BIPOLAR BATTERY
TECHNICAL FIELD
This invention in general relates to lead acid batteries and in particular relates to bipolar lead acid batteries, and to bipolar plates for use in bipolar lead acid batteries.
BACKGROUND
A lead-acid battery is a secondary electrochemical device that stores energy and makes it available in an electrical form. Lead acid batteries are the most widely used secondary batteries, extensively employed in automotive applications. A lead acid battery may comprise of several cells connected in parallel or series.
The conventional construction of the cell comprises a positive electrode, a negative electrode, and an electrolyte which is dilute sulphuric acid. The positive electrode and the negative electrode are also referred to as the positive and negative plates respectively. A paste, generally comprising lead oxide, lead sulphate, water and certain other additives, taken in a fixed proportion, is applied to a grid to form a plate. Electrical connections are provided between grids acting as positive and negative plates.
In conventional battery designs, the use of single inter cell electrical connection between adjacent cells restricts cell-to-cell current flow. Single inter cell electrical connections contribute to higher internal resistance of a battery and have an adverse impact on high power discharge performance and rapid recharge capabilities of the battery. Further, single inter cell connections are prone to mechanical failure, affecting battery robustness and operational safety.
Another limitation of conventional in a single inter cell connection is non uniform current density over the plate geometry. Due to the non uniform current distribution, the portion of the grid closest to the connector corrodes more compared to the portions further away from the connector.
A bipolar plate battery overcomes some of the drawbacks of the conventional battery. A bipolar plate, also called a biplate, consists of a thin, electrically conducting

acid resistant substrate having negative active material applied to one side of its surface and positive material applied to the other side. The bipolar battery structure typically has a "stack" that employs a sequence of elements, including a negative monopolar terminal plate, a separator, a repeating sequence of bipolar plates and separators, and a positive monopolar terminal plate. Electrical termination is achieved via the monopolar terminal plates.
Since the electrical path between the positive and negative active materials of adjacent cells is extremely short i.e., due the small thickness of the substrate, the resistance is typically very small.
A bipolar plate normally comprises a lead substrate which is pasted with positive paste on one surface and negative paste on the opposite surface. One of the primary problems associated with such bipolar plate constructions is premature failure due to corrosion through the lead substrate, resulting in cell to cell short circuits. Another common, life limiting issue is the poor adherence of the paste material to the flat surfaces of the lead substrate, resulting in eventual loss of contact between the active material and grid. There thus exists a need for bipolar plates with corrosion resistant substrates and of robust construction.
For better retention of active material in bipolar plates US Pat. No. 4,900,643 suggests the use of folded metallic mesh which encloses the sides and top of a conductive plastic substrate and is partially embedded within the side surfaces of substrate. US patent 4,874,681 describes a quasi bipolar lead acid battery wherein a pair of bipolar electrode elements folded into a hairpin configuration over opposite edges of a partition sheet so as to cover opposite surfaces of adjacent halves of the partition sheet. The designs of the aforesaid patents however cause non-uniform current distribution that causes the portion of the grid closest to the connector to corrode more compared to the portions further away from the connector. Another life limiting issue is the poor adherence of the paste material to the flat surfaces of the lead substrate, resulting in eventual loss of contact between the active material and the grid.
US patent 5,441,824 describes biplates formed with a tube of woven lead yarn which is slipped over a non-conductive biplate. This

construction however results in eventual loss of contact between the active material and the grid.
There is thus a need for bipolar plates for use in bipolar lead acid batteries that have uniform current distribution and which maintain uniform contact between the active material and the grid.
The invention described hereinafter overcomes the shortcomings of the prior art.
SUMMARY OF THE INVENTION
This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify essential features of the claimed subject matter, nor is it intended for use in determining the scope of the claimed subject matter.
In an embodiment of the subject matter, a bipolar plate for a lead acid battery is described. The bipolar plate comprises a bipolar grid, a positive paste and a negative paste. The bipolar grid comprises a grid having a first predetermined dimension and a spacer having a second predetermined dimension, such that the surface area of the spacer is half of the surface area of the grid. The grid is folded over the spacer, such that the grid substantially covers both sides of the spacer and the corner points of the grid substantially meet at a single point. The four corner points of the grid can be electrically connected in a manner known to the prior art. A positive conductive surface is formed by pasting the positive paste on one side of the bipolar grid, and a negative conductive surface is formed by pasting the negative paste on other side of the bipolar grid.
Another embodiment of the subject matter describes a bipolar plate for a lead acid battery. The bipolar plate comprises a bipolar grid, a positive paste and a negative paste. The bipolar grid comprises a grid having a first predetermined dimension and a spacer having a second predetermined dimension, such that the surface area of the spacer is substantially half of the surface area of the grid,-the length of the spacer is substantially half of the length of the grid and the width of the spacer is substantially equal to the width of the grid. The grid is placed over the spacer, so as to substantially cover the spacer. The edges of the grid are thereafter electrically connected in a manner known to the prior art.

