Abstract:
The invention relates to a charging device for charging at least one rechargeable energy storage unit, having a defined inner resistance and having a compensation circuit for the compensation of a voltage drop during the charging operation caused by the inner resistance. The compensation circuit has a current detection for detecting a charge current of the rechargeable energy storage unit and a voltage regulator for adjusting the charge voltage to the detected charge current. The charging device is characterized in that the compensation circuit has a voltage distributor between the current detector and the voltage regulator, by the dimensioning of which the compensation of the inner resistance is carried out.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP2008/062124 filed on Sep. 12, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a charger for charging at least one rechargeable energy storage unit. 
     2. Description of the Prior Art 
     In chargers, which for charging rechargeable energy storage units, such as rechargeable lithium-ion, nickel-cadmium, nickel-hydride or lead batteries, employ a fixed charge termination voltage, the internal resistance of the charger results in a voltage drop that is dependent on the charging current. This voltage drop causes a lengthening of the charging time and consequently affects the fill level of the rechargeable energy storage unit. 
     It is known to monitor the charging operation and shorten it by means of a conventional four-point measurement. However, the rechargeable energy storage unit is then permanently connected to the charger, so that a slight current flows via the measuring terminals and leads to discharging. One option for circumventing this problem is the use of a switchover device, which is capable of disconnecting the measurement connections from the rechargeable energy storage unit. However, such a switchover device is as a rule expensive and requires a relatively large amount of space. 
     From German Patent Disclosure DE 44 31 620 A1, a charger for rechargeable energy storage units with a maximum charging voltage of 4.2 V is known. The charger includes a compensation circuit, which compensates for at least the voltage drop that is caused by the internal resistance of the charger, with the exception of the rechargeable energy storage unit. For that purpose, a compensation resistor is integrated as a shunt with the ground line of the charger, and its resistance is equivalent to the internal resistance of the charger. A voltage detection circuit measures the charging voltage at the contacts of the wiring, including of the rechargeable energy storage unit, and a charge control circuit controls the charging operation on the basis of the measured charging voltage in such a way that the charging voltage of the rechargeable energy storage unit does not exceed a prescribed voltage. The magnitude of the charging current is fixed via a further shunt in the power supply line of the rechargeable energy storage unit. 
     In European Patent Disclosure EP 0 795 946 A2, a charger for accelerated charging of lithium-ion cells is also shown, which compensates for the internal series resistance (Electrical Series Resistance or ESR) during the charging operation, which results from the battery terminals, the fuses, the contacts, and the cabling, by means of a suitable compensation circuit, such that the compensation circuit subtracts the voltage drop, generated by the ESR, from the charging voltage contacting the terminals of the rechargeable energy storage unit. To that end, a shunt with a value corresponding to the ESR is disposed between the positive terminal of the rechargeable energy storage unit and the positive supply terminal of a current source of the charger, and measurement of the charging voltage is done via additional sensing lines. The charger shown in EP 0 796 946 A2 is likewise designed for a maximum charging voltage of 4.1 V to 4.2 V. 
     ADVANTAGES AND SUMMARY OF THE INVENTION 
     The charger according to the invention for charging at least one rechargeable energy storage unit, having a defined internal resistance and having a compensation circuit for compensating for a voltage drop caused by the internal resistance during the charging operation offers the advantage over the prior art that the compensation takes place only whenever a charging current in fact is also flowing. To that end, the compensation circuit has a current detector for detecting the charging current of the rechargeable energy storage unit as well as a voltage regulator for adapting the charging voltage to the detected charging current. Advantageously, a simple two-point measurement thus results, with which the aforementioned disadvantages of a four-point measurement can be avoided, so that pronounced shortening of the charging time is attainable. Because the compensation circuit includes a voltage divider disposed between the current detector and the voltage regulator, and the compensation for the internal resistance is effected via the dimensioning of the voltage divider, there is no longer any dependency on a shunt required for current detection and integrated with the supply line and/or the ground line. Moreover, any sensing lines for measurement can be dispensed with entirely. The charger of the invention is furthermore not bound either to a defined maximum charging voltage or to a maximum current limit for the compensation or to any specific technology of the rechargeable energy storage unit. 
     According to the invention, the voltage divider is connected on one side to a reference voltage and on the other to an output of the current regulator, and a center pickup of the voltage divider is connected to at least one input of the voltage regulator. In this way, the current detector influences the voltage regulator via the voltage divider in such a manner that as a function of the detected charging current, the reference voltage for the voltage regulator is varied. The increase in the charging voltage corresponds to the voltage drop caused by the internal resistance. In this respect, especially simple generation of the reference voltage is obtained by means of a Zener diode located at an auxiliary voltage. 
