Abstract:
For medical implants it is not only important to have a battery powering the implant with a long run-time, but to know exactly at any point in time how long the remaining run-time is. On the other hand the implant together with its battery shall be as user-friendly as possible requiring no or hardly any interaction with the device except for recharging of the battery. To better address these concerns, a method for determining the state of charge of a battery is suggested comprising the steps: charging the battery, discharging of the battery, predicting the state of charge of the battery using a recharge prediction unit, wherein the recharge prediction unit is disconnected from the battery during the discharging of the battery. The prediction of the state of charge of the battery during the discharge of the battery, i.e. during powering of a device connected to the battery, without carrying out any measurements during the discharging of the battery, enables a local separation of the battery or the implant powered by the battery and the recharge prediction unit during the discharge cycle of the battery.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the field of methods and devices for determining the state of charge of a battery. In particular the invention relates to a method and a device for determining the state of charge of a battery used for powering a medical device implanted into a human or animal body. 
       BACKGROUND OF THE INVENTION 
       [0002]    U.S. Pat. No. 6,901,293 B2 discloses a power source longevity monitor for an implantable medical device. An energy counter counts the amount of energy used by the implantable medical device. An energy converter converts the energy used into an estimate of remaining power source longevity and generates an energy longevity estimate. A voltage monitor monitors the voltage of the power source. A voltage converter converts the voltage monitored by the voltage monitor into an estimate of remaining longevity of the power source and generating a voltage longevity estimate. A calculator is operatively coupled to the energy converter and to the voltage converter and predicts the power source longevity using the energy longevity estimate early in the useful life of the power source and using the voltage longevity estimate later in the useful life of the power source. The functionality of the device disclosed in U.S. Pat. No. 6,901,293 B2 is based on an operative coupling between the power source longevity monitor and the power source and therapeutic delivery device, respectively. The prediction of the remaining power source longevity relies on measurements of the implant&#39;s energy consumption as wells as the battery voltage during discharge of the battery. 
       SUMMARY OF THE INVENTION 
       [0003]    It would be advantageous to provide a method and a device for determining the state of charge of a battery which requires little action by the user of the battery. 
         [0004]    It would also be desirable to keep the complexity of the battery itself or the system powered by the battery, which is possibly implanted into the patient&#39;s body, low. 
         [0005]    To better address one or more of these concerns, in a first aspect of the invention a method for determining the state of charge (SoC) of a battery is provided comprising the steps: charging the battery, discharging of the battery, predicting the state of charge of the battery using a recharge prediction unit, wherein the recharge prediction unit is disconnected from the battery during the discharging of the battery. The prediction of the state of charge of the battery during the discharge of the battery, i.e. during powering of a device connected to the battery, without carrying out any measurements during the discharging of the battery, enables a local separation of the battery or the device powered by the battery, i.e. a medical implant, and the recharge prediction unit (RPU) during the discharge cycle of the battery. 
         [0006]    In terms of the present application “disconnected” shall mean that no connection, including wired or wireless communication, for providing energy or signal transmission between the RPU and the battery is established. 
         [0007]    Although in an embodiment of the present invention the battery and the RPU shall be disconnected during the entire discharging of the battery, there may be alternative embodiments, in which the battery and the RPU may remain connected shortly after charging or shortly before charging the battery. For example when charging a medical implant one could leave the battery and the RPU connected for a certain period of time after the charging has been completed and discharging of the battery commences. If compared to the overall run-time of the freshly charged battery keeping the connection between the battery and the RPU upright shortly after charging still implies that the battery and the RPU are disconnected during almost the complete discharging process. 
         [0008]    In an embodiment of the present invention the state of charge of the battery and therefore the remaining run-time of the battery until a recharge would be necessary, is calculated based on an accurate model of the discharging process that is based on battery status and drainage characteristic of the device powered by the battery, i.e. the device&#39;s power consumption. 
         [0009]    In a further embodiment the model takes into account the aging of the rechargeable battery for which the remaining run-time shall be predicted. This helps to consider effects due to a battery changing its capacity over its overall life time. 
