Abstract:
A battery charger monitors an open-circuit voltage across the battery and the rate of change of temperature of the battery to determine a time to terminate the process of charging the battery. Charging proceeds in four stages. In the first stage the open-circuit voltage of the battery is monitored until such voltage crosses a threshold value. In the second stage, the rate of change of battery temperature is monitored to determine a reference value, for example, a minimum of the monitored rate. In the third stage, the rate of change of battery temperature is again monitored to identify a time when such rate exceeds the reference value by a predetermined amount. In the fourth stage, power supplied to charge the battery is limited immediately after stage three or a predetermined time after stage three for example by reducing the charging current to a trickle-charge level or by reducing the voltage by about 100 mV. The predetermined time may be a function of the elapsed charging time, for example a predetermined percentage of about 25%.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method and an apparatus for charging a rechargeable battery. More particularly, the present invention is directed to the control of the termination of the charging process.  
         BACKGROUND OF THE INVENTION  
         [0002]    Generally, when charging a rechargeable battery or a secondary battery, including for example NiCd (Nickel-Cadmium) or NiMH (Nickel-Metal-Hydride), it is known to have a rapid charging process wherein a relatively high constant current is applied to the battery until a certain event occurs. A typical method of detecting this event is to measure the increase in battery temperature as a function of time in order to detect when the battery temperature rate of change (dT/dt or delta_T/delta_t) reaches a predetermined high limit, see for example U.S. Pat. Nos. 3,852,652 to Jasinski, 5,329,219 to Garrett, and 5,550,453 to Bohne et. al.  
           [0003]    A common drawback of the above mentioned known charging processes is the use of a constant predetermined reference value to be reached for the measured battery temperature rate of change when terminating the charging process. Use of a predetermined reference value which is constant throughout the battery life sometimes results in undercharge of the battery (leading to a poor battery capacity) or overcharge of the battery (leading to a decreased battery lifetime). Therefore, the need exists for a battery charging method and apparatus that avoid undercharge and overcharge of the battery.  
           [0004]    Another drawback of known charging processes in which a characteristic of the battery is monitored for the detecting of an event that indicates the termination of a rapid charging stage, is the appearance of peak values of the characteristic of the battery at the initial stage of charging. To avoid a premature termination of the charging process due to a rise in such a characteristic, the need exists for a battery charging method and apparatus that avoids the detection of the event during the initial charging stage and yet allows the detection of a fully charged battery in order to avoid overcharging of a battery which has already been fully charged.  
         SUMMARY OF THE INVENTION  
         [0005]    Accordingly, a method in one embodiment of the present invention for charging a rechargeable battery includes the steps of: providing a supplied power to charge the battery; measuring a first characteristic of the battery to provide a first value; measuring a second characteristic of the battery to provide a second value after the first value has crossed a first threshold; and limiting the supplied power after the second value has crossed a second threshold. In alternate methods, the first and second characteristics are each selected from the group consisting of a battery voltage, a charging current, a battery temperature, a rate of change of battery voltage, a rate of change of charging current, and a rate of change of battery temperature and are not the same characteristic. For example the first characteristic may be a battery voltage and the second characteristic may be a rate of change of battery temperature that crosses a threshold based on a minimum of rate of change of battery temperature measured after the battery voltage has crossed a voltage threshold. By limiting supplied power in response to the second value that is measured after the first value had crossed a threshold, premature termination of the charging process is avoided. In a variation of such an alternate method, supplied power is limited in further response to a reference value determined in response to measurements of the second characteristic. Such a reference value accurately accounts for battery technology, battery use, and battery degradation to avoid undercharging the battery and avoid overcharging the battery.  
           [0006]    A method in another embodiment of the present invention for charging a rechargeable battery includes the steps of: supplying a charging current to the battery; determining a first plurality of values of rate of change of battery temperature during charging; determining a reference value based on the first plurality of values; determining further values of rate of change of battery temperature; comparing the reference value and these further values; and controlling termination of charging based on the comparison. In an alternate method, the reference value is based on a minimum of the first plurality of values. In another alternate method, the reference value is based on a sum of a minimum of the first plurality of values and a constant.  
           [0007]    A method in yet another embodiment of the present invention for charging a rechargeable battery includes the steps of: providing a supplied power to charge the battery, measuring a rate of change of temperature of the battery to provide a first plurality of values and a second value, determining a reference value in response to the first plurality of values, and limiting the supplied power in response to the reference value and the second value. Such a reference value accounts for battery technology, battery use, and battery degradation to avoid undercharging the battery and avoid overcharging the battery.  
           [0008]    An apparatus in one embodiment of the present invention for controlling power supplied for charging a rechargeable battery cell includes a circuit that measures a first characteristic of the cell (for example cell voltage), measures a second characteristic of the cell (for example rate of change of cell temperature), and provides a signal for controlling power supplied in response to the second characteristic being measured after the first characteristic crosses a threshold. Operation of the apparatus accounts for battery technology, battery use, and battery degradation to avoid undercharging the cell and avoid overcharging the cell.  
