Abstract:
A method and apparatus for measuring battery cell DC impedance by controlling charging of the battery cell. The method includes real-time characterization of a battery, (a) measuring periodically a DC impedance of the battery to determine a measured DC impedance; (b) ratioing the measured DC impedance to a reference DC impedance for the battery to establish an impedance degradation factor; (c) obtaining, during use of the battery and responsive to a set of attributes of the battery, an operational reference impedance for the battery; and (d) applying the impedance degradation factor to the operational reference impedance to obtain a real-time effective impedance for the battery.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to estimations of health for batteries used in heavy power use applications, and more particularly to a state of health for electric vehicle and hybrid vehicle batteries and battery packs. 
         [0002]    It is important for users and manufacturers of vehicles using batteries that the health and performance of those batteries be monitored. Batteries lose power and capacity (the specific mechanisms of that loss vary based upon cell chemistry) therefore it is important that the power and capacity be known to aid in service diagnostics and power limit algorithms during usage. The following description is specifically focused on lithium-ion chemistry but other chemistries may be benefited from the following description. 
         [0003]    Current techniques for monitoring the battery health include battery capacity measurements and estimates. This is an important battery parameter, but it is the case that a battery having sufficient capacity may cause a user, under certain conditions, to experience a sudden loss in available power from the battery. 
         [0004]    One way to estimate available power is based upon measurement of AC impedances of the batteries, using conventional techniques such as Kalman filtering. Knowledge of AC impedance allows accurate real-time DC power estimation. However, since the real-time impedance is a function of state of charge and health, this estimation does not indicate the relative degradation in power to a fresh pack. 
         [0005]    A current method for estimating available power uses a look-up table. The table uses information about temperature, state of charge (SOC), and battery age to predict and estimate available power. This predictive method fails to account for varying degrees of degradations in battery chemistry that occur over long periods that result from varying operating environments. For example, a user operating an electric vehicle in a hot climate may experience shorter battery life due to high temperature usage. 
         [0006]    What is needed is an apparatus and method to measure battery degradation in contrast to conventional techniques of predicting the battery degradation. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Disclosed is a method and apparatus for measuring battery cell DC impedance by controlling charging of the battery cell. The method includes real-time characterization of a battery, (a) measuring periodically a DC impedance of the battery to determine a measured DC impedance; (b) ratioing the measured DC impedance to a reference DC impedance for the battery to establish an impedance degradation factor; (c) obtaining, during use of the battery and responsive to a set of attributes of the battery, an operational reference impedance for the battery; (d) applying the impedance degradation factor to the operational reference impedance to obtain a real-time effective impedance for the battery; and (e) managing the battery temperature to a warm temperature so that the user does not notice a delay in charging time when measuring the DC impedance. 
         [0008]    The apparatus includes a battery charging system for a battery having a charger coupled to the battery, a controller, and a battery data acquisition and monitoring subsystem wherein the controller: measures periodically a DC impedance of the battery to determine a measured DC impedance; ratios the measured DC impedance to a reference DC impedance for the battery to establish an impedance degradation factor; obtains, during use of the battery and responsive to a set of attributes of the battery acquired by the battery data acquisition and monitoring subsystem, an operational reference impedance for the battery; and applies the impedance degradation factor to the operational reference impedance to obtain a real-time effective impedance for the battery. Various functions and structures of this system may be divided or integrated together into different elements than described herein. 
         [0009]    Embodiments of the present invention provide apparatus and method to measure battery degradation directly. Knowing both available power and current capacity helps the operator of an apparatus, like an electric vehicle, avoid dangerous uses of the apparatus that they may be otherwise able to avoid should they have a better representation of available power. In terms of manufacturers and maintenance, accurate impedance measurement, in cooperation with battery capacity, provides a more reliable indicator of the state of health of the battery as compared to battery capacity alone. 