         [0010]    In an embodiment of the present invention the model used for predicting the state of charge of the battery may be based on an assumption of the amount of charge available from the battery directly after recharging. This overall amount of charge available from the battery could either be measured as laid out in detail below or based on a model without measurements assuming the general behavior of the battery. 
         [0011]    In a further embodiment of the present invention the step of predicting the state of charge of the battery is carried out using a parameter indicating a period of time having elapsed since recharging of a battery was finished. This consideration of the time having elapsed since recharging allows an exact prediction of the state of charge of the battery, i.e. the remaining run time of the battery during the current discharge cycle. 
         [0012]    An embodiment of the present invention is desirable in which the step of predicting the state of charge of the battery is carried out by using the following parameters alternatively or in any combination thereof: A parameter indicating a characteristic of the battery, e.g. its capacity, its nominal current or its nominal voltage, a parameter indicating environmental parameter of the battery and/or the device powered, e.g. temperature, a parameter indicating the power consumption of the device powered by the battery, the number of operating cycles of the device power, etc. 
         [0013]    In an embodiment of the invention the step of predicting the state of charge of the battery is carried out modeling the aging of the battery. 
         [0014]    In an embodiment the method according to the present invention further comprises the steps: Connecting the recharge prediction unit to the battery before charging, charging the battery, measuring battery parameters during the charging of the battery, disconnecting the recharge prediction unit from the battery before discharging the battery, predicting the state of charge of the battery using the battery parameters measured during charging of the battery. 
         [0015]    In terms of the present application a measurement “during the charging of the battery” means includes intervals directly prior and directly after charging. 
         [0016]    Further to the previously described embodiments this embodiment helps to predict the state of charge of the battery during the discharge cycle more accurately due to the incorporation of parameters measured during the charging cycle of the battery. 
         [0017]    During the charging of the battery the battery must be connected to a battery charger in order to deliver energy to the battery. Therefore during the charging cycle the battery and thus the device connected to the battery must be brought into contact with the charging device in any case. Thus the requirement to bring the recharge prediction unit into contact with the battery does not add any further inconvenience or complexity to the usage of the system comprising the battery, the device powered by the battery, the battery charger and the recharge prediction unit. 
         [0018]    However, a measurement of battery parameters during the charging of a battery allows to accurately determine the state of charge at the end of the charging cycle, i.e. determining the overall amount of charge available from the battery after recharging and furthermore allows to take into account aging effects of the battery not only by modeling, but by detecting the actual state of the battery with respect to its overall lifetime. 
         [0019]    An example of an embodiment of the present invention providing a model for determining the state of charge of a battery is given in the following description. The state of charge and the maximum charge available from a battery are updated during recharging of the battery. To achieve this the following steps are carried out: The state of charge prior to charging is measured, the amount of charge flowing into the battery during the charging cycle is determined, the state of charge is measured after the charging cycle has successfully been completed. 
         [0020]    The state of charge prior to charging can be obtained by measuring the battery voltage when the battery is in a relaxed or stand-by state, operating at very low or no drain currents, i.e. the so called equilibrium voltage value (EMF). From the EMF the state of charge may be calculated using a look-up table. However the battery drain in many practical applications, e.g. medical devices, is so low that in practice the battery operates very near its stand-by-state, so a measured battery voltage directly yields EMF. Thus in embodiments of the present invention EMF and thus SoC si  can be determined directly prior to onset of charging process without any entire relaxation of the battery. 
         [0021]    In an embodiment the state of charge SoC si  before the beginning of a recharging of the battery is determined. 
         [0022]    The amount of charge Q ch  flowing into the battery during the charging period is simply obtained by integrating the charging current flowing into the battery during the charging period. 
         [0023]    In an embodiment of the present invention the EMF is measured again after charging has been completed. This measurement could be carried out after a “transitional” period in which battery overpotential (due to battery charging) relaxes. From this measured EMF the state of charge SoC sf  after charging is estimated. The overall maximum charge capacity Q max  available from the battery after charging could then be calculated from the following equation: 
         [0000]    
       
