Abstract:
A method includes determining that a battery has entered a transition phase based on the occurrence of a change in direction of current flow through the battery. In response to determining that the battery is in the transition phase, a capacity of the battery is determined based on a transition phase battery capacity model of capacity-vs.-voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation of U.S. patent application Ser. No. 10/982,461, filed Nov. 5, 2004 now U.S. Pat. No. 7,107,161, hereby incorporated herein by reference. 

   TECHNICAL FIELD 
   The present application relates generally to batteries. More particularly, the present application relates to accurately reporting the capacity of a battery. 
   BACKGROUND 
   Many mobile computing and communicating devices rely upon standard battery cells for providing power on which to operate. Though disposable battery cells, such as alkaline cells, are a well-known and reliable technology, it is common in such mobile devices to employ rechargeable battery cells. These rechargeable batteries depend on a number of known cell types, including Ni-Cad, Ni-MH, and Li-Ion cells. All these cells are known to those of skill in the art, as are some of their deficiencies. One of the known deficiencies of the above mentioned rechargeable battery cells is related to the fact that each battery has a finite life span that can be measured in terms of recharge cycles. The process of charging and discharging the cell damages the cell&#39;s charge storage capabilities, causing the stored potential, which is typically measured in mA-hours, to decrease over the life of the battery. As the ability to store charge decreases, so does the battery&#39;s utility. The life of the battery can be drastically curtailed by improperly charging, or over discharging the battery. As a result of these deficiencies, it is crucial that a user be able to determine the capacity of a battery both prior to and during the usage. 
   A state of the art technique for battery capacity reporting relies on the coulomb counter. The principle of operation involved in coulomb counting is computing the difference between the coulombs injected into a battery and the coulombs taken out of the battery. The capacity of the battery is then reported by comparing the coulomb count relative to a reference coulomb count value that corresponds to maximum battery capacity. For instance, if the coulomb count of a battery is half of the reference value, the battery capacity is reported to be 50 percent. Although the coulomb counter addresses battery capacity reporting, it may have several problems. First, the reported capacity may not be meaningful if an accurate reference coulomb count value corresponding to maximum battery capacity is not known. Furthermore, with a coulomb counter it may be difficult to keep an accurate reference coulomb count, particularly when battery capacity decreases over the lifetime of the battery. Further still, with a coulomb counter it may be necessary to know the current battery capacity before beginning the coulomb count. 
   A limitation of the coulomb counting principle is that it may not be applicable to reporting the capacity of a battery of initially unknown battery capacity: if the capacity of a battery is to be reported using the coulomb count system and method, the battery may have to be taken from its unknown capacity state to either a fully charged 100 percent battery capacity state or to a fully discharged 0 percent capacity state before the coulomb count can be used. Because the state of the battery is unknown at a certain point, the only way to charge the battery to 100% capacity is to constantly provide charge over an extended length of time. This can result in an overcharging of the cell, which is known to damage the storage capability of the cell. Conversely, to guarantee that the cell is at 0% capacity, the cell must be completely discharged. Rechargeable batteries are possibly permanently damaged by being overly discharged. 
   Further practical limitations exist with coulomb counting techniques. In practice, coulomb counting works by applying integration over time. The presence of an offset in a coulomb counter may result in the inaccuracy of the coulomb count. This applies even to batteries with an assumed initially known battery capacity, and is compounded with every recharge cycle. This may be especially true if the battery needs to be used for a long period of time between opportunities to reset the coulomb counter. For instance, in a battery that needs to be used for 3 weeks between charges, even small offsets with each charge cycle may accumulate to become large inaccuracies in reported capacity. 
   Other known existing techniques of battery capacity reporting are primarily based on measuring battery voltage. 
   Batteries have known characteristic charge and discharge curves.  FIG. 1  illustrates a charge curve model  130  and a discharge curve model  140  for a battery. These curves relate battery voltage  110  to capacity percentage  120  for a rechargeable battery. Battery capacity percentage  120  is related to battery voltage  110  in either a discharging state, shown by the discharge curve model  140 , or the charging state shown by the charge curve model  130 . Illustrated is a multiplicity of points such as point  132  on the charge curve model  130  and point  142  on the discharge curve. Interpolation can be used to provide capacity values  120  for voltages  110  that lie between points for which values are known. In reference to  FIG. 1 , the relationship between battery voltage  110 , battery charge state and capacity  120  is illustrated by two curve models  130 , 140 . The first curve model  130  corresponds to a positive battery charge current or battery charging state, and the second curve model  140  corresponds to a negative battery charge current or battery discharging state. 
   When the battery is in a charging state, a charge curve corresponding to the charging state is utilized. When the battery is in a discharging state, a discharge curve corresponding to the discharging state is utilized. The charge and discharge curves are such that given a battery voltage value and a charge curve or a discharge curve, it is possible to obtain a corresponding capacity value from the curves. 
