Although the electrical power demands of consumer electronic and electrical devices vary greatly, they are generally satisfied by a relative few standard battery and cell configurations. In the past, meeting the needs of a particular device has generally consisted of selecting a particular cell configuration (usually based on space constraints), determining the number of cells needed, and connecting them in an appropriate circuit to provide the desired voltage and capacity. This approach works best in situations where the required performance can be met with only a few numbers of cells. However, in applications such as laptop computers where capacity requirements are many times greater than the capacity of traditional cell configurations, a large number of cells are needed. For these devices, cylindrical cell configurations such as what are commonly known in the industry as"AA","4/3 A"and"18650"cells are typically used.

These may be assembled individually into a device or may be first integrated into a packaged power supply. Packaged power supplies or battery packs generally use housings or enclosures of light weight plastic as it is not a principal function to protect the cells. Particularly with devices such as computers where the total lifetime energy consumption is many times the capacity of their battery pack, secondary cells have become the dominant choice over primary cells. At the same time, the design evolution of electronic devices to be ever smaller and lighter has demanded higher energy density and more efficient use of space-power supplies are being driven

smaller and lighter. As a consequence, designs of power supplies have attempted to fit standard configuration cells into smaller and smaller packages. However, some of the characteristics of cylindrical secondary cells hinder this objective. Large numbers of cylindrically shaped cells cannot be efficiently grouped or stacked as inter-cell space is wasted. In addition, the dimensions of standard cylindrical cells are too large to fit in the smaller electronic devices.

One reason these secondary cells are cylindrical in shape is because they include an outer can or container that must function as a pressure vessel. The cell chemistries used in the most common secondary cells (such as nickel-cadmium and nickel-metal hydride systems) produce gases during operation that must be contained. The resulting pressures cannot be practically and efficiently contained in enclosures having large planar sides. Cylindrical pressure vessels are more efficient for this purpose, but they increase the overall weight of the power supply and decr. ease spatial efficiency.

Cells contained within prismatic containers are more spatially efficient than cylindrical cells as less inter-cell space is wasted. However, known prismatic containers containing cells subject to gassing and resultant internal pressure are required to be manufactured from materials having sufficient strength to withstand such pressures, thereby compromising the efficient use of volumes available for accommodating power supplies.

To provide low weight power supplies having greater energy density, a new family of secondary electrochemical cells has been developed. They are based on various chemistries that do not produce significant gases during operational life and therefore do not require pressure containment. One benefit of this characteristic is direct-reduced weight and volume from elimination of the containment structure and consequently increased energy density. A second benefit is the ability to produce planar geometry cells. Without the necessity of fitting the cell electrodes into a cylindrical container, the electrodes may be made flat or planar resulting in a low

profile high capacity cell that can fit into many small electronic devices. of particular interest are power supplies for portable laptop computers, cellular phones and other portable communication and computing devices, which are typically thin in one dimension. Examples of this family of cells are what are known as lithium-ion, lithium-polymer and lithium-sulphur cells.

A drawback of cells without rigid containment structures is their inherent greater susceptibility to damage. The family of cells discussed above typically includes a metal foil or polymer envelope sealed about the electrode assembly. The purpose of this envelope is to prevent escape of the electrolyte and isolate the cell from contact with external air and environment. The envelope is generally very thin and vacuum sealed or otherwise tightly drawn or heat sealed about the cell to reduce volume. This envelope provides little protection to the cell from physical contact with external objects that might damage the cell structure. This type of envelope is discussed in U. S. Patent No 5,487,958 to Tura.

Handling of these cells, either during assembly into a powered device or during the cell's operational lifetime (swapping power supplies or batteries for recharging) can lead to damage to the electrodes and other functional elements from impact with the external environment.

To monitor and control charge and discharge operations, these cells typically include connected active or passive circuit elements. These may take the form of discrete devices or elements on a circuit board or similar structure. These charge and discharge control devices are also subject to damage from contact with external objects and require protection.