           [0009]    An apparatus for charging a rechargeable battery in a second embodiment of the present invention includes the apparatus discussed above for controlling supplied power, and includes a power supply. The power supply, in response to the signal, limits the supplied power. By limiting supplied power in response to the second value that is measured after the first value had crossed a threshold, premature termination of the charging process is avoided. In a variation of this second embodiment, supplied power is limited in further response to a reference value determined in response to the plurality of second values. Such a reference value accurately accounts for battery technology, battery use, and battery degradation to avoid undercharging the battery and avoid overcharging the battery.  
           [0010]    An alternate apparatus for charging a rechargeable battery includes: a power supply that provides a supplied power to charge the battery and a circuit that: measures rate of change of a temperature of the battery, determines a minimum of the measured rate of change, determines a present measured rate of change, provides a comparison of the minimum rate of change and the present rate of change, and provides a signal to the power supply in response to the comparison. The power supply, in response to the signal, limits the supplied power.  
           [0011]    Yet another alternate apparatus for charging a rechargeable battery includes a power supply that provides a supplied power to charge the battery and a circuit that: measures rate of change of temperature of the battery to provide a first plurality of values and to provide further values of the measured rate of change of the battery temperature, determines a reference value in response to the first plurality of values, compares the further provided values with the reference value, and provides a signal to the power supply in response to this comparison. The power supply limits the supplied power in response to the signal.  
           [0012]    Practice of the methods and operation of the apparatus of the present invention reduce or eliminate drawbacks of the prior art.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]    The structure and operation of exemplary embodiments of the invention, together with further objects and advantages thereof, may best be appreciated by reference to the following detailed description taken in conjunction with the accompanying drawing, in which:  
         [0014]    [0014]FIG. 1 is a functional block diagram of a battery charging apparatus according to an embodiment of the present invention;  
         [0015]    [0015]FIG. 2 is a flow chart of a method of charging a rechargeable battery in one embodiment of the invention;  
         [0016]    [0016]FIG. 3 is a graph of a charging process according to the method of FIG. 2 as applied to an initially fully discharged battery;  
         [0017]    [0017]FIG. 4 is a graph, of a charging process according to the method of FIG. 2 as applied to a battery having a higher initial temperature than in FIG. 3;  
         [0018]    [0018]FIG. 5 is a graph of a charging process according to the method of FIG. 2 as applied to a battery having about 90% of its final charge capacity at the beginning of the process; and  
         [0019]    [0019]FIG. 6 is a graph of a charging process according to the method of FIG. 2 as applied to a battery having a lower initial battery temperature and as performed at a higher ambient charging temperature than in FIG. 3.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    A functional block diagram of a battery charger according to the present invention is illustrated in FIG. 1. FIG. 1 shows battery pack  10  which is to be charged by battery charger apparatus  20 . Battery pack  10  comprises a number of series connected individual cells  11 , battery temperature sensing thermistor  12  (NTC thermistor), battery type resistor  13 , and battery pack output terminals  14 ,  15 ,  16 , and  17 . Battery output voltage is provided across terminals  14  and  17 . The charging current supplied to the battery is sensed by sense resistor  23  connected to battery terminal  17  and ground. Thermistor  12  has current supplied through pull-up resistor  21 , senses battery temperature, and provides a related output at terminal  16 . Type resistor  13  has current supplied through pull-up resistor  22  and provides a battery type related voltage at terminal  15 .  
         [0021]    In a variation of battery charger  20  and methods of charging rechargeable batteries according to the present invention, type resistor  13  and thermistor  12  are omitted from battery pack  10 . An alternate thermistor is located to sense battery temperature when the battery is being charged. And, battery type is presumed or is identified by operator input, by a conventional circuit, or by a conventional mechanical arrangement.  
         [0022]    Battery charger  20  includes power supply  24 , a charge controller  25 , and signal conditioning circuit  26 . Power supply  24 , preferably a switch mode power supply, has power input  27  which is supplied with a DC voltage, preferably in the range of 12-15 Volts DC. Power supply  24  provides supplied power to charge the battery via output terminal  28 , connected to terminal  14  through switch  29 . Supplied power is controlled via control output  30  of charge controller  25 . When power supply  24  is a switch mode power supply, control output  30  is preferably a pulse width modulated (PWM) signal that may be fed to a filter for converting the PWM signal to a variable analog voltage. The analog voltage may then be used for the control of power supply  24 . When using a PWM signal at control output  30 , charge controller  25  controls the supplied power to battery  10  via terminal  28  by controlling the duration of on- and off-periods of the PWM signal.  
         [0023]    Signal conditioning circuit  26  converts voltage input signals representing the battery terminal voltage, the battery type, the battery temperature, and the charging current, to voltage output signals being suitable as input signals for analog to digital (A/D) converter inputs  32  of charge controller  25 . Preferably, current sense resistor  23  has a very low value which may be about 0.1 ohm and conditioning circuit  26  may then include an operational amplifier to provide a suitable output. The supply voltage for charge controller  25  is preferably about 5 volts. Since the battery terminal voltage may exceed 5 volts, conditioning circuit  26  may also include a voltage divider for providing a suitable output signal for the battery terminal voltage.  
         [0024]    Charge controller  25  preferably includes switch control output  31  for operating switch  29  on and off. Switch  29  may be turned off at short time intervals during the charging process to measure an open-circuit voltage of the battery, thereby decreasing the effect of the voltage drop from the internal loss resistance when measuring the battery terminal voltage.  
         [0025]    Charge controller  25 , may include a processor, for example a COP 8ACC marketed by National Semiconductor, programmed to implement battery charging in accordance with the present invention. Charge controller  25  controls the power delivered from power supply  24  to battery  10  based upon the input signals from conditioning circuit  26 . These input signals represent characteristics of the battery including battery type, battery terminal voltage, battery temperature, and battery charging current.  
         [0026]    Battery charger apparatus  20  in operation performs one or more methods of charging a rechargeable battery according to the present invention. Such methods are described below with reference to FIGS. 2 through 4.  
         [0027]    [0027]FIG. 2 presents a method for charging a rechargeable battery according to one embodiment of the present invention. Such a method begins at step  40 . At step  40 , battery pack  10  is connected to charger  20  and charge controller  25  is initialized.  
         [0028]    During initialization, charge controller  25  reads the battery type and battery voltage and uses these values for obtaining predetermined charging parameters stored in charge controller  25 . Such parameters may represent a maximum charging current (Imax), an initial value for an end time period (End_Time), a maximum change in the rate of change of battery temperature (dT/dt_add), an initial value for the determined minimum value of the battery temperature rate of change (Min_dT/dt), an initial voltage limit (VoltLimit), and an initial time period (TimeLimit).  
         [0029]    End_Time defines a safety time at which the charging process will be stopped unless a new value of End_Time is determined and stored during the charging process.  
         [0030]    The value dT/dt_add defines a maximum allowed change in the rate of change of battery temperature compared to Min_dT/dt.  
         [0031]    Min_dT/dt is a variable determined during the charging process. The initial value of Min_dT/dt is preferably set to a large value.  
         [0032]    VoltLimit defines a minimum limit that the measured battery voltage should reach before updating the predetermined initial value of Min_dT/dt. The value of VoltLimit will typically be 1.4 Volts per battery cell.  
         [0033]    TimeLimit defines a time period that expires before updating the predetermined initial value of Min_dT/dt. The value of TimeLimit will typically be 5 minutes for NiCd or NiMH batteries.  
         [0034]    Exemplary initial values of End_Time, Min_dT/dt, and dT/dt_add are given below with the description of FIGS. 3 through 5.  
         [0035]    After initialization at step  40  the charging process is begun at process step  41 . The charging process is controlled based on measured values of the open-circuit battery voltage (Vopen), the charging current (Ichar), the battery temperature (Tbat), and the elapsed time since charging began (Time). From values of Tbat, values of the battery temperature rate of change (dT/dt) are calculated. During the first stage of the charging process, it is preferred to increase the charging current Ichar until the predetermined Imax has been reached. The magnitude of Imax is predetermined at a value that will quickly charge the battery as opposed to a trickle-charge amount. When Imax has been reached, a second charging stage is entered, in which the output of the power supply is preferably controlled so as to charge at a constant charging current, i.e. the output of the power supply is controlled so that Ichar is close to Imax. In a preferred embodiment, the value of Imax is chosen to be within the range of 0.5 Amp through 1.5 Amp for NiCd and NiMH batteries.  
         [0036]    If the measured battery voltage reaches VoltLimit before TimeLimit has expired, the predetermined initial value of Min_dT/dt will be updated and updating will continue from the point in time when VoltLimit had been reached. Otherwise, updating of the predetermined initial value of Min_dT/dt will begin when the TimeLimit period has expired.  
         [0037]    At decision step  42 , it is determined whether the charging time has reached the stored value of TimeLimit and dT/dt is smaller than the stored value of Min_dT/dt. If so, then at process step  43 , the stored value of Min_dT/dt is updated (replaced) with the measured value of dT/dt and the process continues with decision step  44 . If the requirements at step  42  are not fulfilled, the process continues directly with decision step  44 .  
         [0038]    At decision step  44 , it is determined whether the measured open-circuit battery voltage (Vopen) has reached the stored value VoltLimit. If not, the process continues with decision step  49 .  