A positive conductive surface is formed by pasting positive paste on one side of the bipolar grid and a negative surface is formed by pasting negative paste on the other side of the bipolar grid.
Yet another embodiment of the subject matter describes a bipolar plate for a lead acid battery. The bipolar plate comprises a bipolar grid, a positive paste and a negative paste. The bipolar grid comprises two grids having a predetermined dimension and a spacer having substantially the same predetermined dimension. The two grids are placed on the two sides of the spacer. The edges of the two grids are thereafter electrically connected in a manner known to the prior art. A positive conductive surface is formed by pasting positive paste on one side of the bipolar grid and a negative surface is formed by pasting negative paste on the other side of the bipolar grid.
In yet another embodiment of the subject matter a bipolar plate stack assembly is described. The bipolar plate stack assembly comprises a positive monopolar terminal plate, a negative monopolar terminal plate and a plurality of bipolar plates of previous embodiments. The bipolar plates are stacked with a separator placed between adjacent bipolar plates. The positive monopolar terminal plate and the negative monopolar terminal plate are placed at the two ends of the plurality of bipolar plates with a separator between the monopolar terminal plate and the next bipolar plate.
In yet another embodiment a lead acid battery is described. The lead acid battery comprises a battery container having a positive terminal and a negative terminal, and one or more bipolar plate stack assembly of the preceding embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1A provides an illustration of a rectangular grid 100. Fig. IB and 1C provide an illustration of a method of folding the grid of Fig.lA over a spacer in accordance with one embodiment of the invention. Fig. ID illustrates a bipolar grid formed by the method of folding of Fig. IB and 1C.
Fig. 2A provides an illustration of a grid 100. Fig. 2B and 2C provide an illustration of a method of folding the grid of Fig.2A over a spacer in accordance with

one embodiment of the invention. Fig. 2D illustrates a bipolar grid formed by the method of folding of Fig. 2B and 2C.
Fig. 3A provides an illustration of two grids 100. Fig. 3B illustrates a spacer 200 and Fig. 3C provides an illustration of a method of placing the grids of Fig.3A above and below a spacer in accordance with one embodiment of the invention. Fig. 3D illustrates a bipolar grid formed by the method of Fig. 3C.
Fig. 4 illustrates a bipolar plate of the previous embodiments.
Fig.5 provide an illustration of bipolar plate stack assembly comprising the bipolar plates of any of the previous embodiments.
Fig. 6A provides an illustration of a lead acid battery comprising one or more bipolar plate stack assembly of Fig.5. Fig. 6B provides an illustration a top view of a lead acid battery comprising one or more bipolar plate stack assembly of Fig.5.
DETAILED DESCRIPTION
Aspects of the bipolar plate, bipolar grid, bipolar plate stack assembly and 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 by way of the following examples. These examples, as would be obvious to the person skilled in the art, are not limiting the scope of the described subject matter.
The subject matter describes, in one aspect bipolar plates for a lead acid battery. The subject matter in another aspect describes a bipolar grid for a bipolar plate. In another aspect the subject matter describes a bipolar plate stack assembly comprising bipolar plates. In yet another aspect, the subject matter describes, a lead acid battery comprising bipolar plate stack assembly.
Example 1

