Abstract:
Methods and apparatus are provided for estimating the remaining state-of-charge of a non-rechargeable power source, such as a back-up battery for an electronic module normally connected to a primary power source. A processor within the electronic module implements a prediction algorithm, in order to monitor the temperature of the back-up attery, and to calculate various current draws of the back-up battery. The processor adjusts the nominal self-discharge rate of the back-up battery in accordance with the measured temperatures, and calculates an adjusted self-discharge quantity. The processor also accumulates current draw data and reduces the initial state-of-charge value of the back-up battery by the temperature-adjusted self-discharge quantity and the cumulative current draw quantities to determine a remaining state-of-charge of the back-up battery. At a predetermined state-of-charge threshold level, an indicator can be activated to advise a user to replace the back-up battery.

Description:
TECHNICAL FIELD 
     The present invention generally relates to a non-rechargeable power source, and more particularly relates to an algorithm for estimating the state-of-charge of a non-rechargeable power source. 
     BACKGROUND 
     Recent developments in automotive technology have introduced various devices to enhance the safety and convenience of the driving experience. For example, air bags are now commonly deployed in vehicles to reduce the likelihood of injury in an accident, and on-board navigation systems are used to help the driver find his/her way to a destination. Another recent development is an on-board telematics system, such as the “OnStar” system. This type of telematics system enables the driver to access many types of services via a wireless communication from the vehicle to an affiliated call center. The call center can then respond to a driver&#39;s request for service, which typically includes navigation instructions, roadside assistance, emergency services, and other types of information. 
     In general, a telematics system is configured as an electronic module installed in a vehicle, and connected to a primary power source, which is typically the main vehicle battery. This type of system is generally intended to provide a driver with various types of call center services, and also to provide an automatic notification capability to a call center in the event of an emergency situation, such as the deployment of an air bag in the vehicle. Therefore, it is generally desirable that a back-up power source be available for the electronic module in the event of primary power disruption. The back-up power source is generally in the form of a relatively low power battery, designed to provide sufficient power to the electronic module to accommodate an emergency situation. 
     Over lengthy time periods, however, a back-up battery is typically subject to a gradual loss of power capability, even when it is not used to power an electronic module. For example, a typical battery will experience a nominal self-discharge rate (shelf life), and may also experience loss of charge due to various types of leakage currents when connected to any type of electronic circuitry. Therefore, it is desirable to monitor the state-of-charge of a back-up battery in order to implement a timely replacement if the state-of-charge falls below a predetermined threshold level. 
     Various techniques can be used to anticipate the state-of-charge replacement/threshold level, such as estimation calculations or periodic test sampling. Typically, a simple estimation calculation is based primarily on the nominal self-discharge rate, or shelf life, of the battery. However, this type of calculation may not take other factors into consideration, such as temperature changes and miscellaneous current drains. For example, according to the Arrhenius rate law, chemical reaction rates rise exponentially with reagent temperature. As such, a battery self-discharge rate would be related to temperature. 
     Another technique involves the periodic sampling of battery voltage, but this intrusive type of testing typically causes current drains from the back-up battery, which can further shorten the life of the battery. Moreover, this technique is generally not a reliable measure of state-of-charge because battery cells typically have a flat voltage curve until nearly discharged. 
     Accordingly, it is desirable to provide a method of predicting a replacement threshold level for a back-up battery that is minimally intrusive, and that adjusts the predicted self-discharge rate of the back-up battery in accordance with measured temperature values. In addition, it is desirable to provide a prediction method that approximates temperature values during periods when actual temperature measurements cannot be made, such as, for example, when the electronic module is turned off. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     BRIEF SUMMARY 
     According to various exemplary embodiments, devices and methods are provided for estimating the remaining state-of-charge of a non-rechargeable power source that is typically used as a back-up battery for an electronic module, in the event of a loss of primary power. One exemplary method comprises the steps of:
     a) calculating an adjusted self-discharge rate of the non-rechargeable power source, based on its nominal self-discharge rate being adjusted for temperature;   b) calculating a quantity of parasitic leakage current draw from the non-rechargeable power source;   c) calculating a quantity of voltage sampling current draw from the non-rechargeable power source;   d) calculating a quantity of transient current draw from the non-rechargeable power source; and   e) calculating an estimated remaining state-of-charge of the non-rechargeable power source by reducing the initial rating of the non-rechargeable power source in accordance with the temperature-adjusted self-discharge rate, the quantity of parasitic leakage current draw, the quantity of voltage sampling current draw, and the quantity of transient current draw.   

