Abstract:
A comparator ( 10 ) compares the voltage of a battery ( 2 ) with a switchable reference voltage (Uref 1 A, Uref 1 B,  18 ) and supplies a signal (IS) if the voltage of the battery passes the reference voltage. The switching point of the reference voltage is defined by means of a first resistor ( 32 ). By means of the first resistor the number of cells of the battery is set to one or two. A second resistor ( 6 ) in series with the battery enables an additional voltage to be obtained in order to correct the reference voltage for the type of battery cell used.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a voltage indicator for supplying an indication signal to indicate that a voltage of a battery passes a given value, comprising: a battery terminal for connection to the battery, a reference voltage source for supplying a reference voltage, and a comparator for comparing the reference voltage with a voltage on the battery terminal, which comparator has a first input terminal coupled to the reference voltage source, a second input terminal coupled to the battery terminal, and an output for supplying the indication signal in response to the comparison. 
     SUMMARY OF THE INVENTION 
     Such a voltage indicator is known from U.S. Pat. No. 5,705,913, particularly FIG. 2, and is used for indicating whether a rechargeable battery is nearly empty. For this purpose, the comparator of the known voltage indicator compares a fraction of the battery voltage with a reference voltage and, when the battery voltage passes the reference voltage, the output voltage of the comparator changes to another extreme value. In the development of electrical and electronic apparatuses having rechargeable batteries, the type of rechargeable battery as well as the number of cells of the rechargeable battery is not a predetermined fact. The voltage levels at which the cells are nearly empty are not the same for various cells, such as NiCd (Nickel-Cadmium), NiMH (Nickel-Metal Hydride) and the like. Moreover, the level of this voltage changes as a result of the developments in the cell formulation. Furthermore, there are apparatuses using one, two or more rechargeable cells in series. 
     It is an object of the invention to provide a voltage indicator by means of which the voltage level which is indicative of a nearly-empty state of a battery having one cell or more cells in series can be adjusted with the aid of two resistors. To achieve this object, the voltage indicator of the type defined in the opening paragraph is characterized in that the voltage indicator further comprises: 
     a further reference voltage source for supplying a further reference voltage; 
     a first resistor; 
     a second resistor connected between the battery terminal and the second input terminal of the comparator; 
     a voltage-to-current converter having a first current output and a second current output for supplying a first current and a second current, respectively, which currents have current intensities which are proportional to the further reference voltage and inversely proportional to the value of the first resistor, the second current output being coupled to the second input terminal of the comparator; 
     means coupled to the first current output, for supplying a switching signal in response to the first current; and 
     means for switching the reference voltage from the first-mentioned reference voltage source between at least two different values in response to the switching signal. 
     The first resistor defines the number of cells in series, for example, one or two. For this purpose, the first current of the voltage-to-current converter is compared with a threshold value. Upon passage of the threshold value the reference voltage with which the battery voltage is compared is changed to a value which is a multiple of said reference voltage, the multiple corresponding to the number of cells of the battery. The second current of the voltage-to-current converter flows through the second resistor which is arranged in series with the second input terminal of the comparator and which is connected to the battery. Thus, a correction voltage is added to the battery voltage so as to enable the nearly-empty indication to be adapted to the cell type, which correction voltage is adjustable by the appropriate choice of the value of the second resistor. When it is assumed that the voltage indicator, particularly the reference voltage sources of this indicator, is constructed as an integrated circuit, the voltage indicator in accordance with the invention offers the possibility of defining the nearly-empty indication voltage by means of two external resistors. 
     If the choice is only between a battery having one cell or two cells in series the voltage indicator can be implemented by using a further comparator having first and second inputs coupled respectively to the first current output and to bias voltage, and an output for supplying the switching signal. Depending on the value of the first resistor the output signal of the further comparator assumes either of two values. For the one value the reference voltage is switched to a reference value corresponding to a single cell. For the other value the reference voltage is switched to the double reference value corresponding to two cells in series. It is alternatively possible to switch from the full reference voltage to half the reference voltage. 
     If a distinction is to be made between three or more cells in series, the first current of the voltage-to-current converter is compared with a plurality of threshold values and the reference voltage is switched between three or more values which are multiples of each other. 