Additional risk is created by the high-energy nature of lithium-based electrodes used in many of these cells. The lithium-based materials used can release large amounts of energy if ignited. Fire may originate from an external source or from internally generated heat from an electrical short or other incident due to a damaged cell. As a consequence, both

physical protection of lithium-based electrodes and containment in case of fire is desired.

What is needed is a packaged power supply incorporating high capacity electrochemical cells in an enclosure capable of providing protection from external forces. Such a power supply should also be adaptable to thin planar electrochemical cells.

Summary of the Invention The present invention provides a high energy density power supply including an energy storage device such as an electrochemical cell in a durable low volume package. By providing a storage device with an enclosing structure that is an effective shield from external forces and also has low volume, overall energy density is maximized.

A pair of thin metal covers are spaced apart and retained at their edges by a surrounding frame. A highly protected cavity is formed between the covers. The means of interconnecting the frame and covers minimizes the frame volume. In one embodiment, the frame includes slots, which receive lips formed on the perimeters of the covers. The use of metal as a cover material allows a reduced thickness resulting in a strong but low volume structure. Insulating sheets or other insulators are provided to separate the storage device from the conductive covers. Alternatively, the covers act as contacts and are electrically connected to the storage device. The energy storage device incorporated in the present power supply is preferably a thin planar electrochemical cell of a type otherwise unprotected by a rigid containment structure. Such cells are considered exposed and potentially damaged from contact with external objects. In order to enable their use in rough handling environments, a durable external structure is placed around them. Most preferably, a cell is selected from lithium based secondary cells. Alternative configurations using devices such as capacitors for energy storage are also provided.

A particular advantage of the power supply for devices such as laptop computers is the small thickness dimension obtainable with framed metal covers. Using thin planar electrochemical cells of large capacity, such as is possible with lithium-based cells, a thin but high capacity power supply is constructed. The invention includes electrically powered devices incorporating the durable power supplies as described herein. From the following illustrations and examples, additional advantages and embodiments of the present invention will become clear to those skilled in this craft.

Description of the Drawings Figure 1 is an exploded perspective view of a power supply according to one embodiment of the present invention.

Figure 2 is a cross section view of the embodiment of Figure 1.

Figure 3 is a cross section view of a battery pack in which one cover is not insulated from the enclosed cell.

Description of the Preferred Embodiments Figures 1 and 2 depict a power supply 10 according to one embodiment of the invention that is formed by positioning a first sheet metal cover 12 on a top (in this particular orientation) first side 16 of a frame 20. A second cover 14 is similarly positioned on the bottom second side 22 of the frame 20. The frame is formed of elongated legs 24 of minimized cross-sectional area. The inside surfaces of the legs 24 define an inner frame perimeter 26 that, together with the inner faces 28 of the covers, define a cavity 30. A planar cell 32 and a control circuit board 34 reside within the cavity 30. The first cover 12 has a cutaway 36 that exposes contacts 38 on the circuit board 34. Insulator sheets 40 are positioned between the covers 12,14 and the cell 32 as a means of electrically insulating the covers 12,14 from the cell 32.

Many materials and means for insulating such devices are known and the selection of a proper insulator will be clear to those skilled in making these devices. A securing means such as adhesive or double-sided tape (not shown) may be used to attach the cell 32 and circuit board 34 to one of the covers to reduce relative motion. The figures are not drawn to scale and the space between the cell 32 and frame 20 is shown enlarged for clarity. In the actual device, the frame 20 and covers 12,14 are sized to tightly fit the dimensions of the cell 32.

To optimize energy density, the frame and cover dimensions, materials, and interconnections must be selected to maximize the available internal space while minimizing the total power supply volume. The volume of the frame and cover structures themselves must be minimized. The frame 20 is preferably formed of a lightweight electrically insulating plastic that is injection moldable. The frame's principal function is as a spacer and anchor to secure and retain the covers. The frame may be formed by injection molding or other similar process. The height or thickness of the frame is determined by, and defines, the spacing required to contain the cell 32 and any insulating elements. The width of each leg 24 of the frame 20 should be as small as possible to maximize the available space within. The frame width must also be sufficiently large to provide for interconnection with the covers 12,14. Figure 2, a cross-section view of a frame leg 24 depicts a slot 46 provided to receive a turned lip 48 formed on the perimeter of each cover 12,14. The slot 46 is preferably slightly wider than the thickness of the lip 48 to receive a quantity of bonding agent for securing the cover by adhesion to the frame 20. These structures are but one of many alternative means of joining the covers and frame. Other alternatives include threaded fasteners passing through holes in the covers and secured in receptors in the frame; plastic rivets passing through holes in the covers and enlarged on the cover exterior faces; tab-and-slot type mechanism ; as well as others which are known to those skilled in the art. The