   Though it is possible to determine the capacity of a battery by measuring the voltage of the battery and examining the curves, it should be noted that the existence of two distinct curves presents a problem. For example, when a battery voltage is 3.8 V and a power source is plugged into the battery at this time, according to the discharge state curves, there is an abrupt drop of the reported battery capacity from 52% to 17%. The reported result is not correct. Actually, a battery enters a transition phase P 1  from discharging to charging when a power source is plugged in while the battery is discharging. After the transition phase P 1 , the battery goes into the charging state. Similarly, when a power source is removed while charging a battery, for example, at a battery voltage 3.9V, there is an abrupt rise of the reported battery capacity from 49% to 71% based on the charging curve and the discharging curve of  FIG. 1 . Actually, a battery enters a transition phase P 2  from charging to discharging when a power source is removed while charging the battery. After the transition phase P 2 , the battery goes into the discharging state. Under the above circumstances, the reported result will not be correct if the discharging curve and the charging curve of  FIG. 1  are used to report the battery capacity in the transition phases. 
   SUMMARY 
   A method includes determining that a battery has entered a transition phase based on the occurrence of a change in direction of current flow through the battery. In response to determining that the battery is in the transition phase, a capacity of the battery is determined based on a transition phase battery capacity model of capacity-vs.-voltage. 
   The end of the transition phase can be determined based on the lapse of a predetermined time period from the start of the transition phase. It can also be determined based on the battery&#39;s voltage having changed by a predetermined voltage value. It can further be determined based on the transition phase battery capacity model and a non-transition battery capacity model of capacity-vs.-voltage yielding the same capacity value for a given measured voltage of the battery. The method can be performed by a handheld device. 
   Another method includes monitoring the output voltage of a power source configured to charge a battery. The occurrence of the battery having entered a transition phase is determined based on sensing an abrupt change in the output voltage to or from a predetermined value. In response to determining that the battery is in the transition phase, a capacity of the battery is determined based on a transition phase battery capacity model. 
   A transition phase battery capacity model can be determined from both a non-transition battery capacity charge model and a non-transition battery capacity discharge model, all three models defining a battery&#39;s capacity as a function of its voltage. Each capacity value of the transition phase model for a given voltage is calculated based on a weighted average of the two capacity values yielded by the two non-transition models for the given voltage. The transition phase model can then be used to determine capacity of a battery. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates two example curve models, a charge and a discharge curve model, relating battery voltage to capacity percentage for a rechargeable battery. 
       FIG. 2  is a block diagram of an example mobile communication device. 
       FIG. 3  illustrates an example method for reporting battery capacity. 
       FIG. 4  illustrates an example of a transition battery capacity model curve for reporting a battery capacity in a transition phase from discharging to charging. 
       FIG. 5  illustrates an example of a transition battery capacity model curve for reporting a battery capacity in a transition phase from charging to discharging; and; 
       FIG. 6  is a flowchart illustrating an example method to carry out step  360  of  FIG. 3  according to the transition battery capacity models of  FIGS. 4 and 5 . 
   

   DETAILED DESCRIPTION 
   Generally, the present application provides a method and system for reporting battery capacity accurately by means of a battery capacity transition phase model when an event occurs. The event may be the attachment or disconnection of the battery to a battery charger or power source, the occurrence of a fault condition such as power failure to the battery charge when the battery is attached, or the like. 
   The battery capacity transition phase model may be described as a function or may be described through interpolation of values stored in a look up table or array. 
   An example method for reporting battery capacity reports the battery capacity based on a transition phase battery capacity model during a transition phase. A transition phase battery capacity model relevant to the transition phase from discharging to charging and a transition phase battery capacity model relevant to the transition phase from charging to discharging are predetermined. Once it is determined that the battery is in a transition state, a discharging state, or a charging state, then a transition phase battery capacity model curve, a discharge curve or a charge curve is selected respectively. A voltage of the battery is then read, and a battery capacity is determined by using the selected curve. The transition phase battery capacity model is preferably a function associated with battery voltage, discharge curve and charge curve. This function may be expressed as an equation, a set of equations, a look up table, or the like. 
   Alternatively, considering battery temperature effects, temperature compensations regarding a transition phase battery capacity model curve, a discharge curve and a charge curve may be performed to obtain accurate battery capacities under different temperatures. 
   Alternatively, a plurality of transition phase battery capacity model curves, discharge curves and charge curves corresponding to a plurality of battery operating temperatures or a plurality of battery operating temperature ranges may be provided, so that a corresponding curve for reporting battery capacity cab be selected based on a current battery operating temperature to obtain an accurate battery capacity. 
     FIG. 2  is a block diagram of a mobile communication device  10  that may implement a system and method for accurately reporting battery capacity, as described herein. The mobile communication device  10  is preferably a two-way communication device having at least voice or data communication capabilities. The device preferably has the capability to communicate with other computer systems on the Internet. Depending on the functionality provided by the device, the device may be referred to as a data messaging device, a two-way pager, a cellular telephone, a wireless Internet appliance or a data communication device (with or without telephony capabilities). It should be understood, however, that battery capacity reporting and measurement may have applications other than in the field of mobile communicating and computing devices. 
