Battery Pack

This invention relates to an electrical power supply for an electrical apparatus particularly, but not exclusively, a hand-held portable radio telephone, hereinafter referred to as a portable telephone.

Portable telephones, as with other portable electrical apparatus operate from an internal, removable, power supply generally in the form of a rechargeable battery pack. Typically, the battery pack may comprise six nickel cadmium (NiCd) cells.

Work is continuously being carried out in developing new cells for battery packs, which provide smaller, more lightweight, more efficient and generally more versatile power supplies. Recently developed cells use a polymer electrolyte. One such cell is called a Lithium Solid State (LSS) cell.

LSS cells typically are of a layered structure comprising a lithium metal electrode, a polymer electrolyte, a carbon collector, and, a metallic electrode, for example, of aluminium or nickel foil. This layered structure is, typically, of the order of less than 1 mm thickness. To provide the required voltage output, a number of the layered structures--in the form of rectangular sheets--may be stacked and suitably interconnected. LSS cells are described in Electronics Times, Power Special Report, 16 Jul. 1992, pages 21-22. LSS cells have the advantage of being lightweight, small and rechargeable time after time, and, because they are in the form of very thin laminated structures can be designed into virtually any shape.

Despite these advantages, one drawback is that their voltage output is proportional to the charge remaining i.e. they exhibit a noticeable voltage drop during use, which makes them not entirely suitable for use in portable electrical apparatus. For example, a two cell battery will have an output of around 6.3 V at full charge and a 3.8 V output when discharged, but between these points, the output drops noticeably over time as shown in FIG. 1A. The discharge rate is generally linear, the exact rate being dependant upon the load coupled to the battery. This is unlike a conventional NiCd battery which exhibits an initial sharp voltage drop, with a substantially more constant voltage output thereafter until, at discharge, there is another sharp drop in output voltage.

To overcome the problem exhibited by LSS batteries, a Direct Current-to-Direct Current (DC/DC) conversion system could be provided in the electrical apparatus to provide a stable output voltage before it is supplied to the electronic circuitry of the apparatus. FIG. 1B shows how the voltage output from an LSS battery can be stabilized as a function of time by providing such a DC/DC conversion system. However, this has the problem of electromagnetic interference (EMI), i.e. the electromagnetic radiation emitted by the DC/DC converter that "disturbs" other circuitry in the apparatus e.g. the radio frequency (RF) section of a telephone's electronic circuitry.

In addition, providing a DC/DC converter in the apparatus is an additional cost when it may not be needed, for example, if the apparatus is also capable of using a conventional NiCd battery as its power source. It also means that LSS battery cannot be used in "older" apparatus which do not have a DC/DC converter.

According to the present invention, there is provided a battery pack for coupling to an electronic apparatus for supplying a current thereto, the battery pack comprising a housing enclosing a plurality of cells providing the current supply for the electronic apparatus therein, and means coupled to the cells for maintaining a substantially uniform voltage output therefrom as the charge state of the cells diminishes, the voltage maintaining means being present within said housing.

This has the advantage, that by locating the voltage stabilizing means in the battery pack, the battery will itself supply a constant voltage even when LSS cells are used. The battery may also have the advantages of LSS cells e.g. that it is lightweight, more compact, and configurable into virtually any shape required. Thus, a battery pack can readily be made to the same shape as an existing battery pack for a known apparatus to enable it to be retrofitted to pre-existing apparatus.

The voltage stabilizing means may be provided within and electrically conductive screen disposed on the battery housing.

This has the advantage that because the voltage stabilising means is located at a distance from other circuitry in the apparatus (i.e. it is in the battery pack), and because an electrically conductive screen is provided then the effect of EMI within the apparatus is reduced or eliminated.

FIGS. 1A and 1B are illustrative graphs of output voltage of a lithium solid state battery pack as a function of time without and with a DC/DC conversion unit respectively;

FIG. 2 is a schematic diagram illustrating the interior of battery pack according to the invention;

FIG. 3 is a schematic representation of the battery of FIG. 1, illustrating its connections to a telephone;

FIG. 4 is a schematic representation of a second embodiment of the invention; and

FIG. 5 is a schematic representation of a third embodiment of the invention; and

FIG. 6 is a schematic diagram for supply of a control voltage to the battery pack for controlling the output voltage thereof.

In the following description, an embodiment of the invention will be described byway of example only and as designed for use with a known portable radio telephone.

