Abstract:
A rechargeable power supply including voltage protection means through voltage dependent disconnecting means. A rechargeable power supply comprises at least one rechargeable cell connectable to a power source, a Zener diode connected to the at least one cell, and a bipolar transistor or a FET for selectively disconnecting the Zener diode from the cell in order to reduce any discharge of the cell through the voltage protection means. The Zener diode and the transistor are connected together in series and jointly connected across the cell in parallel.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a National Phase Patent Application of International Application Number PCT/GB02/02090, filed on May 7, 2002, which claims priority of British Patent Application Number 0111107.9, filed on May 5, 2001. 
     FIELD OF THE INVENTION 
     The present invention relates to a power supply and particularly, but not exclusively, to a rechargeable power supply for powering electronic equipment. 
     BACKGROUND OF THE INVENTION 
     Rechargeable power supplies generally consist of a number of rechargeable batteries or cells arranged in series, in parallel or in a series/parallel combination. It is common to provide Zener diodes connected in parallel across each cell to prevent the recharging voltage supplied to the cell from exceeding a predetermined level, which could result in damage to the cell, and to allow the power supply to continue to supply power to any connected equipment in the event of one or more of the cells failing in an open circuit condition. 
     In such an arrangement, the Zener diodes act as voltage regulators whereby if the voltage applied to the cell exceeds a certain level, break down of the Zener diode will occur causing current to leak through the Zener diode in the reverse direction. As the applied voltage increases the current leaking through the Zener diode also increases. This has the effect of clamping the voltage over the cell to a predetermined level. 
     To ensure that the cell is not subjected to a voltage higher than its maximum rating, the Zener diode used must be capable of leaking a reverse current which is at least equal to the charging current of the power supply at a voltage which is less than or equal to the cell&#39;s maximum voltage rating. However, Zener diodes generally do not have a stepped on-off break down characteristic and thus permit leakage current through the diode, to greater or lesser extent, over a range of voltages. Thus, a Zener diode having a 2.5 mA leakage current at a breakover voltage of 3.3 V may still permit 50% of that current to pass through at an applied reverse-biased voltage of 3.2 volts. Consequently, once the power supply is fully charged and the charging current removed, the leakage current through the Zener diodes will continue until the voltage over the cell has been reduced to a point where the Zener diode has zero leakage current. During this time, the leakage current through the Zener diode will cause discharging of the cell. 
     Since rechargeable cells are effective for only a limited number of charge/discharge cycles, this number usually being a function of the percentage depth of each cycle, the discharge effect of the Zener diode on the cell means that the potential life of the cell, and therefore power supply as a whole, is significantly reduced, both in terms of each charge/discharge cycle and the overall life of the power supply. This is a particular problem in low power applications (for example smoke detectors/alarms), where the leakage current through the Zener diodes is often a high multiple of the supply current required to power the equipment. This has the effect of reducing the life of the rechargeable power supply to a fraction of its potential life. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide an improved power supply. 
     Accordingly, the present invention provides a rechargeable power supply comprising:
     a rechargeable cell connectable to a power source;   voltage protection means connected to said cell; and   disconnecting means for selectively disconnecting said voltage protection means from said cell thereby to reduce any discharge of said cell through said voltage protection means.   

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is an electrical circuit diagram of a first form of power supply according to the invention; 
         FIG. 2  is an electrical circuit diagram of a second form of power supply according to the invention; 
         FIG. 3  is an electrical circuit diagram of a third form of power supply according to the invention; and 
         FIG. 4  is an electrical circuit diagram of a fourth form of power supply according to the invention 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a preferred form of power supply according to the invention is shown generally at  10 . The power supply  10  comprises a single battery or cell  12 , the positive terminal of the cell  12  being connected to a power source or charging current source in the form of a power rail  14  via a diode  15  and a parallel combination of a resistor  16  and a Schottky diode  18 . The negative terminal of the cell  12  is connected to the earth or zero volt rail  17  of the charging current source. 
