Technical field

The present invention generally relates to a positive and/or negative flat electrode for a lead acid battery and a battery equipped thereof. It also relates to a process for manufacturing such positive and/or negative flat electrode.

Background Art

Lead acid batteries are widely used and include adjacent positive and negative plate electrodes immersed in an electrolyte and spaced by separators. The latter are used to prevent short circuits occurring between the electrodes while enabling ions exchange and electrolyte flow.

The lead acid battery is charged and discharged alternatively. This operation provokes the electro-chemical transformation of an active mass made of lead dioxide into lead sulphate on the positive plate electrodes and lead into lead sulphate on the negative plate electrodes during discharge and vice versa during charge. During these reactions, the active mass contracts or expands. This phenomenon is called breathing.

The electrode plate can either be flat or tubular. Tubular electrodes comprise pencils made of current carrying spines surrounded by the active mass. Flat electrodes comprise a grid and active mass located in cells defined by the grid, the active mass being formed by a paste. In both cases, breathing could lead to active mass shedding and/or mossing which has a major impact on the life time of the battery. To avoid such phenomenon the active mass is protected and retained by tubular gauntlet in tubular type electrodes. For flat electrodes the solutions used to avoid shedding and/or mossing are since long focused on the characteristics of the active mass and/or of the connection provided between the active mass and the grid like in patents US5302476 or US20100015531 for instance.

However, such improvements lead to select components and/or materials having high costs which in turn lead to increase the cost of the battery to the final customer whereas such products need to be used to address all kinds of market.

Moreover, the choices made to increase the performances of the battery, especially its electrical efficiency, such as its capacity and/or its charging capability can lead to increase the shedding and/or mossing. For example, for negative plates, a carbon based additives have been proposed to decrease the density of the active mass but the paste adhesion to the grid is reduced and the shedding and/or mossing increases. For the positive plate it is known to increase porosity (decrease density) but the paste has not sufficient consistency and adherence to the grid. As a consequence, the active mass tends to shed during the formation or operation of the batteries.

The increase of the quality of the battery by improving its electrical efficiency is therefore often limited by the increase of the shedding and/or mossing.

As a consequence, there is still an attempt for batteries having both sufficiently high performances and limited costs.

General description of the Invention

In order to reach the above-mentioned attempt, the present invention proposes a positive and/or negative flat electrode for a lead acid battery including a grid and an active mass connected to the grid, the grid and/or the active mass being chosen among components and/or materials likely to generate excessive shedding and/or mossing, the flat electrode further comprising a protection tightly fitted on said grid and/or said active mass to limit said excessive shedding and/or mossing.

By “the grid and/or the active mass being chosen among components and/or material likely to generate excessive shedding and/or mossing” it must be understood that the electrode plate comprising said grid and said active mass, if used without the protection in a battery, would lead to shedding and/or mossing above standard tolerated quantities.

Examples of such electrode plates are electrode plates with weak connection of the active mass to the grid resulting in active mass shedding and/or mossing off the grids. Such electrode plates are especially flat electrode having an active mass with a pore volume, a specific active surface, an hardness and/or a consistency outside the ranges commonly considered as being efficient.

The present invention allows using electrode plate having limited quality as regards shedding and/or mossing without loosing active mass thanks to the protection placed around said electrode plate. The invention thus improves the life cycle of the flat electrode and the battery. Thanks to the invention, it is therefore possible to use an electrode plate having standard electrical efficiency or improved electrical efficiency even if the shedding and/or mossing capability of the active mass is increased because the shedding and/or mossing of the electrode itself will be reduced thanks to the protection.

