Method and apparatus for determining a capacity of a battery

Abstract

Some embodiments of the present invention provide a system that accurately and reliably updates a full charge capacity of a battery. During operation, the system charges the battery from an initial state to a rest point prior to reaching a fully charged state. The system then interrupts the charging process to allow the battery to relax to a resting voltage. Next, the system measures the resting voltage. The system then resumes the charging process toward the fully charged state. The system subsequently estimates the capacity of the battery based on the measured resting voltage and one or more other parameters.

Claims

1. A method for estimating a capacity of a battery, comprising: determining, using a controller, a current state of charge of the battery; charging, using a charger, the battery to a first state of charge when the current state of charge is lower than a threshold state of charge; measuring, using a voltmeter, a first resting voltage corresponding to the first state of charge; charging, using the charger, the battery from the first state toward a second state of charge; measuring, using the voltmeter, a second resting voltage corresponding to the second state of charge; and estimating, using the controller, the capacity of the battery based on at least the first and second resting voltages.

2. The method of claim 1, wherein the capacity is a full charge capacity.

3. The method of claim 1, wherein charging the battery to the first state of charge involves charging the battery with a constant current during a time period immediately preceding the first state of charge.

4. The method of claim 3, wherein measuring the first resting voltage involves immediately dropping the charging current to zero.

5. The method of claim 3, wherein measuring the first resting voltage involves reducing the charging current to a low level above zero.

6. The method of claim 1, wherein estimating the capacity of the battery based on at least the first resting voltage and the second resting voltage involves: computing, using the controller, the first state of charge of the battery corresponding to the first resting voltage; computing, using the controller, the second state of charge of the battery corresponding to the second resting voltage; determining a coulomb count between the first state of charge and the second state of charge; and determining the capacity of the battery based on the first state of charge, the second state of charge, and the coulomb count.

7. The method of claim 6, wherein determining the coulomb count between the first state of charge and the second state of charge involves: initiating a coulomb counting from the first state of charge after resuming the charging process; concluding the coulomb counting when the second state of charge is reached; and determining the coulomb count based on the coulomb counting between the first state of charge and the second state of charge.

8. The method of claim 6, wherein each of the first resting voltage and the second resting voltage is an open circuit voltage (OCV).

9. The method of claim 1, wherein prior to charging the battery, the method further comprises selecting the first state of charge based at least on one or more of the following parameters: a time required for the battery to relax at the first state of charge; an error associated with a coulomb counting between the state of charge and the second state of charge; a likelihood of a user discharging the battery below the first state of charge; and an open-circuit-voltage measurement accuracy required to compute a state of charge associated with the first state of charge.

10. The method of claim 9, wherein selecting the first state of charge involves ensuring that the time required for the battery to relax at the first state of charge is significantly shorter than a time to charge the battery from the current state to the second state of charge without interruption.

11. The method of claim 9, wherein the state of charge is in the vicinity of or above a 60% state of charge of the battery.

12. The method of claim 9, wherein the first state of charge is in the vicinity of or below an 80% state of charge of the battery.

13. A non-transitory computer-readable storage medium storing instructions that when executed by a controller cause the controller to perform a method for accurately estimating a capacity of a battery, the method comprising: determining, at the controller, a current state of charge of the battery; charging, using a charger, the battery to a first state of charge when the current state of charge is lower than a threshold state of charge; measuring a first resting voltage corresponding to the first state of charge; charging, using the charger, the battery from the first state of charge toward a fully charged state corresponding to a second resting voltage; and estimating, at the controller, the capacity of the battery based on at least the first and second resting voltages.

14. The non-transitory computer-readable storage medium of claim 13, wherein the capacity is a full charge capacity.

15. The computer-readable storage medium of claim 13, wherein charging the battery to the first state of charge involves charging the battery with a constant current during a time period immediately preceding the first state of charge.

16. The non-transitory computer-readable storage medium of claim 15, wherein interrupting the charging process involves immediately dropping the charging current to zero.

17. The non-transitory computer-readable storage medium of claim 13, wherein after reaching the second state of charge, the method further comprises: allowing the battery to relax to a second resting voltage; and measuring the second resting voltage corresponding to the second state of charge.

