Method and device for checking the charge state of a battery, in particular a rechargeable battery for a cellular mobile telephone

Abstract

The amount of charge passing through a measurement resistor connected to a rechargeable battery is measured by integrating in an analog manner an overall current. This overall current is equal to the sum of the resistor current and of a reference current that selectively takes one of two opposite values. The results of the integration are compared with a reference voltage, and one of two opposite values of the reference current is selected depending on each result of the comparison. The number of times where the positive opposite value of the reference current is selected furnishes an indication on the amount of charge during the integration time.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to the field of power supplies, and in particular, to checking the charge state of a rechargeable power-supply. The present invention is especially, but not exclusively, applicable to determining at any instant the remaining operational time of a rechargeable power-supply in a cellular mobile telephone.

BACKGROUND OF THE INVENTION

[0002] To check the charge state of a battery, and consequently, to be able to possibly determine the remaining operational time of this battery, the amount of charge passing through a resistor connected in series with the battery is measured. One approach currently used includes connecting a voltage-frequency converter to the terminals of the resistor. However, so that the energy consumed in the resistor is as small as possible so as not to cause problems in measuring the amount of charge, it is necessary to have a low enough resistance.

[0003] This then requires the use of a high-accuracy converter. However, even in this case, the offset voltage of the converter may prove to be problematic and lead to false information. Moreover, a converter of this type is sensitive to the supply voltage. The result of this is, in particular, a variation in the gain of the converter with a variation in the supply voltage.

[0004] Another approach includes using a conventional converter of the delta-sigma type. A converter of this type is found to be very accurate provided a high enough sampling frequency is used. However, this contributes to an increase in the current consumption of the apparatus. To overcome this drawback, it is possible to reduce the sampling frequency, but at the risk of then losing information.

SUMMARY OF THE INVENTION

[0005] In view of the foregoing background, an object of the present invention is to check the charge state of a rechargeable power-supply while minimizing any loss of information, and while doing so with low current consumption.

[0006] Another object of the present invention is to provide a checking device of this type which overcomes variations in the supply voltage.

[0007] These and other objects, advantages and features according to the present invention are provided by a method of checking the charge state of power-supply, and in particular, a rechargeable battery, by measuring an amount of charge passing through a resistor connected in series with the power-supply.

[0008] According to a general characteristic of the invention, the measurement of the amount of charge may comprise:

[0009] an analog integration over a predetermined integration time of an overall current equal to the sum of a current called a resistor current representative of that passing through the resistor, and of a predetermined reference current capable of selectively taking two opposite values of absolute value greater than the value of the resistor current;

[0010] successive comparisons throughout the integration time of the corresponding successive results of the integration with a comparison reference, for example, a zero voltage;

[0011] successive selections of one of the two opposite values of the reference current depending on the successive results of the comparisons; and

[0012] a count over the integration time of the number of times where one of the two opposite values was selected, for example, the number of times where the positive value of the reference current was selected, and the result of this count is representative of the amount of charge.

[0013] According to one embodiment of the method, the counting step involves the incrementation of a counter each time the value in question is selected (for example, the positive value of the reference current) and the non-incrementation of the counter each time the opposite value (for example, the negative value of the reference current) is selected. Half the value of the integration time is then subtracted from the counter value. The result of this subtraction furnishes a value representative of the amount of charge.

[0014] The result of this subtraction can be obtained simply by taking the two's complement of the contents of the counter. This two's complement is obtained by inverting the binary value of the significant bit of the counter value.

[0015] According to one embodiment of the method applicable to a differential measurement of the current passing through the resistor, the integration phase involves reversing the polarity of the differential measurement at half the integration time and continuing the integration for the second half of the integration time with the polarity inverted in this way. Moreover, during the polarity reversal, the values of the results of the comparisons are also reversed. This embodiment makes it possible, when the integration is carried out by an operational amplifier, to compensate for the offset voltage of this operational amplifier.

[0016] It is also possible to determine the remaining operational time of the power-supply at a given instant from the accumulation at this instant of the amount of charge measured during successive integration times, and from a calculation of the current consumption at this given instant. The result of the count over a predetermined number of previous integration times furnishes, for example, a calculation of this current consumption.

[0017] The subject of the invention is also a device for checking the charge state of a power-supply. The device comprises a resistor connected in series with the power-supply, and measurement means connected to the terminals of the resistor for measuring the amount of charge passing through the resistor.

[0018] According to a general characteristic of the invention, the measurement means or circuitry may comprise:

[0019] an input interface connected to the terminals of the resistor and delivering a resistor current;

[0020] a reference interface delivering a predetermined reference current capable of selectively taking two opposite values with an absolute value greater than the value of the resistor current;

[0021] provision means capable of providing a predetermined integration time;

[0022] analog integration means capable of carrying out an integration over the integration time, of an overall current equal to the sum of the resistor current and of the reference current;

[0023] comparison means capable of carrying out successive comparisons during the integration time of corresponding successive results of the integration with a comparison reference;

[0024] selection means capable of successively selecting one of the two opposite values of the reference current depending on the successive results of the comparisons; and

[0025] counting means capable of counting, over the integration time, the number of times when one of the two opposite values has been selected, the result of this count being representative of the amount of charge.

[0026] According to one embodiment of the invention, the integration means may comprise an operational amplifier fed back by a capacitor, receiving the overall current and carrying out a continuous analog integration of the overall current. The provision means may comprise a first counter which is incremented in time with a clock signal up to a maximum counting value corresponding to the integration time. The comparison means may comprise an analog comparator followed by a flip-flop regulated by the clock signal.

[0027] According to one embodiment of the invention, the counting means may comprise a second counter incremented each time the value in question is selected, and non-incremented each time the opposite value is selected. Moreover, the counting means may comprise means capable of subtracting half of the maximum counting value of the first counter from the value of the second counter. The result of this subtraction furnishes a value representative of the amount of charge.

[0028] The subtraction means may advantageously comprise an inverter connected to the output of the second counter delivering the significant bit. The first counter and the second counter are advantageously asynchronous counters, that is, they are counters whose internal flip-flops do not all switch at the same time. This makes it possible to obtain a very low consumption for these counters.

[0029] The device according to the invention advantageously may comprise an analog block and a digital block. The analog block may comprise the input interface, the reference interface, the integration means, and part of the comparison and selection means. The digital block may comprise the means for preparing the integration time, the other part of the comparison and selection means, and the counting means.

[0030] The analog block may advantageously have a differential structure which makes it possible to overcome stray capacitances, and consequently, effects of variations in the supply voltage. In a differential structure, the input interface comprises a differential input formed from two terminals. It is thus particularly advantageous that the measurement means furthermore comprise chopper means capable of inverting the two terminals at half the integration time. The integration continues during the second half of the integration time with the two terminals thus inverted. The chopper means also is capable of inverting the output signal of the comparator during the second half of the integration time. An embodiment of this sort makes it possible to compensate for the offset of the operational amplifier forming the integrator.

[0031] The chopper means may comprise, for example, an inverter connected to the two terminals of the input interface. This inverter is controllable by the value of the significant bit of the first counter, that is, the counter which determines the integration time.

[0032] Moreover, the chopper means may comprise an EXCLUSIVE OR logic gate. The logic gate includes a first input connected to the output of the significant bit of the first counter, a second input connected to the output of the comparison means, and an output connected to an input of the second counter controlling the incrementation or the non-incrementation of this second counter. This EXCLUSIVE OR logic gate makes it possible to invert the output of the comparator in the second half of the integration time.

[0033] To calculate the remaining operational time of the power-supply means, the device may advantageously comprise an accumulation register capable of accumulating the successive count results. Means then calculate the remaining operational time of the power-supply means from the contents of the accumulation register, and from a predetermined number of previous count results.

[0034] The subject of the invention is also an apparatus operating with a rechargeable power-supply. In particular, the apparatus may be a cellular mobile telephone incorporating a charge-checking device as defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Other advantages and characteristics of the invention will become apparent on examining the detailed description of the embodiment, which is in no way limiting, and of the appended drawings in which:

[0036] FIG. 1 is a block diagram illustrating a cellular mobile telephone comprising a charge-checking device according to the present invention;

[0037] FIG. 2 is a block diagram illustrating the operating principle of an analog block of a charge-checking device according to the present invention;

[0038] FIG. 3 is a schematic diagram illustrating an embodiment of an analog block with a differential structure according to the present invention;

[0039] FIG. 4 is a schematic diagram of the internal architecture of a digital block according to the present invention;

[0040] FIG. 5 is a graph illustrating a count of the amount of charge over one integration time according to the present invention;

[0041] FIG. 6 is a timing diagram illustrating an embodiment of the method according to the present invention; and

[0042] FIG. 7 is a block diagram illustrating means for calculating the remaining operational time of a power-supply according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] In FIG. 1, the reference TP denotes a cellular mobile telephone comprising a set of electronic components BLE powered by a rechargeable battery BT. To check the charge state of the battery BT, the amount of charge passing through a resistor RM connected in series with the battery is measured using measurement means MMS. As will be seen in more detail below, the measurement means illustratively comprise an analog block BAN and a digital block BNM.

[0044] As illustrated in FIG. 2, a current called the overall current Ig, equal to the sum of a current Iin representative of the current passing through the resistor RM and of a reference current which may take two opposite values −Iref and +Iref, is continually integrated in an integrator INT during an integration time Tc. The output voltage of the integrator is continually compared with a comparison reference, in this case, for example, a zero voltage, in an analog comparator CMP. The output signal CMPO of the comparator is then sampled in a flip-flop B6 with a clock signal CLK. The output of the flip-flop B6 therefore furnishes periodically, for example, every 30.5 μs, the results of successive comparisons during the integration time of corresponding successive results of the integration with the reference voltage.

[0045] A switch, for example, a multiplexer MUX1, then successively selects one of the two opposite values −Iref, +Iref of the reference current depending on the successive results of the comparisons. The number of times where one of the two opposite values has been selected, for example, the number of times where the value +Iref has been selected, will then be counted over the integration time Tc (for example, about 250 ms). This count will be carried out in the digital block BNM, and the result of this count is representative of the amount of charge which has passed through the resistor during the integration time.

[0046] This is because, at the end of each period of the clock signal, the output voltage Vc of the integrator is compared to the reference voltage, in this case 0 volts. If the voltage Vc is positive, then the value +Iref is selected for the following period. Otherwise, the value −Iref is selected.

[0047] The loop then tends to keep the voltage Vc close to the reference value, that is, close to 0 volts. To this end, it is desired for the absolute value of the reference current to be greater than the absolute value of the current Iin. The upper limit of the current Iref will be determined by a person skilled in the art based upon the characteristics of the integrator, and especially the value of the capacitor which is part of this integrator.

[0048] As will be seen in more detail below, a counter CPT2 is incremented by 1 each time the value Iref is selected, and is not incremented when the value −Iref is selected. After N periods of the clock signal CLK defining the integration time, the value d of the counter CPT2 is noted. If D and the difference d-N/2 are noted, then it is shown that D is proportional to the integral of the current Iin over the integration time, and consequently, proportional to the integral of the current having passed through the measurement resistor RM. D therefore furnishes a picture of the amount of charge having passed through the resistor RM.

[0049] FIG. 3 illustrates in more detail a differential embodiment of the analog block BAN. More specifically, the block BAN comprises an input interface. In this case, the input interface comprises two terminals BPS and BMS respectively connected to the two terminals of the measurement resistor RM. Moreover, the input interface comprises two resistors Rs. The input interface therefore delivers the current Iin which is representative of the current passing through the measurement resistor RM.

[0050] The analog block BAN moreover comprises a reference interface comprising a first terminal connected to the reference voltage Vref, and a second terminal connected to ground. Moreover, this reference interface comprises a resistor Rr delivering the current Iref.

[0051] The multiplexer MUX1 of the selection means is in this case connected between the two terminals of the reference interface, and is consequently capable of reversing the connection of these two terminals so as to selectively deliver the two opposite values of the reference current, that is, the values +Iref and −Iref.

[0052] The gain is fixed with respect to the resistors Rs/Rr. It is equal to Vref/|Vin,max| where Vin,max denotes the maximum voltage at the terminals of the measurement resistor. The ratio Rs/Rr must be chosen such that |Iref|>|Iin|. By way of example, a ratio of Rr/Rs equal to 20 is chosen, with Rr equal to 4 MΩ and Rs equal to 200 kΩ. The respective output terminals of the input and reference interfaces are connected together such that the overall current Ig is equal to the sum of the current Iin and of the current Iref.

[0053] The integrator INT is formed in this case from an operational amplifier OPA fed back by a capacitor C. The operational amplifier OPA has a reference input for receiving a reference voltage which makes it possible to set the common mode voltage. To this end, it is possible to choose the voltage Vref as the reference voltage.

[0054] The differential output voltage Vc of the integrator is delivered to the two input terminals of an analog comparator CMP. Consequently, the comparator compares the voltage difference Vc at its two input terminals with the value 0. The output signal CMPO of the comparator CMP is then delivered to the input of the flip-flop B6 of the digital block BMN illustrated in FIG. 4. In this embodiment, the flip-flop B6 is controlled on the rising edges of the signal NCLK, that is, on the falling edges of the clock signal CLK.

[0055] Moreover, the digital block BNM comprises means called chopper means CHP, the meaning and function of which will be covered in more detail below. This being so, these means CHP, although advantageous, are not indispensable. Initially, it will be assumed that they are not incorporated in the block BNM. In this case, the output of the flip-flop B6 which delivers the selection signal RFMX to the multiplexer MUX1 is also directly connected to the incrementation/non-incrementation input CNC of the counter CPT2.

[0056] This counter CPT2 is regulated by the clock signal CLK. Consequently, it is capable of being incremented in time with the rising edges of the clock signal CLK depending on the value of the signal RFMX at the input CNC. In the example described here, if the voltage difference Vc is positive, then during the following period of the clock signal CLK, the resistor Rr will be connected to +Vref. If Vc is negative, then Rr will be connected to −Vref. In other words, the two input terminals of the reference interface will be reversed using the multiplexer MUX1.

[0057] Under these conditions, the counter CPT2 is incremented by 1 each time the resistor Rr is connected to −Vref, and is not incremented each time the resistor Rr is connected to +Vref. Moreover, it is assumed in the example described here, that the word contained in the counter CPT2 is a word Q of 13 bits. The 13 data outputs of the counter CPT2 are connected to the 13 inputs of a register RG. However, the output of the significant bit of the word contained in the counter CPT2, that is, in this case the output Q<12>, is connected to the corresponding input of the register RG via an inverter IV. Consequently, the given word D contained in the register RG is equal to the two's complement of the difference between the word Q contained in the counter CPT2 and N/2, where N denotes the maximum counting value of a counter CPT1 which will determine the integration time.

[0058] More specifically, this first counter CPT1 is incremented by 1 in time with the rising edges of the clock signal CLK. The counting word C, also over 13 bits in the example described and contained in the counter CPT1, is compared in a comparator CMP2 with the maximum counting value N, which is in this case equal to 1FFF, that is, to 8192.

[0059] The output of the comparator CMP2 is connected, via a flip-flop B1, to the validation input of the register RG. When the maximum counting value of the counter CPT1 is reached, the end-of-counting signal EOC, delivered by the flip-flop B1, changes over to the high state and the word contained in the counter CPT1 is then transferred into the register RG. The word D contained in the register RG thus represents the amount of charge having passed through the measurement resistor RM during the integration time which has just elapsed.

[0060] In FIG. 5, the 8192 edges of the clock signal which control the counter CPT2 are shown on the y-axis. In the left part of FIG. 5, the 8192 edges of the clock signal which control the counter CPT1 are shown on the x-axis. The value d shows the final value of the counter CPT2 at the end of the integration time, that is, when the counter CPT1 has reached the final counting value N (1FFF). C1 shows the change in the contents of the counter CPT2, where the value +Iref of the reference current is continually selected throughout the integration time.

[0061] The curve C3 shows the change in the value of the counter CPT2 when the value −Iref is continually selected throughout the integration time. The curve C2 shows the change in the value of the counter CPT2 with a zero input current Iin, moreover, by assuming that there is no offset of the operational amplifier. It is therefore seen that, for the curve C1, the final value d of the counter CPT2 is equal to 8191. It is equal to 4096 for the curve C2 and to 0 for the curve C3. In this case, D represents the two's complement value of the value d. This value D also corresponds to d-N/2. This value D is the value of the register RG when the signal EOC takes the value 1. It is obtained by reversing the significant bit of the word d.

[0062] By way of example, the right part of FIG. 5 gives corresponding indicative examples of the voltage Vin at the terminals of the measurement resistor RM and of the current Iin. The operation of the digital block BMN will now be described, particularly with reference to FIG. 6.

[0063] When the end-of-counting signal EOC takes the value 1, that is, when the final counting value 1FFF is reached, the contents Q of the counter CPT2 is transferred to the register RG. Moreover (FIG. 4), the output of the flip-flop B1 is connected to the return-to-zero input RS of the counter CPT2 via a flip-flop B4 which introduces a delay between the input signal EOC and the output signal ENS. This makes it possible, first, to transfer the result contained in the counter CPT2 into the register RG before resetting the counter CPT2 to zero. Moreover, it can be seen that, in time with the falling edges of the clock signal CLK, the signal RFMX may take two distinct values depending on the sign of the value of the signal CMPO.

[0064] The structure and the function of the chopper means CHP will now be covered. These chopper means CHP comprise a flip-flop B2 connected to the output of the significant bit of the counter CPT1, and delivers a chopper signal CHPX. Another flip-flop B3 is connected between the output of the flip-flop B2 and one input of a logic gate PL of the EXCLUSIVE OR type. The other input of this logic gate PL is connected to the output of the flip-flop B6, and consequently, receives the signal RFMX. The output of the logic gate PL is connected to the incrementation/non-incrementation input CNC of the counter CPT2. The logic gate PL therefore carries out an EXCLUSIVE OR function between the signal CHPD coming from the logic gate B3 and the signal RFMX coming from the flip-flop B6.

[0065] The chopper signal CHPX controls a second multiplexer MUX2 connected to the two terminals BPS and BMS of the input interface, and is capable, depending on the value of the signal CHPX, either of reversing or not reversing the connection of these two terminals. Thus, the chopper means CHP will invert the connection of the two terminals BPS and BMS via the multiplexer MUX2 and the signal CHPX, at half the integration time. This instant is defined by the value of the significant bit C<12> of the word contained in the counter CPT1.

[0066] Moreover, the logic gate PL makes it possible to compensate for the effect of this reversal with regard to the decision of whether or not to increment the counter CPT2. These chopper means CHP thus make it possible to compensate for the offset of the operational amplifier OPA.

[0067] Moreover, the device according to the present invention may comprise an accumulation register ACC (FIG. 7) capable of accumulating the successive count results contained in the register RG. This accumulation is carried out with the rising of the end-of-counting signal EOC to 1.

[0068] Moreover, means MDR make it possible to calculate the remaining operational time DR of the battery from the content of the accumulation register, and from at least the last count result, that is, the last content of the register RG. This is because this last content is representative of the current consumption over the last integration time. Dividing the content of the accumulation register by the content of the register RG, carried out in the means MDR, makes it possible to determine this remaining time. With this being so, in practice and especially in a mobile telephone application, it will be preferred to use the mean of a predetermined number of previous count results to calculate the current consumption.

Claims

1. Method of checking the charge state of a power-supply means, comprising a measurement of the amount of charge passing through a resistor (RM) connected in series with the power-supply means, characterized in that the measurement of the said amount of charge comprises an analog integration over a predetermined integration time of an overall current (IG) equal to the sum of a resistor current (Iin) representative of that passing through the said resistor and of a predetermined reference current capable of selectively taking two opposite values of absolute value greater than the value of the resistor current, successive comparisons throughout the said integration time of the corresponding successive results of the said integration with a comparison reference, successive selections of one or other of the two opposite values of the reference current depending on the successive results of the comparisons, and a count (CPT2) over the said integration time of the number of times where one of the two opposite values was selected, the result of this count being representative of the said amount of charge.

2. Method according to claim 1, characterized in that the count step involves the incrementation of a counter (CPT2) each time the said value in question is selected, and the non-incrementation of the counter each time the opposite value is selected, and in that half (N/2) of the value of the integration time is subtracted from the value (d) of the counter (CPT2), the result of this subtraction furnishing a value (D) representative of the said amount of charge.

3. Method according to claim 2, characterized in that the result (D) of the said subtraction is obtained by taking the two's complement of the contents (d) of the counter (CPT2), this two's complement being obtained by inverting the binary value of the significant bit of the counter value (CPT2).

4. Method according to one of the preceding claims, characterized in that the measurement of the current passing through the resistor (RM) is a differential measurement, in that the integration phase involves reversing the polarity of the differential measurement at half the integration time and continuing the integration for the second half of the integration time with the polarity reversed in this way, and in that during the polarity reversal, the values of the results of comparison are also reversed.

5. Method according to one of the preceding claims, characterized in that the remaining operational time (DR) of the power-supply means is determined at a given instant from the accumulation at this instant of the amounts of charge measured during successive integration times, and from a calculation of the current consumption at this given instant.

6. Method according to claim 5, characterized in that the result of the said count over a predetermined number of previous integration times furnishes a calculation of the said current consumption.

7. Device for checking the charge state of a rechargeable power-supply means, comprising a resistor (RM) connected in series with the power-supply means and measurement means (MMS) connected to the terminals of the resistor and capable of measuring the amount of charge passing through the said resistor, characterized in that the measurement means (MMS) comprise an input interface (RS) connected to the terminals of the resistor and delivering a resistor current (Iin), a reference interface (Rn) delivering a predetermined reference current (Iref) capable of selectively taking two opposite values with an absolute value greater than the value of the resistor current, provision means (CPT1) capable of providing a predetermined integration time, analog integration means (INT) capable of carrying out an integration over the said integration time, of an overall current equal to the sum of the resistor current and of the reference current, comparison means (CMP) capable of carrying out successive comparisons during the said integration time of corresponding successive results of the said integration with a comparison reference, selection means (MUX1) capable of successively selecting one or other of the two opposite values of the reference current depending on the successive results of the said comparisons, and counting means (CPT2) capable of counting, over the said integration time, the number of times where one of the two opposite values has been selected, the result of this count being representative of the said amount of charge.

8. Device according to claim 7, characterized in that the integration means (INT) comprise an operational amplifier (OPA) fed back by a capacitor (C), receiving the said overall current and carrying out a continuous analog integration of the overall current, in that the provision means comprise a first counter (CPT1) which is incremented in time with a clock signal (CLK) up to a maximum counting value (N) corresponding to the said integration time, and in that the comparison means comprise an analog comparator (CMP) followed by a flip-flop (B6) regulated by the said clock signal.

9. Device according to claim 8, characterized in that the counting means comprise a second counter (CPT2) incremented each time the said value in question is selected, and non-incremented each time the opposite value is selected, and means capable of subtracting half of the maximum counting value of the first counter from the value of the second counter, the result of this subtraction furnishing a value representative of the said amount of charge.

10. Device according to claim 9, characterized in that the subtraction means comprise an inverter (IV) connected to the output of the counter delivering the significant bit.

11. Device according to claim 9 or 10, characterized in that the first counter (CPT1) and the second counter (CPT2) are asynchronous counters.

12. Device according to one of claims 7 to 11, characterized in that it comprises an analog block (BAN) comprising the input interface, the reference interface, the integration means and part of the comparison and selection means, and a digital block (BNM) comprising the means for preparing the integration time, the other part of the comparison and selection means and the counting means.

13. Device according to claim 12, characterized in that the analog block (BAN) has a differential structure.

14. Device according to claim 13, characterized in that the input interface comprises a differential input formed from two terminals (BPS, BMS), and in that the measurement means furthermore comprise chopper means (CHP) capable of reversing the two terminals (BPS, BMS) at half the integration time, the integration continuing during the second half of the integration time with the two terminals thus reversed, and of reversing the output signal of the comparison means during the second half of the integration time.

15. Device according to claim 14 taken in combination with claim 9, characterized in that the chopper means (CHP) comprise an inverter (MUX2) connected to the two terminals of the input interface, this inverter being controllable by means of the value of the significant bit of the first counter (CPT1), and an EXCLUSIVE OR logic gate (PL), a first input of which is connected to the output of the significant bit of the first counter, the second input of which is connected to the output of the comparison means, and the output of which is connected to an input (ENC) of the second counter (CPT2) controlling the incrementation or the non-incrementation of the counter.

16. Device according to one of claims 7 to 15, characterized in that it comprises an accumulation register (ACC) capable of accumulating the successive count results, and means (MDR) capable of calculating the remaining operational time (DR) of the power-supply means from the contents of the accumulation register and from a predetermined number of previous count results.

17. Apparatus operating with a rechargeable power-supply means, in particular a cellular mobile telephone, characterized in that it comprises a device according to one of claims 7 to 16.