         
           
             
               Q 
               max 
             
             = 
             
               
                 100 
                 
                   
                     SoC 
                     sf 
                   
                   - 
                   
                     SoC 
                     si 
                   
                 
               
                
               
                 Q 
                 ch 
               
             
           
         
       
     
         [0024]    This approach corrects for battery aging and takes into account remaining battery capacity prior to the charging. The charge available for operation of the device Q av  is calculated from 
         [0000]        Q   av   =Q   max *SoC sf . 
         [0025]    In an embodiment of the present invention the battery is recharged while the device powered by the battery is still operated. Under these circumstances it could be the case that the battery discharge rate during charging is too high to allow a direct determination of the EMF of the battery. Then the method for recharging the battery will be started by replacing the battery as the power source by an external recharger delivering the power needed to operate the device which had previously been powered by the battery. The battery is then put on stand-by while the external power source, i.e. the recharger, continuous to power the device. The battery voltage then relaxes to the EMF. The EMF and thus the state of charge of the battery can then be determined either from the dynamics of relaxation into the relaxed state of the battery or by the end value of the EMF. After relaxation of the battery into the stand-by mode charging of the battery is started as described above. 
         [0026]    In an embodiment of the present invention after charging the battery may be put into stand-by again before determining the EMF. The final state of charge of the battery is then determined before the device is once again powered by the freshly recharged battery. 
         [0027]    In an embodiment the battery voltage and charging current are measured during charging. From the relation between battery voltage and charging current Q max  is derived using a mathematical model of the battery. 
         [0028]    In an embodiment the battery charging is interrupted one or more times to let the battery voltage relax to EMF leading to an intermediate SoC estimate during the charging. By combining the one or more intermediate measurements of the integrated charging current and SoC and the final measurements, more accurate estimates of Q max  using the methodology described above can be obtained. 
         [0029]    In an embodiment of the invention the state of charge determined during the measurements is compared to a corresponding model value and the model for the battery is adaptively updated to the battery used. 
         [0030]    In an embodiment of the present invention the SoC values, Q max  values and recharge time stamps from one or more, or at least the last, recharging sessions are stored. This storage in an embodiment occurs in the battery or a device associated with the battery, such that the storage is disconnected from the RPU during discharging of the battery. 
         [0031]    In an embodiment the measured SoC si  from the current charging session and the stored SoC sf  and Q max  from the previous charging session and the elapsed time between the charging events are used to estimate the average battery drain I drain . 
         [0032]    In a further embodiment the voltage of the battery V bat  is measured at the onset of the discharging of the battery. Then the difference between V bat  and the EMF after charging is determined. This difference is referred to as the overpotential. Typically V bat  will be lower than the EMF. The overpotential is influenced e.g. by the ageing of the battery or by temperature. Thus the overpotential may help to determine the amount of charge that can be withdrawn from the battery under predetermined conditions. The charge that can be withdrawn from the battery is distinct from the available charge in the battery. According to an empirical relationship the State-of-Charge left SoC l  can be calculated at a given C-rate discharge current (C), from the following equation 
         [0000]    
       
         
           
             
               SoC 
               l 
             
             = 
             
               
                 
                   [ 
                   
                     
                       C 
                        
                       
                         ( 
                         
                           
                             SoC 
                             st 
                           
                           100 
                         
                         ) 
                       
                     
                     
                       ζ 
                        
                       
                         ( 
                         
                           ϑ 
                           - 
                           C 
                         
                         ) 
                       
                     
                   
                   ] 
                 
                 
                   γ 
                   + 
                   
                     δ 
                      
                     
                         
                     
                      
                     T 
                   
                 
               
               
                 α 
                 + 
                 
                   β 
                    
                   
                       
                   
                    
                   T 
                 
               
             
           
         
       
     
         [0033]    where SoC st  in % denotes the SoC at the beginning of the discharge at C-rate current C and temperature T in ° C. ζ, ν, γ, δ, α, β are parameters determined empirically by the fitting of experimentally measured SoC l  data. When the SoC l  function described by the above equation is substituted in the following relation 
         [0000]    
       
         
           
             
               
                 t 
                 r 
               
                
               
                 [ 
                 min 
                 ] 
               
             
             = 
             
               
                 0.06 
                  
                 
                   
                     Q 
                     max 
                   
                   100 
                 
                  
                 
                   ( 
                   
                     
                       SoC 
                       d 
                     
                     - 
                     
                       SoC 
                       l 
                     
                   
                   ) 
                 
               
               
                 I 
                 d 
               
             
           
         
       
     
         [0034]    wherein SoC d  and I d  are the state-of-charge and the current immediately after discharging has started, respectively, the remaining run-time t r  at any point in time can be accurately predicted, immediately after discharging has started. The overall maximum charge capacity Q max  available from the battery after charging may be calculated as stated above from the following equation: 
         [0000]    
       
         
           
             
               