     Switching over of the reference voltage can be effected by choosing from a plurality of reference voltages by means of a selector in response to the switching signal. Alternatively, the reference voltage can be switched over by attenuating a reference voltage by a factor of two, three or more by means of a voltage divider having a switchable voltage dividing factor. 
     The second resistor, which defines the correction voltage, further has the advantage that in combination with a capacitor it can serve as a smoothing filter for possible ripple voltages on the battery voltage to be measured. 
     The invention can be used in apparatuses having rechargeable batteries or having non-rechargeable (primary) batteries, in which it is desired to have an indication or to show that the batteries are nearly empty. The voltage indicator may activate a visual or acoustic signal which warns the user that the battery is nearly empty. The voltage indicator may also form part of a complex battery management system which controls and monitors the charging and discharging process and which, for example, inhibits further discharging of the battery when the battery is nearly empty. Examples of apparatuses of this kind are in the fields of personal care such as shavers and toothbrushes, of telecommunication such as portable phones, of computers such as laptops and organizers, and of portable audio and video equipment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in more detail with reference to the accompanying drawings, in which: 
     FIG. 1 shows an embodiment of a voltage indicator in accordance with the invention; and 
     FIG. 2 shows a block diagram of a shaver having a rechargeable battery and a voltage indicator in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an embodiment of a voltage indicator in accordance with the invention, which supplies an indication signal IS as an indication that the voltage of a rechargeable battery passes a given value. The rechargeable battery  2  has its positive terminal connected to a battery terminal  4 , which is connected to one terminal  8  of a comparator  10  via a resistor  6  having a value R2, the indication signal IS being supplied by an output of said comparator. The battery  2  has its negative terminal connected to ground. An optional capacitor  12  is connected between the input terminal  8  and ground. The comparator  10  has its other input terminal  14  selectively coupled to one of the reference voltage sources  16 A and  16 B, which supply a reference voltage Uref 1 A and Uref 1 B, respectively. The reference voltage is selected by means of a selector  18  under control of a switching signal supplied by the output  20  of a further comparator  22 . The voltage indicator further comprises a voltage-to-current converter  24 , which converts the reference voltage Uref 2  from a further reference voltage source  26  into a first current I 1  and a second current I 2 , which are available on a first current output  28  and a second current output  30 , respectively. The conversion factor of the voltage-to-current converter  24  is determined by a resistor  32  having a value R1, which resistor is connected between ground and a connection terminal  34  of the voltage-to-current converter  24 . A first transistor  36  has its main current path connected between the first current output  28  and the connection terminal  34  and a second transistor  38  has its main current path connected between the second current output  30  and the connection terminal  34 . By way of example unipolar (MOS) transistors are shown but it is also possible to use bipolar transistors. The control electrodes, in the present case the gates, of the transistors  36  and  38  are coupled to an output  40  of a differential amplifier  42  which has its inverting input  44  coupled to the connection terminal  34  and which has its non-inverting input  46  connected to the further reference voltage source  26 . The first output  28  is coupled to a positive supply voltage via a load. The load is a current source  48  which supplies a current Ic, but a load that takes the form of a resistor is also possible. The node between the first current terminal  28  and the current source  48  is connected to one input  50  of the further comparator  22 , which has its other input  52  connected to a bias voltage source  54 . The dimensions (W/L) of the transistors  36  and  38  are in a ratio of 1:9 but other ratios are also possible. As a result of this, the currents I 1  and I 2  are also in a ratio of 1 to 9. The sum of the currents I 1  and I 2  flows through the resistor  32 . In the case of an adequate gain of the differential amplifier  42 , the voltage drop across the resistor  32  is substantially equal to the reference voltage Vref 2  from the further reference voltage source  26 . The current I 1  is then equal to 0.1*Uref 2 /R1 and the current I 2  is equal to 0.9*Uref 2 /R1. If the current I 1  is larger than the current Ic or, in other terms, if the resistance value R1 of the resistor  32  is smaller than 0.1*Uref 2 /Ic, the sign of the voltage difference between the inputs  50  and  52  of the further comparator  22  is reversed, as a result of which the switching signal on the output  20  assumes another value. As a result of this, the reference voltage is switched from the value Uref 1 A of the reference voltage source  16 A to the value Uref 1 B of the reference voltage source  16 B. The value Uref 1 B is, for example, twice as great as the value Uref 1 A. Thus, the reference voltage on the input terminal  14  of the comparator  10  is changed by a factor of two. It is alternatively possible to use a single reference voltage source  16  and to attenuate its reference voltage by a factor of two by means of a voltage divider  56 ,  58 , which is switched over by means of a switching transistor  60  under command of the switching signal. 
