Battery power prediction for high load events

Abstract

The present disclosure provides "Battery Power Prediction for High Load Events". A system for a vehicle includes: an engine and an electric machine, each configured to propel the vehicle; and a controller configured to reduce power to the electric machine and activate the engine in response to a resistance of a traction battery and a change in the resistance being greater than a respective threshold during battery discharge.

Description

Technical Field

[0001] The present disclosure relates to systems and methods for analyzing the available charge of a traction battery during a predefined sustained discharge period. Background Art

[0002] The term "electric vehicle" may be used to describe a vehicle having at least one electric motor for vehicle propulsion, such as a battery electric vehicle (BEV), a hybrid electric vehicle (HEV), and a plug-in hybrid electric vehicle (PHEV). A BEV includes at least one electric motor, wherein the energy source for the motor is a battery that can be recharged from an external power grid. An HEV includes an internal combustion engine and one or more electric motors, wherein the energy source for the engine is fuel and the energy source for the motor is a battery. In an HEV, the engine is the primary energy source for vehicle propulsion, wherein the battery provides supplemental energy for vehicle propulsion (the battery buffers fuel energy and recovers kinetic energy in electric form). A PHEV is like an HEV, but the PHEV has a larger capacity battery that can be recharged from an external power grid. In a PHEV, the battery is the primary energy source for vehicle propulsion until the battery is depleted to a low energy level, at which point the PHEV operates like an HEV in terms of vehicle propulsion. Summary of the invention

[0003] A system for a vehicle includes: an engine and an electric machine, each configured to propel the vehicle; and a controller configured to reduce power to the electric machine and activate the engine in response to a resistance of a traction battery and a change in the resistance being greater than a respective threshold during battery discharge.

[0004] A method for a vehicle includes, by a vehicle controller, reducing power from a traction battery to an electric machine and activating an engine in response to a resistance of the traction battery and a change in the resistance being greater than respective thresholds during battery discharge, wherein each of the electric machine and the engine is configured to propel the vehicle.

[0005] A system for a vehicle includes: an engine and a motor, each of which is configured to propel the vehicle; and a controller, which is configured to reduce the battery power provided to the motor and activate the engine in response to the magnitude of the battery discharge current being greater than a current threshold and the rate of decrease of the battery discharge voltage being greater than a rate of change threshold. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1A to Figure 1B is a graph showing changes in the measured battery voltage;

[0007] Figure 1C is a block diagram illustrating an exemplary plug-in hybrid electric vehicle;

[0008] Figure 1D is a block diagram illustrating an exemplary battery cell configuration;

[0009] FIG. 2A to FIG. 2C is a graph illustrating exemplary predicted power during a battery discharge event;

[0010] Figure 3 is a flow chart showing an algorithm for responding to a non-linear decrease in measured voltage;

[0011] FIG. 4A to FIG. 4D is a graph showing exemplary resistance changes based on measured voltage and current;

[0012] FIG. 5A to FIG. 5B is a graph showing an exemplary variation of voltage measured during a drive cycle;

[0013] Figure 6 is a flow chart illustrating an algorithm for predicting available battery propulsion power;

[0014] Figure 7 is a flow chart illustrating an algorithm for predicting available power based on non-linear changes in measured voltage; and

[0015] FIG. 8A to FIG. 8B is a graph showing predicted available battery power. DETAILED DESCRIPTION

[0016] Embodiments of the present disclosure are described herein. However, it should be understood that the disclosed embodiments are merely examples, and other embodiments may take various and optional forms. The drawings are not necessarily drawn to scale; some features may be enlarged or minimized to show the details of specific components. Therefore, the specific structural and functional details disclosed herein are not interpreted as limitations, but are merely interpreted as representative bases for teaching those skilled in the art to adopt the present invention in various ways. As will be understood by those skilled in the art, the various features shown and described with reference to any one of the figures may be combined with the features shown in one or more other figures to produce embodiments that are not explicitly shown or described. The combination of features shown provides representative embodiments for typical applications. However, specific applications or implementations may expect various combinations and modifications of features consistent with the teachings of the present disclosure.