advantage of the present joining means is a particuiarly narrow frame leg width. In alternative configurations, additional intermediate legs are present partitioning the cavity into multiple regions. These multiple regions may be used to contain circuitry or other elements separated from the energy storage device, or for other purposes.

The covers have two principal functions. The primary function is to provide adequate stiffness and rigidity to the power supply to prevent damage to the cell and maintain operation of the cell. To do this, the covers must have sufficient strength and stiffness to prevent general bending of the cell and localized deformation of the covers themselves.

Typically in the prior art, enclosures for encasing electrochemical cells or a group of cells have been made of thin walled plastic. Plastic enclosures have a benefit in being relatively light weight. However, plastic has been found to provide insufficient strength unless of a greater thickness that significantly adds to the overall thickness dimension of the power supply. The second function of the covers is to act as a fire barrier to both prevent an external fire from reaching the cells and to limit the release of energy from the cell in the case of a fire initiated within the cell.

Industry standards have been established for testing materials for this purpose. Most plastic covers must be at least about 1.5 mm thick (0.059 inch) to be capable of performing this function. In very thin power supplies where the overall thickness may be only 2.5 mm (0.098 inch) the relatively large thickness of a plastic cover greatly reduces energy density of the power supply. Both cover functions are satisfied in the present invention with a thin sheet metal cover. Although a metal cover adds weight compared with a plastic cover, its benefit in strength and fire resistance outweighs this disadvantage. By using much thinner sheet metal covers, separated by a frame, a power supply is formed which provides greatly improved protection from bending and impact forces and also meets fire safety requirements. In the embodiment of the

figures, the covers 12,14 are formed of stainless steel of 0.3 mm (0.012 inch) thickness. A minimum practical thickness in stainless is about 0.15 to 0.20 mm (0. 0059 to 0.0079 inch).

The practical maximum, above which advantages over other materials diminish, is about 0.5 mm (0.020 inch). Other metals having comparable characteristics are also available.

An additional advantage of metal covers is the increased heat dissipation possible due to the relatively high thermal conductivity and capacity of most metals. Sheet metal covers in contact or close proximity to a hot cell provide significant conduction and thermal capacity to dissipate heat buildup that may be due to operation or incidental events. Sheet metal covers also assist in dissipating heat generated outside of the power supply to the periphery of the power supply and away from the cells. Further, the closely spaced metal covers can be manufactured to provide electromagnetic field shielding which is a factor with many electronic devices.

In Figure 1 a control circuit board 34 is shown connected to the terminals 50 at one end of the cell 32. The structure and necessary functions of the circuit board 34 are dependent upon the requirements of the particular storage device and will be known to the designer. The contacts 38 are used to connect with an external device to be powered. The power supply 10 can be snapped into an external device receptor having mating contacts to electrically connect the power supply 10. There are other alternative means of electrical connection. In one alternative (not shown) extended wire leads are attached to the circuit board 34 and exit the power supply through a small aperture in the frame 20 or cover 12. The leads are then connected to leads or contacts to an external device. In such a leaded configuration, the cutaway 36 of the cover is unnecessary. In another alternative, wire leads connect the circuit board to rigid contacts on the exterior of the frame or covers.

In a similar alternative, the covers 12,14 themselves function as rigid electrical contacts providing connection to

the circuit board or cell. In one such embodiment shown in Figure 3, the upper cover 12 is not insulated from the cell.

The cover is connected with a conductive envelope 53 of the cell 32 by direct contact or through an intermediate conductor.