   Where the device  10  is enabled for two-way communications, the device may incorporate a communication subsystem  11 , including a receiver  12 , a transmitter  14 , and associated components such as one or more, preferably embedded or internal, antenna elements  16  and  18 , local oscillators (LOs)  13 , and a processing module such as a digital signal processor (DSP)  20 . The particular design of the communication subsystem  11  is dependent upon the communication network in which the device is intended to operate. For example, a device  10  may include a communication subsystem  11  designed to operate within the Mobitex™ mobile communication system, DataTAC™ mobile communication system, General Packet Radio Service (GPRS) communication subsystem, CDMA communication system, and iDEN communication system. 
   Network access requirements may also vary depending upon the type of network  19 . For example, in the Mobitex™ and DataTAC™ networks, mobile devices are registered on the network using a unique personal identification number or PIN associated with each device. In GPRS networks however, network access is associated with a subscriber or user of a device  10 . A GPRS device therefore requires a subscriber identity module, commonly referred to as a SIM card, in order to operate on a GPRS network. Without a SIM, a GPRS device will not be fully functional. Local or non-network communication functions (if any) may be operable, but the device  10  may be unable to carry out functions involving communications over network  19 . When required network registration or activation procedures have been completed, a device  10  may send and receive communication signals over the network  19 . Signals received by the antenna  16  through a communication network  19  are input to the receiver  12 , which may perform such common receiver functions as signal amplification, frequency down conversion, filtering, channel selection and analog-digital conversion. Analog to digital conversion of a received signal allows complex communication functions, such as demodulation and decoding, to be performed in the DSP  20 . In a similar manner, signals to be transmitted are processed, including modulation and encoding for example, by the DSP  20  and input to the transmitter  14  for digital to analog conversion, frequency up conversion, filtering, amplification and transmission over the communication network  19  via the antenna  18 . 
   The DSP  20  not only processes communication signals, but also provides for receiver and transmitter control. For example, the gains applied to communication signals in the receiver  12  and transmitter  14  may be adaptively controlled through automatic gain control algorithms implemented in the DSP  20 . 
   The device  10  preferably includes a microprocessor  38  which controls the overall operation of the device. Communication functions, including at least one of data and voice communications, are performed through the communication subsystem  11 . The microprocessor  38  also interacts with further device subsystems such as the display  22 , flash memory  24 , random access memory (RAM)  26 , auxiliary input/output (I/O) subsystems  28 , serial port  30 , keyboard  32 , speaker  34 , microphone  36 , a short-range communications subsystem  40  and any other device subsystems generally designated as  42 . 
   Some of the subsystems shown in  FIG. 2  perform communication-related functions, whereas other subsystems may provide “resident” or on-device functions. Some subsystems, such as keyboard  32  and display  22  for example, may be used for both communication-related functions, such as entering a text message for transmission over a communication network, and device-resident functions such as a calculator or task list. 
   Operating system software used by the microprocessor  38  may be stored in a persistent store such as flash memory  24 , which may instead by a read only memory (ROM) or similar storage element. Discharge curves, charge curves and transition phase battery capacity models as discussed below may be pre-stored in memory  24 . The operating system, specific device applications, or parts thereof, may be temporarily loaded into a volatile store such as RAM  26 . Received communication signals may also be stored to RAM  26 . 
   The microprocessor  38 , in addition to its operating system functions, enables execution of software applications on the device. A predetermined set of applications which control basic device operations, including at least data and voice communication applications for example, will normally be installed on the device  10  during manufacture. One example application that may be loaded onto the device is a personal information manager (PIM) application having the ability to organise and manage data items relating to the device user such as, but not limited to e-mail, calendar events, voice mails, appointments, and task items. One or more memory stores may be available on the device to facilitate storage of PIM data items on the device. Such PIM application may have the ability to send and receive data items, via the wireless network. The PIM data items may be seamlessly integrated, synchronized and updated, via the wireless network, with the device user&#39;s corresponding data items stored or associated with a host computer system thereby creating a mirrored host computer on the mobile device with respect to the data items at least. This may be especially advantageous in the case where the host computer system is the mobile device user&#39;s office computer system. Further applications may also be loaded onto the device  10  through the network  19 , an auxiliary I/O subsystem  28 , serial port  30 , short-range communications subsystem  40  or any other suitable subsystem  42 , and installed by a user in the RAM  26  or a non-volatile store for execution by the microprocessor  38 . Such flexibility in application installation increases the functionality of the device and may provide enhanced on-device functions, communication-related functions, or both. For example, secure communication applications may enable electronic commerce functions and other such financial transactions to be performed using the device  10 . 
   In a data communication mode, a received signal such as a text message or web page download is processed by the communication subsystem  11  and input to the microprocessor  38 , which may further process the received signal for output to the display  22 , or alternatively to an auxiliary I/O device  28 . A user of device  10  may also compose data items such as email messages for example, using the keyboard  32 , which may be a complete alphanumeric keyboard or telephone-type keypad, in conjunction with the display  22  and possibly an auxiliary I/O device  28 . Such composed items may then be transmitted over a communication network through the communication subsystem  11 . 
   For voice communications, overall operation of the device  10  is substantially similar, except that received signals may be output to a speaker  34  and signals for transmission may be generated based on an input received through a microphone  36 . Alternative voice or audio I/O subsystems such as a voice message recording subsystem may also be implemented on the device  10 . Although voice or audio signal output may be accomplished primarily through the speaker  34 , the display  22  may also be used to provide an indication of the identity of a calling party, the duration of a voice call, or other voice call related information for example. 