     One exemplary device comprises an electronic module connected to a primary power source, with the electronic module having a back-up non-rechargeable power source with an initial ampere-hour rating and a nominal self-discharge rate. The exemplary device includes a processor that is configured to monitor the temperature of the back-up non-rechargeable power source, and to calculate the various cumulative current draws from the back-up non-rechargeable power source. These current draws typically include parasitic leakage current (when the back-up power source is connected to the electronic module), check current (during voltage sampling), and transient current (when the electronic module is operating at reduced power from the back-up power source, due to the loss of primary power). 
     The processor is further configured to calculate an estimated remaining state-of-charge of the back-up non-rechargeable power source, based on the reduction of the initial ampere-hour rating due to a temperature-adjusted amount of self-discharge, a cumulative parasitic leakage current draw, a cumulative check current draw, and a cumulative transient current draw. A predetermined state-of-charge threshold is typically set to enable the activation of some type of indicator when the estimated remaining state-of-charge value falls below the predetermined threshold level. As such, a user can be advised to replace the back-up non-rechargeable power source on a timely basis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIG. 1  is a block diagram of an exemplary electronic module connected to a primary power source and a back-up power source; and 
         FIG. 2  is a flow diagram of an exemplary algorithm for predicting an estimated state-of-charge of a non-rechargeable power source. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     Various embodiments of the present invention pertain to the area of estimating the state-of-charge of a non-rechargeable power source. When this type of power source is used as an emergency back up for a primary power source, it is desirable to predict the end of useful life of the back-up power source in order to replace it in a timely fashion. In order to make as accurate a state-of-charge prediction as possible, certain ancillary factors, such as temperature variations and various types of leakage current draws, can be considered in addition to the nominal shelf-life rating of the back-up power source. Therefore, a prediction algorithm that includes the shelf-life rating and the relevant ancillary factors can be developed to make an accurate estimation of the state-of-charge of a non-rechargeable power source. 
     According to an exemplary embodiment of a system for predicting the state-of-charge of a non-rechargeable power source, as shown in  FIG. 1 , an electronic module  102  is installed within a vehicle  100 . Electronic module  102  is normally powered by a primary power source  104 , such as the main battery of vehicle  100 . A non-rechargeable power source  106  is typically connected to electronic module  102  via some type of electronic switching circuit (e.g., a transistor) that enables non-rechargeable power source  106  to provide emergency operating power to electronic module  102  in the event of a loss of primary power source  104 . In addition, a temperature-sensing device  108 , such as a thermistor, is typically coupled to non-rechargeable power source  106 , as will be described more fully below. 
     In this exemplary embodiment, electronic module  102  represents a telematics communication device between a driver and a service call center, e.g., the OnStar system. This type of system typically enables a driver to access a variety of services (navigation instructions, roadside assistance, emergency help, etc.) by simply pressing an appropriate button associated with the electronic module  102 , which can automatically provide a wireless connection to an OnStar call center. In the event of a loss of primary power source  104 , electronic module  102  can continue to operate for a limited period of time with power from back-up power source  106 . For example, a typical back-up power source  106  can operate electronic module  102  at essentially full power for approximately 300 seconds, although other embodiments may have widely varying parameters. During this limited operating time, the driver can generally use the OnStar system to request emergency assistance. When no call is being made, electronic module  102  is typically switched into a low power mode in order to conserve the charge state of back-up power source  106 . This low power mode is typically designated as a “monitor” mode, in which electronic module  102  can still receive certain signals, such as an airbag release signal. 
     Back-up power source  106  is typically a battery, with an initial ampere/hour rating and a nominal self-discharge rate, as generally supplied by the battery manufacturer. The nominal self-discharge rate, however, is quite sensitive to temperature variations. For example, a typical nominal self-discharge rate is approximately 1.3% per year at an ambient temperature of 20 degrees C. However, this exemplary rate typically doubles for every 10 degree increase, and typically halves for every 10 degree decrease in temperature, in accordance with the previously noted Arrehnius rate law. That is, at 30 degrees C., the rate would be approximately 2.6%, and at 10 degrees C., the rate would be approximately 0.65%. As such, an accurate determination of self-discharge rate generally depends on the corresponding temperature of the back-up battery. 
     According to an exemplary embodiment of a prediction algorithm for the state-of-charge of a back-up battery (BUB)  106 , a nominal self-discharge rate (R) can be adjusted for temperature variations, as typically measured by a temperature-sensing device  108  coupled to BUB  106 , and electrically connected to measuring circuits within electronic module  102 . That is, for any temperature sample (j), a temperature adjustment factor can be expressed as follows, based on a nominal self-discharge rating specified at 20 degrees C., and in accordance with the temperature relationship described above:
 