     The cell  12  has a series combination of voltage protection means, in the form of a Zener diode  20 , and disconnecting means, in the form of a field effect transistor (FET)  22  connected in parallel across the cell  12 . The Zener diode  20  is connected across the cell  12  in a reverse biased direction, i.e. with its cathode connected to the positive terminal of the cell  12 . The anode of the Zener diode  20  is connected to the drain electrode of the FET  22  whilst the source electrode of the FET  22  is connected to the negative terminal of the cell  12 . 
     It will be appreciated that any electrically equivalent connection of the Zener diode and FET, for example with the anode of the Zener diode being connected to the negative terminal of the cell  12 , the cathode of the Zener diode being connected to the source of the FET and the drain of the FET being connected to the positive terminal of the cell, will function equally well. 
     The gate electrode of the FET  22  is connected directly to the power rail  14  of the charging current source. 
     In operation, when a voltage is applied to the power rail  14  of the charging current source, thereby to charge the cell, current flows through the diode  15  and resistor  16  and through the cell  12  to earth. The purpose of the resistor  16  is to limit the current through the cell to a value appropriate for charging the cell. At the same time, the voltage on the power rail  14  is applied to the gate electrode of the FET  22  which is thereby switched on. This connects the Zener diode  20  across the cell  12 . 
     As the cell  12  is charged, the voltage across the cell rises. If the voltage across the cell  12 , and thus across the Zener diode  20 , rises above the breakover voltage of the Zener diode  20 , the Zener diode  20  will break down and begin to conduct leakage current therethrough The flow of leakage current through the Zener diode  20  prevents the voltage across the cell  12  from rising further and thus the voltage is effectively clamped at or around the breakover voltage of the Zener diode. It is normal, therefore, for the breakover voltage of the Zener diode to be chosen to correspond substantially to the maximum voltage rating of the cell. It will be appreciated, therefore, that the presence of the Zener diode prevents overcharging of the cell  12 . 
     When the cell is sufficiently charged and the voltage on the power rail  14  from the charging current source is switched off, current flows from the charged cell  12  through the Schottky diode  18  to the power rail  14  and out to any connected electronic equipment or circuit. Current is prevented from returning to the charging current source by the diode  15 . The Schottky diode  18  is included to provide a low impedance path between the cell  12  and the connected equipment or circuit However, it is not essential to the invention and its inclusion is entirely optional. 
     As described above, however, with the Zener diode  20  connected across the cell  12 , leakage current will continue to flow through the Zener diode  20  thus gradually discharging the cell  12 , possibly at a rate greater than that caused by the electronic equipment itself. To prevent this, therefore, when the voltage on the power rail  14  from the charge current source is switched off, the voltage applied to the gate electrode of the FET  22  is reduced to zero such that the FET  22  is switched off and presents an open circuit. The Zener diode  20  is therefore effectively disconnected from the cell  12  and thus no leakage current can flow therethrough. All current from the cell  12  is thus applied to the electronic equipment or circuit and any unwanted discharging to the Zener diode  20  is effectively eliminated. 
       FIG. 2  illustrates a similar arrangement but where the power supply comprises three, series-connected cells  12 , hereafter termed collectively as the “battery”. In this embodiment, each cell  12  in the battery has a Zener diode  20  connected in parallel with it and a FET  22  connected in series with the Zener diode. It is clear from the drawings that the arrangement within the dashed box  10  in  FIG. 1  is repeated for each cell in the battery of  FIG. 2 . 
     In this embodiment, when the voltage across any one of the cells  12  in the battery exceeds the breakover voltage of the cell&#39;s respective Zener diode  20 , leakage current will be passed through the Zener diode and prevent the voltage on the cell from rising further. 
     In  FIG. 3 , the power supply is shown having a different arrangement of cells  12 . In this case, the battery is comprised of 4 cells  12  in a two by two, series-parallel arrangement. Each pair of parallel connected cells  12  is provided with a respective Zener diode  20  and FET  22  connected in parallel over the cell pair. 
