Abstract:
A method for determining the momentary capacity of a battery cell of a handheld power tool, including the following steps: measuring a first open-circuit voltage of the battery cell; determining the state of charge of the battery cell at the measured first open-circuit voltage as a function of a predetermined ratio of the open-circuit voltage to the state of charge of the battery cell; changing the charge stored in the battery cell in order to provide a changed state of charge; measuring a second open-circuit voltage at an actual value of the changed state of charge, and calculating the momentary capacity of the battery cell as a function of a nominal capacity of the battery cell, as a function of a target value of the changed state of charge, and as a function of the measured second open-circuit voltage.

Description:
[0001]    The present invention relates to a diagnostic method and to a diagnostic device for determining the momentary capacity of a battery cell for a battery-operated handheld power tool, and it also relates to a battery-operated handheld power tool, especially an electric handheld power tool such as, for example, an electric screwdriver, a handheld power drill. 
       BACKGROUND 
       [0002]    In the conventional diagnosis of a battery cell such as, for instance, a rechargeable battery or an accumulator, the battery cell is completely charged and subsequently completely discharged in order to determine the momentary capacity. Therefore, conventional methods for diagnosing battery cells require at least a complete charging and subsequent discharging of the battery. In this context, the maximum possible charging current as well as the maximum possible discharging current are limited by the battery cell. This translates into a time-consuming procedure for diagnosing the battery cell. 
         [0003]    Here, a value of about 1 C (simple, one-hour charging current) is typical, whereby the battery cell is charged within one hour. The discharging can be regularly carried out at a multiple of 1 C, whereby the discharging time is shortened accordingly. At 4 C, for instance, the battery cell is discharged within 15 minutes. All in all, these conventional methods can give rise to a diagnosis time of more than one hour. High charging and discharging currents of, for example, more than 1 C, are normally avoided since they cause the battery to age prematurely. 
         [0004]    Moreover, a disadvantage is the need for complex power electronics that are designed to appropriately charge and discharge the battery cell. 
       SUMMARY OF THE INVENTION 
       [0005]    The method according to the present invention for determining the momentary capacity of a battery cell of a handheld power tool encompasses the following steps: measuring a first open-circuit voltage of the battery cell; determining the state of charge of the battery cell at the measured first open-circuit voltage as a function of a predetermined ratio of the open-circuit voltage to the state of charge of the battery cell; changing the charge stored in the battery cell in order to provide a changed state of charge; measuring a second open-circuit voltage at an actual value of the changed state of charge, and calculating the momentary capacity of the battery cell as a function of a nominal capacity of the battery cell, as a function of a target value of the changed state of charge, said target value having been determined on the basis of the determined state of charge, and as a function of the measured second open-circuit voltage. 
         [0006]    The target value of the changed state of charge is especially determined as a function of the determined state of charge at the measured first open-circuit voltage and at the degree of change of the charge stored in the battery cell. 
         [0007]    According to the invention, it is not necessary to completely charge and to subsequently completely discharge the battery cell in order to determine its momentary capacity. This reduces or minimizes the time needed to determine the momentary capacity. As a result, the diagnosis time is reduced to a minimum according to the invention. Here, diagnosis times of less than five minutes are possible. 
         [0008]    Moreover, according to the invention, there is no need for the conventional use of complex power electronics. For instance, discharging takes place via the usual consumers of the handheld power tool, for example, via the motor of the handheld power tool. In particular, for this purpose, the handheld power tool can be switched on for a predetermined period of time, for instance, one minute. This markedly reduces the technical resources needed by the present invention in comparison to conventional diagnostic methods. 
         [0009]    In one embodiment, a target open-circuit voltage is determined at a target value of the changed state of charge as a function of the predetermined ratio. Then the momentary capacity of the battery cell can be calculated as a function of the nominal capacity of the battery cell, as a function of the determined target open-circuit voltage and as a function of the measured second open-circuit voltage. 
         [0010]    In another embodiment, the calculated momentary capacity is stored in a memory associated with the battery cell or with the handheld power tool. The memory, for example, an EEPROM, is suitable for storing information. 
         [0011]    In another embodiment, the calculated momentary capacity is provided by means of the memory to at least one device that has been authenticated vis-à-vis the battery cell. 
         [0012]    The authenticated device is, for instance, a receiving device associated with the user of the handheld power tool or a receiving device associated with a customer service. 
         [0013]    In another embodiment, the charge stored in the battery cell is changed by discharging the battery cell or by charging the battery cell. 
         [0014]    In this context, the charge of the battery cell is changed by a charge quantity that, at the maximum, amounts to 10% of the nominal capacity of the battery cell. 
         [0015]    In another embodiment, the predetermined ratio of the open-circuit voltage to the state of charge of the battery cell is provided by measuring the battery cell prior to its use in the handheld power tool. 
         [0016]    Consequently, prior to being used in the handheld power tool, the battery cell is measured in order to generate the predetermined ratio of the open-circuit voltage to the state of charge. The momentary capacity can be determined every time as a function of this predetermined ratio, which is valid during the entire operating time of the battery cell. 
         [0017]    In another embodiment, the predetermined ratio of the open-circuit voltage (y), expressed in volts, to the state of charge (x), expressed as a percentage, of the battery cell is a polynomial of the n th  order, wherein n=3 or n=5. For example, for n=5: y=ax 5 +bx 4 +cx 3 +dx 2 +ex+f. 
         [0018]    In the case of a lithium-ion battery with five cells and a nominal capacity of 2240 mAh, the following polynomial is obtained: y=(2E−09)x 5 −(5E−07)x 4 +(6E−05)x 3 −0029x 2 +0.0777x+13.589. 
         [0019]    In another embodiment, the predetermined ratio of the open-circuit voltage to the state of charge of the battery cell is stored in a look-up table (LUT) that is stored in a memory that is associated with the battery cell or with the handheld power tool. 
         [0020]    In yet another embodiment, the equation 
         [0000]    
       
