This invention relates generally to methods for determining state of charge for secondary batteries, and, more specifically, to using battery current and voltage parameters in real time to determine same.
Real-time estimation of battery current and voltage parameters can be used to provide additional information about a cell's state-of-charge and state-of-health.
State of charge (SOC) is the equivalent of a fuel gauge for the battery pack in a battery electric vehicle (BEV), hybrid vehicle (HEV), or plug-in hybrid electric vehicle (PHEV). SOC is usually expressed as a percentage of full charge (e.g., 0%=empty; 100%=full). An alternate form of the same measure is the depth of discharge (DoD), the inverse of SOC (e.g., 100%=empty; 0%=full). SOC is normally used when discussing the current state of a battery in use, while DoD is most often used when discussing the capacity utilization of a cell during performance rating or cycle life testing.
State-of-charge (SOC) and state-of-health (SOH) are important parameters for monitoring and controlling battery cells, but they can be difficult to determine in many cases. SOH is typically estimated by tracking a cell's accessible capacity.
For battery chemistries where the open-circuit voltage (OCV) decreases continuously during discharge, there is a reasonable correlation between the open-circuit voltage and the SOC. However, this method requires the system to be disconnected from load periodically and in real world applications where a battery system may be under operation for the majority of its life, this is impractical. In chemistries where the OCV changes significantly with SOC, and in which the deviations from OCV under load conditions are relatively small, voltages under load can be used as a close proxy for the OCV. Thus, the voltage along with the amount of current passed into and out of the cell can be used to make an estimate of the SOC. For such battery chemistries, these estimates are often good enough for most purposes.
But for some other battery chemistries, the open-circuit voltage does not decrease continuously during discharge. For example, in a cell with a lithium metal anode and a LiFePO4 cathode, the open-circuit voltage decreases at the very beginning of discharge and then remains stable throughout most of the discharge until it finally drops at the end. As the cell continues to discharge, the SOC decreases whereas the open-circuit voltage remains nearly constant. This relatively flat open-circuit voltage curve is not useful in trying to determine the SOC of such a cell. Additional methods to mitigate these factors include application of Kalman Filter theory, accurate Coulomb counting (current integration), and/or a priori determined complex RC circuit model fitting. However, these additional methods have further limitations and can be impractical to implement depending on the battery chemistry, pack design, or available electronics.
Another method, known as current accounting or Coulomb counting, calculates the SOC by measuring the battery current and integrating it over time. Problems with this method include long-term drift, lack of a reference point, and, uncertainties about a cell's total accessible capacity (which changes as the cell ages) and operation history. Only fully-charged or fully-discharged cells have well-defined SOCs (100% and 0%, respectively).
SOH determination is similarly convoluted—accurate capacity determination is difficult in dynamic usage scenarios due to errors in Coulomb counting. These problems are particularly compounded in lithium-polymer cells in which transport limitations give rise to significant cell polarization, which obscures voltage end-point determination under load.
Some methods of SOC determination involve fitting complicated resistor-capacitor (RC) circuit models to a priori tests in order to model dynamic cell behavior. However, those methods are very complicated, computationally intensive, and are indirect, all of which can contribute to errors and cost. Moreover, such methods are set up in advance, and are not used to predict real-time status indicators.
What is needed is a simple, direct, accurate method to determine the SOC for rechargeable batteries.
The foregoing aspects and others will be readily appreciated by the skilled artisan from the following description of illustrative embodiments when read in conjunction with the accompanying drawings.
A method of determining the state of charge for a battery cell in real time is disclosed. The method involves:
The battery cell may have lithium metal as an anode, lithium iron phosphate as a cathode, and a polymer electrolyte as a separator.
In step (b), the time periods may range from about 0.5 to 60 seconds. In step (c), determining a nominal resistance involves using the following expression to calculate the nominal resistance (Rnom):
wherein OCVnom is a nominal open circuit voltage, Vavg is a time-averaged voltage value, and Iavg is a time-averaged current value. Vavg may be given by:
where ΔT is the time period (in step (c)) and integration is performed over a time bounded discharge region.
In step (e), the previously-populated look-up table may be created by:
In another aspect of the invention, a computer-readable medium is disclosed. The medium has code which, upon execution by a computer processor, implements a method, the method comprising:
The embodiments are illustrated in the context of State-of-Charge (SOC) measurements in a battery with a lithium metal anode and a LiFePO4 cathode. The skilled artisan will readily appreciate, however, that the materials and methods disclosed herein will have application in a number of other contexts where accurate SOC determination is desirable, particularly where there is not an obvious functional relationship between open circuit voltage and SOC.
These and other objects and advantages of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings.