                 Q 
                 max 
               
               = 
               
                 
                   100 
                   
                     
                       SoC 
                       sf 
                     
                     - 
                     
                       SoC 
                       si 
                     
                   
                 
                  
                 
                   Q 
                   ch 
                 
               
             
             , 
           
         
       
     
         [0035]    wherein SoC si  and SoC sf  are the state of charge before the beginning of a recharging of the battery and after charging, respectively. Q ch  is the amount of charge flowing into the battery during the charging period, which can be obtained by integrating the charging current flowing into the battery during the charging period. This way the prediction of the remaining run-time t r  according to an embodiment accounts for the overpotential after recharging of the battery and thus for the ageing of the battery. 
         [0036]    In an embodiment of the method according to the present invention the average battery drain is adaptively calculated using stored values from at least one previous charging session. Calculation of the average battery drain is carried in the RPU. 
         [0037]    In an embodiment the device powered by the rechargeable battery or a device associated with the battery contains means to measure average battery drain and means to transmit this information to the RPU. 
         [0038]    The autonomy time T a  of the device powered by the rechargeable battery may be calculated from the available charge in the device, the battery drain, and a critical state of charge level SoC min  where recharging becomes desirable: 
         [0000]        T   a   =Q   max (SoC sf −SoC min )/ I   drain    
         [0039]    The remaining run-time is calculated as the difference between the autonomy time and the time elapsed since the last recharge. 
         [0040]    The remaining run-time may be displayed to the user and a warning signal may be given out alternatively or in addition to the remaining run-time when the remaining run-time drops below a critical pre-stored value. 
         [0041]    In an embodiment the method according to the present invention may be used in order to provide an indication for the state of charge of a battery used to power a medical implant and thus to determine the remaining run-time of that battery. Batteries in medical implants do have the advantage of being operated at well known conditions, i.e. at constant temperature and constant and reproducible power consumption such that drainage of the battery during the discharge operation can be very accurately modeled. 
         [0042]    To better address one or more of the above concerns, in a further aspect of the invention a recharge prediction unit is provided, comprising a processor, and a connecting device, wherein the processor is arranged for predicting of the state of charge of a battery during the discharging of the battery while the recharge prediction unit being disconnected from the battery, and wherein the connecting device is arranged to establish a connection between the recharge prediction unit and the battery. 
         [0043]    In an embodiment the connecting device may be a male or female connector. However, a connecting device in terms of the invention could also be any other means for the transfer of energy or information such as a radio frequency receiver/transmitter or an inductive coupling. 
         [0044]    In a further embodiment the recharge prediction unit may comprise a memory for storing at least one parameter for the prediction of the state of charge of the battery. The at least one parameter may in an embodiment be one of the parameters used for the prediction of the state of charge as discussed above. 
         [0045]    The recharge prediction unit according to an embodiment of the invention may comprise an up-dating device for up-dating the at least one parameter. The updating device updates the parameter when the connecting device is connected to the battery or any device associated with the battery during charging of the battery. The updated parameter may be stored in the memory. The updated parameters may be parameters determined during the charging of the device, but could also be parameters which have been obtained and stored by the battery or any associated device during discharging, which are only communicated to the recharge prediction unit during the charging operation. 
         [0046]    A recharge prediction unit enabling a prediction of the state of charge of a battery using the method according to an embodiment of the present invention may be integrated in a battery charger. Alternatively the recharge prediction unit may be a unit being separate from the battery as well as from the battery charger. 
         [0047]    In a further embodiment it may be the implantable device itself which comprises a recharge prediction unit according to an embodiment of the present invention. However, the recharge prediction unit according to an embodiment of the present invention may alternatively be integrated into the battery itself. 
         [0048]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF EMBODIMENTS 
         [0049]      FIG. 1  diagrammatically shows the components of a recharge prediction unit according to an embodiment of the present invention. 
           [0050]      FIG. 2  schematically shows a method performed by a recharge prediction unit according to an embodiment of the present invention to estimate the remaining run-time of a battery. 
           [0051]      FIG. 3  shows a rechargeable implanted medical device with a recharge prediction unit according to an embodiment of the present invention. 
           [0052]      FIG. 4  schematically shows a smoke detector with a separate recharge prediction unit according to an embodiment of the present invention. 
           [0053]      FIG. 5  schematically shows an alarm clock in combination with a separate recharge prediction unit according to an embodiment of the present invention. 
           [0054]      FIG. 6  is a flow chart schematically illustrating a process according to an embodiment of the invention for estimating the energy content of a battery. 
           [0055]      FIG. 7  is a flow chart schematically illustrating a process according to an alternative embodiment of the invention for estimating the remaining run-time of a battery. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0056]      FIG. 1  schematically shows the components of a recharge prediction unit  1  according to an embodiment of the present invention. It comprises four elements C 1 -C 4 : 
         [0057]    Denoted by C 1  (Initial energy content estimator) is the initial energy content estimator. Element C 1  estimates the content of energy of the electrical source of energy, i.e. a battery, before usage of a device being powered by the rechargeable energy source after a recharge operation occurred. This estimation of the energy content of the electrical energy source requires the prediction of the overall charge available from the energy source. 
         [0058]    The initial energy content estimator may either be based on an accurate modeling of the rechargeable energy source or additionally on measurements taken during the recharging process of the energy source. 
         [0059]    In an embodiment C 1  estimates the content of the energy source by the process described now with reference to  FIG. 6 . When in step  10  (RPU: Battery needs to be recharged) the recharge prediction unit (RPU)  1  determines that the battery needs to be recharged, the operator, e.g. a patient having an electrically powered implant or a medical doctor, connects the recharger to the battery in step  11  (Connect recharger/RPU to battery). In the described embodiment, the recharger as well as the recharge prediction unit, are integrated into a single device. In step  12  (Recharger takes over energy delivery) the recharger being connected to the battery and thus to the device being powered by the battery takes over energy delivery to the powered device such that the battery in step  13  (Battery relaxation) relaxes into a standby mode showing any or a very low battery discharge current. After the battery has relaxed, the battery voltage (EMF) of the relaxed battery is measured in step  14  (RPU: Measure EMF). However, in an alternative embodiment a relaxation of the battery can be omitted if the battery drain during operation is very small such that the battery is in a quasi relaxed state such that the measured voltage is a good approximation of the EMF. The measured EMF is used to determine the state of charge SoC si , i.e. a percentage of the maximum charge available from the battery in the fully charged condition, from a look-up table in step  15  (RPU: Determine SoC si ). The battery is then in step  16  (Charge battery) recharged by the recharger. In step  17  (RPU: Determine amount of charge Q ch ) the overall amount of charge Q ch  going into the battery during the charging period is determined by integrating the charging current into the battery during the charging period. After charging has been completed, the EMF is measured again in step  18  (RPU: Measure EMF) for the charged state of the battery and thus the state of charge SoC sf  is determined from the look-up table in step  19  (RPU: Determine SoC sf ). In step  20  (RPU: Calculate Q max ) the overall maximum charge capacity Q max  being available from the battery after recharging is then calculated from the following equation: 
         [0000]    
       