         [0039]    At decision step  49 , it is determined whether the charging time has reached the stored value of End_Time. If not, the process returns to step  41  for further charging. If End_Time has been reached, then the normal charging process is stopped and the charging current is reduced at step  50  to a trickle-charge current (for example a low, maintenance current) to maintain the charged status of the battery.  
         [0040]    At step  50 , the trickle-charge current is preferably set in the range of 0.05 C through O.I C, where 1 C is equivalent to a charging current in Amps that would theoretically fully charge the battery in one hour.  
         [0041]    Here it should be noticed that passing directly from step  44  to step  49  and then to step  50  is not the route of a normal charging process. However, the battery to be charged might be a defective battery or there might be a faulty connection to the battery terminals, leading to the result that the measured battery voltage did not reach the stored VoltLimit value within the initial safety value of End_Time. Thus, the charger will automatically terminate the charging process at expiration of the initial End Time period.  
         [0042]    If the measured battery voltage has reached VoltLimit at step  44 , the process passes on to decision step  45 . At step  45 , it is determined whether the measured value of dT/dt is smaller than the stored value of Min_dT/dt. If so, then at process step  46 , the stored value of Min_dT/dt is updated (replaced) with the measured value of dT/dt and the process continues with decision step  47 . If the requirement at step  45  is not fulfilled, then the process continues directly with decision step  47 .  
         [0043]    At decision step  47 , it is determined whether the measured value of dT/dt is larger than the sum of the presently stored minimum value of the change in dT/dt and the predetermined maximum allowed value of the change in dT/dt. That is, whether the measured value of dT/dt has reached the sum Min_dT/dt plus dT/dt_add. If not, the charging process has not yet reached the normal stage of termination and the process continues with decision step  49 , described above. If so, the process continues with process step  48 .  
         [0044]    At step  48 , a third stage of the charging process is entered where the remaining part of the charging process is controlled so that the measured battery voltage does not exceed a maximum allowed battery voltage (MaxVolt). The value of MaxVolt is not a predetermined value, but is a function of the measured battery voltage Vopen dT/dt  at the point in time where dT/dt has reached the sum Min_dT/dt plus dT/dt_add. In a preferred embodiment, MaxVolt is defined as fl(Vopen), where fl(Vopen) is defined as (Kl*Vopen dT/dt −k 2 ). The constant kl may be set to 1 and the constant k 2  may be set in the range of 0 through 100 mV per cell, preferably about 40 mV, per battery cell. Thus, for a 4 cell battery the value of MaxVolt may preferably be set to Vopen dT/dt −160 mV.  
         [0045]    For NiMH batteries it is preferred to have such a reduction in the charging voltage in order to avoid overheating of the battery, since such overheating might damage the battery and decrease the battery lifetime.  
         [0046]    The constant k 2  may be set in the range of 0 to 50 mV. The constant k 2  may be set to zero for NiCd batteries. However, it is preferred to set k 2  to about 50 mV for NiCd batteries.  
         [0047]    At process step  48 , the stored initial value of End_Time is updated (replaced) with a new End_Time value. The new End_Time value is determined as a function of the total charging time Time dT/dt  up to the point in time where dT/dt has reached the sum of Min_dT/dt plus dT/dt_add. In a preferred embodiment, the new End_Time value is defined as f 2 (Time) where f 2 (Time) is defined as (k 3 *Time dT/dt +k 4 ). The constant k 3  may be set to about 1.25 and the constant k 4  may be set to zero. In a variation, k 3  can be set in the range of 1 through 2 and k 4  can be set to represent a fixed time period in the range of 0 through 20 minutes.  
         [0048]    At process step  48 , it is determined how the charging process is to be terminated, i.e. a final charging period and the maximum allowed battery voltage are determined. Here it should be mentioned that in a preferred embodiment the measured battery temperature is compensated for variations related to the present rate of change in the battery temperature. Using a maximum allowed battery voltage results in a decrease in the charging current during the final charging period.  
         [0049]    The termination of the charging process is illustrated by the loop comprising process step  51  and decision step  52 . After the determination of MaxVolt and the new End_Time value, the charging process is continued as described above until the total charging time reaches the stored value of End_Time in step  52 .  
         [0050]    At step  52 , when the total charging time (Time) reaches the stored value of End_Time, the charging process is stopped and the charging current is reduced as already described in step  50  above.  
         [0051]    The predetermined charging parameters, including variables, constants, and functions, for the above described preferred embodiment of a method of charging a rechargeable battery are shown Table I.  
