LEAD ACID BATTERY TECHNICAL FIELD
This invention in general relates to a lead acid battery and in particular relates to compression of a cell in a lead acid battery. BACKGROUND
Lead acid batteries are the most widely used secondary batteries, extensively employed in applications like automobiles. A battery has several cells electrically connected in parallel or series. The basic construction of a cell comprises a positive plate, a negative plate, and an electrolyte.
Battery capacity is increased by electrically connecting a plurality of positive and negative plates in parallel to form a cell and, if required, subsequently electrically connecting a plurality of such cells in series.
The application for which the battery is being used determines the battery voltage and capacity i.e. the number of plates per cell, number of cells and the type of connections, i.e. series or parallel, to be made between the cells.
Direct contact between the positive and negative plates would lead to an electrical short circuit. A separator is a physical insulation provided between two opposite polarity plates of a cell. The separator provides simultaneous electronic insulation and ionic conduction. The material forming the separator can be made from any of various-microporous, acid resistant materials having an affinity, i.e. high wettability for the liquid electrolyte. Examples of materials used as a separator are microporous rubber, porous plastic polymers and thin ion exchange resin membranes.

One function of the separator is to immobilize the electrolyte within the separator pores as in absorptive glass mats (AGM) used in sealed recombinant batteries.
Lead Acid batteries as is known to a person skilled in the art can be classified in many ways. In one classification, the Lead Acid batteries can be classified as Valve Regulated Lead Acid (VRLA) batteries, and conventional Flooded flat plate and tubular batteries. In VRLA batteries, the gases generated during battery operation are retained in the battery. These batteries have a safety valve to permit discharge of the gases above a certain pressure.
Fig. 1 and 1A provides an illustration of a conventional lead acid battery of the prior art. The battery comprises a container 100 comprising a plurality of compartments 102, a positive terminal 104 and a negative terminal 106. The battery further comprises a plurality of cells comprising a first cell 121, a last cell 125 and one or more intermediate cells 123. Each of the above cells comprises one or more positive plates 131, one or more negative plates 133 and one or more separator 135. The separator 135 is placed between the positive plate 131 and the negative plate 133. Each cell is placed in a compartment 102 of the container 100. The positive plates 131 are electrically connected in parallel to each other to form a positive plate group and the negative plates 133 are electrically connected in parallel to each other to form a negative plate group. The positive plate group of the first cell 121 is electrically connected to the positive terminal 104 while the negative plate group of the first cell is electrically connected to the positive plate group of the adjacent intermediate cell 123. The negative plate group of the last cell 125 is electrically connected to the negative terminal 106 and the positive plate group of the last cell 125 is electrically connected to the negative plate group of the adjacent intermediate cell 123. The positive or

negative plate groups of intermediate cells 123 are electrically connected to each other and to the positive or negative plate group of the first cell 121 and the last cell 125 in either series or parallel connection or a combination of the two, as required.
The battery compartments 102 are separated by a wall. The battery container 100 is typically made of acid resistant plastic material such as polypropylene or acrylonitrile-butadiene-styrene (ABS) and other materials known to the prior art.
For the proper functioning of the battery good contact between the separator 135 and adjacent plates is necessary. It is important that each separator 135 remains completely in contact with the entire surface area of its adjacent plates. The loss of contact between the plate surface and the separator 135 degrades battery performance.
Further in case different plates are compressed to different pressures the electrolyte and paste usage in different plates differs? with consequent effect on battery life and efficiency.
It is therefore necessary to ensure that the plate assemblies of a lead acid battery are maintained at the same uniform pressure throughout battery life.
EP5846397 ('397) suggests a conventional lead acid battery wherein the battery casing is compressed using external straps. It is however experienced that during battery operation, gases produced due to the electrochemical reactions in the cell, apply pressure on the walls of the battery container and cause bulging of the container walls. The bulging of the battery container walls results in non-uniform compression between the inner and outer cells. Even if the plates are initially supported by the container walls, the gas formation would cause deformation of the walls and the straps would be unable to totally prevent such deformation. There would

thus be non uniform plate compression during battery operation degrading performance and life.
US5187031 ('031) suggests a conventional lead acid battery wherein a flat rigid sheet is inserted between each end cell element and the corresponding container end wall of a battery. This protects a battery from the flexing of the container end walls during battery operation. The plate assemblies of the battery of the '031 patent however are likely to be at different pressures with consequent deterioration in performance.
DE7002943 ('943) discloses the use of a diaphragm of an elastic electrolyte resistant plastic in a conventional Lead Acid battery. The diaphragm in its un-stretched state has the form of a leaf spring. The diaphragm serves to compress the plate assemblies. The drawback of the battery of '943 patent is that the diaphragm will not be sufficient to ensure the uniform and equal compression of the plate assemblies. There will thus be deterioration in battery performance. Additionally, differences between individual diaphragms will lead to non uniform compression between cells in a battery.
DEI9738087 ('087) discloses a mono-block container for a conventional Lead Acid battery wherein the middle part of the container is elastically deformable. The middle part of the container exerts pressure on the plate assemblies. A drawback of the battery of the '087 patent is that the middle part of the container is unlikely to ensure uniform and equal compression of the plate assemblies. There would thus be deterioration in battery performance.