18. The non-transitory computer-readable storage medium of claim 17, wherein estimating the capacity of the battery based at least on the first resting voltage involves: computing a first state of charge of the battery corresponding to the first resting voltage; computing a second state of charge of the battery corresponding to the second resting voltage; determining a coulomb count between the first state of charge and the second state of charge; and determining the capacity of the battery based on the first state of charge, the second state of charge, and the coulomb count.

19. The non-transitory computer-readable storage medium of claim 18, wherein determining the coulomb count between the first state of charge and the second state of charge involves: initiating a coulomb counting from the first state of charge after resuming the charging process; concluding the coulomb counting when the second state of charge is reached; and determining the coulomb count based on the coulomb counting between the first state of charge and the second state of charge.

20. The non-transitory computer-readable storage medium of claim 13, wherein prior to charging the battery, the method further comprises selecting the first state of charge based at least on one or more of the following parameters: a time required for the battery to relax at the first state of charge; an error associated with a coulomb counting between the first state of charge and the second state of charge; a likelihood of a user discharging the battery below the first state of charge; and an open-circuit-voltage measurement accuracy required to compute a state of charge associated with the first state of charge.

21. The non-transitory computer-readable storage medium of claim 20, wherein selecting the first state of charge involves ensuring that the time required for the battery to relax at the first state of charge is significantly shorter than a time to charge the battery from the initial state to the second state of charge without interruption.

22. A battery with a capacity estimation mechanism, comprising: a cell; a voltage sensor configured to measure a voltage for the battery; a current sensor configured to measure a current for the battery; and a controller configured to receive inputs from the voltage sensor and the current sensor, and to generate a capacity estimate; wherein the controller is configured to: determine a current state of charge of the battery; charge the battery to a first state of charge when the current state of charge is lower than a threshold state of charge; measure a first resting voltage corresponding to the first state of charge; charge the battery from the first state toward a second state of charge; measure a second resting voltage corresponding to a second state of charge; and estimate the capacity of the battery based on at least the first and second resting voltages.

23. The battery of claim 22, wherein the controller is configured to charge the battery to the first state of charge by charging the battery with a constant current during a time period immediately preceding the first state of charge.

24. The battery of claim 23, wherein the controller is configured to interrupt the charging process by immediately dropping the charging current to zero.

25. The battery of claim 22, wherein after reaching the second state of charge, the controller is configured to: allow the battery to relax to a second resting voltage; and measure the second resting voltage corresponding to the second state of charge.

26. The battery of claim 25, wherein the controller is configured to determine the capacity of the battery by: computing a first state of charge of the battery corresponding to the first resting voltage; computing a second state of charge of the battery corresponding to the second resting voltage; determining a coulomb count between the first state of charge and the second state of charge; and determining the capacity of the battery based on the first state of charge, the second state of charge, and the coulomb count.

27. The battery of claim 26, wherein the controller is configured to determine the coulomb count between the first state of charge and the second state of charge by: initiating a coulomb counting from the state of charge after resuming the charging process; concluding the coulomb counting when the second state of charge is reached; and determining the coulomb count based on the coulomb counting between the first state of charge and the second state of charge.

28. A method for evaluating a state of charge of a battery during a charging process, the method comprising: determining, by a controller, whether a current state of charge of a battery is greater than or equal to a first state of charge corresponding to a predetermined first rest point; in event the current state of charge is lower than the first state of charge: charging, using a charger, the battery to the first predetermined rest point; allowing the battery to relax from the first rest point to a resting voltage; measuring, using a voltmeter, the resting voltage; charging, using the charger, the battery toward a second rest point corresponding to a second state of charge; and determining a state of charge of the battery at the first rest point based at least on the resting voltage.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 illustrates a coulomb counting based Q.sub.max measurement technique for a battery in accordance with some embodiments herein.

(2) FIG. 2 presents a flowchart illustrating a process of charging a battery that includes a controlled Q.sub.max update in accordance with some embodiments herein.

(3) FIG. 3 presents a flowchart illustrating a modified charging process which uses an inserted rest point for a controlled Q.sub.max update in accordance with some embodiments herein.

(4) FIG. 4 illustrates a charging profile that employs a constant-current constant-voltage charging profile in accordance with some embodiments herein.

(5) FIG. 5 illustrates a charging profile of a controlled Q.sub.max update that includes a single rest point in accordance with some embodiments herein.

