ESTIMATION OF THE STATE OF CHARGE OF BATTERY CELLS

Abstract

Vehicle (12) comprising: a traction battery (24) having a plurality of battery cells (72); and at least one battery controller (76) programmed to output, in response to a difference between a cell voltage (218) and an average cell voltage (224), a cell state of charge (264) based on a voltage offset derived from the difference and in relation to a reference voltage associated with the state of charge of a traction battery (24), and to balance the battery cells (72) according to the cell state of charge (264) of the battery cells (72), and wherein the battery controller (76) determines the voltage offset for each battery cell (72) in the form of an open circuit voltage difference (254) of the respective battery cell (72), wherein the open circuit voltage difference (254) depends on a difference between a reference charge state (222) of the traction battery (24) and a respective cell charge state (264) of the respective battery cell (72).

Description

[0001] This application generally refers to estimating the state of charge of a traction battery.

[0002] Hybrid electric vehicles and pure electric vehicles rely on a traction battery to provide power for propulsion. Typically, the traction battery has a number of battery cells connected in various configurations. To ensure optimal vehicle operation, various properties of the traction battery can be monitored. One useful property is the battery's state of charge (SOC), which indicates the amount of charge stored in the battery. The state of charge can be calculated for the traction battery as a whole and for each of its cells. The traction battery's state of charge provides a useful indication of the remaining charge. The state of charge for each individual cell provides information useful for balancing the state of charge between cells.Balancing cells is useful because differences in the state of charge between cells can cause some cells to become overcharged or undercharged relative to the others.

[0003] WO 2014 / 085507 A1 and DE 10 2011 113 754 B4 disclose generic vehicles and control methods.

[0004] The objective, technical problem to be solved can be seen as eliminating or at least mitigating the disadvantages of the prior art. This problem is solved according to the invention by the subject matter of the independent patent claims.

[0005] A vehicle includes a traction battery having a plurality of cells and at least one controller. The at least one controller is programmed to output, in response to a difference between a cell voltage and an average cell voltage, a cell state of charge based on a voltage offset derived from the difference and relative to a reference voltage associated with a state of charge of a traction battery. The at least one controller is programmed to balance the cells according to the cell state of charge of the cells. The at least one controller may be further programmed to output the cell state of charge based on the reference voltage associated with the state of charge of the traction battery in response to there being no difference between the cell voltage and the average cell voltage.The average cell voltage may be a quotient of a total voltage of the traction battery and a total number of cells of the traction battery. The voltage offset may further be derived from a battery current. The cell state of charge may further be based on an open-circuit voltage difference of the cell derived from a cell model, wherein the difference and a battery current are input values ​​to the cell model. The cell state of charge may further be based on an open-circuit voltage and a state of charge characteristic such that the cell state of charge is the state of charge of the traction battery offset by a state of charge difference associated with the open-circuit voltage difference.

[0006] A battery management system includes at least one controller programmed to operate a traction battery having a plurality of cells according to a cell state of charge of the cells based on a voltage difference between an open circuit voltage and an average open circuit voltage of the cell and relative to a cell voltage associated with a state of charge of the traction battery. The average open circuit voltage of the cell may be based on a total voltage of the traction battery and a total number of cells of the traction battery. The open circuit voltage of the cell may be based on a measured cell terminal voltage. The voltage difference may be based on a battery current. Operating the traction battery may include balancing the cells based on the cell state of charge of the cells.The voltage difference may be based on an output value of the cell model, which has a difference between a measured cell terminal voltage and an average cell terminal voltage as an input value. The voltage difference may be a difference between a first output value of the cell model with a measured cell voltage as an input value and a second output value of the cell model with an average cell terminal voltage as an input value.

[0007] A battery control method includes operating, by a controller, a traction battery having a plurality of cells according to a cell state of charge of the cells based on a voltage offset derived from a difference between a measured cell voltage and an average cell voltage and relative to a cell reference voltage associated with a state of charge of the traction battery. The voltage offset may further be derived from a battery current. The method may further include balancing the cells of the traction battery according to the cell state of charge of the cells. The average cell voltage may be based on a total voltage of the traction battery.The cell state of charge may further be based on an open circuit voltage and a state of charge characteristic such that the cell state of charge is an average state of charge of the traction battery offset by a state of charge amount associated with an open circuit voltage difference of the cell. Fig. is a diagram of a hybrid vehicle illustrating typical powertrain and energy storage components. Fig. is an illustration of a possible battery pack arrangement comprising multiple cells and monitored and controlled by a Battery Energy Control Module. Fig. is a representation of an example battery cell equivalent circuit. Fig. is a graphical representation illustrating a possible relationship between open-circuit voltage (Voc) and battery state of charge (SOC) for a typical battery cell. Fig. is a block diagram illustrating a possible battery management system for calculating cell charge levels. Fig. is a block diagram illustrating a possible battery cell estimation system for calculating a state of charge compensation for a battery cell. Fig. is a block diagram illustrating another battery cell estimation system for calculating a state of charge compensation for a battery cell.

[0008] As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The drawings are not necessarily to scale; some features may be exaggerated or reduced to show details of particular components. Therefore, specific details disclosed herein are not to be considered limiting, but merely as representative principles intended to provide guidance to one skilled in the art to variously employ the present invention.

[0009] Embodiments of the present disclosure are described below. However, it should be understood that the disclosed embodiments are merely examples, and other embodiments may take various and alternative forms. The drawings are not necessarily to scale; some features may be exaggerated or reduced to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be considered limiting, but merely as a representative basis for providing guidance to one skilled in the art to variously employ the present invention.As one of ordinary skill in the art will understand, various features illustrated and described with reference to any of the figures may be combined with features illustrated in one or more other figures to create embodiments not expressly illustrated or described. The illustrated combinations of features represent representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of this disclosure may be desired for particular applications or implementations.

[0010] Fig. shows a typical plug-in hybrid-electric vehicle (HEV). A typical plug-in hybrid-electric vehicle 12 may include one or more electric machines 14 mechanically connected to a hybrid transmission 16. The electric machines 14 may be capable of functioning as a motor or a generator. Further, the hybrid transmission 16 is mechanically connected to a prime mover 18. The hybrid transmission 16 is also mechanically connected to a driveshaft 20, which is mechanically connected to the wheels 22. The electric machines 14 may provide propulsion and deceleration potential when the prime mover 18 is turned on or off. The electric machines 14 also function as generators and may provide fuel economy benefits by recovering energy that would normally be lost as heat in the friction braking system.The electric machines 14 may also reduce vehicle emissions by enabling the engine 18 to operate under more efficient conditions (engine speeds and loads) and by enabling the hybrid electric vehicle 12 to operate in an electric mode in which the engine 18 is off under certain conditions.

[0011] A traction battery or battery pack 24 stores energy that can be used by the electric machines 14. A vehicle battery pack 24 typically provides a high-voltage direct current output. The traction battery 24 is electrically connected to one or more power electronics modules. One or more contactors 42, when open, can isolate the traction battery 24 from other components and, when closed, can connect the traction battery 24 to other components. The power electronics module 26 is also electrically connected to the electric machines 14 and provides the ability to transfer power between the traction battery 24 and the electric machines 14 in either direction. For example, a typical traction battery 24 may provide a direct current, while the electric machines 14 require three-phase alternating current to operate properly.The power electronics module 26 can convert the DC voltage into a three-phase AC voltage used by the electric machines 14. In a regenerative mode, the power electronics module 26 can convert the three-phase AC current from the electric machines 14, which function as generators, into the DC voltage used by the traction battery 24. The present description is equally applicable to a pure electric vehicle. In a pure electric vehicle, the hybrid transmission 16 can be embodied as a transmission connected to an electric machine 14, and the prime mover 18 can be absent.

[0012] In addition to providing power for propulsion, the battery pack 24 can provide power to other electrical systems in the vehicle. A vehicle may include a DC-to-DC converter module 28 that converts the high-voltage DC output of the traction battery 24 to low-voltage DC power compatible with other vehicle loads. Other high-voltage electrical loads 46, such as compressors and electric heaters, may be directly connected to the high voltage without using a DC-to-DC converter module 28. The electrical loads 46 may have an associated controller that operates the electrical load 46, if necessary. The low-voltage systems may be electrically connected to an auxiliary battery 30 (e.g., a 12V battery).

[0013] The vehicle 12 may be an electric vehicle or a plug-in hybrid vehicle in which the traction battery 24 may be charged by an external power source 36. The external power source 36 may be embodied as a connection to an electrical outlet. The external power source 36 may be electrically connected to the electric vehicle supply equipment (EVSE) 38. The EVSE 38 may provide the circuitry and controls to regulate and manage the transfer of power between the power source 36 and the vehicle 12. The external power source 36 may provide direct current or alternating current to the EVSE 38. The EVSE 38 may have a charging connector 40 for plugging into a charging port 34 of the vehicle 12. The charging port 34 may be any type of connector configured to transfer power from the EVSE 38 to the vehicle 12.The charging port 34 may be electrically connected to a charger or a built-in power conversion module 32. The power conversion module 32 may condition the power supplied by the EVSE 38 to provide the appropriate voltage and current levels for the traction battery 24. The power conversion module 32 may be interconnected with the EVSE 38 to coordinate the delivery of power to the vehicle 12. The EVSE connector 40 may have pins that engage corresponding recesses of the charging port 34. Alternatively, various components described as being electrically connected may transfer power using a wireless inductive coupling.

[0014] One or more wheel brakes 44 may be provided for decelerating the vehicle 12 and preventing movement of the vehicle 12. The wheel brakes 44 may be actuated hydraulically, electrically, or by a combination of these methods. The wheel brakes 44 may be part of a braking system 50. The braking system 50 may include other components that cooperate to operate the wheel brakes 44. For simplicity, the figure shows a single connection between the braking system 50 and one of the wheel brakes 44. A connection between the braking system 50 and the other wheel brakes 44 is implied. The braking system 50 may include a controller for monitoring and coordinating the braking system 50. The braking system 50 may monitor the braking components and control the wheel brakes 44 to decelerate or steer the vehicle.The braking system 50 can respond to driver commands and can also operate autonomously to implement features such as stability control. The controller of the braking system 50 can implement a method of applying braking force upon request from another controller or subfunction.

[0015] The various components discussed may have one or more associated controllers to control and monitor their operation. The controllers may communicate via a serial bus (e.g., a Controller Area Network (CAN)) or via discrete conductors. In addition, a system controller 48 may be present to coordinate the operation of the various components.

[0016] A traction battery 24 can be constructed based on various chemical formulas. Typical chemical compositions of battery packs can be lead acid, nickel-metal hydride (NIMH), or lithium-ion. Fig. shows a typical traction battery pack 24 with a simple series arrangement of N battery cells 72. However, other battery packs 24 may be composed of any number of individual battery cells connected in series or parallel, or a combination of these circuit arrangements. A typical system may include one or more controllers, such as a Battery Energy Control Module (BECM) 76, that controls and monitors the performance of the traction battery 24. The BECM 76 may monitor several battery pack-level properties, such as pack current 78, pack voltage 80, and pack temperature 82. The BECM 76 may include non-volatile memory such that data may be retained when the BECM 76 is in a powered-off state. Retained data may be available at the next ignition cycle.A battery management system may consist of components other than the battery cells and may include the BECM 76, measurement sensors (78, 80, 82), and sensor modules 74. The function of the battery management system may be to operate the traction battery safely and efficiently.

[0017] In addition to the properties at the pack level, there may be properties at the battery cell 72 level that are measured and monitored. For example, the terminal voltage, current, and temperature of each cell 72 may be measured. A system may use a sensor module 74 to measure the properties of the battery cell 72. Depending on the functionality, the sensor module 74 may measure the properties of one or more battery cells 72. The battery pack 24 may contain up to N cSensor modules 74 may be used to measure the characteristics of each of the battery cells 72. Each sensor module 74 may transmit the measurements to the BECM 76 for further processing and coordination. The sensor module 74 may transmit signals to the BECM 76 in analog or digital form. In some embodiments, the functionality of the sensor module 74 may be integrated internally into the BECM 76. That is, the hardware of the sensor module 74 may be integrated into the BECM 76 as part of the circuitry, and the BECM 76 may handle the processing of raw signals.

[0018] The battery cell 72 and pack voltages 80 may be measured using a voltage sensor. The voltage sensor circuitry within the sensor module 74 and the pack voltage measurement circuitry 80 may include various electrical components to scale and sample the voltage signal. The measurement signals may be routed to the inputs of an analog-to-digital converter in the sensor module 74 and the BECM 76 for conversion to a digital value. These components may be shorted or opened, causing the voltage to be measured improperly. Furthermore, these problems may occur intermittently over time and appear in the measured voltage data. The sensor module 74, the pack voltage sensor 80, and the BECM 76 may include circuitry to determine the status of the voltage measurement components.In addition, a controller in the sensor module 74 or the BECM 76 may perform signal threshold checks based on expected signal levels.

[0019] In a battery consisting of many connected cells, the state of charge of the cells can be unbalanced for many reasons, such as manufacturing variations, different rates of cell decay due to the temperature distribution within the traction battery, and different rates of internal leakage due to the chip design. Battery cell imbalance can be defined as the difference between the SOC of the cells. A battery controller 76 can have a cell balancing function. Cell balancing is a process that attempts to equalize the SOC of the cells by adding or subtracting charge from the affected cells.

[0020] Various methods for performing cell balancing are possible. A switch may be provided above each battery cell, which can selectively switch a circuit element for the cell. The circuit element may allow the cell to discharge. Alternatively, a switch may selectively connect cells together so that one cell discharges while another cell is charged. Cell balancing may be achieved by selectively charging and discharging the cells of the battery until all cells have approximately the same state of charge. The battery controller 76 may include switches and interface circuits to control and activate the switches. The battery controller 76 may implement software to perform cell balancing. The efficiency of cell balancing may be improved by using an accurate SOC estimate for each cell.

[0021] A battery cell can be modeled in a variety of ways. For example, a battery cell can be modeled as a circuit. Fig. shows a possible battery cell equivalent circuit model (ECM). A battery cell may be modeled as a voltage source (Voc) 100 with an associated impedance. The impedance may include one or more resistors (102 and 104) and a capacitance 106. Voc 100 represents the open-circuit voltage (OCV) of the battery. The model may include an internal resistance, r1 102, a charge-transfer resistance, r2 104, and a double-layer capacitance, C 106. The voltage V1 112 is the voltage drop across the internal resistor 102 due to the current 114 flowing through the circuit. The voltage V2 110 is the voltage drop across the parallel combination of r2 and C due to the current 114 flowing through the combination. The voltage Vt 108 is the voltage at the terminals of the battery (terminal voltage).

[0022] Due to the battery cell impedance, the terminal voltage, Vt 108, may not be identical to the open-circuit voltage, Voc 100. The open-circuit voltage, Voc 100, may not be easily measurable because only the terminal voltage 108 of the battery cell is accessible for measurement. If no current 114 flows for a sufficiently long period of time, the terminal voltage 108 may be identical to the open-circuit voltage 100. A sufficiently long period of time may be required for the internal dynamics of the battery to reach a steady state. If current 114 flows, Voc 100 may not be easily measurable, and the value may need to be derived based on the SOC, as in Fig. The parameter values, r1, r2, and C, can be known or unknown. The value of the parameters may depend on the cell design and the chemical properties of the battery.

[0023] In a typical lithium-ion battery cell, there is a relationship between the SOC and the open circuit voltage (Voc) such that Voc = f(SOC). Fig. shows a typical curve 124 representing the open-circuit voltage Voc as a function of SOC. The relationship between SOC and Voc can be determined by analyzing the battery characteristics or by testing the battery cells. The exact shape of curve 124 can vary depending on the exact formulation of the lithium-ion battery. The voltage Voc changes as the battery charges and discharges.

[0024] A possible model can be the equivalent circuit model from Fig. The governing equations for the equivalence model can be formulated as follows: V˙2=−1r2CV2+1C*i Voc=Vt+V2+r1*i where i is the current and V̇2 is the time-based derivative of V2. The model can represent a single cell or the traction battery as a whole (e.g., a series connection of numerous cells). The various impedance values ​​(r1, r2, and C) of the model can be estimated or predefined. Various impedance parameter estimation systems can be used.

[0025] Referring to the model from Fig. Different voltage and current values ​​can be measured per cell or for the entire pack. For example, the terminal voltage, Vt 108, can be measured for each cell of the traction battery. The current, i 114, can be measured for the entire traction battery, since the same current can flow through each cell. Different pack configurations can use different combinations of measurements.

[0026] Using the model, a relationship or function can be determined that relates the terminal voltage to the open-circuit voltage. The terminal voltage can be expressed as Vt = g(Voc, i). An inverse function can be determined that relates the open-circuit voltage 100 to the terminal voltage 108. This open-circuit voltage 100 can be expressed as Voc = g -1 (Vt, i). The above function and inverse function depend on the chosen model equations. The example from Fig. is a possible model, and the procedures described are not limited to this particular model.

[0027] The battery state of charge value can be used to estimate the battery's usable energy and performance. As the accuracy of the SOC value increases, the accuracy of battery control operations can also increase. An accurate SOC value can improve cell balancing, which can increase the battery's usable energy and reduce potential risks from excessive battery operation.

[0028] One state-of-the-art method for calculating SOC is current integration, also known as ampere-hour integration or coulomb counting. This approach requires very accurate current sensors to ensure the desired accuracy of the SOC estimate. Any measurement noise can lead to inaccurate SOC estimates. More sophisticated methods with increased accuracy can also be used. Other approaches can use voltage information to estimate SOC. Such approaches can rely on complex battery models and identification systems. Some SOC estimation systems may rely on running a sophisticated identification model for each cell in the battery pack. Running these time-intensive algorithms for each battery cell may be beyond the capacity of the battery management system due to limited computing power and hardware resources.

[0029] A traction battery may consist of battery cells with the same or sufficiently similar properties. The battery dynamics for each cell may be similar to the other battery cells in the battery pack. Because the cells may be similar, the computational processes performed for each cell may result in the same or similar values. Performing the same computational processes may be redundant and unnecessary if the input values ​​are similar. A more efficient computational method may be to estimate a cell SOC from an estimated reference SOC, which corresponds to an average battery cell SOC and an SOC difference between each cell and the reference state of charge.

[0030] One advantage of this method can be a reduction in computational complexity. Estimating the reference SOC can use sophisticated and computationally intensive methods, while estimating each cell's SOC uses simple and fast instructions. This reduces complexity because the computationally intensive method is only performed to calculate the reference SOC and is not repeated for each cell. Each cell's SOC can be adjusted depending on the reference SOC using less computationally intensive methods.

