Abstract:
A method and system for charging a rechargeable battery, such as a Lithium based battery, by applying a voltage charge signal, and monitoring a battery charging current and a varying internal resistance of the rechargeable battery and changes in open circuit voltage of the rechargeable battery. The voltage charge signal is dynamically established as a function of the varying internal resistance of the rechargeable battery during charging and the changes in open circuit voltage of the rechargeable battery. The voltage charge signal is a function of a state of charge (SOC) of the rechargeable battery.

Description:
CLAIM OF PRIORITY 
     This application claims priority under 35 U.S.C. Section 119(e) of U.S. Provisional application 61/987,297 entitled “Method and Apparatus for Fast Charging Li Polimer Based Rechargable Batteries” filed May 1, 2014, and of U.S. Provisional application 61/987,290 entitled “Method and Apparatus for Fast Charging Li Polimer Based Rechargable Batteries” filed May 1, 2014, the teaching of each is incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure is generally directed to charging rechargeable batteries, and more particularly to charging Lithium (Li) based rechargeable batteries. 
     BACKGROUND 
     Conventional batteries are based on a plurality of technologies, such as lead acid, nickel cadmium, and Lithium just to name a few. An advantage of Lithium based batteries is the high charge capacity for a unit size, and the life of the battery. 
     Efficiently and quickly charging batteries remains one of the key challenges in battery technology. While a constant voltage constant current (CCCV) charging signal is acceptable, it is not usually the most efficient or quickest charging algorithm, and may limit the number of times a battery can be charged, referred to as charge cycles, thus reducing the life of the battery. Pulse charging a battery is sometimes more efficient, wherein a battery voltage and/or current charging signal is pulsed. Pulse charging may increase the charge rate (and thus reduce charge time) and extend the useful life of a battery. Care must be taken to minimize the generation of heat in the battery during charging, which heat reduces the useful life of the battery. 
     A Lithium based battery is a more complex battery, thus, advanced charging signal algorithms may help increase the charging rate of the battery, thus reducing charge times, reduce heating of the battery, and increase battery life. 
     SUMMARY 
     A method and system for charging a rechargeable battery, such as a Lithium based battery, by applying a voltage charge signal, and monitoring a battery charging current and a varying internal resistance of the battery. The voltage charge signal is dynamically established as a function of the measured varying internal resistance of the battery during charging. The voltage charge signal is a function of a state of charge (SOC) of the battery. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates a system level diagram of a battery charger configured to charge a rechargeable battery as a function of a charging algorithm; 
         FIGS. 2 a -2 b    illustrate a battery charging algorithm; 
         FIGS. 3 a -3 d    illustrate signal waveforms, including a charge signal voltage waveform, a battery voltage waveform, and a current charging waveform, and a battery internal resistance determination for the beginning of the charge; 
         FIGS. 4 a -4 c    illustrate signal waveforms, including a charge signal voltage waveform, a battery voltage waveform, and a current charging waveform used during the main charge; and 
         FIG. 5  illustrates a battery current waveform during measurement corrections. 
     
    
    
     DETAILED DESCRIPTION 
     Definitions 
     
         
         Ub is the actual voltage of the battery in the given situation 
         Ib is the actual current going through the battery in the given situation 
         C means the nominal capacity of the battery. (for example, if the battery is a 10 Ah battery then C=10). 
         Imax is the factory defined maximum current 
         Umax is the factory defined maximum voltage 
         Tmax is the factory defined maximum charge temperature 
         OCV is the Open Circuit Voltage of the battery 
         OCVb is the OCV of the battery at the beginning of a given charge cycle 
         OCVe is the OCV of the battery at the end of a given charge cycle 
         OCVTempMultiplier is typically 0.8-1 
         OCVTempCorrection is the correction value dependent on temperature rise 
         CycleCount is between 10 to 1000 depending on implementation 
         Charge Signal is an arbitrary charging signal. The signal starts at zero point. The signal has one maximum value and one maximum point. It is monotonously increasing until the maximum point, then monotonously decreasing to zero point. The frequency of the charging signal is typically 1 Hz to 10 kHz. 
         Tr is the rest time, when the Diode does not allow the battery to be discharged
 
Description of Charging Process
 
       
    
       FIG. 1  illustrates a charger  10  for charging a battery  12 . The charger  10  has a controller  14  which comprises one or more processors, a shunt resistor  16  for measuring battery current, and a battery temperature sensor  18  for measuring a temperature of battery  12 . Battery current is measured by the controller  14  measuring the voltage drop across the shunt resistor  16  having a known resistance R, where I=V/R. The diode provides reverse current protection. 
       FIG. 2  illustrates a method  20  performed by the controller  14  of charger  10  in  FIG. 1  to charge the battery  12  according to one embodiment 
     This method  20  assumes that the battery  12  is in chargeable condition i.e. not “dead”. The charging of the battery  12  takes place according to the following charging algorithm. 
     Start of Charge 
     When the battery  12  is put on the charger  10 , the battery open circuit voltage OCV is measured in the following manner. The controller  14  applies a ChargeSignal comprising a voltage to the battery  12  as shown in  FIG. 3 a   . The ChargeSignal voltage minimal value is zero, and the ChargeSignal voltage maximal value is Umax. High negative current from the battery is prevented by the diode. OCVb0 is defined as the first Ub value during the measurement cycle when Ib&gt;zero as seen in  FIGS. 3 a   - 3   c.    
     The ChargeSignal has a monotonously increasing first portion and a monotonously decreasing second portion. The ChargeSignal may look like a triangle, but can be of any shape, such as a semicircle. 
     The controller  14  repeats this cycle N times, where N is typically 3-10. This defines OCVb0 through OCVbN values. 
     The controller  14  considers these N values and determines the following cases: 
     1. All values are monotonously decreasing (CASE1) 
     2. All values are monotonously increasing (CASE2) 
     3. Other cases (CASE3) 
     The controller  14  determines if the battery can or cannot be charged. 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 CASE1 
                 N.A - Battery or system bad. 
                 STOP all activites 
               