[0017] For charge-depleted PHEV operation, during sustained discharge conditions, high current or power demand may cause the apparent battery cell resistance to increase rapidly, i.e., the rate of change is greater than a threshold rate, and the battery cell voltage may drop to a voltage less than a predefined voltage. In such conditions, the nonlinear increase in the apparent resistance of the battery cell may cause the battery cell discharge power capacity to decrease. If the voltage change is greater than a predefined threshold, this nonlinearity of the voltage response may make it difficult to accurately estimate the actual available battery power and react quickly enough. As an example, during a highway passing maneuver, when the vehicle is already traveling at a high cruising speed, the power demand may increase from moderate to high. During this situation, if the power demand causes a nonlinear voltage change rate within the battery cell, the available discharge power of the battery may be reduced.

[0018] As an example, Figure 1C A vehicle system 10 is shown for a plug-in hybrid electric vehicle 12. The vehicle 12 may include one or more electric machines 14 mechanically coupled to a hybrid transmission 16. The electric machines 14 are capable of acting as motors or generators. Additionally, the hybrid transmission 16 may be mechanically coupled to an engine 18. The hybrid transmission 16 may also be mechanically coupled to a shaft 20, which is mechanically coupled to wheels 22. Although Figure 1C A typical plug-in hybrid electric vehicle is depicted, but the description herein is equally applicable to pure electric vehicles or hybrid electric vehicles of different configurations (such as but not limited to series hybrids). For pure electric vehicles, such as battery electric vehicles (BEVs), the hybrid transmission 16 may be a gearbox connected to the motor 14, and the engine 18 may not be present.

[0019] The motor 14 can provide propulsion and deceleration capabilities when the engine 18 is running or turned off. The motor 14 can act as a generator and provide fuel economy benefits by recovering energy that is normally lost as heat in the friction braking system. The motor 14 can additionally provide a counter torque against the output torque of the engine 18 to generate electricity when the vehicle is running to recharge the traction battery 24. The motor 14 can further reduce vehicle emissions by allowing the engine 18 to operate near the most efficient speed and torque range. When the engine 18 is turned off, the vehicle 12 can operate in a pure electric mode (e.g., EV mode) using the motor 14 powered by traction battery electricity as the sole propulsion source.

[0020] The traction battery 24 stores energy that can be used by the electric machine 14. The traction battery 24 generally provides a high voltage direct current (DC) output. In some cases, one or more contactors 42 can isolate the traction battery 24 from the DC high voltage bus 54A when open, and couple the traction battery 24 to the DC high voltage bus 54A when closed. Although the contactors 42 are shown as separate components, in some examples, the one or more contactors 42 may include a bus electrical center (BEC) 44 that is configured to communicate with the battery controller 50 to allow electrical energy to be supplied to and extracted from the traction battery 24. In response to an operating parameter associated with the traction battery 24 reaching a predefined threshold, the battery controller 50 may issue a command to the BEC 44 to operate a plurality of switches, such as opening or closing a contactor or relay, or otherwise manipulate one or more electrical components that control the transfer of energy to the traction battery 24.

[0021] The traction battery 24 is electrically coupled to one or more power electronic controllers 26 via a DC high voltage bus 54A. The power electronic controller 26 is also electrically coupled to the motor 14 and provides the ability to transfer energy bidirectionally between the alternating current (AC) high voltage bus 54B and the motor 14. For example, the traction battery 24 can provide a DC output, while the motor 14 can function using three-phase AC operation. The power electronic controller 26 can convert the DC output of the traction battery 24 to the three-phase AC input necessary to operate the motor 14. In the regenerative mode, the power electronic controller 26 can convert the three-phase AC output from the motor 14 acting as a generator to a DC input compatible with the traction battery 24.