         [0010]    Knowing the available power is important in other aspects of use of the battery. For electric vehicles, there is a specification regarding available sustained peak power which is directly related to available power. In application of an electric vehicle, for those without the embodiments of the present invention, it may be the case that a user initiates a maneuver with increased risk (e.g., passing another vehicle) and the duration of the sustained power peak cannot be maintained for the expected duration. The user experiences what appears to be a sudden loss in power which can have many different consequences depending upon the situation and how the user reacts. Embodiments of the present invention may be used as a feed-forward control path to pre-restrict operation to power levels that can be sustained for specified periods. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a representative charging system; 
           [0012]      FIG. 2  is a control diagram for the charging system shown in  FIG. 1 ; 
           [0013]      FIG. 3  is a second control diagram for the charging system shown in  FIG. 1  for dampening effects of noise, temperature variation, and measurement inaccuracies during the process described in  FIG. 4 ; and 
           [0014]      FIG. 4  is a process diagram for the charging system shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Embodiments of the present invention provide a system and method for measuring battery degradation in contrast to conventional techniques of predicting the battery degradation, particularly for lithium-ion battery cells. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
         [0016]      FIG. 1  is a preferred embodiment for a charging system  100 , such as may be used in an electric vehicle. System  100  includes a battery  105 , a charger  110  coupled to battery  105  and a battery management system (BMS)  115  and a battery data acquisition and monitoring subsystem  120 . A communication bus  125  couples subsystem  120  to BMS  115  and a communication bus  130  couples BMS  115  to charger  110 . A communication bus  135  couples battery data from battery  105  to subsystem  120 . 
         [0017]    Battery  105  is shown as a series-connected group of battery cells, however the arrangement of cells may be a combination of parallel/series connected cells of many different arrangements. Charger  110  of the preferred embodiment provides the charging current applied to battery  105 . BMS  115  controls the charging current according to a profile established by the embodiments of the present invention. Subsystem  120  acquires the desired data as described herein regarding battery  105 . For example, voltage, SOC, temperature, and other applicable data used by BMS  115 . In some embodiments, subsystem  120  may be part of BMS  115  and BMS  115  may be part of charger  110 . One or more of charger  110 , BMS  115 , and subsystem  120  control a switch  140 . Again, the organization, arrangement, and distribution of the functions described herein may provided other than as described in this exemplary embodiment. 
         [0018]      FIG. 2  is a control diagram  200  for the charging system shown in  FIG. 1 . Diagram  200  describes a typical control system as may be used for charging lithium ion cells. A target voltage  205  and a maximum cell voltage  210  are subtracted and used by a controller  215  to produce a charging current  220 . In prior art systems, current  220  is constant or compensates for an internal resistance (IR) drop of battery  105 . As described above, the preferred embodiments of the present invention describe a measurement system for actual DC impedance. In broad terms, the measurement of real-time DC impedance permits calculation of available current/available power which may be used in a variety of ways including representing a state-of-health of a battery and providing a feed-forward parameter that may be used to indicate/control a current level of sustained peak power. 
         [0019]    Lithium ion batteries are common in consumer electronics. They are one of the most popular types of battery for portable electronics, with one of the best energy-to-weight ratios, no memory effect, and a slow loss of charge when not in use. In addition to uses for consumer electronics, lithium-ion batteries are growing in popularity for defense, automotive, and aerospace applications due to their high energy and power density. However, certain kinds of treatment may cause Li-ion batteries to fail in potentially dangerous ways. 
         [0020]    One of the advantages of use of a Li-ion chemistry is that batteries made using this technology are rechargeable. Traditional charging is done with a two-step charge algorithm: (i) constant current (CC), and (ii) constant voltage (CV). In electric vehicles (EVs), the first step could be constant power (CP). 
         [0021]    Step 1: Apply charging current limit until the volt limit per cell is reached. 
         [0022]    Step 2: Apply maximum volt per cell limit until the current declines below a predetermined level (often C/20 but sometimes C/5 or C/10 or other value). 
         [0023]    The charge time is approximately 1-5 hours depending upon application. Generally cell phone type of batteries can be charged at 1 C, laptop types 0.8 C. The charging typically is halted when the current goes below C/10. Some fast chargers stop before step 2 starts and claim the battery is ready at about a 70% charge. (As used herein, “C” is a rated current that discharges the battery in one hour.) 
         [0024]    Generally for consumer electronics, lithium-ion is charged with approximate 4.2±0.05 V/cell. Heavy automotive, industrial, and military application may use lower voltages to extend battery life. Many protection circuits cut off when either &gt;4.3 V or 90° C. is reached. 