According to different aspects of the invention which can be taken alone or in combination:
- said protection is made of a shrinkable material,
- the total pore volume of the active mass of the negative flat electrode is less than 0.08cm3.g-1 or greater than 0.09cm3.g-1 and/or the total pore volume of the active mass of the positive flat electrode is less than 0.08cm3.g-1 or greater than 0.12 cm3g-1,
- the total pore volume of the active mass of the negative flat electrode is less than 0.07cm3.g-1 or greater than 0.1cm3.g-1 and/or the total pore volume of the active mass of the positive flat electrode is less than 0.04cm3.g-1 or greater than 0.16 cm3g-1,
- the specific active surface of the active mass of the negative flat electrode is less than 3 m2/g or greater than 4 m2/g and/or the specific active surface of the active mass of the positive flat electrode is less than 4 m2/g or greater than 5 m2/g,
- the specific active surface of the active mass of the negative flat electrode is less than 2 m2/g or greater than 5 m2/g and/or the specific active surface of the active mass of the positive flat electrode is less than 3 m2/g or greater than 6 m2/g,
- the paste density of the active mass of the negative flat electrode, is less than 4,10 g/cm3 or greater than 4,50 g/cm3 and/or the paste density of the active mass of the positive flat electrode is less than 3,90 g/cm3 or greater than 4,20 g/cm3.
- the moisture content of the active mass of the negative and/or positive flat electrode is less than 10% or greater than 12%. These values are obtained before the curing step of the flat electrode manufacturing process. After the curing and the drying steps of the flat electrode manufacturing process, the moisture content of the active mass of the negative and/or positive flat electrode is greater than 0,2%. Moisture content is determined from the weight loss after 1 hour at 110°C. Outside this range, which is commonly considered to be efficient, the consistency is not satisfying

According to an aspect of the invention, which can used alone or in combination with the preceding features, said protection comprise two flat faces, said flat faces having two lateral sides, a lower side and an upper side.

Advantageously, said flat faces are joined together along said lateral sides according to a first kind of assembly, said protection being open between said upper sides, said protection being further open between said lower sides or being closed along said lower sides by a join assembling said flat faces according to another kind of assembly.

By another kind of assembly, we especially mean a technique providing a different joining strength. By open, we especially mean open along the whole length of said lower sides but the invention also encompasses protection being partially opened. Such features have enabled shrinkage of the sleeve or pocket which has provided surprisingly good results, especially as regards battery lifetime.

According to different embodiments of such aspect of the invention which can be taken alone or in combination:
- said lateral sides are joined through weaving, sewing or knitting;
- said lateral sides are joined through sewing according to a first kind of stitches;
- said protection are open or closed along said lower sides by sealing;
- said protection are open or closed along said lower sides by sewing according to another kind of stitches;
- said first kind of stitches are double lockstitches and said another kind of stitches are serger stitches;
- said flat faces are configured to have a first ability to shrink along said lateral sides and another ability to shrink in a direction perpendicular to said lateral sides;
- said protection is reinforced along said lateral sides;
- said protection can be partially closed along said upper sides;
- said protection comprises a foil of said shrinkable material, woven or non woven and folded to form the two flat faces such as the fold is forming one of the lateral sides, the other lateral sides being closed as described previously.

According to a first variant of the invention, said protection comprises two non woven sheets of said shrinkable material, said two sheets being assembled together to correspondingly define said flat faces when being joined together along said lateral sides through sewing.

According to different aspects of said first variant which can be taken alone or in combination:
- said sheets are assembled together by ultrasonic welding along said lower sides;
- said protection comprises an insulation resin along said lateral sides;
- said protection comprises an insulation resin along said lower side;
- said shrinkable material is made of polyester, polypropylene or a mixture thereof.

Any kind of non woven fabric can be used. It can be a mechanically bonded fabric as a needled or hydro entangled or calendared fabric, for instance a flat thermo-bonded or point-bonded fabric. It can also be a chemically bonded fabric. From another perspective it can be a fabric using spunbond or meltblown or staple fibers, maid by wet laid process or dry laid process.

According to a second variant of the invention said protection comprises a woven sheet of said shrinkable material, said woven sheet forming a sleeve defining said flat faces when being joined together along said lateral sides through weaving.

