Abstract:
A computer program is provided. The computer program product includes a medium readable by a computer, the medium (1) having means for inputting a signal representative of a temperature range in which a temperature of an operating environment of a battery resides; and (2) means for estimating a service life of the battery based on the signal. Other means are also provided.

Description:
This application is a divisional of U.S. patent application Ser. No. 09/415,652, filed Oct. 12, 1999 now U.S. Pat. No. 6,191,556, which is incorporated herein in its entirety by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to battery technology, and more particularly to a method and apparatus for estimating the service life of a battery. 
     BACKGROUND OF THE INVENTION 
     For years single use and rechargeable nickel-cadmium (NiCd) batteries have been employed to power portable radios, shavers, laptop computers, non-volatile memory components, etc. NiCd batteries exhibit good capacity (i.e., the length of useable time between charges), reusability (i.e., the ability to be recharged) and service life (i.e., the length of time a battery may be used before a minimum useful battery capacity, such as 60% of the battery&#39;s maximum or “initial” capacity, is unachievable by re-charging). However, due to environmental concerns over the disposal of heavy metals such as cadmium, alternative battery technologies have been developed. 
     Nickel metal hydride (NiMH) batteries offer a more environmentally friendly alternative to NiCd batteries. NiMH batteries have better capacity than NiCd batteries, but suffer from poor battery service life in hot operating environments (e.g., above 36° C.). For example, for operating environments having a temperature of about 28° C. or less, the service life of a NiMH battery is about 36 months. However, for operating environments having a temperature of about 36° C. or higher, the service life of a NiMH battery is only about 19.5 months. 
     Because batteries typically are rated based on the worst case operating environment (e.g., about 40° C. for NIMH batteries) even though actual battery operating environments rarely approach the worst case operating environment, NiMH batteries often are changed prematurely. The high costs associated with service calls, service disruption and battery disposal that accompany battery replacement necessitate longer battery service life. 
     SUMMARY OF THE INVENTION 
     To address the need for longer battery service life, a method and an apparatus for estimating the service life of a battery are provided. Specifically, battery service life is estimated based on the actual temperature of the operating environment of a battery rather than on a worst case operating environment temperature. Preferably battery service life is estimated periodically or is otherwise “integrated” during battery operation to account for any changes in the operating environment temperature of the battery during its operation and to account for both the present and the past operating environment temperatures of the battery. By estimating the service life of a battery based on its actual operating environment temperature, a worst case service life need not be assumed for the battery, and the battery need not be replaced prematurely. Maximum service life is therefore extracted from every battery. 
     In a first aspect of the invention, an apparatus is provided for estimating the service life of a battery. The apparatus includes a temperature measurement circuit that measures a temperature of an operating environment of the battery and outputs a signal representative of the temperature of the operating environment. The apparatus further includes a controller coupled to the temperature measurement circuit that receives the temperature signal therefrom and estimates the service life of the battery based on the temperature signal. Preferably, the temperature measurement circuit outputs a signal representative of a temperature range in which the measured operating environment temperature resides and the controller estimates the service life of the battery based on the temperature range signal output by the temperature measurement circuit. 
     In a second aspect of the invention, a method is provided for estimating the service life of a battery by measuring a temperature of an operating environment of the battery and by estimating the service life of the battery based on the measured operating environment temperature. Preferably, measuring the temperature of the operating environment of the battery includes outputting a signal representative of a temperature range in which the measured operating environment temperature resides. The service life of the battery is estimated based on the temperature range signal. 
     In a third aspect of the invention, a computer program product is provided for estimating the service life of a battery as described above. The inventive computer program product is carried by a medium readable by a computer (e.g., a carrier wave signal, a floppy disc, a hard drive, a random access memory, etc.). 
     Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit of a reference number identifies the drawing in which the reference number first appears. 
     FIG. 1 is a schematic diagram of a novel service life estimation circuit for estimating the service life of a battery in accordance with the present invention; and 