     In an alternative feature, a temperature sensor is provided for varying the reference voltage. It is thus possible to compensate for a temperature-caused increase in the internal resistance by means of a corresponding increase in the reference voltage. 
     A very economical and simple embodiment is advantageously obtained if the voltage regulator is embodied an operational amplifier, whose inverting input is connected to the center pickup of the voltage divider and whose noninverting input is connected to a center contact of a further voltage divider, and the further voltage divider is located between a positive pole of the rechargeable energy storage unit and an electrical ground and furnishes an actual signal of the voltage of the rechargeable energy storage unit. 
     The current detection is effected by means of a shunt that can be connected to the rechargeable energy storage unit. Advantageously, the shunt is connected on one side to a node point, at which both an electrical ground and a ground contact of the rechargeable energy storage unit are located, and is connected on the other side to the current detector. This shunt can also be used for the compensation, so that this task need not be taken on by an additional resistor in the charging current circuit. 
     The defined internal resistance includes at least the various resistances of the relevant conductor tracks of a printed circuit board, wiring that may be present, the contacts for making contact between the rechargeable energy storage unit and the charger, and/or a disconnection device for disconnecting the rechargeable energy storage unit from the charger, respectively. If in addition the ohmic resistance of the individual cells used in the rechargeable energy storage unit and of the corresponding contacts and the cabling is known, then the defined internal resistance can also include that resistance for the sake of compensation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described below in form of examples in conjunction with  FIGS. 1 and 2 , and in the drawings, the same reference numerals indicate the same components with the same mode of operation. The drawing figures, their description, and the claims include numerous characteristics in combination. One skilled in the art will also consider these characteristics individually and put them together to make further useful combinations. In particular, one skilled in the art will also put the characteristics of different exemplary embodiments together to make further useful combinations. 
       In the drawings: 
         FIG. 1  shows a first block circuit diagram of the charger of the invention; and 
         FIG. 2  shows a second, detailed block circuit diagram of the compensation circuit, according to the invention, of the charger. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1 , a first block circuit diagram is shown of a charger  10  of the invention, for charging a rechargeable energy storage unit  12  that is connected to the charger  10  via a first and a second contact  14  and  16 , respectively. This connection may be made for instance by means of a plug contact, not shown in detail, which is known to one skilled in the art. 
     The charger  10  has a charging circuit  18 , which among other elements includes an energy source, not shown, that can be connected to a power network and the electronics required for the charging operation of the rechargeable energy storage unit  12 . Since the construction of the charging circuit  18  may vary for different types of energy storage units and for different requirements made of the charging operation, and is furthermore of subordinate importance to the invention, a detailed description will be dispensed with. 
     The two contacts  14  and  16  are connected to the charging circuit  18  via a power supply line  20  and a ground line  22  that is at an electrical ground GND. Reference numerals  24 ,  26  and  28 , respectively, identify the resistances of the following: the conductor tracks, relevant to the charging operation, of a printed circuit board, not shown; a disconnection device for disconnecting the rechargeable energy storage unit  12  from the charger  10 ; and wiring, which may be present, that connects the contacts  14 ,  16  to the printed circuit board. A relay, semiconductor, or the like may be used in a known manner as the disconnection device. The resistances  24 ,  26  and  28  together form an internal resistance  30 , which leads to a voltage drop dependent on the charging current I. This voltage drop on the one hand affects the fill level of the rechargeable energy storage unit  12 , or in other words its maximum charging voltage, and on the other lengthens the charging operation. 
     To shorten the charging operation, according to the invention a compensation circuit  32  is provided in the charger  10 ; it includes a current detector  34  for detecting the charging current I of the rechargeable energy storage unit  12  and a voltage regulator  36  for adapting the charging voltage U to the detected charging current I. The voltage regulator  36  is connected on one side to the power supply line  20  and on the other to the current detector  34 , and the current detector  34  is at least partially integrated with the ground line  22 . Via a connecting line  38 , the voltage regulator  36  has an influence on the charging circuit  18  such that the charging voltage U is increased as a function of the charging current I by the amount of the voltage drop generated by the internal resistance  30 , thus shortening the charging time accordingly. 