According to different aspects of said second variant which can be taken alone or in combination:
- said sleeve is assembled by ultrasonic welding along said lower sides;
- said sleeve has a denser weaving along said lateral sides;
- said shrinkable material is made of polyester yarn, polypropylene yarn or a polyester/polypropylene mixture yarn;
- the yarn for the weft is a thermo shrinkable yarn;
- the yarn for the weft is perpendicular to said lateral sides;
- said protection comprises an insulation resin along said lateral sides;
- said protection comprises an insulation resin along said lower side.

The invention also concerns a lead acid battery comprising a positive and/or negative flat electrode as described previously.

According to an aspect of the invention, the battery is adapted to run under high temperature, particularly in a temperature higher than 50 °C, under partial state of charge (PSOC), for example between 5 and 40% state of charge (SOC), and/or under deep cycle application, particularly more than 80% depth of discharge (DOD). Thanks to the protections provided to the electrodes, the battery is especially fit for such severe conditions which are often met, for instance in domestic and/or automotive devices.

By the way, the invention concerns as well an uninterruptible power supply (UPS) system, especially domestic, often called inverter system, comprising a lead acid battery as described previously.

The invention is also related to a power system to drive a reversible or non reversible electrical motor of an automotive vehicle like the power systems known under the name “start and stop” systems and so called deep cycle application such as golf cart, three-wheelers, sweepers, scrubbers, wheelchairs, comprising a lead acid battery as described previously.

The invention also concerns a vehicle like golf cart, three-wheelers, sweepers, scrubbers and wheelchairs comprising a power system as described previously.

According to another aspect of the invention there is provided a process for manufacturing a positive and/or negative flat electrode for use in a lead acid battery, comprising the steps of:
- providing a grid and an active mass connected to said grid to form an electrode plate, the grid and/or the active mass being chosen among components and/or materials which would lead the electrode plate to suffer excessive shedding and/or mossing if used as such in a battery,
- placing said electrode plate into a protection made of a shrinkable material,
- shrinking the protection in order to have said protection tightly fitted on said grid and/or said active mass.

Said protection may present any of the features already mentioned above.

The invention is further described in the accompanying drawing and the description below. Other features, objects and advantages of the invention will be apparent from the description, the drawing and the claims.

Description of drawing

Fig. 1 is a perspective view schematically showing a positive or negative flat electrode according to a first embodiment of the invention.

Detailed description

As illustrated, the invention is related to a positive and/or negative flat electrode for a lead acid battery and to the lead acid battery itself. The lead acid battery of the invention comprises at least one of these flat electrodes and in particular both of it. However, the invention is also related to a lead acid battery comprising one of these flat electrodes, which could be the negative or the positive electrode, the other one being for example tubular. Even if not detailed, the flat electrode comprises a grid 2 filled with active mass, said grid 2 being shown mainly in doted lines.

The electrode plate made of the grid and the active mass is voluntary chosen as having potential shedding and/or mossing issues, especially to limit the cost thereof. We understand here that electrode plate especially has a weak connection of the active mass to the grid likely to lead to a loss of active mass during its breathing.

More precisely, said electrode plate having shedding and/or mossing issues is an electrode plate having, for instance, an active mass with a pore volume, a specific active surface, an hardness and/or a consistency outside the ranges generally used to achieve high grade results, called thereafter “common ranges”. With at least one of these characteristics outside said range and without a protection, the active mass would shed off the grid excessively.

The common range used for the pore volume of the active mass for a negative flat electrode is from 0.08cm3.g-1 to 0.09cm3.g-1 and the one of a positive flat electrode is from 0.08cm3.g-1 to 0.12 cm3g-1.

The common range for the specific active surface of the active mass of the negative flat electrode is from 3 m2/g to 4 m2/g and the one of the positive flat electrode is from 4 m2/g to 5 m2/g.

The common range for the hardness of the active mass can be derived from adequate paste density of the active mass which is typically comprised between 4,10 and 4,50 g/cm3 for a negative flat electrode and between 3,90 g/cm3 and 4,20 g/cm3 for a positive flat electrode. The paste density of the active mass can characterized its hardness. Outside these values of density, the hardness of the active mass is not satisfying.