[0022] In some cases, during cold climate operation, battery cell resistance tends to be higher (1 to 2 orders of magnitude higher than normal temperatures), and nonlinear reductions in battery cell discharge power capacity may occur more frequently. This may occur at least in part due to diffusion limitations of electrolyte and electrode phases within the battery cells. Such events may affect PHEV vehicle performance, drivability, and fuel economy. Early detection of possible events of nonlinear power capacity reduction can improve battery power capability predictions by providing the control system with additional time to respond with more gradual power restrictions. For example, in response to detecting that the voltage reduction rate is greater than a predefined rate threshold, the predicted gradual reduction in battery power capacity can allow the control system to implement changes in the predicted available power in a linear rather than nonlinear manner, and can prevent the battery power limit from being activated at an unexpected time.

[0023] The battery controller 50 may perform power prediction and forecasting of the PHEV during a sustained driving event during a charge depletion period. The battery controller 50 may be connected to one or more sensors and may receive signals corresponding to voltage, current, resistance change rate, and other parameters of one or more of the plurality of battery cells within the traction battery 24, such as reference to at least Figure 1D As described. In some examples, a system controller 48 may be present to coordinate the operation of various components, including those operations related to non-linear power capacity reduction. The system controller 48 and the battery controller 50, although each shown as a single controller, may also be implemented as one or more controllers. The system controller 48 may, for example, monitor the operating conditions of the traction battery 24, the power conversion controller 32, and the motor 14. The system controller 48 and / or the battery controller 50 may be configured to receive signals indicating the magnitude and direction of the current through the traction battery 24, the voltage level between the terminals of the traction battery 24, etc.

[0024] The battery controller 50 may enable detection of events associated with a non-linear decrease in voltage during sustained discharge operation. The battery controller 50 may determine that the predicted available battery power is less than the predicted available battery power determined using an existing control algorithm, thereby preventing (or delaying or gradually changing) activation of a battery power limit due to increased power demand. Consistent with the PCP / PLT power distribution strategy, the battery controller 50 may forecast a smaller predicted available battery power so that activation of power limits on the vehicle 12 is more gradual and EV mode operation may be maintained.

[0025] For example, Figure 1D A block diagram of an exemplary arrangement 60 of one or more portions of a traction battery 24 is shown. Traction battery 24 may include a plurality of monocrystalline battery cells 61, each having a cathode layer 62 and an anode layer 66 (also referred to as a positive electrode and a negative electrode, respectively) separated by a separator 64. Separator 64 enables current to flow between cathode layer 62 and anode layer 66 of monocrystalline battery cells 61. Each monocrystalline battery cell 61 may also have a predefined nominal voltage.

[0026] A predefined number of monocrystalline cells 61 (e.g., 20) may be connected in series or in parallel relative to one another to define a battery cell 68 including a positive terminal 70 and a negative terminal 72. The monocrystalline cells 61 and the battery cell 68 may be, for example, electrochemical cells, capacitors, or other types of energy storage device implementations. The monocrystalline cells 61 and the battery cell 68 may be arranged in any suitable configuration and configured to receive and store electrical energy for use in the operation of the vehicle 12. Each battery cell 68 may provide the same or different nominal voltage thresholds. Although the traction battery 24 is described as including, for example, electrochemical cells, other types of energy storage device implementations, such as capacitors, are also contemplated.

[0027] The battery cells 68 may also be arranged into one or more arrays, sections or modules that are further connected in series or in parallel. For example, a plurality of battery cells 68 connected together in series may include a battery module 74. The battery module 74 may include a data connection 76 that allows one or more controllers of the vehicle 12 (e.g., the BEC 44) to enable and disable energy flow into and out of the battery module 74. Additionally or alternatively, the data connection 76 may include one or more battery sensors connected to one or more controllers of the vehicle 12 (e.g., the battery controller 50). The battery sensors may, for example, include one or more of a temperature sensor, a voltage sensor, a current sensor, and the like.