               
                   
                 CASE2 
                 Battery impedance is high 
                 Battery good, can be charged 
               
               
                   
                 CASE3 
                 Battery impedance is low 
                 Battery is either full or cannot be 
               
               
                   
                   
                   
                 charged. Stop charge. 
               
               
                   
                   
               
             
          
         
       
     
     Internal Battery resistance Rb is measured in the following manner as shown in  FIG. 3   d:    
     A small current is applied to the battery, C/10 Ampere (A)(I0b), for 150 msec and the battery voltage is measured (U0a). Then, for another 150 msec, current C/20 A (I0b) is applied and battery voltage is measured (U0b). The internal battery impedance is Rb=(U0a−U0b)/(I0a−I0b). 
     Charging 
     Uamax is defined by the controller  14  at the beginning of charge:
 
 Ua max=OCV b+Rb*I max
 
The maximum value for Uamax is Umax.
 
At the beginning of charge (first cycle) OCVeN equals OCVb as measured before.
 
OCVTempMultiplier is 1 at the beginning of charge
 
     The ChargeSignal voltage signal is applied by the charger  10  to the battery  12 . The starting voltage value is the last measured OCV value (OCVeN). Then, the controller  14  increases the ChargeSignal voltage and after a while current response of the battery  12  starts to increase. After reaching its maximum value of Uamax, the controller  14  starts to decrease the ChargeSignal voltage symmetrically as seen in  FIGS. 4 a -4 c   . Then, the controller  14  starts the whole cycle again. 
     Voltage Ub and current Ib is measured by the controller  14  constantly. Voltage OCVe is defined here as the first point after the maximum point of the ChargeSignal voltage when current Ib=0. This OCVe*OCVTempCorrection becomes the starting voltage for the next cycle. 
     When voltage OCVe is determined to reach the value of Uamax, then the controller  14  applies a constant voltage of Umax to the battery until said battery&#39;s current decreases below C*k, where k is typically 0.05 to 0.5 and C is the nominal capacity of the battery. This is the traditional CV charging of the battery. 
     This cycle is repeated by the controller Cycle Count times. 
     Measurement Corrections 
     Correction Based on Internal Resistance Rb of the Battery. 
     A new Rb value of the battery is determined. Again, a small current is applied to the battery, C/10 Ampere (A)(I0b), for 150 msec and the battery voltage is measured (U0a). Then, for another 150 msec, current C/20 A (I0b) is applied and battery voltage is measured (U0b). The internal battery impedance is Rb=(U0a-U0b)/(I0a−I0b). From this point on this new Rb value is used and the process described in paragraph [0023] is repeated, and iterated. Therefore, the charger  10  dynamically changes the ChargeSignal voltage signal provided to the battery  12  during charging based on the tendency of the changing battery impedance Rb increasing or decreasing. 
     Maximal Current Signal Modification 
     In all phases of charging, battery&#39;s current is monitored by the controller  14 . In case battery&#39;s current reaches or exceeds Imax, the following will happen: 
     Instead of ChargingSignal, the controller  14  applies Uamax to the battery for t time as seen in  FIG. 5 . 
     where
 
 t=tc− 2* ta  
 
where
 
tc equals the cycle time of Charging Signal
 
ta equals the time from the beginning of the current cycle
 
     After t time, ChargingSignal resumes its slope. 
     Temperature Control 
     Temperature is measured by controller  14  using temperature sensor  18  at every 10 to 60 seconds and the values are stored. 
     If Tn exceeds Tmax, then the charge is stopped immediately. 
     If any 5 successive temperature measurements are determined by the controller  14  to show an increase greater than a first limit, then the controller  14  decreases the OCVTempCorrection and OCVTempMultiplier is decreased to 0.95. The new OCVTempCorrection is the old OCVTempCorrection multiplied by OCVTempMultiplier. The first limit may be defined as more than 1 degree Celsius difference between any two measurement points. 
     If any 5 successive temperature measurements are determined by the controller  14  to show an increase greater than a second limit being greater than the first limit then the charge is stopped immediately. The second limit may be defined as is more than 2 degree Celsius difference between any two measurement points. Different first limits and second limits may be established, and limitation to these limits is not to be inferred. 
     The charging voltage signal is applied by the controller to the battery as a function of a measured state of charge (SOC) of the battery.