   The serial port  30  in  FIG. 2  may be implemented in a personal digital assistant (PDA)-type communication device for which synchronization with a user&#39;s desktop computer may be desirable, the serial port  30  may enable a user to set preferences through an external device or software application and extend the capabilities of the device by providing for information or software downloads to the device  10  other than through a wireless communication network. The alternate download path may for example be used to load an encryption key onto the device through a direct and thus reliable and trusted connection to thereby enable secure device communication. 
   A short-range communications subsystem  40  may be included to provide for communication between the device  10  and different systems or devices, which need not necessarily be similar devices. For example, the subsystem  40  may include an infrared device and associated circuits and components or a Bluetooth™ communication module to provide for communication with similarly-enabled systems and devices. 
   A charging subsystem  44  may be included to provide power for the device  10  and different subsystems or devices. For example, the charging subsystem  44  may determine the presence of detachable power source device  46  and associated circuits, such as an AC adapter, USB cable, or car adapter to provide power for the device and to charge battery  48 . Additionally, charging subsystem  44  may determine the absence of power source device  46 , and consequently obtain power for the device  10  from battery  48 . Generally speaking, when power source device  46  is disconnected to charging subsystem  44  and battery  48  powers device  10  alone, battery  48  is said to be in a discharging state. Conversely, when power source device  46  is connected to charging subsystem  44  and powers device  10 , and charging subsystem  44  charges battery  48 , battery  48  is said to be in a charging state. Actually, there is a transition phase from charging to discharging before battery  48  enters into a discharging state from a charging state, and a transition phase from discharging to charging before battery  48  enters into a charging state from a discharging state. The present application describes an example system and method for reporting the capacity of a battery, such as battery  48 , during transition phases. 
   The battery capacity reported is a function of several factors, including battery voltage, battery charging current, and so on. The relationship between battery voltages, battery charging currents, and battery capacity is modelled using charge curves such as those illustrated in  FIG. 1 . Therefore, before describing embodiments of the method and system in detail, several concepts will be defined for greater certainty. 
   As used in this description and in the appended claims, the battery voltage is defined as the voltage differential between positive and negative terminals of the battery. 
   As used in this description and in the appended claims, battery charging current is defined as a current flowing into the battery. Battery charging current is capable of taking on a signed value, with a positive value meaning current being delivered into the battery and a negative value meaning current drawn out of the battery. 
   As used in this description and in the appended claims, a state of a battery is one of a charging state corresponding to a positive battery charging current value and a discharging state corresponding to a negative battery charging current value. A discharge curve model or a charge curve model is defined as the relationship between battery voltage, battery charging current and capacity so that given battery voltage and battery charging current, capacity can be determined by applying the capacity curve model. 
   When there is a change in the direction of a battery charging current or a change in the sign of a battery charging current value, for example, if the change is from delivering into a battery to drawing out of the battery or from a positive current value to a negative current value, it is determined that a battery enters a transition phase from a charging state; if the change is from drawing out of the battery to delivering into the battery or from a negative current value to a positive current value, it is determined that a battery enters a transition phase from a discharging state. Alternatively, as shown in  FIG. 2 , by monitoring a battery&#39;s power source connector, when there is an abrupt voltage change from 0V to a predetermined voltage value, it is determined that the battery  48  enters into a transition phase from discharging to charging. When there is an abrupt voltage change from the predetermined voltage value to 0V, it is determined that the battery  48  enters into a transition phase from charging to discharging. It should be understood that there are various methods to determine if a battery is in a transition phase or in a charging state or discharging state. 
   Referring to  FIGS. 1 and 2 , the example method may use a system, such as system  10  of  FIG. 2 , including a charging subsystem  44 , to assist in determining values for the battery voltage  110  and battery capacity  120 . The charging current can be used to determine the state and select either one of the curve models  130 ,  140 . The charging subsystem  44  may be capable of performing several operations such as constant current charging operation and constant voltage charging operation. 
     FIG. 3  illustrates an example method for reporting battery capacity. At step  305 , a battery identification (ID) is provided to identify the type of the battery. At step  310 , a discharge curve model, such as  140 , corresponding to the battery ID is provided. At step  320 , a charge curve model, such as  130 , corresponding to the battery ID is provided. At step  330 , with respect to the battery ID, a transition phase battery capacity model F 1  corresponding to a transition phase P 1  from discharging to charging and a transition phase battery capacity model F 2  corresponding to a transition phase P 2  from charging to discharging are provided for reporting battery capacities during the transition phases P 1  and P 2 . A transition phase P 1  and a transition phase P 2  are defined and provided. Models F 1  and F 2  may have a variety of forms from simple to complicated. More complicated models may more accurately report the capacity with less error at the expense of higher computational complexity. Models F 1  and F 2  may be of different or the same form. 