2 (Temp(j)−20)/10 
 
     That is, for a temperature reading (Temp(j)) of 20 degrees C., this temperature-adjustment expression would yield a value of 1; for a reading of 30 degrees C., the value would be 2, and for a reading of 10 degrees, the value would be ½. As noted above, these temperature-adjustment values would be in agreement with the exemplary specifications cited. Where there may be a different known relationship between temperature and self-discharge rate, a mathematical expression corresponding to that relationship can be used in a similar manner to determine an appropriate temperature adjustment factor. 
     An exemplary self-discharge factor (E 1 ), based on the above described temperature-adjustment expression, can be developed for a number of measurements made over a period of time, where an exemplary period of time is one year, to correspond to the typical self-discharge rating. For example, if a measurement is taken once every 10 minutes (measurement interval D=10), the number of intervals of the time duration (Y) during a year would equal 525,960/D, or 525,960/10 minutes. Therefore, an exemplary temperature-adjusted self-discharge factor E 1  can be expressed in accordance with the previously described temperature/self-discharge relationship as follows:
 
 E   1 =1 /Y* 2 (Temp(j)−20/10   (Equation 1)
 
     For a nominal self-discharge rate R and an initial ampere/hour rating C 0  in the exemplary algorithm embodiment, the loss of battery charge quantity due to the temperature-adjusted self-discharge factor E 1  can be expressed on the basis of averaging a summation of temperature-adjusted self-discharge factors over a given period of time:
 
C 0 *SUM(R*E 1 ).
 
     The preceding loss of charge expression is dependent on actual temperature measurements, which generally require the electronic module  102  to be in an “ON” state. When electronic module  102  is “OFF”, as is normally the case when the vehicle is parked, for example, the temperature measuring circuitry is typically inoperative. Since there is still a self-discharge of BUB  106  during electronic module  102  OFF time, a calculation of an average self-discharge value can be used in place of actual self-discharge measurements. Therefore, an exemplary embodiment of a calculated temperature-interpolated self-discharge factor E 2  can be expressed as follows:
 
 E   2   =T   off *(SUM(1 /Y* 2 (Temp(k)−20)/10 )/ V +SUM(1 /Y* 2 (Temp(m)−20)/10 )/ V ))/2  (Equation 2)
         where T off =the number of hours that electronic module  102  is in the Off state for the measured fraction of a year, and V is a non-OFF state interval between 1 and 24 hours, and   where Temp(k)=the temperature values measured in a V period before the Off State;   Temp(m)=the temperature values measured in a V period after the Off State;   k=the sample interval in a V period before the Off State; and
           m=the sample interval in a V period after the Off State.   
               

     Therefore, given the nominal self-discharge rate R and initial ampere/hour rating C 0  in the exemplary algorithm embodiment, the loss of battery charge quantity due to the self-discharge factor E 2  can be expressed on the basis of averaging a summation of temperature-adjusted self-discharge factors prior to and after an electronic module  102  OFF period of time as:
 
C 0 *SUM(R*E 2 ).
 