     FIG. 2 is a flow chart of a service life estimation algorithm for use with the service life estimation circuit of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic diagram of a novel circuit (e.g., service life estimation circuit  100 ) for estimating the service life of a battery in accordance with the present invention. The service life estimation circuit  100  comprises a temperature measurement circuit  102  for measuring the temperature of an operating environment of a battery and for outputting a signal representative of the measured temperature; and a controller  104  coupled to the temperature measurement circuit  102  for receiving the temperature signal and for estimating the service life of the battery based thereon. 
     In the preferred embodiment of the present invention, the temperature measurement circuit  102  measures an operating environment temperature of a battery and outputs to the controller  104  a temperature range signal that indicates in which of a plurality of temperature ranges the measured operating environment temperature resides. Within the controller  104  a counter is associated with each of the plurality of temperature ranges set by the temperature measurement circuit  102 . Each counter counts the amount of time the battery is operated within the temperature range associated with the respective counter. The battery&#39;s total operation time thereby can be obtained by adding together the counts of each counter. 
     Moreover, to determine if the battery&#39;s service life has been exceeded, and thus whether the battery should be replaced, a pro-rated operation time (rather than the actual operation time) of the battery is computed (e.g., by multiplying any battery operation time at elevated temperatures by a pro-rate factor which is greater than one) to account for elevated temperature operation of the battery. The pro-rated operation time of the battery is compared to a maximum service life of the battery (e.g., the service life of the battery if the battery is operated in a low temperature environment such as 28° C. or less for a NiMH battery). If the pro-rated operation time exceeds the maximum service life of the battery, an alarm is generated to indicate that the battery should be replaced. Thus, although battery life varies depending on the environmental temperature in which the battery operates, the invention allows for an accurate estimate of battery life regardless of the operating environment temperature. 
     With reference to FIG. 1, the temperature measurement circuit  102  comprises a thermistor  106  (e.g., a Mitsubishi Materials Corporation Model No. TH20-3V103F thermistor or any other suitable thermistor) coupled to an encoder circuit  108  via a first voltage divider circuit  110 , and a second voltage divider circuit 112 coupled to the encoder circuit  108 . The first voltage divider circuit  110  comprises a first resistor  114  coupled between a power supply rail (V cc ) and a thermistor node  116 , and a second resistor  118  coupled between the thermistor node  116  and ground. The thermistor  106  also is coupled between the thermistor node  116  and ground. 
     The second voltage divider circuit  112  comprises a third resistor  120  coupled between the power supply rail (V cc ) and a first reference node  122 , a fourth resistor  124  coupled between the first reference node  122  and a second reference node  126 , a fifth resistor  128  coupled between the second reference node  126  and a third reference node  130 , and a sixth resistor  132  coupled between the third reference node  130  and ground. As described further below, each voltage divider circuit  110 ,  112  generates one or more reference voltages based on a common voltage supply (e.g., the power supply rail (V cc ). Accordingly, the power supply rail (V cc ) need not be tightly regulated because any voltage fluctuations present thereon will have an equivalent affect on each voltage divider circuit (and any reference voltage or voltages generated thereby). It will be understood that regulated reference voltages and conventional regulation circuitry associated therewith (e.g., Zener diodes and the like) may be employed to generated the various reference voltages described herein. 
     The encoder circuit  108  comprises a first comparator  134 , a second comparator  136  and a third comparator  138  each having a positive input terminal coupled to the thermistor node  116 . The negative input terminals of the first comparator  134 , the second comparator  136  and the third comparator  138  are coupled to the first reference node  122 , to the second reference node  126  and to the third reference node  130 , respectively. The output of the second comparator  136  serves as an “X” output of the encoder circuit  108  and the outputs of the first comparator  134  and the third comparator  138  are combined via an XOR gate  140  to serve as a “Y” output of the encoder circuit  108 . The comparators  134 - 138  may comprise any conventional comparator, but preferably comprise comparators having hysteresis (such as Maxim Corporation Model No. MAX8214 comparators) so as to limit toggling of the X and Y outputs of the encoder circuit  108  as described below. Note that a decoupling capacitor  141  preferably is coupled between the power supply rail (V cc ) and ground for noise reduction purposes as is known in the art. 
     In operation, the temperature measurement circuit  102  generates a two bit output signal (e.g., outputs X, Y of the encoder circuit  108 ) that indicates in which of four temperature ranges the temperature (as measured by the thermistor  106 ) of an operating environment of a battery resides. Specifically, as shown in TABLE 1 below, the thermistor  106  has a resistance (R 106 ) that varies with temperature from about 10 Kohms at 25° C. to about 6.258 Kohms at 36° C. In response to the resistance variations of the 
     