The common range for the consistency of the active mass of a negative and/or positive flat electrode can be derived from an adequate moisture content which is typically comprised between 10% and 12% measured before the curing step of the flat electrode manufacturing process. After the curing and the drying steps of the flat electrode manufacturing process, the adequate moisture content of the active mass is typically less than 0,2%. Moisture content is determined from the weight loss after 1 hour at 110°C. Outside these ranges, the consistency and potentially also the density is not satisfying, and as a consequence, the risk of shedding and/or mossing without any protection is high.

Active mass consistency can also be measured with penetrometers by checking the penetration depth into the paste of a cone with a given angle and a given weight.

Electrode plates according to the invention are plates with one or several of such features outside said common ranges.

Nevertheless to limit said excessive shedding and/or mossing, the flat electrode according to the invention further comprises a shrinkable protection 1. With such kind of protection, it has been observed by the applicant that shedding and/or mossing can be significantly decreased even with electrode plates made of optimised cost material and/or components.

For instance, thanks to the protection of invention, it is possible to use a positive flat electrode with a paste density of its active mass comprised between 4,30 and 4,60 cm/g3 to improve the electrode’s capacity or between 3,70 and 3,80 cm/g3 to improve the electrode’s life cycle.

Said protection 1 here comprises two flat faces 3a, 3b, each being made of a shrinkable material. The basic material of said flat faces 3a, 3b is, for instance, a fabric which can comprise polyester or polypropylene or mixtures thereof.

The fabric can be impregnated by a protective resin aiming at creating a protective film enhancing the resistance of said fabric against oxidation. Said protective resin can be an aqueous or solvent dispersion of a thermoplastic or thermoset polymer or copolymer.

The protective resin can also be made of latex. It can be a fabric using core-shell latex, multiphase latex or latex in stable dispersion.

The quantity of protective resin applied is in the range of 5 to 40 % weight, preferably 13 to 30 % weight. The sheet is dried in an oven down to a moisture content less than 1 % weight while avoiding shrinkage. The sheet is wound on rolls.

Said protective resin is chosen, for instance, among the group consisting of: acrylic resins like e.g. methyl methacrylate resins or butyl acrylate/methyl acrylate copolymer resins, styrene-butadiene resins, phenolic resins or mixtures thereof.

When a thermoplastic resin is used, said protective film can be obtained by coating, especially by liquid or by powder coating.

The fabric and the protective film can be a cross-linked system as a three dimensional network.

Said flat faces 3a, 3b show a permeability distinguishing them from the separator. As an example, their greater pore size is over 50 µm, more precisely over 10 µm.

Said flat faces have two lateral sides 4, 5, a lower side 6 and an upper side 7.

The protection is open between the upper sides 7 of said flat faces 3a, 3b, at least before insertion of the electrode plate 2.

As illustrated, the protection 1 can remain open after insertion of the electrode plate 2. In other words, the upper sides 7 of said flat faces 3a, 3b can remain distant. In another embodiment, the protection is partially closed along said upper sides and defines an aperture for a connector 8 of the electrode plate 2.

Said flat faces 3a, 3b are joined together along said lateral sides 4, 5 according to a first kind of assembly.

According to a first aspect of the invention, corresponding to the solution shown on Fig. 1, said protection is open between said lower sides 6 which mean that said lower sides 6 are distant from each other. In other words, the protection here defines a sleeve.

According to another aspect of the invention, not shown, said protection is closed along said lower side 6 by a join assembling said flat faces 3 according to another kind of assembly. In other words, the protection here defines a pocket.

As a first example, said lateral sides 4, 5 are joined through weaving, sewing or knitting while the lower sides 6 are joined by sealing. By sealing, we for instance mean welding, gluing, thermo fusing and/or any equivalent sealing techniques. As another example, said lateral sides 4, 5 are joined through sewing according to a first kind of stitches while said lower sides 6 are joined by sewing according to another kind of stitches. Said first kind of stitches can be double lockstitches and said another kind of stitches can be serger stitches.

From a general perspective, it is to be understood that while the lateral sides 4, 5 of both flat faces 3a, 3b are respectively assembled in a first way, the lower sides 6 thereof remain at least partially separated or unassembled or are assembled in a different way. It was found by the applicant that such solution provides a protection showing interesting results after shrinkage on the electrode plate.

The protection has the following advantages:
• It is elastic, i.e. the pocket expands and contracts with the change in volume of the active mass during the charge and discharge of the battery.
• It exerts a certain compression on the active mass.
• It has a tenacity to withstand the forces exerted by the active mass.
• It has a low electrical resistance.
• It is porous to allow an easy ion exchange.

Said flat faces 3a, 3b can be configured to have a first ability to shrink along said lateral sides 4, 5 and another ability to shrink in a direction perpendicular to said lateral sides 4, 5. More precisely, their ability to shrink along the direction perpendicular to said lateral sides 4, 5 can be greater than their ability to shrink along their lateral sides 4, 5. As an example their ability to shrink along the direction perpendicular to said lateral sides 4, 5 is between 4 to 15% and/or their ability to shrink along said lateral sides 4, 5 is below 2%. The fabric has thus an advantageous ability to shrink around the electrode plate 2 in order to keep the active mass in good contact with the grid.

The width of the pocket corresponds to the width of the plate plus between 1 and 3.5 times the thickness of the electrode plate 2, preferably between 1.05 and 2.6 times and most preferably between 1.08 and 2.2 times the thickness of the plate. The pocket is shrunk by applying heat, for example by passing it in front of an Infrared source or by heating it in an oven at a temperature greater than 160°C during between 20 seconds and 10 minutes.

The electrode plate 2 may be provided with feet inserted in the join.

As will be detailed below, the protection can be reinforced along said lateral sides 4, 5, and/or along said lower side 6, for instance over a width inferior to 20 mm. Such feature aims at avoiding short circuits in the battery between adjacent electrodes.

In a first variant, said protection comprises two non woven sheets of said shrinkable material, said two sheets being assembled together to correspondingly define said flat faces 3a, 3b when being joined together along said lateral sides 4, 5 through sewing.

To obtain such non-woven sheets, extruded polyester filaments are laid down on a belt and then calendared.

The fabric is preferably made of flat calendared polyester spunbond material. Such fabrics are sold e.g. by Freudenberg under the trade name TerbondTM, by JohnsManville under the trade name DuraspunTMor under the name Mopet by Mogul.

As already mentioned, the non-woven fabric can be impregnated with said protective resin. The fabric can be impregnated thereof between the calendaring step and a winding step, and/or the impregnation can be made of line.

To obtain the sleeves or pockets, two layers of fabric as described above are sewn together along a line at a certain distance of the outmosts. This distance is given by the width and the thickness of the battery plate which has to be protected and an additional space to allow an easy insertion of the electrode plate while ensuring envelopment of the electrode plate after shrinkage.

Said sheets are here assembled together by ultrasonic welding along said lower sides 6 to form said join. For example, the welding forming said join is rectangular, its long sides being parallel to the lower side 6 of the protection and its short sides being parallel to the laterals sides 4, 5 of the protection. Alternatively, the welding forming said join is curved and comprises one ore two wings raised in the direction of the laterals side 4, 5 of the protection. In this case, the welding forming said join could have the form of an arc with an apex oriented toward the lower side 6.

Said protection can further comprise an insulation resin along said lateral sides 4, 5 and/or along said lower side 6, which can be realized preferably by the application of hot melt, but any other mean allowing closing the pores on the lateral sides and/or on the lower side 6 can be used. A band of said insulation resin can have a width from 1 to 4 mm for the lateral sides 4, 5 and a width from 5 to 20 mm for the lower side 6.

In an another variant, said protection can comprise a woven sheet of said shrinkable material, said woven sheet forming a sleeve defining said flat faces when being joined together along said lateral sides 4, 5 through weaving.

According to a preferred embodiment the woven sheet is made of polyester yarn, polypropylene yarn or a polyester/polypropylene mixture yarn.

The yarns for warp and weft have different properties; the most important difference is that the yarn for the weft is a thermo shrinkable yarn from for example Diolen sold under the reference HT57Z130. The yarn for the weft is, for instance, perpendicular to said lateral sides.

For the warp direction, the yarn can consist of polyester or polypropylene or mixtures thereof in the range of 250 – 700 dtex, preferably of polyester of 400 – 550 dtex. This yarn is sold for example by Yamatex under the reference NE 24/2.

The difference in yarns allows the pocket to shrink in the transverse direction to maintain the active mass while in longitudinal direction the shrinkage is minimal. This is obtained by the thermo shrinkability of the above-mentioned yarn from Diolen.

For the weft direction, the thermo shrinkable yarn consists of polyester or polypropylene or mixtures thereof in the range of 300 – 800 dtex, preferably of polyester of 500 – 600 dtex.

For the warp direction, the number of yarns should be in the range of 140 – 225 per 100 mm, preferably between 165 and 205 per 100 mm. On both outmosts of the pocket, the number of yarns should be increased to 250 to 500, preferably from 350 – 410 per 100 mm and this on a width of 5 – 30 mm, preferably from 7 – 20 mm in case of one single pocket woven in width. In other words, said sleeve can have a denser weaving in a band 10 along said lateral sides 4, 5. Said band can have a width from 15 to 25 mm. As it is the case for the non woven sheets, said protection comprising woven sheets can further or alternatively comprise an insulation resin along said lateral sides 4, 5 and/or along said lower side 6, which can be realized preferably by the application of hot melt, but any other mean allowing closing the pores on the lateral sides and/or on the lower side 6 can be used. A band of said insulation resin can have a width from 1 to 4 mm for the lateral sides 4, 5 and from 5 to 20 mm for the lower side.

For the weft direction, the number of yarns should be in the range of 80 – 230 per 100 mm, preferably between 130 and 180 per 100 mm.

The sheet coming out of the weaving machine is preferably impregnated with the protective resin either on line or off line in a subsequent process step.

Said sleeve is here assembled by ultrasonic welding along said lower sides 6.

The two flat faces of shrinkable material, woven or non woven, can also be formed by a single folded foil. The fold is therefore forming one of the lateral sides 4, 5, the other one being closed as described above.

As the shrinkable material used to provide said protection forms a sleeve or pocket after assembly of the lateral sides 4, 5 of the protection, the fabric can be cut according to the length requested by the customer and adapted to the length of the plate. It is then assembled along its lower sides 6 or left open.

The electrode plate is inserted by hand or by an automated process in the protection. Said protection can be supplied individually. It can also be supplied in rolls and unwound before insertion of the electrode plate.

The plate and the protection pass in front of infrared heaters or are heated by other means to the fabric shrinkage temperature range, typically around 180 °C to allow the fabric to shrink. After shrinkage the electrode plate is well insulated and ready for use. The plate and the protection can also be inserted in a boot, especially in case of protections made of sleeve.

The battery manufacturing process can further comprise the following steps:
- put at least one of the electrodes with the protection inside separator sleeve or pocket, or
- in a battery where only the negative or the positive electrodes has the protection, put the other one, i.e. the electrode without protection, inside separator sleeve or pocket, or
- in case of leaf separator, laying the leaf separator between a positive flat electrode and a negative flat electrode, the positive and/or negative flat electrode comprising said protection.

The electrodes with their protections and separators are then stacked.

By the way the protection used according to the invention must be differentiated from the well known separator especially as they do not share the same function. More precisely, the protection according to the invention aims at retaining the active mass of the electrode plates on the grid while the separator aims at providing an electrical insulation between two adjacent positive and negative plates. In that view, separators have a porosity far below protections porosity. As an example, said protections show a porosity with a greater pore size over 50 µm, more precisely over 10 µm while the greater pore size of a separator is typically below 10 µm.