         
           
             
               C 
               momentary 
             
             = 
             
               
                 C 
                 new 
               
               · 
               
                 
                   Δ 
                    
                   
                       
                   
                    
                   
                     U 
                     target 
                   
                 
                 
                   Δ 
                    
                   
                       
                   
                    
                   
                     U 
                     actual 
                   
                 
               
             
           
         
       
     
         [0000]    is employed for purposes of calculating the momentary capacity. C momentary , refers to the momentary capacity of the battery cell. C new  refers to the nominal capacity of the battery cell. ΔU target  refers to the voltage differential between the measured first open-circuit voltage OCV 1  and the determined target open-circuit voltage U target . ΔU actual  refers to the voltage differential between the measured first open-circuit voltage OCV 1  and the measured second open-circuit voltage OCV 2 . 
         [0021]    In another embodiment, the equation 
         [0000]    
       
         
           
             
               C 
               momentary 
             
             = 
             
               
                 C 
                 new 
               
               · 
               
                 
                   Δ 
                    
                   
                       
                   
                    
                   
                     C 
                     actual 
                   
                 
                 
                   Δ 
                    
                   
                       
                   
                    
                   
                     C 
                     target 
                   
                 
               
             
           
         
       
     
         [0000]    is employed for purposes of calculating the momentary capacity. C momentary  refers to the momentary capacity of the battery cell. C new  refers to the nominal capacity of the battery cell. ΔC target  refers to the difference between the state of charge SOC 1  of the battery cell at the measured first open-circuit voltage OCV 1  and the target value C target  of the changed state of charge. ΔC actual  refers to the difference between the state of charge SOC 1  of the battery cell at the measured first open-circuit voltage OCV 1  and the actual value C actual  of the changed state of charge. 
         [0022]    In another embodiment, the actual value C actual  of the changed state of charge at the measured second open-circuit voltage OCV 2  is determined as a function of the predetermined ratio. 
         [0023]    Consequently, the equation above can also be formulated as follows: 
         [0000]    
       
         
           
             
               C 
               momentary 
             
             = 
             
               
                 
                   C 
                   new 
                 
                 · 
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       C 
                       actual 
                     
                   
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       C 
                       target 
                     
                   
                 
               
               = 
               
                 
                   C 
                   new 
                 
                 · 
                 
                   
                     
                       SOC 
                        
                       
                           
                       
                        
                       1 
                     
                     - 
                     
                       C 
                       actual 
                     
                   
                   
                     
                       SOC 
                        
                       
                           
                       
                        
                       1 
                     
                     - 
                     
                       C 
                       target 
                     
                   
                 
               
             
           
         
       
     