                   TABLE I                           Variables:           Voltage or   Open-circuit battery voltage       Vopen       MaxVolt   Maximum allowed voltage across the battery       Ichar   Charging current       Time   Time elapsed since charging began       End_Time   The time when normal charging is to be stopped (initialized           with a safe value and set to an optimized value calculated           during the charging process)       Tbat   Battery temperature       dT/dt   Rate of battery temperature change       Mm_dT/dt   Minimum value of dT/dt during the charging process       Constants:       Imax   Maximum allowed charging current       End_Time   Predetermined initial charging time safety value       Initial       Mm_dT/dt   Predetermined initial minimum value of dT/dt       Initial       dT/dt_add   The maximum allowed dT/dt is Mm_dT/dt + dT/dt_add       TimeLimit   Mm_dT/dt is updated when Time reaches TimeLimit       Initial   (typically about 5 minutes)       VoltLimit   Mm_dT/dt is updated when Vopen reaches VoltLimit       Initial   (typically 1.4 V for single cell batteries)       Functions:       F1(Vopen)   Example: k1 * Vopen · k2 (typically k1 = 1 and           k2 = 40 mV for single cell batteries)       F2(Time)   Example: k3 * Time + k4 (typically k3 = 1.25 and k4 = 0)                  
 
         [0052]    [0052]FIG. 3 shows a charging process controlled as described above with reference to the flow diagram of FIG. 2 and as applied to charge a fully discharged 1600 mAh NiMH battery with 6 cells. In FIG. 3, the battery is fully discharged before the charging process is begun, and the battery is charged at room temperature with an initial battery temperature about 23° C. In FIG. 3 the thick solid line waveform represents the measured open-circuit battery voltage, the dashed line waveform represents the measured charging current and the thin solid line waveform represents the measured battery temperature.  
         [0053]    For the process shown in FIG. 3, predetermined charging parameters are set as follows. Imax is set to 900 mA. The initial End_Time value is set to 160 minutes. The initial value of Min_dT/dt is set to a high value of 10° C./minute, thereby disabling the effect of the rate of temperature change during the upstart of the charging process. The value of dT/dt_add is set to 0.5° C./minute. The value of TimeLimit is set to 5 minutes. And, the value of VoltLimit is set to 8.25 volts. The function f 1  for MaxVolt is set to Vopen dT/dt −240 mV, and the function f 2  for End_Time is set to 1.25*Time dT/dt .  
         [0054]    Here it should be mentioned that the optimal value of dT/dt_add will vary as a function of battery capacity and the maximum charging current. Thus, the value of dT/dt_add should be larger both for a smaller nominal battery capacity and for a higher charging current. In order to measure a relative change in dT/dt of 0.5° C./minute, it is necessary to measure changes in the battery temperature at a relatively high resolution. In a preferred embodiment the temperature is measured using a 10 bit A/D converter resulting in a resolution in change of temperature of about 0.10° C.  
         [0055]    It is preferred when comparing Vopen to VoltLimit that VoltLimit be compensated for battery temperature at the time Vopen is measured. Such compensation might be 20 mV/°C. added to VoltLimit for temperatures below 35° C.  
         [0056]    The first stage of charging in FIG. 3 is rather short and the charging current reaches Imax within a short time period. During the second stage of charging the current is controlled to approximate Imax. During the third stage of charging, equivalent to the final charging period, the charging current is decreased.  
         [0057]    During the second stage of charging, the value of TimeLimit (5 minutes) is smaller than the time at which the compensated voltage Vopen reaches VoltLimit (about 35 minutes). Thus, after TimeLimit has been reached, new values of Min_dT/dt are stored according to steps  42  and  43 .  
         [0058]    In a preferred embodiment, none of the new values of Min_dT/dt are used for the control of termination of the charging process before VoltLimit has been reached according to step  44 . During the charging process dT/dt is measured at regular intervals, but the value of Min_dT/dt is not updated before Time equals (or exceeds) TimeLimit. Further, when Voltage (Vopen) reaches (or exceeds) VoltLimit, the measured values of dT/dt are used for determining termination. These processes can be seen in steps  42  through  45  of FIG. 2.  
         [0059]    The battery voltage does not reach the compensated value of VoltLimit until Time is about 35 minutes. At Time equal about 35 minutes, battery temperature is about 27.5° C., and the corresponding voltage compensation is about +150 mV. When Vopen reaches VoltLimit, VoltLimit has a compensated value of about 8.4 volts (8.25+150 mV).  
         [0060]    [0060]FIG. 4 shows a charging process controlled as described above with reference to the flow diagram of FIG. 2 and as applied to charge a 1600 mAh NiMH battery with 6 cells. In FIG. 4, the battery is charged at the same ambient temperature of about 23° C. as the battery of FIG. 3, but the battery of FIG. 4 has been stored at a higher temperature before being charged, resulting in an initial battery temperature of about 27° C.  
         [0061]    In FIG. 4, the predetermined charging parameters are the same as for the charging process of FIG. 3.  