     A typical BUB  106  is also subject to loss of charge due to various current draws. These draws can include, for example, parasitic leakage current (I L ), voltage check current (I C ), and BUB  106  monitor current (I M ), as will be explained below. For each type of exemplary current draw, the corresponding active time period can be multiplied by the respective current draw to determine the cumulative current draw loss of charge factors. 
     Parasitic leakage current (I L ) generally occurs whenever BUB  106  is electrically connected to electronic module  102 , for a time period T L . Voltage check current (I C ) can occur whenever BUB  106  is checked for electrical connection to electronic module  102 , for a time period T C . BUB monitor current (I M ) can occur when primary power source  104  is inactive, and electronic module  102  is operating from BUB  106  in a low power (BUB monitor) mode, for a time period T M . Typically, the values of I L , I C , and I M  are predetermined by the electronic module  102  manufacturer, and are generally used as static values in the exemplary algorithm. Time period T L  is typically measured by a timer running continuously in electronic module  102 , time period T C  is generally predetermined as a static value by the electronic module  102  manufacturer, and time period T M  is typically accumulated by a timer in electronic module  102  whenever electronic module  102  is operated in the BUB monitor mode. Therefore, the above described exemplary loss of charge terms can be expressed as (I L *T L ), (I C *T C ), and (I M *T M ). 
     In accordance with the above described loss of charge factors, a general exemplary prediction algorithm for the estimated state-of-charge of BUB  106  can be expressed as follows:
 
 C ( N )= C   0 *{1−SUM[ R*E   1   +R*E   2   ]}−I   L   *T   L   −I   M   *T   M   −I   C   *T   C   (Equation 3)
 
where N is the point in time a measurement is taken.
 
     The exemplary prediction algorithm (Equation 3) is depicted as a flow diagram  200  in  FIG. 2 . Initially, the predetermined parameters are set: C 0  in step  202 , R in step  204 , I L , I M , I C , T C  in step  206 , and j in step  208 . Step  209  indicates that the parasitic leakage term I L *T L  is being accumulated whenever electronic module  102  is connected to BUB  106 . 
     Step  210  represents the state of electronic module  102 . When electronic module  102  is ON (“Yes” in step  210 ), and if primary power source  104  is supplying power to electronic module  102  (“Yes” in step  212 ), the temperature-adjusted self-discharge quantity C 0 *SUM(R*E 1 ) can be calculated (step  214 ), as described above, and applied to Equation 3, as part of step  222 . 
     Step  213  represents an I C *T C  value, which typically occurs when a voltage check is made at ignition turn on to ascertain whether or not BUB  106  is present. When the voltage check is made, I C *T C  can be calculated by the processor in electronic module  102  and applied to Equation 3, as part of step  222 . 
     If there is a loss of primary power source  104  (“No” in step  212 ), electronic module  102  is automatically switched over to a BUB  106  power level (step  215 ). If full power operation of electronic module  102  is required, the exemplary algorithm no longer applies (“End”), since BUB  106  will operate at its full capacity. Otherwise, electronic module  102  is placed in the “BUB Monitor” state, as previously described, and the I M *T M  term can be calculated (step  216 ) and applied to Equation 3, as part of step  222 . 
     Referring again to step  210 , if electronic module  102  is in the OFF state (“No” in step  210 ), the temperature-interpolated self-discharge quantity C 0 *SUM(R*E 2 ) can be calculated, as previously described (step  220 ), and applied to Equation 3, as part of step  222 . Finally, in step  222 , a predicted value of the remaining state-of-charge of BUB  106  (C(N)) can be calculated in accordance with Equation 3. That is, the initial charge C 0  is reduced by the various discharge factors, as implemented by the exemplary algorithm represented by Equation 3:
 
 C ( N )= C   0 *{1−SUM[ R*E   1   +R*E   2   ]}−I   L   *T   L   −I   M   *T   M   −I   C   *T   C .
 
     Accordingly, the shortcomings of the prior art have been overcome by providing an improved prediction algorithm for estimating a remaining state-of-charge of a back-up power source (BUB). The exemplary algorithm adjusts and interpolates the nominal self-discharge rate of the BUB for temperature variations during the self-discharge period. The exemplary algorithm also calculates relevant current draws from the BUB during various modes of operation, in order to arrive at an accurate state-of-charge estimate without the use of intrusive voltage measurements that can shorten the life of the BUB. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.