(6) FIG. 6 illustrates an exemplary cell relaxing profile after a constant current charging process has been applied up to a rest point at a high SOC in accordance with some embodiments herein.

(7) FIG. 7 provides a chart illustrating different state of charge regions which may be used to guide the choice of a proper rest point.

(8) FIG. 8 illustrates a rechargeable battery that supports a controlled Q.sub.max update during charging in accordance with some embodiments herein.

DETAILED DESCRIPTION

(9) The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

(10) The data structures and code described in this detailed description are typically stored on a computer-readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. The computer-readable storage medium includes, but is not limited to, volatile memory, non-volatile memory, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs), DVDs (digital versatile discs or digital video discs), or other media capable of storing computer-readable media now known or later developed.

(11) The methods and processes described in the detailed description section can be embodied as code and/or data, which can be stored in a computer-readable storage medium as described above. When a computer system reads and executes the code and/or data stored on the computer-readable storage medium, the computer system performs the methods and processes embodied as data structures and code and stored within the computer-readable storage medium. Furthermore, the methods and processes described below can be included in hardware modules. For example, the hardware modules can include, but are not limited to, application-specific integrated circuit (ASIC) chips, field-programmable gate arrays (FPGAs), and other programmable-logic devices now known or later developed. When the hardware modules are activated, the hardware modules perform the methods and processes included within the hardware modules.

TERMINOLOGY

(12) Throughout the specification, the following terms have the meanings provided herein, unless the context clearly dictates otherwise. The term battery generally means a rechargeable battery which includes a cell pack (with one or more cells). Hence, a term such as full charge capacity of a battery means full charge capacity of the cell pack within the battery; charging a battery means charging the cell pack within the battery; and allowing the battery to relax means allowing the cell pack within the battery to relax. Furthermore, terms rested and relaxed are used interchangeably to refer to a state of a battery wherein the current in the battery is sufficiently small (including near-zero currents) so that dynamic effects are negligible to allow OCV measurements to be taken within a desired accuracy. Thus, terms rested OCV measurement and relaxed OCV measurement within this disclosure are used interchangeably to mean an OCV measurement performed at the aforementioned rested or relaxed state of the battery. In the discussion below, the term OCV measurement generally refers to the aforementioned rested/relaxed OCV measurement, unless the context clearly dictates otherwise. Moreover, the term resting voltage throughout this disclosure refers to the voltage associated with the rested/relaxed OCV measurement, unless the context clearly dictates otherwise.

(13) Overview

(14) The present disclosure provides a technique for accurately and reliably updating the full charge capacity (also referred to as Q.sub.max below) of a battery. The aforementioned problem with infrequent updates of the full charge capacity commonly results from users not letting their systems relax at low states of charge. The proposed solution to this problem involves modifying the charging process by inserting a rest point during charging, thereby allowing a rested OCV measurement to be performed. Note that battery cells can relax at very different rates at different states of charge. To minimize the impact on the user from the rest point measurement, the rest point can be carefully chosen to be at a state of charge when the cells comparatively quickly reach a steady state where a measurement can be acquired. To further shorten the rest time needed, the charging current to the rest point may be controlled at a fixed level. This ensures that there is little or no dynamic behavior during the cell relaxation, thereby allowing the open circuit voltage measurement to be accelerated by extrapolating quickly to the resting voltage. This controlled charging process is described in more detail below.

(15) Coulomb Counting Based Q.sub.max Measurement

(16) FIG. 1 illustrates a coulomb counting based Q.sub.max measurement technique for a battery in accordance with some embodiments herein. As illustrated in FIG. 1, two rest measurements are performed on the battery. The first rest measurement measures a first open circuit voltage (OCV) value of the battery corresponding to an intermediate-level state-of-charge (SOC) q.sub.prev, wherein q.sub.prev can be derived from the first OCV value based on a predetermined OCV vs. SOC relationship which has been calibrated for the battery. For example, the OCV vs. SOC relationship shown in FIG. 1 can be used. In the example shown, q.sub.prev is between 50% and 75% SOC. In an ideal case, the first rest measurement occurs when the battery is allowed to relax (i.e., unplugged and unused) for a sufficiently long period of time until that the first OCV value can be reliably measured.