Alternatively, where no insulation is required between the cell envelope and either cover, no insulators are present.

In Figure 2, the cell 32 is shown to include an electrode assembly 55 encased in a flexible envelope 52. The use of cells having rigid containment structures such as those structures required to accommodate electrochemical cells that gas is not contemplated in the present invention. The cell 32 combined in the present invention is one that is otherwise unprotected; the active elements such as electrodes are subject to the external forces exerted from the surrounding environment. Due to the thin flexible nature of the envelope 52, it provides insignificant protection to the electrode assembly from external forces.

Many high energy density secondary electrochemical cell designs use lithium-based anodes. A"lithium-based"cell here means that the anode (negative electrode on discharge) of the cell comprises in principal part one or more of the following: lithium foil and mixtures, alloys, composites, intercalated carbons, and intercalated conductive polymers of lithium. There are many lithium-based cell designs that provide both high energy density and for which thin planar cell constructions are possible. A planar electrochemical cell of the present invention includes any cell that can be configured so as to assume a planar geometry. Such cells may include one of a group of secondary cells sharing the following characteristics: lithium based and has only planar electrodes; relatively thin, and employ energy storage chemistries that do not require significant containment of gases during their operational life. These secondary cells typically have electrodes wrapped in a flexible envelope and are particularly adaptable to the advantages in the present invention. Other secondary cell designs use sheet electrodes, where they are

formed into non-flat configurations, such as wound electrode assemblies. The assemblies are then compressed so the cell assumes a planar geometry. Such cells are equally contemplated in the within invention. Planar electrodes, as used here, are electrodes in which the active material (and any nonactive accompanying structure such as supporting films or foils) of each electrode lies in a geometric plane. This includes folded electrodes in which each fold lies substantially in a single geometric plane. Such folded electrodes form a planar electrochemical cell as used herein. A planar electrochemical cell further includes a cell having a multiple of stacked individually planar electrodes.

The benefits of the present design are greatest when used with relatively thin planar cells, and the preferred power supply has a minimum aspect ratio of at least one to eight (1: 8). A power supply"aspect ratio"is, for the purposes here, defined as: the ratio of the overall thickness of the power supply to the lesser of the overall width and length dimensions (thickness being the minimum of the dimensions between all pairs of opposing sides). A clarifying example is: a power supply having outside dimensions of 3 x 30 x 40 mm (0.12 x 1.2 x 1.57 inches) has an aspect ratio of one to ten (1: 10). At high aspect ratios, the energy density retaining advantages of the thin metal covers are greatest compared to alternative structures due to the relative volumes of the covers and frame. However, the protective benefits gained from the combination of power supply structures exist also in small aspect ratio devices.

In an example high energy density power supply incorporating a planar lithium-polymer electrochemical cell, the cell thickness is 2.6 mm thick (0.10 inch). Allowing room for insulating sheets on either side or the cell and additional thickness dimension for tolerances and variation, the necessary cavity dimension is about 3. 2 mm (0. 13 inch). The difference between the cavity dimension and the total component dimensions within the cavity (the gap 57 in Figure 2) should be as small

as possible-preferably less than 0.5 mm (0.020 inch). Using covers of 0.3 mm (0. 012 inch) thick stainless steel, the overall power supply thickness is 3.8 mm (0.15 inch)-only 146 percent of the cell thickness. This compares to about 240 percent for the same configuration if using plastic covers of 1.5 mm (0.059 inch) thickness as might be required for a fire barrier. The result is that the overall power supply energy density with metal covers is higher. Regardless of the energy capacity and volume of the particular energy storage device, the overall energy density of a power supply is maximized when the nonactive structural elements have reduced volume as in the present invention.

The preceding discussion is provided for example only.

Other variations of the claimed inventive concepts will be obvious to those skilled in the art. For example, although the power supply cell is preferably a secondary electrochemical cell of the type described above, in alternative embodiments the cell is a planar high-energy capacitor or other energy storage device requiring rigid shielding from external physical environments. Adaptation or incorporation of known alternative devices and materials, present and future is also contemplated. The intended scope of the invention is defined by the following claims.