         
           
             
               Q 
               max 
             
             = 
             
               
                 100 
                 
                   
                     SoC 
                     sf 
                   
                   - 
                   
                     SoC 
                     si 
                   
                 
               
                
               
                 
                   Q 
                   ch 
                 
                 . 
               
             
           
         
       
     
         [0000]    The charge available for powering the device is given by Q max *SoC sf . The discharger and RPU, respectively, are then disconnected from the battery in step  21  (Disconnect charger/RPU). 
         [0060]    Referring to  FIG. 1  component C 2  (Energy drain estimator) then takes over operation of the recharge prediction unit in order to determine the actual state of charge of the freshly recharged battery during energy drain of the battery. C 2  therefore takes into account the environmental parameters of the battery and the device powered, e.g. temperature, as well as the energy consumption of the device powered by the battery. As the RPU which is connected to the recharger determines when the recharging process is finished and thus the battery once again takes over powering of the device from the recharger, the recharge prediction unit can determine the time having elapsed since recharging of the battery was completed 
         [0061]    From the initial energy content estimated by C 1  as well as the energy estimated to have drained from the battery during the discharge operation by C 2 , component C 3  (Remaining time estimator) calculates the remaining run-time of the battery. This remaining run-time is indicated by component C 4  (Recharge indication to user) to the user in terms of a display indicating the remaining run-time in days, hours and minutes. Component C 4  furthermore activates a warning signal in case the state of charge of the battery has reached a critical level, in the example shown 20% of the initial state of charge, indicating the user to recharge the battery. This may be performed in a graded manner, i.e. increasing warnings are issued when the state of charge becomes more and more critical. 
         [0062]    In the example described in detail above the battery is a Li-ion (liquid or polymer) battery. However, similar solutions could be achieved when using other rechargeable power sources/batteries, e.g. all-solid-state batteries. 
         [0063]    In  FIG. 2  the estimated energy level E available from the battery versus time t is exemplarily shown. When the overall energy level available from the battery drops below a critical energy level E C  at a given time  8  the user of a battery powered device is informed by the recharge prediction unit that a recharge of the battery is needed. This occurs well before the end of operation  9 . 
         [0064]    An alternative embodiment of a method for determining the state of charge and thus the remaining run-time of a battery according to the present invention is depicted schematically in  FIG. 7  in the form of a flow chart. This alternative embodiment takes into account the so-called overpotential, defined as the difference between the EMF immediately after charging and the voltage of the battery directly at the onset of the discharging of the battery. The overpotential is strongly influenced by the ageing of the battery. Thus taking into account the overpotential allows for the consideration of the ageing of the battery when determining the remaining run-time t r  of the battery. 
         [0065]    The method according to this embodiment relies on an empirical relationship between actual the state of charge left SoC l  in a battery and the state-of-charge at the beginning of the discharge SoC st : 
         [0000]    
       