(17) Also shown in FIG. 1, the second rest measurement measures a second OCV value of the battery at a full or near full SOC q.sub.ful, wherein q.sub.ful can be derived from the second OCV value based on the predetermined OCV-SOC relationship. In the example shown, q.sub.prev is at 100% SOC. Typically, the second rest measurement occurs when the battery reaches the full SOC and the system remains plugged in for a sufficiently long period of time until the second OCV value can be reliably measured.

(18) Separately, a coulomb counting is performed during a charging process from the lower SOC q.sub.prev to the full SOC q.sub.ful to determine the amount of charge flow Q during charging. Note that the system can use any current-sensing-based coulomb counting techniques to obtain Q. In one embodiment, the Q measurement takes place between the two rest measurements. In some embodiments, the system measures Q prior to or after both q.sub.prev and q.sub.ful have been determined. Finally, the system determines the full charge capacity Q.sub.max for the battery based on q.sub.prev, q.sub.ful, and Q. In one embodiment, Q.sub.max is computed by:

(19) $Q_{\max }=\frac{Q}{q_{\mathrm{ful}}-q_{\mathrm{prev}}}.$

(20) In practice, a user rarely rests long enough at low states of charge to allow q.sub.prev and Q to be measured. As a result, the conventional Q.sub.max updates based on the above-described approach take place very infrequently. This problem gets even worse on devices with background services that constantly disturb resting, thus preventing the battery from reaching a relaxed state.

(21) A Modified Charging Operation with Inserted Rest Points

(22) Conventional charging of a battery does not include controlled interruptions in the course of the charging process until the battery is unplugged from the charging source. Proposed embodiments modify the conventional charging process by inserting at least one rest point during the charging process which temporarily interrupts the charging current to allow the battery to relax, thereby allowing a rested OCV measurement to be performed during the interruption of the charging process. In one embodiment, the at least one rest point is associated with a predetermined state of charge. We refer to this modified charging process which includes at least one inserted rest point prior to reaching a fully charged state as a controlled Q.sub.max update. FIG. 2 presents a flowchart illustrating a process of charging a battery that includes a controlled Q.sub.max update in accordance with some embodiments herein.

(23) Typically, the process starts when the system detects that the system is plugged in, for example, when the user plugs in the system to a charger or a power source (step 202). Note that the system described herein may include a battery management unit (BMU). The system then determines if it is necessary to perform a controlled Q.sub.max update (step 204). In one embodiment, the system determines if it is necessary to perform a controlled Q.sub.max update based on a predicted uncertainty associated with Q.sub.max. More specifically, the system compares the uncertainty of the most recently updated Q.sub.max with a threshold uncertainty. In this case, the system triggers a controlled Q.sub.max update if the Q.sub.max uncertainty exceeds the threshold uncertainty. On the other hand, the system bypasses the controlled Q.sub.max update if the Q.sub.max uncertainty is below the threshold uncertainty. Note that Q.sub.max uncertainty generally increases with time. Hence, the system can determine if a controlled Q.sub.max update is necessary based on how much time has elapsed since the last Q.sub.max update has taken place. In one embodiment, the system triggers a controlled Q.sub.max update when a predetermined time period (e.g., one month) since the last Q.sub.max update has been reached.

(24) If it is determined that performing a controlled Q.sub.max update is not necessary, the system proceeds to conventionally charging the battery to full, without stopping at a rest point during the charging process (step 210). However, if it is determined that performing a controlled Q.sub.max update is necessary, the system additionally determines if performing a controlled Q.sub.max update is possible (step 206). In some embodiments, performing step 204 is optional and the system directly proceeds to step 206 from step 202.

(25) In one embodiment, determining if performing a controlled Q.sub.max update is possible involves determining if the initial state of charge of the battery is below the state of charge of the battery associated with a predetermined rest point (also referred to as the target state of charge below). Note that one requirement of a controlled Q.sub.max update is that the system stops at the predetermined rest point corresponding to a higher state of charge (relative to the initial state of charge) during the charging process. Consequently, one prerequisite associated with the controlled Q.sub.max update is that the battery has discharged to a state of charge below the target state of charge. For example, if the initial state of charge is 50% whereas the target state of charge is 60%, the controlled Q.sub.max update is deemed possible. On the other hand, if the system determines that the initial state of charge of the battery is above the target state of charge, the controlled Q.sub.max update is deemed not possible.