This process allows the manufacturing of electrode stackings with alternate positive and negative plate electrodes ready for insertion in a battery box or container, while necessitating a reduced number of discrete operating steps. The enveloping of the electrode with the sleeve/pocket material can actually be achieved in a single step, which essentially consists in introducing the electrode plate into a shrinkable sleeve/pocket and shrinking the pocket by applying heat, for example by passing it in front of an Infrared source or by heating it in an oven at a temperature greater than 160°C during between 20 seconds and 10 minutes.

It also greatly facilitates the implementation of the process in a fully automated continuous system and, as a consequence, the reliability of the manufacturing process and its operational availability.

Hence, the present process is particularly advantageous for the battery manufacturer as high quality batteries may be obtained without complicated and expensive machinery and above all without sophisticated active mass.

The invention is also related to a lead acid battery comprising flat electrodes as described above.

CLIAMS:Claims

1. Positive and/or negative flat electrode for a lead acid battery including a grid (2) and an active mass connected to the grid (2), the grid (2) and/or the active mass being chosen among components and/or material likely to generate excessive shedding and/or mossing, the flat electrode further comprising a protection (1) tightly fitted on said grid and/or said active mass to limit said excessive shedding and/or mossing.

2. Positive and/or negative flat electrode according to claim 1, wherein said protection is made of a shrinkable material.

3. Positive and/or negative flat electrode according to claim 1 or 2, wherein the total pore volume of the active mass of the negative flat electrode is less than 0.08cm3.g-1 or greater than 0.09cm3.g-1 and/or the total pore volume of the active mass of the positive flat electrode is less than 0.08cm3.g-1 or greater than 0.12 cm3g-1.

4. Positive and/or negative flat electrode according to claim 3, wherein the total pore volume of the active mass of the negative flat electrode is less than 0.07cm3.g-1 or greater than 0.1cm3.g-1 and/or the total pore volume of the active mass of the positive flat electrode is less than 0.04cm3.g-1 or greater than 0.16 cm3g-1.

5. Positive and/or negative flat electrode according to any of claims 1 to 4, wherein the specific active surface of the active mass of the negative flat electrode is less than 3 m2/g or greater than 4 m2/g and/or the specific active surface of the active mass of the positive flat electrode is less than 4 m2/g or greater than 5 m2/g.

6. Positive and/or negative flat electrode according to claim 5, wherein the specific active surface of the active mass of the negative flat electrode is less than 2 m2/g or greater than 5 m2/g and/or the specific active surface of the active mass of the positive flat electrode is less than 3 m2/g or greater than 6 m2/g.

7. Positive and/or negative flat electrode according to any of claims 1 to 6, wherein the hardness of the active mass of the negative flat electrode is less than […] or greater than […] and/or the hardness of the active mass of the positive flat electrode is less than […] or greater than […].

8. Positive and/or negative flat electrode according to any of claims 1 to 7, wherein consistency of the active mass of the negative flat electrode is less than […] or greater than […] and/or the specific active surface of the active mass of the positive flat electrode is less than […] or greater than […].

9. Lead acid battery comprising a positive and/or negative flat electrode according to any of the preceding claims.

10. Lead acid battery according to claim 9, wherein the battery is adapted to run in a temperature higher than 50°C, under a partial state of charge comprised between 5 and 40%, and/or under a depth of discharge higher than 80%.

11. Uninterruptible power supply system comprising a lead acid battery according to claim 9 or 10.

12. Domestic uninterruptible power supply system according to claim 11.

13. Power system to drive a reversible or non reversible electrical motor of an automotive vehicle comprising a lead acid battery according to claim 9 or 10.

14. Vehicle like golf cart, three-wheelers, sweepers, scrubbers and wheelchairs comprising a power system according to claim 13.

15. Process for manufacturing a positive and/or negative flat electrode for use in a lead acid battery, comprising the steps of
- providing a grid (2) and an active mass connected to said grid (2) to form an electrode plate, the grid and/or the active mass being chosen among components and/or material which would lead the electrode plate to suffer excessive shedding and/or mossing if used as such in a battery,
- placing said electrode plate into a protection (1) made of a shrinkable material,
- shrinking the protection (1) in order to have said protection tightly fitted on said grid and/or said active mass.