   Models F 1  and F 2  may be predetermined by experimentation. A transition phase P 1  from discharging to charging and a transition phase P 2  from charging to discharging are defined by means of a battery voltage change amount or by means of time change amount from the point where the charging or discharging state changes, that is, from the point when a battery is connected to a power source or a battery is disconnected from a power source. A transition phase is deemed to be over after a defined transition phase. For example, if a battery voltage change amount is used to define the transition phase, the voltage change amount may range from 0.05V to 0.3V. Similarly, if a time change amount is used to define the transition phase, the time change amount may range from 0.5 hours to 3 hours when the system is in a standby mode. Alternatively, if a battery capacity determined from a transition phase battery capacity model F 1  corresponding to a transition phase from discharging to charging and a battery capacity determined from a charge curve model are same, the transition phase from discharging to charging is deemed to be over. Similarly, if a battery capacity determined from a transition phase battery capacity model F 2  corresponding to a transition phase from charging to discharging and a battery capacity determined from a discharge curve model are same, the transition phase from charging to discharging is deemed to be over. 
   After the transition phase, a battery enters the charging state or the discharging state. The criterions of modeling capacity curves during the transition phase are to make them approach the actually measured capacity curves so as to minimize the capacity reporting error. The transition phase battery capacity model corresponding to the transition phase P 1  and the transition battery capacity model corresponding to the transition phase P 2  may be described by two functions. The transition phase battery capacity functions may be determined based on the discharge curve model  140  and the charge curve model  130  of  FIG. 1 , as described below with reference to  FIG. 4 . 
   At step  340 , battery voltage is provided to determine battery capacity subsequently. At step  350 , battery current is provided. By detecting a change in the direction or a change in sign of battery current value, it may be determined if the battery is in a transition phase, in a discharging state or in a charging state. At step  355 , the battery temperature is measured. At step  360 , the transition phase battery capacity model corresponding to the transition phase P 1  or the transition phase battery capacity model corresponding to the transition phase P 2  is applied to determine a battery capacity based on a battery voltage. Step  360  is described in detail below with reference to  FIG. 6 . 
     FIG. 4  illustrates an example transition battery capacity model curve for reporting a battery capacity in a transition phase from discharging to charging. In this example, the transition battery capacity model is described by a transition phase battery capacity function. 
   In  FIG. 4 , a battery.  48  is assumed to be initially discharging  140  and at voltage  110  of 3.75V corresponding to point  442 . Consequently, a 37% capacity is determined. Next, the battery enters the charging state, for instance if the power source  46  of  FIG. 2  is connected while the battery is in use. 
   A battery that has been discharging and has a voltage reading of 3.75V can be determined to be 37% full by directly mapping from the initial discharge curve, corresponding to a discharging state. If a power source  46  is plugged in at this point, then the battery&#39;s capacity would erroneously be determined to be 10% full, according to the point where 3.75V maps on the new charging curve model  130  corresponding to a charging state. If that value were reported directly, then the user would see an incorrect capacity. Actually, the battery takes some time to reach to the charging curve model  130 ; that is, there is a transition phase P 1  from discharging to charging. A measured relationship curve  440  between capacity and voltage during a transition phase P 1  from discharging to charging starts at point  442  corresponding to the discharging curve  140  and ends at point  434  corresponding to the charging curve model  130 . A transition battery capacity function F 1  corresponding to a transition phase P 1  from discharging to charging is predetermined for reporting the battery capacity during the transition phase P 1 , and the determined function F 1  curve  450  approaches the measured relationship curve  440 . A transition phase battery capacity function F 1  curve  450  starts at point  442  and ends at point  432 . It can be seen from  FIG. 4  that the curve  450  closely matches the behaviour of the measured relationship curve  440  such that any discrepancy in capacity is within 6%. Various transition phase battery capacity functions F 1  may be used as long as the capacity reporting error is less than measured error 6%. More complicated functions may lead to more accurate battery capacity values and less capacity reporting error such as 2%, 1% or less. The following description concerns an example transition phase battery capacity function F 1  based on a discharge curve and a charge curve. 
   In order to create a transition phase capacity function F 1  corresponding to a transition phase P 1  from a discharging state to a charging state, a real transition phase curve is determined through measurement, and then a function with a curve approaching the real transition phase curve is determined as a function F 1  for a transition phase P 1  from a discharging state to a charging state. Model F 1  may be varied with different complexity. More complicated models may more accurately report the capacity with less error at the expense of higher computational complexity. 
   A transition phase battery capacity function F 1  corresponding to a transition phase P 1  from a discharging state to a charging state may be formulated as:
 
 F 1( V )=(1 −a )× F   discharge ( V )+ a×F   charge ( V ), where  a =( V−V   start )/Δ V , wherein
 
   V is a voltage value  110  during the transition phase P 1 , and 
   ΔV defines the transition phase P 1  which is a battery voltage change amount between an end voltage and a start voltage in the transition phase P 1 ; that is, ΔV=V end −V start , wherein V start  is the battery voltage at the start of the transition phase P 1  and V end  is the battery voltage at the end of the transition phase P 1 . In this example, ΔV is a constant, for example, ΔV=0.2 Volts. 
   F discharge  (V) corresponds to the discharge curve model  140 . It is a function of battery voltage and provides a battery capacity corresponding to a battery voltage. 