(26) If a controlled Q.sub.max update is deemed not possible at step 206, the system proceeds to step 210 to conventionally charge the battery to full, without performing the controlled Q.sub.max update. On the other hand, if a controlled Q.sub.max update is deemed possible at step 206, the system proceeds to charge the battery through a modified charging process with the controlled Q.sub.max update (by inserting a rest point) (step 208). Note that the controlled Q.sub.max update typically includes the steps of controlled charging to a predetermined rest point (e.g., a predetermined cell voltage), relaxing at the rest point, and performing a rest measurement at the rest point. A detailed embodiment of the controlled Q.sub.max update is described in conjunction with FIG. 3. After taking the rest measurement at the rest point, the system proceeds to step 210 to conventionally charge the battery to full.

(27) After the system conventionally charges the battery to full and a sufficient rest period has been reached (e.g., when the system remains plugged in for a while), the system obtains an OCV measurement at the fully charged state of the battery (step 212). Note that step 212 may fail to obtain an OCV measurement if the rest period is too short to allow the rest measurement to take place. For example, the user may unplug the external power and start using the battery right away.

(28) Next, the system determines if an OCV measurement at a rest point has been taken during the charging process (step 214). As mentioned above, the system can reach step 214 without going through the controlled Q.sub.max update (step 208) which obtains the OCV measurement at the rest point. If it is determined that the OCV measurement has been taken at the rest point, the system additionally determines if an OCV measurement at the fully charged state has been taken (step 216). If so, the system proceeds to compute an updated Q.sub.max using a coulomb counting based Q.sub.max update technique or other Q.sub.max update techniques (step 218). In this case, the system obtains an updated Q.sub.max through the controlled Q.sub.max update. However, if either the OCV measurement at the rest point or the OCV measurement at the fully charged state does not occur, the charging process completes without a Q.sub.max update.

(29) Note that the above-described Q.sub.max update process assumes that the first rest measurement for the Q.sub.max update is taken at the rest point and the second rest measurement is taken at the fully charged state. In a variation to this embodiment, the system inserts two predetermined rest points during the charging process between the initial state of charge and the full state of charge, and obtains a relaxed OCV measurement at each of the two rest points. In this embodiment, if the relaxed OCV measurement at the fully charged state fails to occur, the system can use both relaxed OCV measurements from the two rest points to compute the updated Q.sub.max. However, if the relaxed OCV measurement at the fully charged state is also taken, the system can choose one of the two OCV measurements from the two rest points and the OCV measurement at the fully charged state to compute the updated Q.sub.max.

(30) FIG. 3 presents a flowchart illustrating a modified charging process which uses an inserted rest point for a controlled Q.sub.max update in accordance with some embodiments herein. Note that FIG. 3 provides a detailed description of step 208 in FIG. 2. The embodiment assumes that a rest point is predetermined prior to the charging process. A more detailed process for determining the rest point is provided below. In one embodiment, the rest point corresponds to a predetermined state of charge of the battery (i.e., the target state of charge).

(31) The process typically starts by conventionally charging the battery with a constant current from the initial state of charge toward the rest point associated with the target state of charge (step 302). During this process, the system constantly evaluates and compares the current state of charge with the target state of charge of the rest point. Note that between the initial state of charge and the target state of charge, the system can use more than one level of constant charging current. For example, FIG. 4 illustrates a charging profile 400 that employs a constant-current constant-voltage charging profile in accordance with some embodiments herein. More specifically, the system first applies a first constant current 402 to the battery in the first constant current region 404 until a first target voltage is reached (e.g., at 4.0V). Next, the system applies the first constant voltage to the battery until the charging current reduces to a second constant current 406. The system then applies the second constant current 406 to the battery in the second constant current region 408 until a second target voltage is reached (e.g., at 4.1V). The system then applies the second constant voltage to the battery until the charging current reduces to a third constant current 410. Next, the system applies the third constant current 410 to the battery in the third constant current region 412 until a third target voltage is reached. The system then applies the third constant voltage to the battery until the full state of charge is reached, and the charging is completed. Note that the embodiment shown does not include a rest point for the controlled Q.sub.max update.

(32) Referring back to FIG. 3, when the target state of charge, i.e., rest point is reached, the system interrupts the charging process by immediately stopping the charging current and allowing the voltage to relax until an OCV measurement can be acquired (step 304). In some embodiments, instead of immediately dropping the charging current to zero and then resting, the system first reduces the charging current to a significantly low level above zero. This process may take a short period of time, e.g., a few minutes. The system then starts resting the battery by stopping the remaining charging current.