[0028] A predefined number of battery modules 74 connected together in series or parallel may define a battery pack 78. The battery pack 78 may include a battery management system (BMS) 80 configured to monitor and manage one or more subcomponents of the battery pack 78, for example, via data connection 76. In one example, the battery management system 80 may be configured to monitor the temperature, voltage, and / or current of the monocrystalline battery cells 61, the battery cells 68, the battery modules 74, etc. The battery management system 80 may communicate with one or more of the BEC 44 and the battery controller 50 and may enable energy to flow into and out of the battery pack 78 in response to signals or commands from the BEC 44 and / or the battery controller 50. In one example, the traction battery 24 may define a reference to the reference battery 76. Figure 1D One or more components similar to those described in the description. In addition, it is also contemplated that the definition not referred to in Figure 1D Additional and / or different components of the traction battery 24 are specifically described.

[0029] In addition to providing energy for propulsion, the traction battery 24 can also provide energy for other vehicle electrical systems. The vehicle 12 may include a DC / DC converter controller 28 that is electrically coupled to the high voltage bus 54. The DC / DC converter controller 28 may be electrically coupled to a low voltage bus 56. The DC / DC converter controller 28 may convert the high voltage DC output of the traction battery 24 and / or the power electronics controller 26 into a low voltage DC power source that is compatible with the low voltage vehicle loads connected to the low voltage bus 56. In one example, the low voltage bus 56 may be electrically coupled to an auxiliary battery 30 (e.g., a 12V battery). In another example, a low voltage system 52 (such as, but not limited to, accessories, lighting, etc.) may also be electrically coupled to the low voltage bus 56.

[0030] One or more high voltage electrical loads 46 may be coupled to the high voltage bus 54A. The high voltage loads 46 may have associated controllers that operate and control the high voltage electrical loads 46 when appropriate. The high voltage loads 46 may include compressors and electric heaters. The various components discussed may have one or more associated controllers to control and monitor the operation of the components. The controllers may communicate via a serial bus (e.g., a controller area network (CAN)) or via discrete conductors.

[0031] During a continuous discharge (ie, charge depletion) operation, the application of a large discharge current / power demand after a sustained discharge operation may cause the voltage in the battery cell 68 and / or the battery 24 voltage to drop to a voltage less than a predefined voltage threshold. Figure 1A An exemplary graph 100-A illustrates a change in measured voltage 102 with respect to drive cycle time 104. In one example, measured voltage 106 may be a first voltage 106a at a first time t1 108 and a second voltage 106b at a second time t2 110 after first time t1 108. In some cases, first time 108 and second time 110 may illustrate first and second voltage measurements received by battery controller 50, respectively, such that the difference between first time 108 and second time 110 may be the time elapsed between two consecutive voltage 106 measurements.

[0032] Figure 1B An exemplary graph 110-B is shown showing the variation of the measured voltage 102 with respect to the drive cycle time 104. In some cases, such as Figure 1B As shown in FIG. , the measured voltage curve 106 can be represented by Figure 1ADetailed view of one or more portions of the measured voltage curve 106 shown in . The battery controller 50 can be configured to detect a non-linear voltage drop based on a decrease in the voltage 106 between certain moments and / or between consecutive measurements being greater than a predefined voltage threshold. As an example, the battery controller 50 can detect a non-linear voltage drop when the voltage 106 changes from a first voltage value measured at time 112 to a second voltage value measured at time 114. The difference between times 112, 114 can indicate a predefined time period that has passed between consecutive measurements. As another example, the battery controller 50 can detect that the voltage drop between times 116 and 118 and / or between times 120 and 122 is less than a predefined voltage threshold and is therefore linear (or non-linear). The time period that passes between times 116 and 118 can be the same or different than the time period that passes between times 120 and 122 and between times 112 and 114.

[0033] Voltage difference V diff The battery controller 50 can calculate the voltage difference V diff is compared with a predefined voltage threshold. In some cases, in response to the voltage difference V diff If the voltage difference V is greater than the voltage threshold, the battery controller 50 may be configured to reduce (or decrease) the predicted available battery power. The battery controller 50 may be configured to reduce (or decrease) the predicted available battery power based on the voltage difference V diff To determine that even if the entire battery cell 68 and / or traction battery 24 is predicted to have a voltage greater than the actual voltage for a given current pulse at the current state of charge (SOC) level, the voltage value may drop to a predefined voltage value during the pulse. Typically, for short duration, medium current pulses, the battery voltage response may be linear. However, at moderate to high speeds (e.g., on-road 'roll-over' maneuvers), the battery may experience a non-linear voltage drop from normal acceleration. During such conditions, if the battery 24 is not operating in a sustained discharge condition, the available battery discharge power may become less than the predicted available battery power corresponding to the current SOC level for a given current pulse.