         [0025]    Battery chargers for charging lithium-ion-type batteries are known in the art. As is known in the art, such lithium ion batteries require constant current (CC) and constant voltage (CV) charging. In particular, initially such lithium ion batteries are charged with a constant current. In the constant current mode, the charging voltage is typically set to a maximum level recommended by the Li-ion cell manufacturer based on safety considerations, typically 4.2V per cell. The charging current is a factor of design level, based on the cell capability, charge time, needs and cost. Once the battery cell voltage rises sufficiently, the charging current drops below the initial charge current level. In particular, when the battery cell voltage Vb approaches the charging voltage Vc, the charging current tapers according to the formula: I=(Vc−Vb)/Rs, where I=the charging current, Vc=the charging voltage, Vb=the battery cell open circuit voltage and Rs=the resistance of the charging circuit including the contact resistance and the internal resistance of the battery cell. As such, during the last portion of the charging cycle, typically about the last ⅓, the battery cell is charged at a reduced charging current, which means it takes more time to fully charge the battery cell. 
         [0026]    The closed-circuit voltage represents the voltage of the battery cell plus the voltage drops in the circuit as a result of resistance in the battery circuit, such as the battery terminals and the internal resistance of the battery cell. By subtracting the closed-circuit voltage from the open-circuit voltage, the voltage drop across the battery resistance circuit elements can be determined. 
         [0027]      FIG. 3  is a second control diagram for the charging system shown in  FIG. 1  for dampening effects of noise, temperature variation, and measurement inaccuracies during the process described in  FIG. 4  below. The control is an implementation of a low-pass filter to help reduce variations from the noise, temperature variation, and measurement inaccuracies. 
         [0028]      FIG. 4  is a diagram for a process  400  implemented by charging system  100  shown in  FIG. 1 . Process  400  calculates power or current available. The calculation shown is described for individual cells of a multi-cell battery pack. For pack power or pack current, all cell values are summed. Process  400  is implemented every one to four weeks when the battery is being charged (preferably at night) and when the battery is about sixty percent state-of-charge (SOC). For the discussion below, the term “battery” is used to simplify the discussion. In the preferred implementation, the battery is a multi-cell pack and when the term “battery’ is used, it may be exchanged for cell and applicable to each cell of the battery pack, unless the context implies differently. 
         [0029]    Process  400  begins with step  405  to set battery temperature. This step warms the battery and keeps the battery (cells) at a desired temperature, in the preferred embodiment this temperature is approximately thirty-five degrees Celsius. This may be managed in multiple different ways, for example based upon coolant flow from the previous drive cycle. Air and/or liquid or other cooling mechanism is used to equalize cell temperature at the desired level before proceeding with process  400 . 
         [0030]    After step  405 , process  400  charges the battery in desired fashion until the average SOC of the battery reaches about a fixed predetermined percent SOC (typically around sixty percent SOC). The particular value chosen is based upon application and other design considerations. There are some trade-offs in selecting the desired SOC for this step. Between 40% to 100% SOC DC impedance of a battery tends to be flat. Also, for Cobalt cells, the OCV/SOC curve is flat at values &lt;55% SOC. It is the case that various applications and usage patterns will help determine some of these values. For example, some patterns of usage may have users discharge their batteries further than other applications before recharging. It is desirable to base the threshold on typical user discharge patterns. In some applications, a value for SOC is chosen &gt;60% when the SOC is accurate and the impedance is flat and users typically operate their vehicle into this SOC. For longer range batteries, some users may only drive the vehicle down to 70% SOC, in which case the selected value of 70% may be more desired. There are disadvantages to choosing too high of an SOC. For example, the battery may start hitting the taper voltage. 
         [0031]    Step  410  has charging system  100  command zero amps from the charger to the battery for a relaxation period. The relaxation period of the preferred implementation is on the order of about five minutes. This relaxation period permits the battery to depolarize. The actual period may vary from five minutes, especially as the battery ages or varies from the desired temperature or when higher charge currents are used. 
         [0032]    After the conclusion of the relaxation period of step  410 , process  400  performs step  415  of storing relaxed parameters. In the preferred embodiment, the relaxed battery voltage is measured and stored. 
         [0033]    Next, after step  415 , process  400  performs step  420  and resumes full-current charging. In the preferred embodiment, the charge current is desirably at least about C/3. This level increases accuracy of the DC impedance measurement reducing cell voltage/current measurement errors. 
         [0034]    Following step  420 , process  400  performs step  425  to sample cell voltages. Preferably, step  425  is implemented after a sustained peak power period lapses. For many electric vehicles in conventional usage, it is determined that the sustained peak power period should be on the order of about ten seconds. For some applications, it may be that the sustained peak power be eighteen seconds. This value is determined by the application of the battery. Step  430 , following step  425 , calculates an impedance for each cell. The impedance R is the change in voltage divided by the change in current (ΔV/ΔI). 