       
         
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 T 
                 R 106   
                 V 116   
                 V 122   
                 V 126   
                 V 130   
                   
                   
               
               
                 (° C.) 
                 (ohms) 
                 (volts) 
                 (volts) 
                 (volts) 
                 (volts) 
                 Y 
                 X 
               
               
                   
               
             
             
               
                 25 
                 10K 
                 2.075 
                 2.007 
                 1.910 
                 1.808 
                 0 
                 1 
               
               
                 28 
                  8.77K 
                 2.005 
                 2.007 
                 1.910 
                 1.808 
                 1 
                 1 
               
               
                 32 
                  7.392K 
                 1.907 
                 2.007 
                 1.910 
                 1.808 
                 1 
                 0 
               
               
                 36 
                  6.258K 
                 1.805 
                 2.007 
                 1.910 
                 1.808 
                 0 
                 0 
               
               
                   
               
             
          
         
       
     
     thermistor  106 , the first voltage divider circuit  110  generates a temperature-sensitive voltage signal at the node  116  (V 116 ) that depends on the resistance (R 106 ) of the thermistor  106 , the resistance (R 114 ) of the first resistor  114 , the resistance (R 118 ) of the second resistor 118 and the power supply rail voltage (V cc ) via equation (1) below:                V   116     =         V   cc            R   TOTAL         R   TOTAL     +     R   114                       where                   R   TOTAL       =         R   106          R   118           R   106     +     R   118                   (   1   )                                
     TABLE 1 lists values for the temperature-sensitive voltage signal (V116) at operating environment temperatures of 25° C., 28° C., 32° C. and 36° C. (for the resistance values listed in TABLE 2). 
     
       
         
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 R 114   
                 R 118   
                 R 120   
                 R 124   
                 R 128   
                 R 132   
               
               
                   
                 (ohms) 
                 (ohms) 
                 (ohms) 
                 (ohms) 
                 (ohms) 
                 (ohms) 
               
               
                   
                   
               
             
             
               
                   
                 6.04K 
                 7.50K 
                 60.4K 
                 1.96K 
                 2.05K 
                 36.5K 
               
               
                   
                   
               
             
          
         
       