         
           
             
               
                 SoC 
                 l 
               
               = 
               
                 
                   
                     [ 
                     
                       
                         C 
                          
                         
                           ( 
                           
                             
                               SoC 
                               st 
                             
                             100 
                           
                           ) 
                         
                       
                       
                         ζ 
                          
                         
                           ( 
                           
                             ϑ 
                             - 
                             C 
                           
                           ) 
                         
                       
                     
                     ] 
                   
                   
                     γ 
                     + 
                     
                       δ 
                        
                       
                           
                       
                        
                       T 
                     
                   
                 
                 
                   α 
                   + 
                   
                     β 
                      
                     
                         
                     
                      
                     T 
                   
                 
               
             
             , 
           
         
       
     
         [0066]    wherein T is the actual constant temperature in ° C. and C is the C-rate discharge current. Therefore before using the method in order to determine the remaining run-time of a battery employed in a real world application, the typical behavior of the respective battery must be measured experimentally for a given temperature T and a C-rate discharge current C in order to determine the five parameters ζ, ν, γ, δ, α, β. 
         [0067]    Thus, in step  200  (Determine SoC i  vs. SoC st ) in  FIG. 7  the state-of-charge left SoC l  in a specific battery is determined from EMF measurement as stated above for different states of charge at the beginning of the discharge SoC st . For those measurements the temperature T as well as the discharge current C are chosen such that they resemble the conditions under which the battery will be employed, e.g. for powering an implant at 37° C. and at a constant current of 50 mV. In step  201  (Fit empirical formula to measured curve) the curve from the measurement is fitted by the above relationship followed by the extraction of the five parameters ζ, ν, γ, δ, α, β in step  202  (Obtain parameters ζ, ν, γ, δ, α, β. After step  202  has successfully been completed the obtained empirical relationship can be employed in order to determine the remaining run-time t r  for the same battery device under operational conditions. 
         [0068]    In step  203  (Determine state of charge of battery before recharging) the stage-of-charge SoC si  of the battery is determined before the beginning of a recharging of the battery in step  204  (Charging of the battery). During the recharging the amount of charge flowing into the battery is determined in step  205  (Determine Q ch ) by integrating the charging current Q ch  flowing into the battery over the complete charging period. After recharging has being completed the state-of-charge SoC sf  is determined in step  206  (Determine SoC sf ). From the three parameters SoC si , SoC sf  and Q ch  determined during charging the maximum charge capacity Q max  available from the battery of is calculated in step  207  (Calculate Q max ) from 
         [0000]    
       
         
           
             
               Q 
               max 
             
             = 
             
               
                 100 
                 
                   
                     SoC 
                     sf 
                   
                   - 
                   
                     SoC 
                     si 
                   
                 
               
                
               
                 
                   Q 
                   ch 
                 
                 . 
               