         [0062]    Because the battery in FIG. 4 has a higher initial battery temperature, the temperature rate of change dT/dt will be smaller for the charging curves of FIG. 4 than for the curves of FIG. 3. To avoid overcharging the battery, the dT/dt termination value needs to be smaller for the charging process of FIG. 4 than the termination value used for the charging process of FIG. 3. By using an updated value of Min_dT/dt that is smaller for the warm battery of FIG. 4 than for the colder battery of FIG. 3, the resulting maximum allowed value of dT/dt (that is the sum Min_dT/dt plus dT/dt_add) will be smaller in the charging process of FIG. 4 than in the charging process of FIG. 3.  
         [0063]    For batteries having lower initial temperatures than the battery of FIG. 3, yet being charged at the same ambient temperature, higher values of dT/dt will be observed during the initial stage of charging, which values might reach the desired termination value of dT/dt. To avoid premature termination of the charging process, the charging process should be controlled so as to avoid termination based on a high value of dT/dt during the initial stage of charging. Such control might be accomplished in a simple way by having a TimeLimit set at a high value, for example 15 minutes. However, setting TimeLimit to a high value might cause overcharging of the battery when an almost fully charged battery is being charged.  
         [0064]    According to a variation of a charging method of the present invention, overcharging an almost fully charged battery is avoided. Use of the parameter VoltLimit to determine when values of dT/dt should be used to control termination of the charging process avoids overcharging of almost fully charged batteries. When charging a battery that is already almost fully charged, the battery voltage will reach a high value such as VoltLimit within a relatively short time period, whereas when charging a battery that is almost fully discharged, the battery voltage will increase much more slowly.  
         [0065]    In FIG. 4, the battery voltage does not reach the compensated value of VoltLimit until Time is about 22 minutes. At Time equal about 22 minutes, battery temperature is about 30° C., and the corresponding voltage compensation is about +100 mV. When Vopen reaches VoltLimit, VoltLimit has a compensated value of about 8.35 volts (8.25+ 100 mV).  
         [0066]    [0066]FIG. 5 shows a charging process controlled as described above with reference to the flow diagram of FIG. 2 and as applied to charge a 1600 mAh NiMH battery with 6 cells. The initial battery temperature for the charging process of FIG. 5 is the same as the initial battery temperature for the charging process of FIG. 3. However, in the process of FIG. 5, the battery is holding 90% of its capacity at the beginning of the charging process. In FIG. 5, the battery voltage reaches VoltLimit within 4 minutes from beginning the charging process compared to 35 minutes for the fully discharged battery of FIG. 3.  
         [0067]    The battery voltage does not reach the compensated value of VoltLimit until Time is about 4 minutes. At Time equal about 4 minutes, battery temperature is about 23.5° C., and the corresponding voltage compensation is about +230 mV. When Vopen reaches VoltLimit, VoltLimit has a compensated value of about 8.48 volts (8.25+230 mV).  
         [0068]    [0068]FIG. 6 shows a charging process controlled as described above with reference to the flow diagram of FIG. 2 and as applied to charge a 1600 mAh NiMH battery with 6 cells. In the process of FIG. 6, the battery is holding 50% of its capacity at the beginning of the charging process. However, the initial battery temperature is very low, about −7° C., and the ambient charging temperature is high, about 35° C.  
         [0069]    In FIG. 6, the predetermined charging parameters are the same as for the charging process of FIG. 3.  
         [0070]    In the charging process of FIG. 6, the battery is charged with a trickle-charge current (for example, a low, maintenance current) until the battery temperature reaches about 5° C., from which point in time a normal charging process is begun. In FIG. 6, the normal charging process is initiated when Time is about 9 minutes. In the process of FIG. 6 the initial value for TimeLimit is set to 5 minutes, as in FIG. 3. Therefore, the value of Min_dT/dt is not updated before Time reaches about 14 minutes (9+5).  
         [0071]    The battery voltage does not reach the compensated value of VoltLimit until Time is about 27 minutes. Due to the low battery temperature, the measured values of Vopen should be required to reach a relatively high value before reaching the compensated VoltLimit value. At 25° C., the corresponding voltage compensation is about +200 mV so VoltLimit has a compensated value of about 8.45 volts (8.25+200 mV). When Time is about 27 minutes, the battery temperature has increased to about 25° C. and Vopen reaches about 8.45 volts.  
         [0072]    For the process of FIG. 6 the value of Min_dT/dt is not updated before Time equals 14 minutes (according to steps  42  and  43  of FIG. 2). However, after Vopen has reached the compensated VoltLimit, values of dT/dt are used not only to update (replace) the value of Min_dT/dt, but also for determining the termination process (according to steps  44  through  47  of FIG. 2). The value of dT/dt decreases through the charging process until the almost fully charged state is reached. During the charging process, the value of Min_dT/dt is also being decreased until this almost fully charged state is reached. Overcharging the battery is avoided by determining the termination process without requiring the value of dT/dt to reach a fixed value. Rather, the termination process is determined when the value of dT/dt reaches a reference value, Min_dT/dt+ dT/dt_add, which is updated (decreased) during the charging process.  