   F charge (V) corresponds to the charge curve model  130  as a function of battery voltage and provides a battery capacity corresponding to a battery voltage. According to the transition phase battery capacity function F 1 , the battery capacity corresponding to the transition phase P 1  can be calculated. As shown in  FIG. 4 , the capacity reporting error is almost within 6% capacity. 
     FIG. 5  illustrates an example of a transition battery capacity model for reporting a battery capacity in a transition phase from charging state to discharging state. In this example, the transition battery capacity model is described by a transition battery capacity function. 
   In  FIG. 5 , a battery  48  is assumed to be charged following curve  130  to voltage of 3.97V corresponding to point  542 . Consequently, a 65% capacity is determined. Next, once the power source is removed, the battery enters the discharging state, for instance, if a power source  46  of  FIG. 2  is disconnected while it is charging the battery  48 . 
   The battery that has been charging with a voltage reading of 3.97V may be determined to be 65% full by directly mapping using charging curve  130 . If the power source  46  is removed at this point, then the battery&#39;s capacity may erroneously be determined to be 81% full, according to where 3.97V maps on the discharge curve model  140  corresponding to a discharging state. If that value was reported directly, then the user would see an incorrect capacity, the battery capacity jumps while the power source is removed. Actually, the battery takes some time to reach the discharge curve model  140 ; that is, there is a transition phase P 2  from the charging state to the discharging state. A measured capacity curve  540  during a transition phase P 2  starts at point  542  on the charging curve model  130  and ends at point  534  on the discharging curve model  140 . A transition battery capacity function F 2  corresponding to the transition phase P 2  from the charging to discharging is determined to report the battery capacity in the transition phase P 2  and approaches the measured curve  540 . The determined function F 2  curve  550  is used to report a battery capacity and corresponds to an example transition battery capacity function F 2  curve that starts at point  542  and ends at point  532 . It can be seen from  FIG. 5  that the function curve  550  is very close to the measured curve  540 . In fact, the capacity discrepancy error may fall within 2%. Various transition battery capacity functions F 2  may be provided such that the capacity reporting error may be less than measured 2%. More complicated functions may more accurately report the capacity at the expense of higher computational complexity. The capacity error may be minimized with a complex transition phase battery capacity function F 2 . To create a function F 2  corresponding to the transition phase P 2  from the charging state to the discharging state, a real transition phase curve is determined through measurement, then a function closely mimicking the behavior of the real transition phase curve is determined as the function F 2  in the transition phase P 2 . A plurality of transition phase battery capacity functions F 2  may be determined and used to report battery capacity. 
   In one example:
 
 F 2( V )= F   discharge ( V )×SQRT(( V   start   −V )/Δ V )+ F   charge ( V )×(1−SQRT(( V   start   −V )/Δ V ))
 
   where V is the battery voltage  110  during the transition phase P 2  from the charging state to the discharging state. 
   F discharge (V) corresponds to the discharge curve model  140 . It is a function of voltage V. It provides the battery capacity corresponding to the battery voltage. 
   F charge (V) corresponds to the charging curve model  130 . It is the function of voltage V, and it provides the battery capacity corresponding to the battery voltage. 
   ΔV=V start −V end , wherein ΔV defines the transition phase P 2  which is a battery voltage change amount between a start voltage and an end voltage in the transition phase P 2 ; V start  is the battery voltage at the start of the transition phase P 2 , and V end  is the battery voltage at the end of the transition phase P 2 . In this example, ΔV is a constant, for example, ΔV=0.15V. 
   Based on the transition phase battery capacity function F 2 , the battery capacity corresponding to the transition phase P 2  can be calculated. As shown in  FIG. 5 , the capacity reporting error is within 2%. The capacity reporting error may be greatly reduced to less than 2% if other functions are used. 
   The transition phase battery capacity functions described provide examples for the purpose of illustration. It should be understood, however, that many linear or non-linear functions can be used. If more complicated functions were used, then the capacity reporting would be more accurate. The transition phase battery capacity function may be a function of several factors, including battery voltage, battery charge curve, and battery discharge curve. It also could be a function of time. When a transition phase is defined by a time change amount, the transition phase battery capacity may be a function of time, charging curve and discharging curve. 
   In one example, a plurality of charge curve models, discharge curve models and transition phase battery capacity models, each having a unique battery ID, are provided. A charge curve model, a discharge curve model or a transition phase battery capacity model is selected for determining battery capacity based on a battery ID. 
   In another example, a plurality of charge curve models  130 , discharge curve models  140  and transition phase battery capacity models wherein each of the models relates to a battery ID and a battery operating temperature or a temperature range are provided. For example, models may be provided for battery operating temperatures such as −20° C., −15° C., −5° C., 5° C., 15° C., 25° C., 35° C., 45° C. and/or 50° C. or battery operating temperature ranges of −20° C. to −10° C., −10° C. to 0° C., 0° C. to 10° C., 10° C. to 20° C., 20° C. to 30° C., 30° C. to 40° C. and/or 40° C. to 50° C. A temperature range such as from −20° C. to 50° C. may be divided into intervals. For example, an interval size of 5° C. or less may be used. Alternatively, the temperature range may be divided unevenly. A charge curve model, a discharge curve model or a transition phase model corresponding to a temperature closest to the current battery operating temperature is selected and used to report battery capacity. Alternatively, a charge curve model, a discharge curve model or a transition phase battery capacity model corresponding to a temperature range such as 20° C. to 30° C. containing a current battery operating temperature such as 24° C. may be selected and used to report battery capacity. 