A battery pack 4 in accordance with the invention, has a number of LSS cells 1 and a DC/DC converter 2. As discussed above, a DC/DC converter 2 maintains a substantially uniform voltage output from the LSS cells 1 as the charge state of the cells diminishes. Preferably, the cells 1 and the DC/DC converter 2 are arranged in a housing 3--illustrated by the broken lines in FIG. 2--to form a battery pack 4 of the required shape. In the example shown in FIG. 2, the battery pack 4 has a substantially rectangular configuration. The battery pack 4 can be formed to resemble a conventional, known battery pack to allow it to be retrofited into pre-existing apparatus. The LSS bells 1 are arranged in a stack of, for example, 3 sets of LSS cells, each set of LSS cells comprising a stack of three to five of the rectangular sheets described in the opening paragraphs above, the sets being connected in series. Each LSS cell of each set will be connected in parallel. The DC/DC converter 2 is located adjacent the LSS cells at one end as illustrated in FIG. 2. Typically, a DC/DC converter would have the dimensions of 40 mm.times.10 mm.times.5 mm and the stack of LSS cells 1 the dimensions of 40 mm.times.50 mm.times.5 mm.

DC/DC converters are, in themselves, well known. DC/DC converters are, in effect, high frequency transformers which use, for example, transistors to step-up the voltage output from a voltage source where the output voltage decreases or varies over time to provide a stable output voltage, whatever the actual output voltage supplied by the voltage source. The DC/DC converter 2 can be connected to the LSS cells 1 in any manner well known to a person skilled in the art.

The cells 1, and the DC/DC converter 2 are provided within a shield 5 to reduce EMI. This shielding is provided in the form of an electrically conductive coating on the internal surface of the battery housing.

The shield 5 can be provided around the whole of the battery pack 4, with additional shielding 6 at the boundary between the DC/DC converter 2 and the cells 1 to provide separate shielding for both the DC/DC converter 2 and the cells 1. Alternatively partial shielding can be provided to shield the DC/DC converter 2 only.

For use with a portable radio telephone 9, the whole battery pack 4 is provided with two output terminals 7 for connection with respective inputs 8 in the telephone 9 for supplying current to the telephone's conventional electronic circuitry including a processor (not shown) provided for operating the telephone, and implementing the various functions.

In a second embodiment of the invention illustrated in FIG. 4, the battery pack 4 can be provided with two sets of output terminals 10, 11, sharing a common terminal 13. The first pair of outputs 10 supplies current directly from the LSS cells 1 and the second pair 11 supplies current via the DC/DC converter 2.

The voltage from the first output pair 10 will be higher, but less stable, than the voltage from the second output pair 11.

The second output pair 11 provides a lower, but more stable, output voltage than the voltage from the first output pair 10, which is preferably applied to those parts of the telephone's electronic circuitry requiring a lower current e.g. the baseband.

While providing a stable current output is preferable, the less stable, but higher, voltage output from the first output pair 10 may be adequate for those parts of the telephone 9 requiring a larger voltage e.g. the power amplifier (PA).

As is well known, the portable radio telephone 9 can, at times, operate at a lower voltage than at other times. For example, when a telephone is operating in a "stand-by" mode i.e. when a telephone is not being used for either making or receiving a call, a lower operating voltage is required than when the telephone is in "call" mode i.e. when receiving and making a call.

In a third embodiment, illustrated in FIGS. 5 and 6 the DC/DC converter 2 can be made voltage controllable so as to provide different output voltages from the output of the DC/DC converter 2 depending upon the requirements of the telephone e.g. depending upon whether the telephone is in "stand-by" or "call" mode.

The appropriate output voltage is selected by supplying a control voltage Vc to the DC/DC converter 2 from the telephone under control of the telephone's microprocessor.

A first control voltage V'c from the telephone's microprocessor is supplied to an FET 12, connected between power supply V and ground OV, to provide an output control voltage Vc out which is switchable between 3 V and 5 V, for example, under control of the first control voltage V'c as shown in FIG. 6.

The drain of the FET 12 is coupled via a resistor R1 to the common node of two series-coupled resistors R2, R3. The first control voltage V'c places the FET 12 in either a conducting or non-conducting state, such that the appropriate 3 V or 5 V output control voltage Vc is coupled to the DC/DC converter 2.

It will be evident, from the foregoing description, to a person skilled in the art, that various modifications are possible, within the scope of the present invention. For example, a linear regulator could be provided in addition to the DC/DC converter. In this case, means can be provided to control whether the current is supplied through the DC/DC converter or the linear regulator depending upon which is the most efficient. Typically linear regulators are used when there is a low current requirement and the DC/DC converter where there is a high-power requirement.

In addition, a battery pack could be provided which not only supplies two different voltages to different parts of the telephone's electronic circuitry, but has the means for switching the voltage input from the DC/DC converter between two selected values under control of a control voltage i.e. a battery pack having both the features of the second and third embodiments.

Although an embodiment of the invention has been described in relation to a battery designed for a radio telephone, a battery pack could be designed, in accordance with the present invention, for use with other electrical apparatus.