     
     The temperature-sensitive voltage signal (V 116 ) is fed to the negative input terminal of each of the comparators  134 - 138 , while the positive voltage terminal of each comparator  134 - 138  is held at a reference voltage V 122 , V 126  and V 130 , respectively (the voltages of the first, second and third reference nodes  122 ,  126 ,  130 ). The reference voltages V 122 , V 126  and V 130  are set by the second voltage divider circuit  112  and are governed by equations (2)-(4) below:                V   122     =       V   CC              R   124     +     R   128     +     R   132           R   120     +     R   124     +     R   128     +     R   132                   (   2   )                 V   126     =       V   CC              R   128     +     R   132           R   120     +     R   124     +     R   128     +     R   132                   (   3   )                 V   130     =       V   CC            R   132         R   120     +     R   124     +     R   128     +     R   132                   (   4   )                                
     where R 120 , R 124 , R 128  and R 132  are the resistances, respectively, of the resistors  120 - 132 . TABLE 1 lists the values of the first, second and third reference voltages V 122 , V 126 , V 130  of the first, second and third reference nodes  122 ,  126 ,  130 , respectively, for the resistance values listed in TABLE 2. 
     With the reference voltage of each comparator  134 - 138  thus fixed, each comparator  134 - 138  outputs a positive voltage level (e.g., V cc ) if the temperature-sensitive voltage signal (V 116 ) is greater than the comparator&#39;s reference voltage, and outputs zero volts if the temperature-sensitive voltage signal (V 116 ) is less than the comparator&#39;s reference voltage. For example, with reference to TABLE 1, if the operating environment temperature of a battery (as measured by the thermistor  106 ) is 25° C. or less, the temperature-sensitive voltage signal (V 116 ) exceeds the first, the second and the third reference voltages V 122 , V 126  and V 130 . Each comparator output is driven high (e.g., to V cc ), and in response thereto, the X output of the encoder circuit  108  is driven high by the second comparator  136  and the Y output of the encoder circuit  108  is driven low by the XOR gate  140 . 
     The temperature-sensitive voltage signal (V 116 ) remains above all three reference voltages V 122 , V 126  and V 130  until a temperature of about 28° C. when the temperature-sensitive voltage signal (V 116 ) no longer exceeds the first reference voltage V 122 , but remains in excess of the second and third reference voltages V 126 , V 130 . With the first reference voltage V 122  no longer exceeded, the output of the first comparator  134  is low while the outputs of the second and third comparators  136 ,  138  remain high. In response thereto, the X output of the encoder circuit  108  remains high (due to the high voltage level output by the second comparator  136 ) while the Y output of the encoder circuit  108  is driven high (e.g., due to the low voltage level and the high voltage level from the first and third comparators  134 ,  138 , respectively, fed to the XOR gate  140 ). 
     Note that because the comparators  134 - 138  preferably have hysteresis associated therewith, once the temperature-sensitive voltage signal (V 116 ) exceeds a reference voltage associated with one of the comparators  134 - 138 , and the comparator&#39;s output voltage switches polarity in response thereto, the comparator&#39;s output polarity will remain in its new polarity state despite small variations in the voltage signal (V 116 ) about the reference voltage. In this manner, small temperature fluctuations about a temperature that represents a temperature range “boundary” of the temperature measurement circuit  102  (e.g., about 28° C., 32° C. or 36° C.) will not cause the X and Y outputs of the encoder circuit  108  to transition. The number of X and Y output transitions thereby is reduced, as is the likelihood that the controller  104  will read a transitorial and potentially false voltage level on either the X or Y output during battery service life estimation (described below). To further reduce potential false voltage level readings by the controller  104 , the X and Y outputs of the encoder circuit  108  switch polarity using a “grey code” scheme wherein only one of the X and Y outputs switches polarity when a temperature range boundary is reached. Accordingly, even if the controller  104  erroneously reads one of the X and Y outputs before or while it transitions, and identifies an improper temperature range based thereon, the improperly identified temperature range will be within one temperature range of the proper temperature range (e.g., minimizing the error). A grey code scheme also eliminates any potential false readings by the controller  104  due to differing switching rates of the X and Y outputs of the encoder circuit  108 . 
     The temperature-sensitive voltage signal (V 116 ) remains above the second and third reference voltages V 126 , V 130  until a temperature of about 32° C. when the temperature-sensitive voltage signal (V 116 ) no longer exceeds the second reference voltage V 126 , but remains in excess of the third reference voltage V 130 . With the first and second reference voltages V 122 , V 126  no longer exceeded, the outputs of the first and second comparators  134 ,  136  are low while the output of the third comparator  138  remains high. In response thereto, the X output of the encoder circuit  108  is driven to zero volts by the second comparator  136  while the Y output of the encoder circuit  108  remains high (as the voltage levels that feed the XOR gate  140  remain unchanged). 
     The temperature-sensitive voltage signal (V 116 ) remains above the third reference voltage V 130  until a temperature of about 36° C. is reached. Thereafter, with no reference voltage exceeded, the output of each comparator  134 - 138  is driven low, as are the X and Y outputs of the encoder circuit  108 . Because the temperature-sensitive voltage (V 116 ) continues to decrease with increasing temperature, the X and Y outputs remain low for all temperatures in excess of 36° C. 
     In summary, the temperature measurement circuit  102  measures a temperature of an operating environment of a battery (via the thermistor  106 ) and outputs a signal (the X and Y outputs) that represents a temperature range in which the measured operating temperature resides. In the preferred embodiment of FIG. 1, an embodiment specifically adapted for use with NiMH batteries, the four temperature ranges in which a measured temperature may reside (and the corresponding X and Y outputs from the encoder circuit  108 ) are listed in TABLE 3. It will be understood that the temperature measurement circuit  102  may be configured for use with other batteries and/or other or more temperature ranges if desired. 
     