   The transition phase battery capacity models described above may be linear or non-linear functions. 
   In a further example, instead of providing a plurality of models as above, a discharge model, a charge model, a transition phase model from charging to discharging, and a transition phase model from discharging to charging corresponding to a reference temperature or a reference temperature range may be provided and set as reference models. The reference temperature may be a particular temperature such as 22° C., and the reference temperature range may be a particular temperature range such as 20° C. to 25° C. A plurality of battery capacity offsets, wherein each corresponds to a battery ID and a temperature range or a temperature, are predetermined for compensating determined battery capacities. If a current battery operating temperature is a reference temperature or within a reference temperature range, no temperature compensation is required; that is, a zero battery capacity offset is applied. Otherwise, a corresponding temperature offset is applied to a battery capacity reported from a reference model. For example, when a battery operating temperature is 30° C., a battery capacity offset 1% is applied to a battery capacity obtained from a reference model corresponding to a reference range such as 20° C. to 25° C. 
     FIG. 6  is a flowchart illustrating an example method to carry out step  360  of  FIG. 3 , according to the transition battery capacity functions of  FIGS. 4 and 5 . 
   At step  630 , a determination is made as to whether the battery is charging or discharging. For instance, if a battery charging current is determined, the state can be derived from the sign of the charging current. At step  630 , if it is determined that the battery is being charged, the process continues to step  620 . At step  620 , the charging subsystem  44  determines if the power source  46  is removed while charging the battery  48 , for example, by checking a change in the direction or sign of battery current. It should be understood that a variety of methods may be implemented for determining if the battery enters a transition phase. If the power source  46  is removed, the battery  48  enters a transition phase P 2  from charging to discharging, and then at step  622  a transition phase battery capacity model F 2  corresponding to the transition phase P 2 , which provides a minimized battery capacity error, is selected, for example, according to a battery ID. Alternatively, a different transition phase battery capacity model F 2  may be selected according to the requirements of capacity reporting error and computational complexity. At step  624 , a battery voltage is read, and the process proceeds to step  626 . At step  626 , according to the selected battery capacity model F 2  at step  622  and the obtained battery voltage at step  624 , a battery capacity is determined, for example, by calculating the selected battery capacity model or by looking up a table corresponding to the selected battery capacity model. A plurality of tables, wherein each table corresponds to a transition phase battery capacity model, may be predetermined and pre-stored in memory  24 . At step  627 , it is determined if the power source  46  is connected. If yes, the battery enters into another transition phase and the process goes to step  612  and proceeds to the subsequent steps. If no, the process proceeds to step  628  where a determination is made to see if the transition phase P 2  is over. For example, if a battery voltage change amount such as 0.2V is used to define the transition phase, it is determined that the transition phase is over when battery voltage is changed by the battery voltage change amount such as 0.2V from the start of the transition phase. If a time change amount such as 0.5 hours is used to define the transition phase, it is determined that the transition phase is over when time is changed by the time change amount such as 0.5 hours from the start of the transition phase. If the transition phase P 2  is not over, the process returns to step  624  to read the next voltage for determining the next battery capacity. 
   If the transition phase P 2  is over, then step  638  is taken where a discharge curve model  140  is selected, and a battery voltage is read at step  639 . At step  640 , a battery capacity is determined by examining the discharge curve model, for example by looking up a pre-stored table corresponding to the discharge curve model. If the power source  46  is not disconnected while the battery is charging at step  620 , the battery  48  is not in a transition phase, then the process continues to step  634  where a charge curve model is selected, and a battery voltage is read  635 . At step  636 , the charge curve model is applied to determine the battery capacity based on the read battery voltage, and then the process returns to step  630  to determine next battery capacity. 
   At step  630 , if it is determined that the battery is not being charged, the process continues to step  610 . At step  610 , it is determined by the charging subsystem  44  if power source  46  is connected to a battery  48  while the battery  48  is discharging. For example, by checking the change in the direction or sign of battery current, the charging subsystem  44  may determine if the power source  46  is connected. It should be understood that various methods of determining if the battery enters a charging state. If the answer is yes at step  610 , the battery  48  enters a transition phase P 1  from discharging to charging, and the process proceeds to step  612 . At step  612 , a transition phase battery capacity function F 1  corresponding to the transition phase P 1 , which provides a minimized battery capacity error, is selected, for example, according to a battery ID. Alternatively, according to the requirements of capacity reporting error and computational complexity, a different transition phase battery capacity function F 1  may be selected. 
   At step  614 , a battery voltage value is read and the process proceeds to step  616 . At step  616 , according to the selected battery capacity model F 2  at step  612  and the battery voltage at step  614 , a battery capacity is determined, for example, by calculating the selected battery capacity model or by looking up a table corresponding to the selected battery capacity model. A plurality of tables, wherein each table corresponds to a transition phase battery capacity model, may be predetermined and pre-stored in memory  24 . At step  617 , it is determined if the power source  46  is disconnected. If yes, the battery enters into another transition phase and the process goes to step  622  and proceeds to the subsequent steps. If no, the process proceeds to step  618 . At step  618 , it is determined if the transition phase P 1  is over. If it is not over, the process returns to step  614  where the next battery voltage is obtained for determining the most recent battery capacity. 