         [0024]    In another embodiment, the actual value C actual  of the changed state of charge at the measured second open-circuit voltage OCV 2  is determined as a function of the charge quantity Q by which the charge stored in the battery cell is changed. 
         [0000]    
       
         
           
             
               Δ 
                
               
                   
               
                
               
                 C 
                 actual 
               
             
             = 
             
               
                 Q 
                 
                   Δ 
                    
                   
                       
                   
                    
                   
                     U 
                     actual 
                   
                 
               
               = 
               
                 Q 
                 
                   
                     OCV 
                      
                     
                         
                     
                      
                     1 
                   
                   - 
                   
                     OCV 
                      
                     
                         
                     
                      
                     2 
                   
                 
               
             
           
         
       
     
         [0025]    In this context, the current, for example, in the battery charger, is measured during the change of the state of charge and integrated in the microcontroller of the battery charger. This yields the charge quantity Q by which the charge stored in the battery cell is changed. In this case, the following equation can be employed to calculate the momentary capacity of the battery cell: 
         [0000]    
       
         
           
             
               C 
               momentary 
             
             = 
             
               
                 
                   C 
                   new 
                 
                 
                   Δ 
                    
                   
                       
                   
                    
                   
                     C 
                     target 
                   
                 
               
               = 
               
                 Q 
                 
                   
                     OCV 
                      
                     
                         
                     
                      
                     1 
                   
                   - 
                   
                     OCV 
                      
                     
                         
                     
                      
                     2 
                   
                 
               
             
           
         
       
     
         [0026]    In this embodiment as well, there are two ways to change the stored charge in the battery cell in order to provide the changed state of charge, namely, charging and thus diagnosis during the charging procedure, or else discharging and thus diagnosis during the discharging procedure. Examples of the diagnosis during the charging procedure and of the diagnosis during the discharging procedure will be given below: 
         [0027]    Diagnosis during the charging procedure 
         [0028]    The predetermined ratio of the open-circuit voltage OCV to the state of charge SOC is stored in the memory of the battery cell. Prior to the start of the charging procedure, the microcontroller of the battery cell measures the momentary open-circuit voltage OCV 1 . The ascertained OCV 1  value is stored in the memory of the microcontroller. On the basis of the table, the state of charge SOC 1  of the battery cell associated with the open-circuit voltage OCV 1  is ascertained. In this context, it is assumed that the ratio of SOC to OCV is not dependent on the capacity. During the charging procedure, the current, for example, in the battery charger, is measured and integrated in the microcontroller of the battery charger. The charged capacity ΔC actual  is results from the difference between the state of charge SOC 1  of the battery cell at the first open-circuit voltage OCV 1  and the actual value C actual  of the changed state of charge. After the charging procedure, the battery charger sends the ΔC actual  value to the microcontroller of the battery cell. After the charging procedure has been completed, the microcontroller of the battery cell measures the open-circuit voltage OCV 2 . Once the OCV 2  value no longer changes, the appertaining state of charge SOC 2  of the battery cell at this second open-circuit voltage OCV 2  is ascertained. 
         [0029]    The following algorithm can be employed for purposes of determining the momentary capacity C momentary : 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       C 
                       target 
                     
                   
                   = 
                   
                     
                       C 
                       new 
                     
                     · 
                     
                       ( 
                       
                         
                           SOC 
                            
                           
                               
                           
                            
                           2 
                         
                         - 
                         
                           SOC 
                            
                           
                               
                           
                            
                           1 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   SOH 
                   = 
                   
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         C 
                         actual 
                       
                     
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         C 
                         target 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   
                     C 
                     momentary 
                   
                   = 
                   
                     SOH 
                     · 
                     
                       C 
                       new 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0030]    SOH and C momentary  can be stored in the memory. Particularly when a completed charging procedure is carried out with a charging start at SOC&lt;5% and a charging end at SOC&gt;95%, the actually charged ΔC actual  value can be additionally stored in the memory of battery cell. 
         [0000]    Diagnosis during the discharging procedure 
         [0031]    The table containing the predetermined ratio of the open-circuit voltage OCV to the state of charge SOC are stored in the memory of the battery cell. When the battery is discharged for the first time after a charging procedure, the microcontroller of the battery cell measures the momentary open-circuit voltage OCV 1 . The ascertained OCV 1  value is stored in the memory of the microcontroller. During the discharging procedure, the current in the handheld power tool is measured and integrated in the microcontroller of the handheld power tool. This value corresponds to ΔC actual . After each discharging procedure, the ΔC actual  value can be transmitted to the microcontroller of the battery cell and stored in its memory. In each case, the ΔC actual  value transmitted by the handheld power tool is added to the C actual  value that is already present, and this is stored in the memory. 
         [0032]    The following algorithm can be employed for purposes of determining the momentary capacity: 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       C 
                       target 
                     