       
         
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 TEMPERATURE RANGE (° C.) 
                 Y OUTPUT 
                 X OUTPUT 
               
               
                   
               
             
             
               
                 &lt;28 
                 0 
                 1 
               
               
                 about 28-32 
                 1 
                 1 
               
               
                 about 32-36 
                 1 
                 0 
               
               
                 &gt;36 
                 0 
                 0 
               
               
                   
               
             
          
         
       
     
     The controller  104  receives the X and Y outputs (e.g., the temperature range signal) from the temperature measurement circuit  102  and estimates the service life of a battery based thereon (as described below). Preferably the controller  104  comprises a microcontroller or microprocessor such as an IBM PowerPC™ 403 processor having program code stored therein that performs battery service life estimation functions. For example, the X and Y outputs of the temperature measurement circuit  102  may be input by an input/output (I/O) module  142  of the controller  104  (e.g., via I/O pins (not shown) of an I/O port  144  of the controller  104 ). The temperature range information encoded by the X and Y outputs (TABLE 3) then may be used by software (e.g., program code) within a random access or read only memory (represented generally by reference number  146 ) of the controller  104  for battery service life estimation as described below. Alternatively, hardware (e.g., discreet counters, logic modules, application specific integrated circuits, etc.) or a combination of hardware and software may be similarly employed. 
     The operation of the controller  104  is now described with reference to FIG.  1  and with reference to FIG. 2 which is a flow chart of a preferred service life estimation algorithm  200 . It is assumed that a new battery such as a NiMH battery (not shown) having its maximum service life remaining (e.g., 36 months for an operating temperature of about 28° C. or less) is monitored by placing the thermistor  106  proximate the battery (e.g., on, next to or sufficiently close to the battery to monitor the operating environment temperature of the battery). 
     With reference to FIG. 2, in step  201 , the service life estimation algorithm  200  begins. In step  202 , a counter is initialized (e.g., is created and is set to zero) for each temperature range to be monitored. For the preferred embodiment of the service life estimation circuit  100  of FIG. 1, four temperature ranges may be monitored as shown in TABLE 3 (e.g., less than about 28° C., between about 28-32° C., between about 32-36° C. and greater than about 36° C.). Accordingly, four counters  148 ,  150 ,  152  and  154  are initialized for (i.e., are associated with) the temperature ranges of less than about 28° C., between about 28-32° C., between about 32-36° C. and greater than about 36° C., respectively. The counters  148 - 154  preferably are software-based, 8-byte counters defined within the memory  146  as is known in the art. It will be understood that the spatial boundaries and arrangement of the counters  148 - 154  in FIG. 1 are arbitrary and are shown merely for convenience. 
     Following initialization of the counters  148 - 154 , in step  203  the controller  104  waits or “sleeps” for a predetermined time period (e.g., to allow the battery to age). The preferred sleep period for the controller  104  is about 5 minutes although any other time period (if any) may be employed. 
     In step  204 , the temperature range signal (e.g., the X and Y outputs) of the temperature measurement circuit  102  is sampled to identify in which of the four temperature ranges the current operating environment temperature of the battery resides (e.g., below about 28° C., between about 28-32° C., between about 32-36° C. or above about 36° C.). Specifically, program code within the memory  146  (represented in FIG. 1 as estimation and processing code  156  for convenience) directs the I/O module  142  to input the voltage levels of the X and Y inputs. Thereafter, in step  205 , the estimation and processing code  156  interprets these voltage levels in accordance with TABLE 3 to determine in which temperature range the current operating environment temperature of the battery resides. The counter associated with the temperature range also is identified. 
     In step  206 , the estimation and processing code  156  adds the sleep time of the controller  104  (e.g., the time between temperature samples) to the count of the counter associated with the temperature range identified by the X and Y outputs of the temperature measurement circuit  102 . For example, if in step  205  the estimation and processing code  156  determines that the operating environment temperature of the battery resides within the temperature range from about 28° C. to 32° C. (e.g., because the X and Y outputs of the temperature measurement circuit  102  have high logic levels), the estimation and processing code  156  increases the count of the second counter  150  by the sleep time of the controller  104  (e.g., by about 5 minutes). 
     In step  207 , the estimation and processing code  156  computes the total operation time of the battery by adding together the count of each counter  148 - 154 . To compensate for the decrease in battery service life from its maximum value (e.g., 36 months for NiMH batteries operated at about 28° C. or less) that accompanies battery operation at elevated temperatures, a pro-rate factor is assigned to each counter  148 - 154  that weights the count of each counter  148 - 154  based on the temperature range associated with the counter. Specifically, the counts of counters having high temperature ranges associated therewith (e.g., counters  152  and  154 ) are weighted more heavily than the counts of counters having low temperature ranges associated therewith (e.g., counters  148  and  150 ) so that when the battery is operated at temperatures above about 28° C. the computed operating time of the battery exceeds the actual operating time of the battery. The battery thereby is aged more quickly when operated at higher temperatures. Preferably, the pro-rate factor for the count of the first counter  148  (C 148 ) is 1.0, the pro-rate factor for the count of the second counter  150  (C 150 ) is 1.22, the pro-rate factor for the count of the third counter  152  (C 152 ) is 1.50 and the pro-rate factor for the count of the fourth counter  154  (C 154 ) is 1.83. The pro-rated operation time (T PRORATED ) of the battery therefore is governed by equation (5) below: 
     