     The reference voltage on the one input terminal  14  of the comparator  10  is compared with the battery voltage on the other input terminal  8 . The current I 2  flows from the battery  2  to the second current terminal  30  via the resistor  6  and produces a voltage drop equal to I 2 *R2 across the resistor  6 . This voltage drop can be used for setting the change-over point of the comparator  10  to the correct value. This change-over point is reached if the battery voltage passes a value Ubat equal to: 
     
       
         Ubat=k*Uref1A+0.9*Uref2*(R2/R1), 
       
     
     where k is the ratio between the reference voltages Uref 1 B and Uref 1 A. By means of R1 the voltage indicator is adapted to operate in conjunction with one or two cells in series and by means of R2 the change-over point of the comparator  10  is set exactly to the correct value. 
     By means of the circuit shown herein it is possible to choose from two reference voltages which are in a ratio of 1:2, enabling a distinction to be made between batteries having one cell, or two cells in series. However, it is also possible to distinguish between batteries having three or more cells by comparing the current I 1  with a plurality of threshold values. This requires three reference voltage sources having progressive reference voltages. Alternatively, a single reference voltage source may be used, whose reference voltage is divided by a factor of three or more when the current I 1  is compared with said plurality of threshold values. 
     The capacitor  12  and the resistor  6  act as a filter for possible ripple voltages on the voltage of the battery  2 , as a result of which the voltage indication is less susceptible to spurious signals. The reference voltage sources  16 A and  16 B, for example, take the form of band-gap reference sources with a voltage of once or twice 1080 mV, respectively. In combination with the voltage, nevertheless drop across the resistor  6  an indication signal IS can be obtained in the range from 1110 mV to 1180 mV within which the cell voltage of a nearly empty NiCd and NiMH battery cell lies. 
     The use of an accurate reference voltage in the proximity of the nearly-empty indication voltage of one, two or more battery cells and the addition of a comparatively small but less accurate voltage to said accurate voltage yet results in an overall accuracy which does not deviate much from the accuracy of the reference voltage source or sources. The less accurate voltage is the voltage across the resistor  6  and, as is apparent from the aforementioned formula, the accuracy of this voltage is determined by the tolerance of the resistance values R1 and R2, the tolerance of the ratio between the currents I 1  and I 2 , and the tolerance of the reference voltage Uref 2 . External 1% resistors, an accuracy of 2.3% for the scaling factor in the dimensions of the transistors T 1  and T 2 , and a 1% accuracy for the reference voltage Uref 2  yield an accuracy of approximately 5% for the voltage across the resistor  6 . A 5% accuracy for a voltage of approximately 100 mV across the resistor  6  and a 1% accuracy for a reference voltage of 1080 mV result in an overall accuracy of approximately 1.5%. Thus, with the aid of an accurate reference voltage source, the voltage indicator in accordance with the invention makes it possible to define the nearly-empty indication voltage by means of two external resistors without an excessive reduction of the overall accuracy. 
     FIG. 2 shows an application of the voltage indicator in a shaver having a rechargeable battery. The rechargeable battery  2  of the shaver is charged by a battery charger BCH. The battery  2  powers the electric motor M which drives the shaving mechanism. The motor M can be switched on and off by means of a switch SW. The voltage indicator of FIG. 1, referenced BVI, is connected to the battery  2 . The indication signal IS from the voltage indicator BVI is applied to a battery management system BMS which controls and monitors the charging and discharging process of the battery  2  and, for example, inhibits further discharging of the battery  2  when the battery is nearly empty. The battery management system BMS, inter alia, drives a display DSP which gives a visual and, if desired, also an acoustic indication that the battery  2  is nearly empty. 
     It will be evident that the voltage indicator can be used not only for rechargeable batteries but also for primary batteries.