     The mode of operation of the embodiment of  FIG. 3  is similar to that of  FIGS. 1 and 2  in that if the voltage over any one of the cells  12  rises above the breakover voltage of the respective Zener diode  20 , then leakage current through the Zener diode will prevent that voltage from rising any further. 
     In  FIG. 4 , a stepped arrangement of voltage protection is provided. In this embodiment, the battery of the power supply comprises three cells  12   a ,  12   b ,  12   c  connected in series. The series combination of cells  12   a  and  12   b  has a first Zener diode  20 (i) and FET  22 (i) connected in parallel thereover. The series combination of cells  12   b  and  12   c  has a second Zener diode  20 (ii) and FET  22 (ii) connected in parallel thereover while the series combination of all three cells  12   a ,  12   b ,  12   c  have a third Zener diode  20 (iii) and FET  22 (iii) connected in parallel thereover. 
     As will be clearly understood by those skilled in the art, if the voltage over the series combination of cells  12   a  and  12   b  exceeds the breakover voltage of Zener diode  20 (i), then leakage current through Zener diode  20 (i) will prevent the voltage over the cells from rising any further. Similarly, if the voltage over cells  12   b  and  12   c  exceeds the breakover voltage of Zener diode  20 (ii) then leakage current through Zener diode  20 (ii) will prevent the voltage over cells  12   b  and  12   c  from increasing any further. Finally, if the combined voltage over all three cells exceeds the breakover voltage of Zener diode  20 (iii) then this diode will break down and leakage current through Zener diode  20 (iii) will prevent the voltage over cells  12   a ,  12   b  and  12   c  from increasing any further. 
     In all illustrated embodiments, the voltage protection provided by the Zener diodes  20  is only effective when the respective FET  22  connected to the Zener diode is switched on thereby to connect the Zener diode across the respective cell or cells. Each FET is switched on only when the power rail  14  of the charge current source is energised, for example when the cells  12  are being charged. When the power rail  14  is de-energised, each FET  22  will switch off thus disconnecting the respective Zener diode from the respective cell or cells  12 , thus preventing any leakage current from passing through the Zener diode and thus avoiding any unwanted discharging of the cell or cells. 
     It will be appreciated that the present invention allows the safe charging of one or more rechargeable cells and avoids the disadvantageous effects of both overcharging of one or more of the cells and excessive discharging of the cells during normal operation. 
     It will also be appreciated, however, that a number of modifications may be made to the invention as desired. For example, the Zener diode  20  may be replaced by resistive devices or any other devices which permit a selected level of leakage current to pass therethrough. In addition, the FET&#39;s  22  could be replaced by any other form of electronic switches or by any other device which upon energising of the power rail  14 , causes the voltage protection means to be connected across one or more of the cells. It is even envisaged that a mechanical arrangement, such as a manually operated switch, may be employed to selectively disconnect the Zener diodes or other voltage protection means from the cell or cells. 
     It will be clear that the invention is not limited to any particular kind of rechargeable cell which may be, for example, nickel-cadmium (Ni—Cd) “NICAD” cells, nickel metal hydride (NiMH) cells or any other chemical rechargeable cells or even solid cells (e.g. lithium polymer cells), capacitors or any other type of rechargeable device capable of storing and delivering electrical energy. 
     It is also possible for the invention easily to be configured for mechanical applications such as fuel cells for gas, liquid or other fuel or pressure containers. In this case the voltage protection means may be replaced by pressure release valves or other such devices. 
     In order to allow the power supply to operate in the event of an open circuit failure of one or more of the cells, conventional diodes may be permanently connected in parallel across each cell or cell pair to ensure a continuous path around the failed cell or cells. Having negligible reverse leakage current, these diodes will not discharge the power supply in any way. A possible arrangement for such diodes  23  is shown in  FIG. 3  whilst in  FIG. 4 , each FET  22 (i,ii,ii) is provided with an integral diode  24 .