       
           T   PRORATED   =C   148 +1.22 C   150 +1.5 C   152 +1.83 C   154   (5)  
       
     
     In this manner, if a battery is operated at a temperature of about 28° C. or less, the battery is aged at its actual rate (so that an effective service life of 35.75 months is employed for the battery), but if the battery is operated at a temperature of about 36° C. or higher, the battery is aged at 1.83 times its actual rate (so that an effective service life of 19.5 months is employed for the battery). The battery is aged at an intermediate rate for intermediate temperature operation. Further, by including the time the battery is operated within each temperature range during battery service life estimation, both the present and the past operating environment temperatures of the battery are considered during battery life estimation. Accordingly, an accurate estimation of battery service life is computed even if a battery is operated at several different operating environment temperatures during its lifetime. Note that for a binary count, pro-rate factors of 1.25, 1.5 and 1.875 may be obtained by appropriate shifting and addition operations as will be apparent to one of ordinary skill in the art (e.g., to generate 1.875X for a count of X, the count X may be added to the count X shifted to the right by 1 bit, shifted to the right by 2 bits and shifted to the right by 3 bits). 
     In step  208 , the estimation and processing code  156  compares the pro-rated operation time of the battery to the maximum service life of the battery (e.g., the service life of the battery if operated at 28° C. or less). If the pro-rated operation time of the battery is less than the maximum service life of the battery, control passes to step  203 , and steps  203 - 207  are repeated (e.g., the controller  104  sleeps, a new temperature range signal is sampled, the controller sleep time is added to the appropriate counter and the pro-rated operation time is computed). This process is repeated until the pro-rated operation time exceeds the maximum service life of the battery, after which time control passes to step  209 . 
     In step  209 , the estimation and processing code  156  generates a signal (e.g., an alarm signal output by the I/O module  142  of the controller  104 ) to indicate that the service life of the battery has been exceeded and that the battery should be replaced. In step  210 , the service life estimation algorithm  200  ends. 
     The foregoing description discloses only the preferred embodiments of the invention, modifications of the above disclosed apparatus and method which fall within the scope of the invention will be readily apparent to those of ordinary skill in the art. For instance, while the present invention has been described primarily with reference to NiMH batteries, other battery technologies may similarly benefit from the invention. Other temperature measurement means than the thermistor  106  such as a silicon pn-junction diode may be employed, as may other resistance values for the resistors  114 - 132 , other temperature ranges and other pro-rate factors. 
     Accordingly, while the present invention has been disclosed in connection with the preferred embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims.