-like structure, impregnating a second finely divided, corrosion-inhibitive thermoplastic resin powder having a particle size smaller than that of said first finely divided, corrosion-inhibitive, thermoplastic resin powder through the pores of said porous layer in the form of a mesh-like structure into said paste-like active material, and heating and melting said impregnated second finely divided, corrosion-inhibitive, thermoplastic resin powder.

2. A process as set forth in claim 1 wherein said second, finely divided, corrosion-inhibitive, thermoplastic resin powder is dispersed in water and then impregnated into said paste-like active material.

3. A process as set forth in Claim 1 wherein said second, finely divided, corrosion-inhibitive, thermoplastic resin powder is suspended in water and then impregnated into said paste-like active material.