   If the transition phase P 1  is over, then step  634  is taken where a charge curve model  130  is selected, and a battery voltage is read  635 . At step  636 , a battery capacity is determined by examining the charge curve model, for example by looking up a pre-stored table corresponding to the charge curve model, and then the process goes to step  630  to determine the next battery capacity. 
   Conversely, if at step  610 , it is determined that the power source  46  is not connected to the battery  48  while the battery  48  is discharging, the process goes to step  638  where a discharge curve model is selected. At step  639 , a battery voltage is read. At step  640 , a discharging curve model is used to look up a battery capacity based on the read battery voltage, and then the process returns to step  630  to determine the next battery capacity. 
   Discharge curve models, charge curve models and transition phase battery capacity models for determining battery capacity as above may be pre-stored in memory  24  as lookup tables correlating battery voltage, battery state and battery capacity. By looking up a table according to a battery voltage and a battery state, a battery capacity may be determined. 
   Alternatively, the charging curve, discharging curve and transition phase models may be calculated on the fly by microprocessor  38  using code stored in memory  24 . 
   In another example, a plurality of transition phase battery capacity models wherein each of models corresponds to a predetermined temperature range is provided so as to minimize battery capacity error. At step  612 , according to a current battery operating temperature, a transition phase battery capacity model F 1  corresponding to the transition phase P 1  is selected from a plurality of transition phase battery capacity models by determining that the current battery operating temperature falls into a predetermined temperature range. A similar process as above is used in steps  622 ,  634 , and  638 . 
   In a further example, a plurality of transition phase battery capacity models, each corresponding to a predetermined battery operating temperature, are provided to more exactly report battery capacity. At step  612 , according to a current battery operating temperature, a transition phase battery capacity model F 1  corresponding to the transition phase P 1  is selected from a plurality of transition phase battery capacity models by determining that the current battery operating temperature closest to a predetermined temperature. The selected transition phase battery capacity model has the predetermined temperature closest to the current battery operating temperature. A similar process as above is used in steps  622 ,  634 , and  638 . 
   In a further example, a plurality of the battery capacity offsets wherein each of them corresponds to a battery operating temperature range is provided. According to a current battery operating temperature, the calculated battery capacity is compensated based on a battery capacity offset immediately after step  616 , by determining if the current battery operating temperature falls into a predetermined temperature range having a battery capacity offset. Similarly, immediately after steps  626 ,  636  and  640 , a similar process as above is applied. 
   In a further example, a plurality of the battery capacity offsets, each of them corresponding to a battery operating temperature, is provided. According to a current battery operating temperature, the calculated battery capacity is compensated based on a battery capacity offset immediately after step  616  by determining if the current battery operating temperature is a predetermined temperature having a battery capacity offset or the current battery operating temperature is closet to a predetermined temperature having a battery capacity offset. Similarly, immediately after steps  626 ,  636  and  640 , a similar process as above is applied. 
   When a battery is in the transition phase from discharging to charging P 1  or the transition phase from charging to discharging P 2 , the battery could enter another transition phase if the battery charging state is changed again. For example, when the battery is connected to a power source while discharging, it enters into the transition phase from discharging to charging P 1 . If the battery is disconnected from the power source during the transition phase P 1 , the battery enters into a third transition phase P 11 . A third transition phase model F 11  may be used to determine a battery capacity. Similarly, when a battery is disconnected from a power source while charging, it enters into the transition phase from charging to discharging P 2 . If the battery is connected to the power source during the transition phase P 2 , the battery enters into a fourth transition phase P 21 . A fourth transition phase model F 21  may be used to determine the battery capacity. F 11  and F 1  may be the same function. Similarly, F 21  and F 2  may be the same function. 
   The above method may be implemented as an embodiment of charging subsystem  44 . The system may include a transition phase determining circuitry operatively connected to the battery for determining if the battery is in a transition phase and battery capacity determining circuitry operatively connected to transition phase determining circuitry for determining the battery capacity based on a transition phase battery capacity model where the battery is in the transition phase. The system further comprises battery ID determining circuitry operatively connected to the battery for determining the battery ID, circuitry for selecting a transition phase battery capacity model from a plurality of transition phase battery capacity models based on the battery ID, and voltage reading circuitry operatively connected to the battery for determining a battery voltage. The system further comprises state determining circuitry operatively connected to the battery for determining a state of the battery where the battery is not in the transition phase. The battery capacity determining circuitry determines the battery capacity by examining a state curve model correlating voltage, state and capacity based on the determined charge state. The state includes a charging state and a discharging state. The state curve model includes a charge state curve model corresponding to a charging state and a discharge state curve model corresponding to a discharging state. 
   This written description uses examples to disclose the invention, including the best mode, and also to enable a person skilled in the art to make and use the invention. The patentable scope of the invention may include other examples that occur to those skilled in the art.