       FIG. 2  shows a detailed block circuit diagram of the compensation circuit  32  of the invention, which as in  FIG. 1  comprises the current detector  34  and the voltage regulator  36 . The voltage regulator  36  includes an operational amplifier  40 , embodied as a current regulator, at the noninverting input of which an actual signal is present in the form of the charging voltage U, reduced by the corresponding resistance ratio by means of a voltage divider  46  that comprises the resistors  42  and  44 . This charging voltage is picked up at the power supply line  20 , by way of which the rechargeable energy storage unit  12  is charged by means of the first contact  14 . The charging voltage U is the result of the difference in potential between the first contact  14  and the second contact  16  connected with via the ground line  22  and the electrical ground GND. Via a resistor  48 , a reference voltage U Ref  is supplied to the inverting inputs of the operational amplifier  40  and is generated by a Zener diode  50  located at an auxiliary voltage U H . It is understood that instead of the Zener diode  50 , other suitable components for generating a reference voltage, such as operational amplifiers or the like, may be employed. 
     The current detector  34  includes a further operational amplifier  52 , wired as an inverting amplifier, to the inverting input of which the voltage drop, which is negative compared to the electrical ground GND, of a shunt  54  disposed in the ground line  22  is applied. This negative voltage drop is amplified by the amplification factor adjusted by means of resistors  56  and  58 , and is added to an offset voltage U Os  that is present at the noninverting input of the further operational amplifier  52 . The offset voltage U Os  results at a center pickup of a voltage divider  64 , which is formed of resistors  60  and  62  and is connected on one side to the reference voltage U Ref  and on the other to the electrical ground GND. The resistors  60  and  62  of the voltage divider  64  are dimensioned such that in the currentless case, a voltage at the level of the reference voltage U Ref  results at the output  68  of the further operational amplifier  52 . 
     The output signal of the further operational amplifier  52  is now fed via a resistor  70  to the voltage regulator  36 , and the resistors  48  and  70  form a voltage divider  72 , whose center pickup  74  is connected to the inverting input of the operational amplifier  40 . As soon as a charging current I flows via the shunt  54 , the voltage at the output of the further operational amplifier  52  is higher than the reference voltage U Ref . In accordance with the divider ratio of the two resistors  48  and  70  of the voltage divider  72 , the voltage at the inverting input of the operational amplifier  40  is higher than the reference voltage U Ref . Conversely, if no charging current I is flowing, the voltage at the inverting input of the operational amplifier  40  is equivalent to the reference voltage U Ref . The magnitude of the influence of the current detector  34  on the voltage regulator  36  is defined by the resistor  70  and depends on the magnitude of the internal resistance  30  ( FIG. 1 ) that has to be compensated for. The current detector  34  moreover influences the voltage regulator  36  via the voltage divider  72  in such a way that, as a function of the charging current I detected, the reference voltage U Ref  for the voltage regulator  36  and accordingly the charging voltage U as well are varied. To that end, with reference to  FIG. 1 , the output of the operational amplifier  40  is connected to the charging circuit  18  via the connecting line  38 . 
     In an alternative feature, besides the internal resistance  30  of the charger  10 , formed of the resistors  24 ,  26  and  28 , the ohmic resistance  76  of the rechargeable energy storage unit  12  is taken into account as well. The ohmic resistance  76  is the result of the resistances of the individual cells used in the rechargeable energy storage unit  12  as well as of the corresponding contacts and the requisite connections. For compensation, it is necessary that the ohmic resistance  76  be known, so that the voltage divider  72  can be adapted to the resultant total internal resistance  30 . 
     In a further alternative feature, it is provided that the reference voltage U Ref  can be influenced by means of a temperature sensor. The resistor  70  can then be replaced by an NTC  78 , for instance. Alternatively, still other components with a corresponding negative temperature coefficient may be used. The use of the NTC  78  is advantageous because the internal resistance  30  of the charger  10  also varies as a function of the temperature. It is thus possible for instance to compensate for a temperature-caused increase in the internal resistance  30  by means of a corresponding increase in the reference voltage U Ref . 
     In conclusion, it should also be pointed out that the exemplary embodiments shown are limited neither to  FIGS. 1 and 2  nor to the electrical components described. For instance, it is conceivable for the circuit shown to be realized either completely or in part in the form of an integrated circuit. Moreover, there is no need to give exact values for the resistances, because among other factors, they depend on the types of rechargeable energy storage units used and on the peripheral values of the charger  10  (maximum charging voltage U, maximum charging current I). One skilled in the art will be capable of selecting these values to suit the field of use. 
     In addition, both the charger  10  of the invention and its compensation circuit  32  offer the advantage that their use can be considered for arbitrary charging voltages and charging currents (the compensation functions for example with charging currents between 0.1 A and 100 A) and for the most various types of rechargeable energy storage units, such as lithium-ion, nickel-hydride, nickel-cadmium, lead, and so forth. The use for rechargeable energy storage units of electrical power tools would be conceivable, for instance, and the invention is not limited to that. 
     The foregoing relates to the preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.