                   
                   = 
                   
                     
                       C 
                       new 
                     
                     · 
                     
                       ( 
                       
                         
                           SOC 
                            
                           
                               
                           
                            
                           1 
                         
                         - 
                         
                           SOC 
                            
                           
                               
                           
                            
                           2 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   SOH 
                   = 
                   
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         C 
                         actual 
                       
                     
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         C 
                         target 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
             
               
                 
                   
                     C 
                     momentary 
                   
                   = 
                   
                     SOH 
                     · 
                     
                       C 
                       new 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0033]    In another embodiment, in order to stabilize the state of charge of the battery cell, there is a predetermined time period between changing the charge stored in the battery cell and measuring the second open-circuit voltage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    The description below explains the invention on the basis of embodiments and figures provided by way of examples. The figures show the following: 
           [0035]      FIG. 1 : a schematic flow chart of a method for determining the momentary capacity of a battery cell of a handheld power tool; 
           [0036]      FIG. 2 : a diagram that depicts the open-circuit voltage of a battery cell as a function of its state of charge; 
           [0037]      FIG. 3 : a table for storing the predetermined ratio of the open-circuit voltage to the state of charge of the battery cell; 
           [0038]      FIG. 4 : an electric screwdriver; and 
           [0039]      FIG. 5 : a battery charger for an electric screwdriver. 
       
    
    
       [0040]    Unless otherwise indicated, identical or functionally equivalent elements are designated in the figures by the same reference numerals. 
       DETAILED DESCRIPTION 
       [0041]      FIG. 1  shows a schematic flow chart of a method for determining the momentary capacity of a battery cell of a handheld power tool. The battery cell  11  is, for instance, a rechargeable battery, especially a battery pack  10 . The handheld power tool is, for example, an electric screwdriver. 
         [0042]    In step S 1 , an open-circuit voltage OCV 1  of the battery cell  11  is measured. 
         [0043]    In step S 2 , the state of charge SOC 1  of the battery cell  11  at the measured first open-circuit voltage OCV 1  is determined. The predetermined ratio of the open-circuit voltage OCV to the state of charge SOC of the battery cell is used for this determination. The predetermined ratio of the open-circuit voltage OCV to the state of charge SOC is generated by measuring the battery cell  11  prior to its use in the handheld power tool  1 . This predetermined ratio is stored in a look-up table that is stored in a memory  26  associated with the battery cell or the handheld power tool. 
         [0044]    The applicant has ascertained that the ratio of the open-circuit voltage OCV to the state of charge SOC of the battery cell remains essentially the same over its service life and is thus predetermined. 
         [0045]    In this regard,  FIG. 2  shows a diagram that depicts the open-circuit voltage OCV of a battery cell as a function of its state of charge SOC.  FIG. 2  depicts two curves K 1  and K 2 . Curve K 1  shows the open-circuit voltage OCV of a new battery cell, whereas curve K 2  shows the open-circuit voltage OCV after 950 charging cycles. All in all,  FIG. 2  illustrates that the curves K 1  and K 2 —except for the negligible area in which the SOC is below 5%—coincide or at least largely coincide, and thus the ratio of the open-circuit voltage OCV to the state of charge SOC is predetermined. 
         [0046]    In step S 3 , the charge stored in the battery cell  11  is changed for purposes of providing a changed state of charge. The battery cell  11  can be charged or discharged in order to change the stored charge. 
         [0047]    In step S 4 , the target open-circuit voltage U target  at a target value C target  of the changed state of charge is determined as a function of the predetermined ratio. 
         [0048]    In step S 5 , a second open-circuit voltage OCV 2  at an actual value C actual  of the changed state of charge is measured. 
         [0049]    In step S 6 , the momentary capacity C momentary  of the battery cell is calculated as a function of a nominal capacity C new  of the battery cell, as a function of the determined target open-circuit voltage U target  and as a function of the measured second open-circuit voltage OCV 2 . The calculated momentary capacity C momentary  can be stored in a memory associated with the battery cell or with the handheld power tool. The calculated momentary capacity C momentary  is provided by means of this memory to at least one device that has been authenticated vis-à-vis the battery cell. 
         [0050]    An example of the calculation of the momentary capacity C momentary  is given below making reference to  FIG. 3 , In this context,  FIG. 3  shows a table for storing the predetermined ratio of the open-circuit voltage OCV to the state of charge SOC of the battery cell of a Panasonic B144 battery. 
         [0051]    The B144 battery has a nominal capacity of 2240 mAh (C new =2240 mAh). 
         [0052]    In step S 1 , the first the open-circuit voltage OCV 1  is measured (OCV 1 =15825 mV). In step  2 , the table of  FIG. 3  is employed to ascertain the state of charge SOC 1  at the first open-circuit voltage OCV 1  (SOC 1 =88%). In step S 3 , the state of charge of the B144 battery is changed in that it is discharged (ΔSOC=12%). Thus, the target value C target  of the changed state of charge is obtained from C target =SOC 1 −ΔSOC=88%−12%=76%. 
         [0053]    In step S 4 , the target open-circuit voltage U target  at the target value C target  of the changed state of charge is determined on the basis of the table from  FIG. 3  (U target =15355 mV). Subsequently, the second open-circuit voltage OCV 2  at the a priori unknown actual value of the changed state of charge is measured (OCV 2 =15255 mV). 
         [0054]    Consequently, C momentary  results as follows: 
         [0000]    
       