[0034] The current sensor and voltage sensor outputs of the traction battery 24 are provided to the battery controller 50. The battery controller 50 may be configured to monitor the SOC based on signals from one or more sensors, such as the current and voltage sensors of the traction battery 24. Various techniques may be utilized to determine the SOC. For example, ampere-hour integration may be implemented, where the current through the traction battery 24 is integrated over time. The SOC may be estimated based on the output of, for example, a traction battery voltage sensor. The specific technique utilized may depend on the chemistry and characteristics of the particular battery.

[0035] An SOC operating range may be defined for the traction battery 24. The operating range may define upper and lower SOC thresholds for the traction battery 24, for example, max and SOC min The traction battery 24 of the vehicle 12 may be recharged by an external power source 36 (i.e., a charger or charging station) electrically coupled to an electric vehicle supply equipment (EVSE) 38. The external power source 36 may be a distribution network or grid provided by an electric utility company. The EVSE 38 may provide circuitry and controls to regulate and manage the transfer of energy between the power source 36 and the vehicle 12. The external power source 36 may provide DC or AC power 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 port configured to transfer power from the EVSE 38 to the vehicle 12, and may be electrically coupled to an on-board power conversion controller 32, which regulates the power supplied from the EVSE 38 to provide appropriate voltage and current levels to the traction battery 24. Additionally or alternatively, the vehicle 12 may be configured to receive wireless power transfer via a wireless connection to the EVSE 38, such as, but not limited to, inductive charging over an air gap.

[0036] From a battery physics perspective, the transport of lithium ions to the reaction sites and the reaction rates determine the apparent resistance of the battery. The ion transport rate and reaction rate vary during battery use and also depend on the previous use history in the same drive profile. During sustained discharge operation, if a large amount of power / current is required from the battery, the transport of lithium ions to the reaction sites at the electrode surface and / or the reaction rates at the electrode surface may reach their respective maximum values ​​before the power / current demand is met. Such an event may result in a nonlinear increase in the apparent battery cell resistance. This may result in the actual available power of the battery being less than the required power.

[0037] The traction battery 24 may include a plurality of battery cells arranged in a series and / or parallel configuration. In some examples, each battery cell of the traction battery may experience a non-linear drop in voltage, and the battery pack voltage may become less than a predefined voltage threshold, thereby causing a non-linear decrease in available battery discharge power. In some other examples, only the weakest battery cell (the battery cell with the smallest capacity and / or highest resistance) may experience a non-linear increase in apparent resistance and / or reach a battery cell voltage less than a threshold voltage. However, this may not be easily observable at the battery pack voltage level, which may cause the power management system to abruptly reduce the expected battery discharge power capacity to avoid over-discharging that particular battery cell because such a battery cell may reach the minimum allowed voltage V minIn yet other examples, multiple but not all cells in a battery pack may experience a non-linear decrease in voltage during a large power / current demand in a sustained discharge operation. This voltage drop for certain cells may or may not be readily observable at the battery pack voltage level, depending on the number of cells experiencing this condition. Again, this may cause the power management system to reduce the battery discharge power capacity to avoid over-discharging the cells with the larger voltage drop.

[0038] During vehicle operation, the battery controller 50 may be configured to determine a predicted battery power P based on at least one of a power limit table (PLT) and a power capacity processing (PCP) algorithm. pack_predicted In one example, the battery controller 50 may select the predicted available battery power P generated by the power limit table and the power capacity processing algorithm. pack_predicted The smaller of the values. Figure 2A An exemplary graph 200-A illustrates changes in predicted battery pack power 202 with respect to time 204 during continuous discharge operation of the battery 24. The power curve 206 may indicate a long-term predicted battery power P determined using a power limit table. pack_long_PLT The power curve 208 may indicate a short-term predicted battery power P determined using a power limit table. pack_shrt_PLT The power curve 210 may indicate a short-term predicted battery power P determined using a power capacity processing algorithm. pack_shrt_PCP .