[0031] Fig. shows a block diagram of a battery management system's state of charge estimator 200 for calculating the state of charge for each of the cells 72 of the traction battery. The state of charge estimator logic 200 may be implemented as a function in a battery controller. The state of charge estimator logic 200 may include a battery pack SOC estimator 202. The pack SOC estimator 202 may use known methods for accurately calculating a pack-level SOC. To estimate the pack SOC, the SOC estimator may use various measurements. The pack SOC estimator 202 may use a battery current measurement 214 and a pack voltage measurement 220 to calculate and output a pack SOC 222. To achieve high accuracy, sophisticated modeling and identification systems may be used to estimate the pack SOC.

[0032] The pack SOC 222 can be viewed as a combination of the SOC values ​​of each of the cells. The pack SOC 222 can be considered an average cell SOC value. If the cells are precisely balanced, the cell SOCs and the overall SOC of the pack can have the same value. In practice, however, there are fluctuations in the cell SOCs, which can be caused by cell fluctuations and operating conditions.

[0033] The battery controller may measure a pack voltage 220. The pack voltage 220 may be the voltage across the terminals of the battery pack. The pack voltage 220 may be the sum of the battery cell voltages 72, assuming that the battery cells 72 are connected in series. Not shown in Fig. are the hardware circuits that may be present to scale and filter the measurement of the pack voltage 220. In addition to measuring the pack voltage 220, the battery controller may measure the voltage across each cell 72 of the battery pack. Fig. shows a battery with N battery cells 72. Each cell voltage can be measured and input into the state of charge subsystem 200. For example, the voltage measurement of the k-th cell 218 can be measured and input. The estimation system can be repeated for each cell of the battery pack.

[0034] An average cell voltage 224 may be the quotient of the pack voltage 220 divided by the number of battery cells, N. This is represented by applying a gain 206 of 1 / N to the pack voltage 220. A voltage difference 226 between the average cell voltage 224 and the k-th cell voltage 218 may be calculated. This process is represented by the difference element 208 of Fig. The calculation of the average cell voltage 224 can be adjusted based on the specific configuration in the case where the cells are not connected in series.

[0035] The controller may perform a ΔSOC estimation process 210 to calculate a deviation of the state of charge from the average SOC 222 for each cell. A cell SOC difference 228 may be calculated for each cell relative to the average SOC for all cells. The cell SOC difference 228 may be added to the total pack SOC 222 to obtain a cell SOC 216 for the cell. This process may be described by the following equation: SOCcell,k=SOCpack+ΔSOCcell,k

[0036] This process is carried out by a second summing point 212 in Fig. This procedure can be repeated for each cell of the traction battery. Each traction battery can have an associated state of charge difference.

[0037] The ΔSOC estimation method 210 may include additional logic as described in Fig. shown. The voltage difference 226 between the cell voltage and the average cell voltage can be used to calculate a cell open circuit voltage difference 254 between a cell open circuit voltage and an average cell open circuit voltage. A relationship between a terminal voltage difference and the open circuit voltage difference can be known based on the selected model. To generate the cell open circuit voltage difference 254, an inverse dynamics model 250 can be used. The model 250 can also utilize the battery current 214. The model can be as in Fig. shown and represented by equations (1) and (2). The model can be executed in the controller, and the output value can be the cell's open-circuit voltage difference 254. Note that other models can be used with similar results.

[0038] The open circuit voltage difference 254 can be input into an inverse SOC curve or function 252. The inverse SOC function 252 can use the battery characteristic curve or function 256. The characteristic is the Fig. described curve. An additional input value to the inverse SOC function 252 may be the estimated SOC 222 of the package. The operation of the inverse SOC function 252 may be understood by reference to the characteristic curve 256.

[0039] An exit point 260 on curve 258 associated with pack SOC 222 is determined. The exit point 260 defines a cell reference voltage associated with the pack or average cell SOC 222. The reference voltage value at the exit point 260 is the average open circuit voltage for cells. The open circuit voltage difference 254 is used as a voltage offset to determine a cell open circuit voltage. By applying the open circuit voltage difference 254, a cell open circuit voltage is determined. The cell open circuit voltage is defined as a sum of the average open circuit voltage and the open circuit voltage difference 254. The resulting cell open circuit voltage defines a point 262 on curve 258 with an associated cell state of charge 264. The change in SOC 228 defines the difference between the cell state of charge 264 and the average SOC 222.