         
           
             
               C 
               momentary 
             
             = 
             
               
                 
                   C 
                   new 
                 
                 · 
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       U 
                       target 
                     
                   
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       U 
                       actual 
                     
                   
                 
               
               = 
               
                 
                   
                     C 
                     new 
                   
                   · 
                   
                     
                       
                         OCV 
                          
                         
                             
                         
                          
                         1 
                       
                       - 
                       
                         U 
                         target 
                       
                     
                     
                       
                         OCV 
                          
                         
                             
                         
                          
                         1 
                       
                       - 
                       
                         OCV 
                          
                         
                             
                         
                          
                         2 
                       
                     
                   
                 
                 = 
                 
                   
                     2240 
                      
                     
                         
                     
                      
                     
                       mAh 
                       · 
                       
                         
                           
                             15825 
                              
                             
                                 
                             
                              
                             mV 
                           
                           - 
                           
                             15355 
                              
                             
                                 
                             
                              
                             mV 
                           
                         
                         
                           
                             15825 
                              
                             
                                 
                             
                              
                             mV 
                           
                           - 
                           
                             15255 
                              
                             
                                 
                             
                              
                             mV 
                           
                         
                       
                     
                   
                   = 
                   
                     1847 
                      
                     
                         
                     
                      
                     mAh 
                   
                 
               
             
           
         
       