The above documents do not provide for the compression of individual plate assemblies in the battery. The documents also do not disclose the pressure to which the individual plate assemblies should be compressed.
Other prior art documents have pointed out the deficiencies associated with batteries wherein individual plate assemblies are not individually compressed.
EP0613201A1 ('201), suggests a method and apparatus for compressing an individual assembly of plates inside a VRLA battery. '201 suggests methods for repeatable compression of plate assemblies in mass production. A drawback of the methods suggested in '201 is that the actual compression achieved during mass production is not measured for individual plate assemblies. It is hoped that the compressing equipment will ensure uniform pressure across plate assemblies. In actual practice, the compressing equipment is unable to ensure uniform compression especially in mass production because of plate to plate thickness variations. As a result, with time, the individual plate assemblies tend to get compressed to different pressures with consequent deterioration in performance. '201 further does not disclose the pressure to which the individual plate assemblies should be compressed.
US4603093 ('093), discloses a Lead Acid Battery in which plate assemblies are compressed by means such as resilient bands are tied to the lid or by joining the tigs. The battery of the '093 patent is complicated to manufacture. Further no means are disclosed by which it can be ensured that the individual plate assemblies are compressed to the same pressure and it is likely that with time the individual plate assemblies will get compressed to different pressures with consequent deterioration in performance. Further the '093 patent does not disclose the pressure to which the individual plate assemblies should be compressed.

US5,618,641 ('641) discloses a bipolar battery with desired uniform constant pressure between each bipolar plate and separator. This document suggests a pressure of about 7 to 20 pounds per square inch (psi) for a bipolar VRLA type battery. The '641 patent while acknowledging that the use of springs within a battery is generally not viewed with favor because of the corrosion of the spring material and possible contamination of the electrolyte still suggests the use of spring loaded clamping device internal to the battery. A drawback of the battery of the '641 patent is that the patent does not disclose the manner in which uniform compression would be ensured in a mass production environment and it is likely that individual plate assemblies will tend to be compressed to different pressures. Further the degradation of springs in a corrosive lead acid environment and non uniformity between individual springs is likely to result in high maintenance costs and non uniform operational conditions..
There is thus need for a lead acid battery of a simple construction wherein individual plate assemblies of the battery are uniformly compressed to the same pressure and the use of springs is avoided. There is also a need to stipulate the pressure at which Lead Acid batteries will optimally perform.
SUMMARY
This summary is provided to introduce a selection of embodiments in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

In an embodiment of the subject matter described herein, a cell for a conventional lead acid battery is described. The cell comprises one or more positive plates; one or more negative plates and one or more separator such that a separator is placed between one positive plate and one negative plate. The plates, with the separators placed in between the plates, are compressed to a predetermined uniform pressure of between 15 psi to 40 psi, preferably 25 psi to 35 psi, in case the plates are intended to be used in a VRLA battery and to 5 psi to 15 psi, preferably 7 psi to 12 psi, in case the plates are intended to be used in other batteries. The pressure is measured at the time of compression in order to ensure that uniform pressure is maintained cell to cell, after compression, the plates are locked in place. The plates of a cell are internally connected electrically as in the prior art.
In another embodiment, a lead acid battery comprises cells of the previous embodiments.
In yet another embodiment a lead acid battery is described. The battery comprises a container of the prior art and a plurality of cells. Each cell comprises one or more positive plate, one or more negative plate and one or more separator. The separator is placed between the positive plate and the negative plate. Each cell is individually compressed to a predetermined uniform pressure of between 15 psi to 40 psi, preferably 25 psi to 35 psi, in case the cell is intended to be used in a VRLA battery and to between 5 psi to 15 psi, preferably 7 psi to 12 psi, in case the cell is intended to be used in other batteries. The pressure is measured at the time of compression in order to ensure that uniform pressure is maintained. After compression each cell is locked in place and the cells are placed in separate compartments of the

container. The plates of a cell are internally connected electrically and the cells are interconnected electrically as in the prior art.
It would be noted that while this invention is exemplified with reference to a conventional plate type Lead Acid battery, the invention can be exemplified, as would be obvious to a person skilled in the art, for non-conventional batteries, VRLA batteries and other batteries too. The teachings of this document are general and are applicable to all batteries.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 provides an illustration of a battery container of a lead acid battery of prior art.
Fig 1A provides an illustration of the top view of a lead acid battery of the prior art.
Fig. 2 provides an illustration of a cell in accordance with an embodiment of the present subject matter.
DETAILED DESCRIPTION
The subject matter describes, in one aspect, cells for a lead acid battery and a lead acid battery comprising such cells. In another aspect the subject matter describes a lead acid battery wherein individual cells in the battery are compressed.
It has been surprisingly found that it is possible to maintain uniform and equal pressure across individual cells in the battery in case each cell is compressed to the same pressure and the pressure is measured at the time of compression.