         [0073]    The present invention provides a method where termination of the charging process can be controlled in an optimum way by using a reference value (Min_dT/dt) which is determined during the charging process based on determined values of the rate of change of battery temperature, whereby variation in the reference value between performances of the charging process, for example as a consequence of battery life, has the effect of varying the termination of the charging process to avoid overcharging the battery.  
         [0074]    In another embodiment of the present invention, a method for charging a rechargeable battery includes:  
         [0075]    connecting an electrical power source to the terminals of the battery and supplying a charging current to the battery,  
         [0076]    determining values of the rate of change of battery temperature during at least part of the process of charging the battery,  
         [0077]    determining and storing a reference value based on the obtained values of the rate of change of battery temperature,  
         [0078]    comparing values of the rate of change of battery temperature with the stored reference value or a function thereof, and  
         [0079]    controlling termination of the charging process based on said comparison.  
         [0080]    When monitoring battery temperature during a charging process the temperature will increase when the battery approaches the fully charged state. It is preferred to have termination of the charging process based on a threshold value. To allow this threshold value to be adaptive, it is preferred to determine the threshold value based on a plurality of values of the battery temperature rate of change.  
         [0081]    For example, termination of the charging process may be initiated when a determined value of the rate of change of battery temperature exceeds a calculated threshold. The threshold value may be calculated by adding a predetermined maximum allowed change in the rate of change of battery temperature (dT/dt_add) to a determined reference value (Min_dT/dt). In order to avoid overcharging of the battery it is preferred that the determined reference value represents a minimum of the obtained values of the rate of change of battery temperature and it is important to use the smallest possible value for the predetermined maximum allowed change. Such a predetermined maximum allowed change may be in the range of 0.25 through 2° C./minute and preferably about 0.5° C./minute. However, the optimal value will depend on battery capacity and battery technology.  
         [0082]    To control termination of the charging process, the power supplied to the battery needs to be limited (reduced). Here it is preferred that termination of the charging process includes reducing the charging current. This reduction may be abrupt by turning the power supply off.  
         [0083]    The termination of the charging process may alternatively include a final charging period, during which period the battery may be charged with a reduced current until the charging process is finally stopped. Here, the duration of the final charging period may be determined as a function of the total charging time passed at the point in time at which termination of the charging process is initiated. As an example, the length of the final charging period may be in the range of 5% through 50%, preferably about 25%, of the total charging time elapsed up to the point in time at which termination of the charging process is initiated.  
         [0084]    The final charging period need not be a function of the charging time but may have a predetermined duration.  
         [0085]    To limit power supplied to the battery during the final charging period, the charging process may be controlled so as to reduce battery terminal voltage by at least a predetermined amount during initiation of the final charging period for avoiding overcharging. As an example, the battery terminal voltage may be reduced at least 100 mV, preferably at least 200 mV, at the initiation of the final charging period, with the battery terminal voltage preferably not being increased during this final period.  
         [0086]    The battery terminal voltage may also be reduced as a function of the number of cells within the battery. Such a reduction may be in the range of 10 mV through 100 mV per cell, preferably in the range of 20 mV through 70 mV per cell, and even more preferred about 40 mV per cell.  
         [0087]    Alternatively, the charging power may be reduced by controlling the charging process so that the battery terminal voltage is not allowed to increase during the final charging period, for example by keeping the voltage constant during this final charging period.  
         [0088]    When charging a battery, the determined rate of change of battery temperature during upstart of the charging process may vary as a function of the initial battery temperature. Thus, for a battery which has been stored at a low temperature but which is being charged at a higher temperature, a high value of the rate of change of battery temperature can be observed during upstart of the charging process until the battery has reached the ambient temperature.  
         [0089]    To avoid the influence of such an initial high rate of change of battery temperature, it is preferred that the control of the termination of the charging process be based on values of the rate of change of battery temperature as determined after a predetermined time period has lapsed or after the value of a characteristic start-up charging parameter measured during an initial stage of the charging process has reached a predetermined value. In a preferred embodiment the characteristic start-up charging parameter is the battery terminal voltage, which may be measured as an open-circuit voltage.  
         [0090]    When the charging process has been stopped, the capacity of the battery may decrease due to self-discharging. Such self-discharging may depend on battery technology (or type) and on individual batteries of the same type.  
         [0091]    If self-discharging might be a problem, it is preferred that the state of charge of the battery be maintained after termination of the charging process by a trickle-charge current (for example, a pulsating current or a low, maintenance current).  