         [0035]    The preferred implementation uses the low pass filter shown in  FIG. 3  to measure the impedance of each cell. Other implementations of the low pass filter may be desirable, depending upon a variety of factors. 
         [0036]    Once the measured impedance for each cell is calculated, process  400  performs step  435  to determine a state-of-health (SoH) impedance degradation factor. This factor is a ratio of the measured impedance from step  430  to a “reference” impedance that represents the impedance of a fresh, newly manufactured battery. This ratio will be a value less than 1.0. 
         [0037]    During operation (e.g., while driving an electric vehicle using the battery), process  440  (at step  440 ) determines a reference impedance for the battery. Preferably step  440  uses a look-up table that is responsive to SoC and battery temperature to establish the reference impedance. Next process  400  performs step  445  to calculate the real DC impedance. The real DC impedance equals the SOH factor from step  435  multiplied times the reference impedance. This real-time DC impedance value may be used for different purposes. It is used in step  450  of process  400  of the preferred embodiment to calculate the available power/current of the battery. (For power, the calculation represents an individual cell so that all cell power limits are summed to get a total power available for the battery pack.) In the following, DchILimit is the Discharge Current Limit, V min  is a value determined by the manufacturer for the particular application, V cell  is the actual voltage level of the battery, and R is the real-time DC impedance value from step  445 . V min  is typically around three volts for consumer batteries and about two and seven tenths volts for electric vehicles. V max  is a manufacturer determined maximum cell voltage. V min  and V max  are typically constant but may vary by temperature. Embodiments of the present invention may use a dynamic value for these voltages when necessary or desirable. 
         [0000]        DchI Limit=( V   min   −V   Cell )/ R    
         [0000]        DchP Limit= DchI Limit* V   min    
         [0000]        ChgI Limit=( V   max   −V   Cell )/ R    
         [0000]        ChgP Limit= ChgI Limit* V   max    
         [0038]    The system above has been described in the preferred embodiment of an embedded automobile (EV) electric charging system. The system, method, and computer program product described in this application may, of course, be embodied in hardware; e.g., within or coupled to a Central Processing Unit (“CPU”), microprocessor, microcontroller, digital signal processor, System on Chip (“SOC”), or any other programmable device. Additionally, the system, method, and computer program product, may be embodied in software (e.g., computer readable code, program code, instructions and/or data disposed in any form, such as source, object or machine language) disposed, for example, in a computer usable (e.g., readable) medium configured to store the software. Such software enables the function, fabrication, modeling, simulation, description and/or testing of the apparatus and processes described herein. For example, this can be accomplished through the use of general programming languages (e.g., C, C++), GDSII databases, hardware description languages (HDL) including Verilog HDL, VHDL, AHDL (Altera HDL) and so on, or other available programs, databases, nanoprocessing, and/or circuit (i.e., schematic) capture tools. Such software can be disposed in any known computer usable medium including semiconductor (Flash, or EEPROM, ROM), magnetic disk, optical disc (e.g., CD-ROM, DVD-ROM, etc.) and as a computer data signal embodied in a computer usable (e.g., readable) transmission medium (e.g., carrier wave or any other medium including digital, optical, or analog-based medium). As such, the software can be transmitted over communication networks including the Internet and intranets. A system, method, computer program product, and propagated signal embodied in software may be included in a semiconductor intellectual property core (e.g., embodied in HDL) and transformed to hardware in the production of integrated circuits. Additionally, a system, method, computer program product, and propagated signal as described herein may be embodied as a combination of hardware and software. 
         [0039]    One of the preferred implementations of the present invention is as a routine in an operating system made up of programming steps or instructions resident in a memory of a computing system as well known, during computer operations. Until required by the computer system, the program instructions may be stored in another readable medium, e.g. in a disk drive, or in a removable memory, such as an optical disk for use in a CD ROM computer input or other portable memory system for use in transferring the programming steps into an embedded memory used in the charger. Further, the program instructions may be stored in the memory of another computer prior to use in the system of the present invention and transmitted over a LAN or a WAN, such as the Internet, when required by the user of the present invention. One skilled in the art should appreciate that the processes controlling the present invention are capable of being distributed in the form of computer readable media in a variety of forms. 