(33) After the battery has sufficiently relaxed, the system obtains a relaxed OCV measurement at the rest point, thereby obtaining the rest measurement at the target state of charge (step 306). Next, the system resumes charging the battery (step 308) and conventionally charging the battery to the full state of charge (i.e., continuing to step 210 in FIG. 2 of the overall charging process). Note that, if more than one rest point is inserted during the controlled Q.sub.max update, the system simply repeats steps 302-308 each time a new rest point is reached.

(34) FIG. 5 illustrates a charging profile 500 of a controlled Q.sub.max update that includes a single rest point in accordance with some embodiments herein. Note that the charging profile 500 with the controlled Q.sub.max update illustrated in FIG. 5 is a variation of charging profile 400 of FIG. 4, which is also shown in FIG. 5 for comparison. More specifically, charging profile 500 with the controlled Q.sub.max update follows the original charging profile 400 (shown in solid lines) initially. After a predetermined rest point at a time mark 502 (near the 150-minute mark), charging profile 500 enters the controlled Q.sub.max update phase which is shown in dotted lines. As is illustrated in FIG. 5, the system associated with charging profile 500 drops the charging current from the third constant current at 0.4 C-rate to zero. The system then waits for the battery to rest during a rest period marked as 504. Next, the system takes the rest measurement at time mark 506. The system then resumes the charging process by restoring to the third constant current and continues to charge the battery to full. Note that the overall charging time increase 508 from the original charging profile 400 as a result of the controlled Q.sub.max update is substantially the same as the rest period 504.

(35) Note that, while more than one level of charging current may be used to charge up toward the target state of charge, it is desirable to have a constant charging current during a time period immediately preceding the rest point. This ensures that there is little or no dynamic behavior during the cell relaxation, thereby allowing the open circuit voltage measurement be accelerated by extrapolating quickly to the resting voltage.

(36) Note that the controlled Q.sub.max update profile is substantially similar to the original charging profile of FIG. 4 except for additional rest period 504 occurring between rest point 502 and rest measurement point 506. When the rest point is chosen properly, the charging time increase such as time 508 is insignificant and will not degrade the user experience. However, if the rest point is chosen improperly, the charging time increase due to the rest measurement may be significant compared with the regular charging time and can negatively affect the user experience.

(37) Choosing Rest Points

(38) As mentioned above, the rest point associated with the controlled Q.sub.max update can be predetermined prior to the charging process. When choosing a rest point for the controlled Q.sub.max update, a number of interrelated considerations are to be balanced. These considerations can include, but are not limited to, the time required for the battery to relax at the rest point; the uncertainty associated with coulomb count Q from the rest point to the full charge capacity; the likelihood of the user discharging below the rest point; and the voltage and curve accuracy required to establish the state of charge at a given rest point.

(39) Among the above considerations, the time required for the battery to relax is one of the more important considerations because this time accounts for an increased charging time for the system, which can negatively impact the user experience. Hence, a rest point associated with a fast time to relax is preferred. The time needed for a cell to relax is state-of-charge dependent and needs to be characterized on a per cell-chemistry basis. For example, at 60% SOC it can take one hour to relax, but at 80% SOC it may take only minutes to relax. FIG. 6 illustrates an exemplary cell relaxing profile 600 after a constant current charging process has been applied up to a rest point at a high SOC in accordance with some embodiments herein. The data shows that the time to relax, or the additional charging time, can be kept to a few minutes, depending on how large an error in full charge capacity can be tolerated. If a rest point is chosen based on this profile, it is reasonable to rest for somewhere between three or four minutes to get an accurate open circuit voltage measurement. While charging a battery to full capacity usually takes on the order of hours, the additional minutes of charging time are probably insignificant to the user experience.