For the purposes of this disclosure, a “Nominal Resistance” is defined as.
wherein OCVnom is a nominal open circuit voltage that can be defined for the specific cell chemistry. In one example, the OCVnom is the average equilibrium voltage over the full SOC range. In another example, the OCVnom is the equilibrium OCV at a single SOC value. Vavg and Iavg are time-averaged values of voltage and current, respectively, and can be determined for a specific load application (e.g., 15 amp pulse for 30 seconds). Equations for determining these values are shown later in this disclosure.
In cell systems at high enough loads, the cell can become transport-limited due to the finite mobility of the charged species of interest intrinsic to the cell electrochemistry. For example, such a transport limitation can result in a concentration gradient that creates an additional voltage in the cell due to the biased polarization of charged species. The concept of nominal resistance has been introduced so that such deviations from nominal open circuit voltage can be taken into account.
Initially, when cells are manufactured, they undergo extensive quality testing. It can be assumed that cells that pass the tests are all starting out about the same. In one embodiment of the invention, one or more cells are subject to additional testing. In one arrangement, a fully-charged cell is connected to a precision battery testing device that is programmed to push current into and pull current out of the cell under various load conditions. Voltage is monitored and Coulombs are counted throughout the process so that SOC can be determined at any time. Nominal resistances are determined for various load durations using equation (1).
In another arrangement, a fully-charged cell is connected to a precision battery testing device that is programmed to push current into and pull current out of the cell as if the cell were experiencing complex dynamic stress loading. An example of a voltage profile (voltage vs time) for such a test is shown in
The data in
Cell polarization is a dynamic and complicated phenomenon that depends strongly on duration, magnitude and direction of the applied power. Rnom is strongly affected by the cell polarization. The skilled artisan will appreciate the foregoing information and understand that dynamic load conditions could be parsed and calculated in different ways—including combining charge and discharge regions and/or binning by net charge or energy passed—to obtain additional information than that presented herein.
Despite the dynamic and complex nature of the load profile in
Results from either simple or complex cell testing conditions can be presented graphically as shown in
A lookup table is an array that replaces runtime computation with a simpler array indexing operation. The savings in terms of processing time can be significant, since retrieving a value from memory is often faster than undergoing an ‘expensive’ computation or input/output operation. The tables may be pre-calculated and stored in static program storage, calculated (or “pre-fetched”) as part of a program's initialization phase (memorization), or even stored in hardware in application-specific platforms.
In other implementations, the look-up table may contain additional or different information, including, but not limited to, average power, net charged passed (determined via Coulomb counting).
The time-averaged voltage Vavg and the time-averaged current Iavg for each region are given by:
where ΔT is the time duration of the load and integration is performed over the time bounded discharge region. In some arrangements, these average values can be calculated over the total duration of the load using common numerical integration methods, such as trapezoidal integration.
Once these values are determined, values for nominal resistance Rnom under various load conditions can be calculated. The Rnom and load duration can be associated with SOC through accurate Coulomb counting. These values can be used to populate a look-up table or create a graph that can be used as a reference during actual cell operation.
Some of the key advantages of the method as described herein include:
The method can be used on the fly, while the battery pack is operating. No down time is required. This is a tremendous advantage. Packs are already designed to monitor voltage for each cell and current and time for the overall pack (which also applies to each cell). All data needed for this method is already collected in a normal battery management system (BMS). The only additional hardware that may be useful are a microprocessor and memory storage. In some embodiments of the invention, the microprocessor and memory storage of the BMS has enough capacity to carry out the SOC determination method without any additional hardware.
It is important to note that the method measures the SOC of each individual cell. In SOC determination methods that employ Coulomb counting for cells in series, errors can and do occur. Such methods cannot measure SOC for each cell, but rather get some kind of composite value. It is difficult to understand what such a composite value really means. For example, in some pack designs a 50% SOC composite value may mean that all cells are at 50% SOC. Or it may mean that half the cells are completely dead and half are at 100% SOC.
In various embodiment of the invention, the SOC dependent deviations using the nominal resistance Rnom and known load conditions are captured as a means of parsing complex and dynamic data to provide a real-world practical method for estimating SOC during operation. This information can be used as a direct measure or to complement existing SOC estimation algorithms. Because this information is readily available throughout the lifetime of operation, there may also be an application in SOH estimation given the initial state of the system and how the deviations look over time.
This invention has been described herein in considerable detail to provide those skilled in the art with information relevant to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by different equipment, materials and devices, and that various modifications, both as to the equipment and operating procedures, can be accomplished without departing from the scope of the invention itself.
Number | Date | Country | Kind |
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2014-10200256.7 | May 2014 | CN | national |
This application claims priority to U.S. Provisional Patent Application 61/939,072, filed Feb. 12, 2014 and to Chinese Patent Application 2014-10200256.7, filed May 13, 2014, both of which are incorporated by reference herein.
Number | Date | Country | |
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61939072 | Feb 2014 | US |