[0040] An alternative ΔSOC estimation system 210' is in Fig. shown. In this example, the open circuit voltage difference is not input directly. In the alternative example, the cell voltage 218 and the average cell voltage 224 are fed separately to the inverse dynamics block 250. The outputs are an open circuit voltage 282 for the cell and an average open circuit voltage 280 for all cells. The open circuit voltage difference 254 can then be the difference between the cell open circuit voltage 282 and the average open circuit voltage 280. The open circuit voltage difference 254 can then be input to the inverse SOC function 252 as described above.

[0041] The aforementioned method can be implemented in a battery controller as part of a battery management system. The cell state of charge values ​​can be used to balance the cells or power the traction battery. An advantage of this method is that it can be less computationally intensive than other methods. A highly accurate package SOC estimation system can be implemented. Each cell SOC is then calculated using less computationally intensive models. The accuracy of the cell SOC values ​​is maintained because the method relies on the highly accurate package SOC estimation to determine the cell SOC values.

[0042] The processes, methods, or algorithms disclosed in this patent application may be provided to or implemented by a processing device, controller, or computer, including any existing programmable electronic control unit or a dedicated electronic control unit. Likewise, processes, methods, or algorithms may be stored as data and instructions executable by a controller or computer in a variety of forms, including, but not limited to, information permanently stored on a non-writable storage medium, such as ROM (read-only memory) devices, or modifiably stored on writable storage media, such as floppy disks, magnetic tapes, CDs, RAM (random access memory), and other magnetic and optical media.The processes, methods, or algorithms may also be implemented in an executable software object. Alternatively, the processes, methods, or algorithms may be implemented in whole or in part using suitable hardware components, such as application-specific integrated circuits (ASICS), field-programmable gate arrays (FPGAs), state machines, controllers, or other hardware components or devices, or a combination of hardware, software, and firmware components.

[0043] Although exemplary embodiments are described above, these embodiments are not intended to describe all possible forms covered by the claims. The terms used in this specification are merely descriptive and imply no limitation. It is understood that various changes may be made without departing from the spirit and scope of the disclosure. As described above, the features of various embodiments may be combined to form further embodiments of the invention not expressly described or illustrated herein.While various embodiments could have been described as advantageous or preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art will recognize that trade-offs may be made for one or more features or characteristics to achieve desired overall system attributes depending on the particular application and implementation. These attributes may include, but are not limited to, cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, usability, weight, manufacturability, ease of assembly, etc.As such, embodiments described as being less desirable with respect to one or more characteristics relative to other embodiments or prior art implementations are not outside the scope of the disclosure and may be desirable for certain applications.