         [0040]    Any suitable programming language can be used to implement the routines of the present invention including C, C++, Java, assembly language, etc. Different programming techniques can be employed such as procedural or object oriented. The routines can execute on a single processing device or multiple processors. Although the steps, operations or computations may be presented in a specific order, this order may be changed in different embodiments. In some embodiments, multiple steps shown as sequential in this specification can be performed at the same time. The sequence of operations described herein can be interrupted, suspended, or otherwise controlled by another process, such as an operating system, kernel, and the like. The routines can operate in an operating system environment or as stand-alone routines occupying all, or a substantial part, of the system processing. 
         [0041]    In the description herein, numerous specific details are provided, such as examples of components and/or methods, to provide a thorough understanding of embodiments of the present invention. One skilled in the relevant art will recognize, however, that an embodiment of the invention can be practiced without one or more of the specific details, or with other apparatus, systems, assemblies, methods, components, materials, parts, and/or the like. In other instances, well-known structures, materials, or operations are not specifically shown or described in detail to avoid obscuring aspects of embodiments of the present invention. 
         [0042]    A “computer-readable medium” for purposes of embodiments of the present invention may be any medium that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, system or device. The computer readable medium can be, by way of example only but not by limitation, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, system, device, propagation medium, or computer memory. 
         [0043]    A “processor” or “process” includes any human, hardware and/or software system, mechanism or component that processes data, signals or other information. A processor can include a system with a general-purpose central processing unit, multiple processing units, dedicated circuitry for achieving functionality, or other systems. Processing need not be limited to a geographic location, or have temporal limitations. For example, a processor can perform its functions in “real time,” “offline,” in a “batch mode,” etc. Portions of processing can be performed at different times and at different locations, by different (or the same) processing systems. 
         [0044]    Reference throughout this specification to “one embodiment”, “an embodiment”, or “a specific embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention and not necessarily in all embodiments. Thus, respective appearances of the phrases “in one embodiment”, “in an embodiment”, or “in a specific embodiment” in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics of any specific embodiment of the present invention may be combined in any suitable manner with one or more other embodiments. It is to be understood that other variations and modifications of the embodiments of the present invention described and illustrated herein are possible in light of the teachings herein and are to be considered as part of the spirit and scope of the present invention. 
         [0045]    Embodiments of the invention may be implemented by using a programmed general purpose digital computer, by using application specific integrated circuits, programmable logic devices, field programmable gate arrays, optical, chemical, biological, quantum or nanoengineered systems, components and mechanisms may be used. In general, the functions of the present invention can be achieved by any means as is known in the art. Distributed, or networked systems, components and circuits can be used. Communication, or transfer, of data may be wired, wireless, or by any other means. 
         [0046]    It will also be appreciated that one or more of the elements depicted in the drawings/figures can also be implemented in a more separated or integrated manner, or even removed or rendered as inoperable in certain cases, as is useful in accordance with a particular application. It is also within the spirit and scope of the present invention to implement a program or code that can be stored in a machine-readable medium to permit a computer to perform any of the methods described above. 
         [0047]    Additionally, any signal arrows in the drawings/Figures should be considered only as exemplary, and not limiting, unless otherwise specifically noted. Furthermore, the term “or” as used herein is generally intended to mean “and/or” unless otherwise indicated. Combinations of components or steps will also be considered as being noted, where terminology is foreseen as rendering the ability to separate or combine is unclear. 
         [0048]    As used in the description herein and throughout the claims that follow, “a”, “an”, and “the” includes plural references unless the context clearly dictates otherwise. Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. 
         [0049]    The foregoing description of illustrated embodiments of the present invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed herein. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes only, various equivalent modifications are possible within the spirit and scope of the present invention, as those skilled in the relevant art will recognize and appreciate. As indicated, these modifications may be made to the present invention in light of the foregoing description of illustrated embodiments of the present invention and are to be included within the spirit and scope of the present invention. 
         [0050]    Thus, while the present invention has been described herein with reference to particular embodiments thereof, a latitude of modification, various changes and substitutions are intended in the foregoing disclosures, and it will be appreciated that in some instances some features of embodiments of the invention will be employed without a corresponding use of other features without departing from the scope and spirit of the invention as set forth. Therefore, many modifications may be made to adapt a particular situation or material to the essential scope and spirit of the present invention. It is intended that the invention not be limited to the particular terms used in following claims and/or to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include any and all embodiments and equivalents falling within the scope of the appended claims. Thus, the scope of the invention is to be determined solely by the appended claims.