         [0092]    In yet another embodiment of the present invention, a method of charging a rechargeable battery includes:  
         [0093]    connecting an electrical power source to the terminals of the battery and supplying a charging current to the battery,  
         [0094]    determining values of a first characteristic charging parameter during at least part of the charging process, and  
         [0095]    controlling termination of the charging process based on values of the first parameter being determined after a point in time at which point in time a second characteristic charging parameter measured during an initial stage of the charging process has reached a predetermined value or fulfills a predetermined criteria.  
         [0096]    Here, the first characteristic charging parameter may be any characteristic of the battery which is suitable for the control of the charging process such as battery terminal voltage, charging current, battery temperature, the rate of change of any of these parameters or any combination of these parameters and/or their rate of change. Preferably, the first characteristic charging parameter is the rate of change of battery temperature calculated from measured values of the battery temperature.  
         [0097]    Similarly, the second characteristic charging parameter may also be selected from any of the above mentioned first characteristic charging parameters with the exception that it should not be the same parameter as the one chosen as the first characteristic parameter. However, it is preferred that the second characteristic charging parameter is the battery voltage.  
         [0098]    In order to terminate the charging process it is preferred that the obtained values of the first parameter be compared with a stored reference value or a function thereof, and the termination of the charging process be based on said comparison. Preferably, the termination of the charging process is initiated when the measured values of the first charging parameter reaches a threshold value being a function of the stored reference value. Thus, for example, termination of the charging process may be initiated when the obtained value of the first parameter exceeds the stored reference value by a predetermined amount.  
         [0099]    In a preferred embodiment the stored reference value is determined during the charging process based on obtained values of the first parameter. The reference value may be determined as a maximum of the obtained values, but it is preferred that the reference value represents a minimum value of the obtained first parameter values.  
         [0100]    When terminating the charging process, it is preferred that the termination includes a final charging period, and it is preferred that the length of the final charging period is determined as a function of the total charging time passed at the point in time at which termination of the charging process is initiated. Furthermore, it is preferred that termination of the charging process includes reducing the charging current or charging with a reduced current during the final charging period.  
         [0101]    In FIG. 1, charge controller  25  includes control logic (including memory for storage of variables, constants, and programmed instructions), an analog to digital converter, and input/output circuits. The control logic includes a general purpose arithmetic logic unit (ALU) such as used in the conventional micro controller. Cooperation of the control logic and programmed instructions accomplishes the decision and processing steps described with reference to FIG. 2, including such operations as addressing, identifying, determining, comparing, detecting when a value has crossed a threshold value, calculating, updating, determining elapsed time, responding to inputs, and providing outputs. Cooperation of the control logic and the A/D converter accomplishes steps involving input values, including such operations as measuring, detecting, monitoring, converting, comparing, obtaining, and sensing. Cooperation of the control logic and the output circuit accomplishes controlling power supply  24  and switch  29 , including such operations as enabling the provision of supplied power to charge the battery, enabling provision of a trickle-charge current, and limiting supplied power. All such cooperation is accomplished by conventional program development techniques in light of the disclosure of the present invention.  
         [0102]    The above description of battery charger  20  of FIG. 1 illustrates a preferred implementation. In alternate implementations, the functions of battery charger  20  are accomplished with analog circuitry, digital circuitry, or any combination of analog and digital circuitry. As one example, charge controller  25  and signal conditioning circuit  26  may be integrated to form a single integrated circuit. As another example, the functions of signal conditioning circuit  26  and the A/D converter portion of charge controller  25  may be combined to form a measurement circuit that provides a digital signal conveying battery temperature or may provide rate of change of battery temperature. Such a measurement circuit may cooperate with a processor that performs all remaining functions of charge controller  25 .  
         [0103]    What has been described above illustrates how a reference value is compared to the rate of change of battery temperature, which reference value is determined and stored during the charging process. The obtained reference value is used when determining a threshold value for control of termination of the charging process, whereby this embodiment of the present invention implements adaptive battery charging. Such adaptive battery charging allows the present embodiments to account for changes or differences in the threshold value of the battery temperature rate of change due to aging or manufacturing tolerances. Furthermore, such adaptive battery charging allows the present embodiments to account for variations in the threshold value due to differences in ambient temperatures.  
         [0104]    The above described embodiments for a charging process also bring a solution which accounts for differences in the initial battery temperature, whereby a premature termination of the charging process due to a high initial temperature rate of change is avoided.  
         [0105]    The above described embodiments of the present invention apply to recharging batteries such as NiMH (Nickel Metal Hydride) batteries and other types of rechargeable batteries including, for example, NiCd (Nickel Cadmium) and Lithium batteries.  
         [0106]    The foregoing description of preferred exemplary embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the description, as will be apparent to those skilled in the art. All such modifications which retain the basic underlying principles disclosed and claimed herein are within the scope of this invention.