Fig.lA describes a rectangular grid 100 having sides of length a and width b. The grid 100 can be of wire mesh made of metallic material or alloy. The grid 100 is typically made of metallic lead or a lead-based alloy. The grid 100 can be made by conventional techniques such as direct casting, stamping, forging or by mechanical working. The bipolar grid 205 further comprises a substantially rectangular spacer 200 having sides which are substantially of length (1/V2)a and width (1/V2)b. The spacer can be made of any non-conductive, impervious and acid resistant material known to the prior art. Typically spacer is made of plastic. The bipolar grid is constructed by folding the grid 100 over said spacer 200 such that the said grid 100 covers substantially both sides of said spacer 200 and the corners of the grid substantially meet at a point as shown in Fig. IB and Fig. 1C. The four corners of the grid are electrically connected by using any technique known to the prior art such as welding or soldering. The bipolar grid 205 of this example is shown in Fig. ID.
The bipolar plate 600 also comprises a positive paste and a negative paste. The positive paste is pasted on one side of the bipolar grid 205 to form a positive conductive surface 605 and the negative paste is pasted on the other side of the bipolar grid 205 to form a negative conductive surface 610. The bipolar plate of this example is shown in Fig. 4.
Example 2
The bipolar plate of the previous example wherein length a is substantially equal to width b.
Example 3
Fig.2D describes a bipolar grid comprising a rectangular grid 100 as shown in Fig. 2A having sides of length a and width b. The grid 100 can be of the type described in Example 1 above. The bipolar grid further comprises a substantially rectangular spacer 200 having sides which are substantially of length a/2 and width b. The spacer can of the type described in Example 1 above.
The bipolar grid is constructed by folding the grid 100 over said spacer 200 such that the said grid 100 substantially covers both sides of the spacer 200 such that the two edges of the grid substantially meet over the spacer 200 as shown in Fig. 2B and Fig. 2C. The

edges of the bipolar grid are electrically connected by using any technique described in Example 1 above. The bipolar grid 205 of this example is shown in Fig. 2D. The bipolar plate also comprises a positive paste and a negative paste. The positive paste is pasted on one side of the bipolar grid 205 to form a positive conductive surface 605 and the negative paste is pasted on the other side of the bipolar grid 205 to form a negative conductive surface 610. The bipolar plate of this example is shown in Fig. 4.
Example 4
The bipolar plate of Example 3 wherein length a is substantially equal to width b.
Example 5
Fig.3D describes a bipolar grid comprising two grids 100 of substantially the same dimensions. Specifically the grids can be rectangular, square, circular or of any other shape. The embodiment of this example is described with reference to a circular grid 100 (more particularly described in Fig. 3A) but the grid of this example can be of any shape as would be obvious to a person skilled in the art. The grid can be of the type described in Example 1 above. The bipolar grid further comprises a spacer 200 having substantially the same predetermined dimensions as the grids. The spacer can of the type described in Example 1 above. The bipolar grid is constructed by placing one grid on one side of the spacer and the other grid on the other side of the spacer. The edges of the two grids are electrically connected by using any technique described in Example 1 above. The bipolar grid 205 of this example is shown in Fig. 3D.
The bipolar plate also comprises a positive paste and a negative paste. The positive paste is pasted on one of the bipolar grid 205 to form a positive conductive surface 605 and the negative paste is pasted on the other side of the bipolar grid 205 to form a negative conductive surface 610. The bipolar plate of this example is shown in Fig. 4.
Example 6
Fig. 4 provides an illustration of bipolar plate 600 comprising bipolar plates of any of the previous examples.

Example 7 Fig.5 provides an illustration of bipolar plates stack assembly 700 comprising bipolar plates of any of the previous examples.
The bipolar plate stack assembly 700 comprises a positive monopolar terminal plate 705, a negative monopolar terminal plate710, a plurality of bipolar plates 600 of the previous example and a plurality of separators715. The bipolar plates 600 are stacked with a separator715 placed between adjacent bipolar plates 600. The positive monopolar terminal plate 705 and the negative monopolar terminal plate710 are placed at the two ends of said plurality of bipolar plates 600 with a separator715 between the monopolar terminal plate and the next bipolar plate 600. Optionally, a pair of compression elements 720 and 725 may be present to compress the bipolar plate stack assembly 700.
Examples
Fig.6A provides an illustration of a lead acid battery 800 comprising one or more bipolar plate stack assembly ofFig.5.
The figure illustrates a lead acid battery 800 comprising a container 805, a positive terminal 810, a negative terminal 815 and one or more bipolar plate stack assembly 700 of the previous examples. The bipolar plate stack assemblies 700 are arranged such that the positive monopolar terminal plates 705 of the bipolar plate stack assemblies 700 present in the lead acid battery 800 are electrically connected to the positive terminal 810. Similarly the negative monopolar terminal plates 710 are electrically connected to the negative terminal 815.
Fig.6B provides an illustration a top view of a lead acid battery 800 comprising one or more bipolar plate stack assembly 700.
The figure illustrates a lead acid battery 800 with two bipolar plate stack assemblies 700. The two bipolar plate stack assemblies 700 are separated by container partitions 810 which are made of the same material as the container 805 or any other acid resistant, acid impervious and non conductive material.