It has also been surprisingly found that lead acid batteries perform better in case each cell of the battery is compressed to an uniform pressure of between 15 psi to 40 psi, preferably 25 psi to 35 psi, in case the cell is used in a VRLA battery and to a pressure between 5 psi to 15 psi, preferably 7 psi to 12 psi, in case the cell is used in other batteries.
Aspects of the lead acid battery described herein can be implemented in any number of different environments, and/or configurations that will be obvious to a person skilled in the art. Different embodiments of the battery are herein described in the context of the following exemplary method(s) only as examples and are not limiting to the scope of the described subject matter.
Example 1
Fig. 2 provides an illustration of a cell which is compressed in accordance with one embodiment of the present subject matter.
The figure illustrates a plurality of plates comprising a cell 200. As depicted in the figure, the cell 200 comprises a multiplicity of positive and negative plates interleaved with separators and placed between two acid resistant compression plates 202 and 204. This assembly is compressed to a predetermined uniform pressure of between 15 psi to 40 psi, preferably 25 psi to 35 psi, in case the plates are intended to be used in a VRLA battery and to a pressure of between 5 psi to 15 psi, preferably 7 psi to 12 psi, in case the plates are intended to be used in other batteries. The pressure is measured at the time of compression in order to ensure that uniform pressure is maintained. The assembly can be compressed using techniques known to the prior art such as the use of hydraulic presses or pneumatic presses. The predetermined pressure can be measured using any techniques known to the prior art such as pressure gauges,

load cells and pressure transducers. The compressed assembly can be locked in place using welded or bolted ties or using any other locking arrangement described in the prior art.
Example 2
This example describes a lead acid battery comprising cells of the embodiments of the previous examples. One cell is inserted into one compartment of the container of the battery. The plates of the cell and the cells can be electrically connected as in the prior art.
Example 3
This embodiment relates to a lead acid battery comprising a container of the prior art and a plurality of cells. Each cell comprises one or more positive plate, one or more negative plate and one or more separator. The separator is placed between the positive plate and the negative plate. Each cell is individually compressed to a predetermined uniform pressure of between 15 psi to 40 psi, preferably 25 psi to 35 psi, in case the cell is intended to be used in a VRLA battery and to a pressure between 5 psi to 15 psi, preferably 7 psi to 12 psi, in case the cell is intended to be used in other batteries. The pressure is measured at the time of compression in order to ensure that uniform pressure is maintained. After compression, the cells are placed in separate compartments and placed in a compartment of the container. The plates of a cell are internally connected electrically and the cells are interconnected electrically as in the prior art.
It may be noted that the construction of a lead acid battery comprises other steps such as curing and drying of the pasted plates, sealing the container, acid filling

of the battery, etc. These processes are known to the prior art. The details of these steps are therefore not described herein.
While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

1/ We claim:
1. A cell for a lead acid battery comprising
one or more positive plates;
one or more negative plates; and
one or more separators;
such that a separator is placed between one positive plate and one negative plate and wherein said plates are compressed to a predetermined uniform pressure, the pressure being measured at the time of compression and said plates are locked in place using compression plates.
2. A cell of claim 1 wherein said cell is compressed to a pressure between 15 psi to 40 psi in case the cell is used in a VRLA battery and to a pressure between 5 psi to 15 psi in case the cell is used in other batteries.
3. A cell of claim 1 wherein said cell is compressed to a pressure between 25 psi to 35 psi in case the cell is used in a VRLA battery and to a pressure between 7 psi to 12 psi in case the cell is used in other batteries.
4. A lead acid battery comprising cells of any of the preceding claims.
5. A lead acid battery comprising:
a container comprising:
a plurality of compartments; a positive terminal; and 12

a negative terminal; and
a plurality of cells
wherein each cell comprises:
one or more positive plates;
one or more negative plates; and
one or more separators such that a separator is placed between said positive plate and said negative plate and wherein said plates, are compressed to a predetermined uniform pressure, the pressure being measured at the time of compression and said plates are locked in place using compression plates; and
one said cell is placed in one said compartment and said plurality of cells are electrically connected.
6. The lead acid battery of claim 5 wherein each said cell is compressed to a pressure between 15 psi to 40 psi in case the cell is used in a VRLA battery and to a pressure between 5 psi to 15 psi in case the cell is used in other batteries.
7. The lead acid battery of claim 5 wherein each said cell is compressed to a pressure between 25 psi to 35 psi in case the cell is used in a VRLA battery and to a pressure between 7 psi to 12 psi in case the cell is used in other batteries.