(40) While it is desirable to choose the rest point at a higher SOC based on the fast-to-relax consideration, a rest point at a very high SOC can cause a decrease in accuracy in determining the Q.sub.max. In the above-described coulomb counting technique, Q.sub.max is determined by taking the difference between the two open circuit voltage measurements and dividing by the coulomb count Q, which itself is determined during the charging process between the two rest measurements. Because the coulomb count has an error associated with it, if the SOC at the rest point is too high, the measured Q is small and the uncertainty in the measurement increases. This uncertainty in Q is then propagated to the uncertainty in Q.sub.max. For this consideration alone, a lower SOC is desirable for the measurement accuracy, but it could be in conflict with the fast-to-relax consideration. When balancing the two considerations, the rest point should be placed above a given low state of charge and below a given high state of charge. In one embodiment, the rest point is between 60% and 80% SOC to balance these two considerations. Below 60% SOC, relaxation times are significantly increased, whereas above 80% SOC, the error from the coulomb count measurement starts to get noticeable.

(41) Note that, in order for the controlled Q.sub.max update to occur, the user has to discharge below the predetermined rest point. In systems with a very large capacity battery, such as the iPad, discharging below a high SOC rest point can take a significant amount of time. Hence, the likelihood of the user discharging below a given SOC associated with a potential rest point has to be balanced against the time required for the battery to relax.

(42) Another factor which needs to be taken into account in determining the rest point is related to the OCV measurement voltage and curve accuracy required to establish state of charge at a given rest point. Referring to FIG. 1, note that the OCV-SOC curve includes linear regions and non-linear regions. If the rest point is chosen right between two relatively linear regions shown as X, the SOC accuracy is likely to suffer. However, if the rest point is chosen at the q.sub.prev in the relatively linear region, the SOC accuracy may be improved.

(43) In addition to the above considerations, other factors, such as temperature and charging/discharging history of the system may be taken into account and balanced with all other considerations in determining a proper rest point. FIG. 7 provides a chart illustrating different state of charge regions which may be used to guide the choice of a proper rest point. Note that the chart shows that the optimal rest point region is approximately between 60% and 80% SOC.

(44) Battery Design

(45) FIG. 8 illustrates a rechargeable battery 800 that supports a controlled Q.sub.max update during charging in accordance with some embodiments herein. Battery 800 includes a battery cell 802. It also includes a current meter (current sensor) 804, which measures a charging current through cell 802, and a voltmeter (voltage sensor) 806, which measures a voltage across cell 802. Battery 800 also includes a thermal sensor 830, which measures the temperature of battery cell 802. (Note that numerous possible designs for current meters, voltmeters and thermal sensors are well-known in the art.)

(46) Rechargeable battery 800 is coupled to a charger 823. Note that, although charger 823 is illustrated in FIG. 8, charger 823 is typically not part of rechargeable battery 800. However, charger 823 is associated with the charging process by providing charging current to cell 802. Note that charger 823 can include any battery charging mechanism now known or later developed. This includes, but is not limited to, a USB charger, a multi-pin charger, a docking station, a portable charger, and a wall charger.

(47) The above-described modified charging process with a controlled Q.sub.max update is controlled by a controller 820, which receives: a voltage signal 808 from voltmeter 806, a current signal 810 from current meter 804, a temperature signal 832 from thermal sensor 830, and a state of charge (SOC) value 834 from SOC estimator 833. Additionally, controller 820 stores one or more predetermined state of charge values 836. These state of charge values are used to generate one or more rest points during the controlled Q.sub.max update. Controller 820 can include a coulomb counter 838 for estimating the amount of charge flow Q based on current 810 during a charging process. Controller 820 can also generate control signals 840 for controlling a charging current of charger 823. Control signals 840 can also control a switch 842. In some embodiments, control signals 840 can be used to turn off switch 842 to decouple charger 823 from cell 802.

(48) During a charging operation, controller 820 controls SOC estimator 833 to determine two SOC values corresponding to the two relaxed OCV measurements at a rest point and at the full charge. SOC estimator 833 receives a voltage 808 from voltmeter 806, a current 810 from current meter 804 and a temperature 832 from thermal sensor 830, and outputs a state of charge value 834. Controller 820 outputs a full charge capacity estimate 844 of cell 802 based on two SOC values corresponding to the two relaxed OCV measurements and an estimated coulomb count between the two relaxed OCV measurements.

(49) Note that controller 820 can be implemented using either a combination of hardware and software or purely hardware. In one embodiment, controller 820 is implemented using a microcontroller, which includes a microprocessor that executes instructions which control the full charge capacity update process.

(50) The foregoing descriptions of embodiments have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present description to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present description. The scope of the present description is defined by the appended claims.