While the invention has been particularly shown and described with reference to the examples above, 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.

We claim:
1. A bipolar plate comprising:
a bipolar grid 205 of Fig.lD comprising
a rectangular grid 100 having sides of length a and width b;
a substantially rectangular spacer 200 having sides which are
substantially of length (1/V2)a and width (1/V2)b;
said grid 100 is folded over said spacer 200 such that the said grid 100
covers substantially both sides of said spacer 200 and the corners of the
grid substantially meet at a point
the four corners of the grid are electrically connected a positive paste;
a negative paste;
a positive conducting surface 605; and
a negative conducting surface 610,
wherein
the positive paste is pasted on one side of the bipolar grid to form a
positive conductive surface 605 and
the negative paste is pasted on the other side of the bipolar grid to form a
negative conductive surface 610.
2. The bipolar plate of claim 1 wherein length a is substantially equal to width b.
3. A bipolar plate comprising:
a bipolar grid 205 of Fig.2D comprising
a rectangular grid 100 having sides of length a and width b;
a substantially rectangular spacer 200 having sides which are
substantially of length a/2 and width b; wherein
the said grid 100 substantially covers both sides of the spacer 200 such that the two edges of the grid substantially meet over the spacer 200; the edges of the grid 100 are electrically connected;

a positive paste; a negative paste; wherein
the positive paste is pasted on one side of the bipolar grid to form a positive conductive surface 605 and
the negative paste is pasted on the other side of the bipolar grid to form a negative conductive surface 610.
4. The bipolar plate of claim 3 wherein length a is substantially equal to width b.
5. A bipolar plate comprising:
a bipolar grid 205 of Fig.3D comprising
Two grids 100 having substantially a predetermined dimension a spacer 200 having substantially the same predetermined dimension as that of the grids; wherein
the first grid 100 is placed on one side of the spacer 200;
the second grid 100 is placed on the other side of the spacer 200;
the edges of the two grids 100 are electrically connected;
a positive paste;
a negative paste; wherein
the positive paste is pasted on one side of the bipolar grid to form a positive conductive surface 605 and
the negative paste is pasted on the other side of the bipolar grid to form a negative conductive surface 610.
6. A bipolar plate stack assembly 700 comprising:
a positive monopolar terminal plate 705; a negative monopolar terminal plate 710;

a bipolar plate 600 of any of the preceding claims;
wherein a plurality of bipolar plates 600 is stacked with a separator 715 placed between adjacent bipolar plates, and said positive monopolar terminal plate 705 and said negative monopolar terminal plate 710 are placed at the two ends of said plurality of bipolar plates 600 with a separator 715 between monopolar terminal plate and adjacent bipolar plate 600.
7. The bipolar plate 600 of claim 1 to 5 wherein said grid is a wire mesh.
8. The bipolar plate 600 of claim 1 to 5 wherein said grid is made of a metallic material alloy.
9. A lead acid battery 800 comprising bipolar plates of claim 1 to 5, 7 or 8.
10. A lead acid battery 800 comprising:
a battery container 805 comprising: a positive terminal 810; and a negative terminal 815; and one or more bipolar plate stack assembly 700 of claim 6 such that positive monopolar terminal plate 705 of each bipolar plate stack assembly 700 is electrically connected to said positive terminal 810 and negative monopolar terminal plate 710 of each bipolar plate stack assembly 700 is electrically connected to said negative terminal 815.