Claims

[1] Vehicle (12) comprising: a traction battery (24) having a plurality of battery cells (72); and at least one battery controller (76) programmed to output, in response to a difference between a cell voltage (218) and an average cell voltage (224), a cell state of charge (264) based on a voltage offset derived from the difference and in relation to a reference voltage associated with the state of charge of a traction battery (24), and to balance the battery cells (72) according to the cell state of charge (264) of the battery cells (72), and wherein the battery controller (76) determines the voltage offset for each battery cell (72) in the form of an open circuit voltage difference (254) of the respective battery cell (72), wherein the open circuit voltage difference (254) depends on a difference between a reference charge state (222) of the traction battery (24) and a respective cell charge state (264) of the respective battery cell (72). [2] The vehicle (12) of claim 1, wherein the at least one battery controller (76) is further programmed to output the cell state of charge (264) based on the reference voltage associated with the reference state of charge (222) of the traction battery (24) in response to there being no difference between the cell voltage (218) and the average cell voltage (224). [3] The vehicle (12) of claim 1, wherein the average cell voltage (224) is a quotient of a battery voltage (220) of the traction battery (24) and a total number of battery cells (72) of the traction battery (24). [4] The vehicle (12) of claim 1, wherein the voltage offset is further derived from a battery current (214). [5] The vehicle (12) of claim 1, wherein the open circuit voltage difference (254) is estimated from a cell model, the difference between the cell voltage (218) and the average cell voltage (224) and a battery current (214) being inputs to the cell model. [6] The vehicle (12) of claim 5, wherein the cell state of charge (264) is further based on an open circuit voltage and a state of charge characteristic such that the cell state of charge (264) is the reference state of charge (222) of the traction battery (24) offset by a state of charge difference associated with the open circuit voltage difference (254) of the battery cell (72). [7] Battery management system, comprising: at least one battery controller (76) programmed to operate a traction battery (24) having a plurality of battery cells (72) according to a cell state of charge (264) of the battery cells (72) based on a voltage offset, wherein the cell state of charge (264) of the battery cells (72) is based on an open circuit voltage difference (254) between an open circuit voltage (282) of the battery cell (72) and an average open circuit voltage (280) of the battery cell (72) and in relation to a cell voltage (218) associated with a reference state of charge (222) of the traction battery (24), wherein the open circuit voltage difference (254) depends on a difference between a reference charge state (222) of the traction battery (24) and a respective cell charge state (264) of the respective battery cell (72), and wherein the battery controller (76) balances the battery cells (72) according to the cell charge state (264) of the battery cells (72). [8] The battery management system of claim 7, wherein the average open circuit voltage of the battery cell (72) is based on a total voltage (220) of the traction battery (24) and a total number of battery cells (72) of the traction battery (24). [9] The battery management system of claim 7, wherein the open circuit voltage (282) of the battery cell (72) is based on a measured cell voltage (218). [10] The battery management system of claim 7, wherein the open circuit voltage difference (254) is based on a battery current (214). [11] The battery management system of claim 7, wherein operating the traction battery (24) comprises balancing the battery cells (72) based on the cell state of charge (264) of the battery cells (72). [12] The battery management system of claim 7, wherein the open circuit voltage difference (254) is based on an output value of the cell model having a difference between a measured cell voltage (218) and an average cell voltage (224) as an input value. [13] The battery management system of claim 7, wherein the open circuit voltage difference (254) is based on a first output value of the cell model with a measured cell voltage (218) as input value and a second output value of the cell model with an average cell voltage (224) as input value. [14] A battery control method comprising: Operating, by a battery controller (76), a traction battery (24) having a plurality of battery cells (72) according to a cell state of charge (264) of the battery cells (72) based on a voltage offset derived from a difference (226) between a measured cell voltage (218) and an average cell voltage (224) and in relation to a cell reference voltage associated with a state of charge of the traction battery (24), wherein the battery controller (76) determines the voltage offset for each battery cell (72) in the form of an open-circuit voltage difference (254) of the respective battery cell (72), and wherein the open-circuit voltage difference (254) depends on a difference between a reference charge state (222) of the drive battery (24) and a respective cell charge state (264) of the respective battery cell (72). [15] The method of claim 14, wherein the voltage offset is further derived from a battery current (214). [16] The method of claim 14 further comprising balancing the battery cells (72) of the traction battery (24) according to the cell charge state (264) of the battery cells (72). [17] The method of claim 14, wherein the average cell voltage (224) is based on a battery voltage (220) of the traction battery (24). [18] The method of claim 14, wherein the cell state of charge (264) is further based on an open circuit voltage and a state of charge characteristic such that the cell state of charge (264) is a reference state of charge (222) of the traction battery (24) offset by a state of charge amount associated with an open circuit voltage difference (254) of the battery cell (72).

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