             
           
         
       
     
         [0069]    In step  208  (Determine SoC st ) the state-of-charge SoC st  of the battery at the beginning of a discharge process with a current C and at environmental temperature T is determined as described above. The value for SoC st  in % derived from measurement is then used to calculate the state-of-charge left SoC l  according to the above relationship in step  209   
         [0070]    (Calculate SoC l ). When discharging of the battery at constant current C has started the state of charge SoC d  as well as the discharge current I d  are immediately measured in step  210  (Determine SoC d  and I d ). 
         [0071]    Having now obtained all necessary parameters Q max , SoC d , SoC l  and I d  the remaining run-time t r  in minutes can be calculated in step  211  (Calculate t r ) from the below expression 
         [0000]    
       
         
           
             
               
                 t 
                 r 
               
                
               
                 [ 
                 min 
                 ] 
               
             
             = 
             
               
                 
                   0.06 
                    
                   
                     
                       Q 
                       max 
                     
                     100 
                   
                    
                   
                     ( 
                     
                       
                         SoC 
                         d 
                       
                       - 
                       
                         SoC 
                         l 
                       
                     
                     ) 
                   
                 
                 
                   I 
                   d 
                 
               
               . 
             
           
         
       
     
         [0072]      FIGS. 3 to 5  show applications of the method as well as the recharge prediction unit for determining the state of charge of a battery according to embodiments of the present invention. An envisaged application is shown in  FIG. 3 . This example relates to a rechargeable deep brain stimulation device  100 . The device is programmed to a particular stimulation program by means of a programming unit  101  which is powered by a rechargeable battery  102 . The devices are charged by a charger  103 . The energy content of a battery is measured by a power management unit in the device and is communicated to the charger  103 . The power management unit corresponding to component C 1  in  FIG. 1  in the example shown is integrated into the program unit  101  while the other components are part of the recharge prediction unit  104  itself. Upon termination of the charging the charger  103  communicates the updated energy content to the recharge prediction unit  104 . Based on the state of charge of the battery  102  after recharging and the parameters of the stimulation program, i.e. the energy consumption of the device  100 , the recharge prediction unit in the following estimates the curve for the battery drainage. The recharge prediction unit  104  resets an internal clock and tracks the time elapsed after recharging has been finished. When the estimated energy level or estimated remaining run-time drops below a critical value (e.g. set by a physician or pre-programmed in the device), the recharge prediction unit provides a message to the user on a display, which might be a computer display or a television set, suggesting the user to recharge the device. The user may be a patient, a physician or a relative of the patient. 
         [0073]    Further applications are shown in  FIGS. 4 and 5 , wherein equal parts have been assigned equal reference numbers. 
         [0074]      FIG. 4  shows a smoke detector  110  running on a rechargeable battery  102 . A physical separate recharge indicator  104  integrated into a recharger  103  predicts the remaining operation time and informs a user when the detector&#39;s battery needs to be recharged. 
         [0075]      FIG. 5  shows an alarm clock  120  being powered by a rechargeable battery  102 . A physical separate recharge indicator  104  integrated into a recharge unit  103  predicts the remaining run-time and informs a user when the clock&#39;s battery needs to be recharged. 
         [0076]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. 
         [0077]    Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practising the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The fact that certain measures are recited in mutually different dependent claims does not indicate that the combination of these measures cannot be used to advantage. Any reference signs and the claims should not be construed as limiting the scope. 
       LIST OF REFERENCE NUMBERS 
       [0000]    
       
           1  recharge prediction unit 
         C 1 -C 4  essential elements 
           10  RPU: Battery needs to be recharged 
           11  connect recharger/RPU to battery 
           12  recharger takes over energy delivery 
           13  battery relaxation 
           14  RPU: Measure EMF 
           15  RPU: Determine SoC si    
           16  charge battery 
           17  RPU: Determine amount of charge Q ch    
           18  RPU: Measure EMF 
           19  RPU: Determine SoC sf    
           20  RPU: Calculate Q max    
           21  disconnect charger/RPU 
           100  stimulation device 
           101  programming unit 
           102  rechargeable battery 
           103  charger 
           104  prediction unit 
           110  smoke detector 
           120  alarm clock 
           200  determine SoC l  vs. SoC s    
           201  fit empirical formula to measured curve 
           202  obtain fitting parameters ζ, ζ, γ, δ, α, β 
           203  determine state of charge of battery before recharging 
           204  charging of the battery 
           205  determine Q ch    
           206  determine SoC sf    
           207  calculate Q max    
         208 determine SoC st    
           209  calculate SoC l    
           210  determine SoC d  and I d    
           211  calculate remaining run-time t r