     
         [0055]    At C momentary =1847 mAh, an actual value C momentary  of the changed state of charge of 74% (C momentary =74%) is obtained. 
         [0056]      FIG. 4  shows an electric screwdriver  1  as an example of a handheld power tool. The electric screwdriver  1  has housing  2  with a handle  3  by means of which a user can hold and guide the electric screwdriver  1 . A pushbutton  4  on the handle  3  allows the user to operate the electric screwdriver  1 . Typically, the user has to continuously hold the pushbutton  4  depressed in order to keep the electric screwdriver  1  in operation. 
         [0057]    The electric screwdriver  1  has a tool socket  5  into which the user can insert a screwdriver bit  6 . When the pushbutton  4  is actuated, an electric motor  7  turns the tool socket  5  around its axis  8 . The electric motor  7  is coupled to the tool socket  5  via a spindle  9  and optionally by other components of a drive train, e.g. clutch, gears. 
         [0058]    The electric motor  7  is supplied with current by means of a battery cell  11 . The battery cell is, for instance, part of a battery pack  10 . The battery pack  10  especially has a plurality of secondary battery cells  11  which are based on lithium chemistry. 
         [0059]    The housing  2  has a holder  12  for the battery pack  10  which is arranged, for example, on one end of the handle  3 . The holder  12  can have rails with an L-shaped profile into which complementary rails on the battery pack  10  can be slid and inserted. A detachable locking element  13  prevents the battery pack  10  from falling out of the holder  12 . A power connector  14  of the handheld power tool  1  is arranged in the holder  12 . The power connector  14  comprises, for example, two or more electric contacts  15 . The battery pack  10  has contacts  16  that are complementary to the power connector  14  of the handheld power tool  1  and that make electrical contact when a battery pack  10  has been inserted into the holder  12 . 
         [0060]    The battery pack  10  can have an autonomous protection mechanism  17 . The protection mechanism  17  comprises, for instance, a voltage sensor  18  that monitors the voltages of the individual battery cells  11 . Whenever the protection mechanism  17  detects a drop in the voltage of one of the battery cells  11  below a critical threshold value, the current output of the battery pack  10  is interrupted. The critical threshold value is selected in such a way that an irreversible discharging of the battery cells  11  is prevented. The threshold value, for instance, of batteries  11  that are based on lithium-ion chemistry is approximately 2.5 V, especially at room temperature. The battery pack  10  can interrupt a current path  20  between the battery pack  10  and the electric motor  7 , for example, by means of a switch  19 , e.g. a FET in the battery pack  10  or in the handheld power tool  1 . The reversible protection mechanism  17  and the associated switch  19  are independent of other systems. With an arrangement of the switch  19  in the battery pack  10 , this is particularly the case when the power supply to the handheld power tool  1  by means of the battery pack  10  is completely interrupted. 
         [0061]    The handheld power tool  1  also has a motor control unit  21  that has one or more switching elements  22  and that sets the power consumption of the handheld power tool  1  in order to regulate the rotational speed to the target value. Moreover, the handheld power tool  1  has a soft starter  23 . 
         [0062]    The motor control unit  21  communicates with the battery pack  10  in order to ascertain its properties. A communication interface  24  of the motor control unit  21  queries, among other things, the inner resistance of the battery pack  19 . 
         [0063]    The communication interface  24  is preferably an electric communication interface whose receiving unit receives from the battery pack  10  information units that are transmitted as electric signals from a memory module  26 . The memory module  26  stores the ratio of the open-circuit voltage to the state of charge of the battery cell  11 . The motor control unit  21  is configured to carry out the method according to  FIG. 1  and thus to diagnose the battery pack  10 . Moreover, the handheld power tool  1  can have a temperature sensor  25 . 
         [0064]      FIG. 5  shows a battery charger  27  for an electric screwdriver  1 . The electric screwdriver  1  is shown by way of an example in  FIG. 4 . The battery charger  27  has a dock to accommodate the battery pack  10  of the electric screwdriver  1  during the charging procedure. The battery charger  27  also has a device  21  to diagnose the battery cells  11  of the battery pack  10 . The device  21  is especially configured to carry out the method according to  FIG. 1 . Moreover, the battery charger  27  has a memory module  26  that stores the ratio of the open-circuit voltage OCV to the state of charge SOC of the battery cell  11 . 
         [0065]    Glossary 
         [0066]    C momentary  momentary capacity of the battery cell 
         [0067]    C actual  actual value of the changed state of charge 
         [0068]    ΔC actual  difference between the state of charge of the battery cell at the measured first open-circuit voltage and the actual value of the changed state of charge 
         [0069]    C new  nominal capacity of the battery cell 
         [0070]    C target  target value of the changed state of charge 
         [0071]    ΔC target  difference between the state of charge of the battery cell at the measured first open-circuit voltage and the target value of the changed state of charge 
         [0072]    OCV 1  first open-circuit voltage 
         [0073]    OCV 2  second open-circuit voltage 
         [0074]    Q charge quantity 
         [0075]    SOC 1  state of charge of the battery cell at the first open-circuit voltage 
         [0076]    SOC 2  state of charge of the battery cell at the second open-circuit voltage 
         [0077]    SOH momentary state of health of the battery cell 
         [0078]    ΔU target  differential voltage between the measured first open-circuit voltage and the determined target open-circuit voltage 
         [0079]    ΔU actual  differential voltage between the measured first open-circuit voltage and the measured second open-circuit voltage 
         [0080]    U meas2  measured second open-circuit voltage at the actual value of the changed state of charge 
         [0081]    U target  target open-circuit voltage at the actual value of the changed state of charge