[0039] One or more battery cells 68 in the battery pack may experience a decrease in power capacity during a discharge operation. In one example, the battery controller 50 may start at time t b 212 and end time t e 214 will be short-term forecast battery power P pack_shrt_PCP 210 decreases from the first predicted power 210a to the second predicted power 210b. If the difference between the first predicted power 210a and the second predicted power 210b exceeds the predefined power decrease rate threshold, the voltage of the entire battery pack may drop at or approximately at the same time as the predicted power decreases. Figure 2B The cell / battery voltage 216 is shown with respect to time 204 during a continuous discharge operation of the battery 24. Figure 2A 200 -B shows an example of a change in the predicted battery pack power 206 at or approximately at the same time as shown in FIG. ... b 212 and end time t e 214 decreases from the first voltage 217a to the second voltage 217b.

[0040] Because the PCP algorithm uses battery pack voltage measurements as one of the inputs to its Extended Kalman Filter (EKF), it can predict the reduction in power availability during linear voltage reduction. However, because the PCP prediction is an instantaneous power prediction based on voltage and current measurements, in some cases the rate of power reduction may be inaccurate when the battery voltage decreases by more than a predefined voltage threshold. Quite large, for example, when the voltage reduction is non-linear, the power reduction rate may be, for example, 30 kW / sec.

[0041] Figure 2C An exemplary graph 200-C is shown in which the predicted power is reduced by the battery controller 50. The graph 200-C may include a change in the predicted battery pack power 202 with respect to time 204 and may also include a change in voltage 216 at or approximately the same time as the change in the predicted battery pack power. Power curves 218 and 220 may indicate the battery power P predicted by the power limit table, respectively. pack_long_PLT and power capacity processing algorithm power P pack_shrt_PCP Voltage curve 226 may indicate changes in the cell / battery pack voltage at or approximately the same time as the predicted change in battery pack power. In some examples, graph 200-C may indicate a detailed view of one or more portions of graph 200-A and / or graph 200-B. Power curve 218 may, for example, represent a portion of power curve 206, power curve 220 may represent a portion of power curve 210, and voltage curve 226 may represent a portion of voltage curve 217.

[0042] During continuous discharge operation, the battery power capacity decreases at a rate The battery controller 50 may respond to the detection of a power capacity processing algorithm power P pack_shrt_PCP Less than the battery power P predicted by the power limit table pack_long_PLT And start to reduce the time t r 222 Reduce the predicted power P predicted If the battery controller 50 continues to provide the predicted battery power P predicted , then when the power capacity decreases at a rate Rates greater than the threshold may result in performance degradation.

[0043] On the other hand, if the battery controller 50 takes into account the power reduction rate If the rate of decrease is greater than a predefined threshold during continuous discharge operation, performance degradation and customer dissatisfaction can be avoided. In one example, considering the rate of decrease in available battery power The battery controller 50 may be enabled to begin reducing the predicted battery power, thereby reducing the battery propulsion power contribution and activating the engine 18 to assist in meeting the power demand. predicted When the battery controller 50 controls the power reduction rate Such considerations may reduce instances where power limiting conditions are invoked during continuous discharge operations and / or increase the length of time that the vehicle 12 is continuously operated in an electric-only mode (ie, EV mode).

[0044] In the event that only the weakest cell 68 in the battery pack 24 (e.g., the cell 68 having the smallest capacity value and / or the largest resistance relative to one or more other cells 68) experiences a non-linear increase in apparent resistance during PHEV operation, the resulting non-linear voltage drop of the cell 68 may have an undetectable effect on the voltage of the entire battery pack 24. As a result, power capacity processing calculations based on the battery pack voltage may not reflect a decrease in the power capacity of the battery pack 24 until the voltage of that particular cell 68 drops to an operational limit voltage. For example, if Figure 2C As shown in FIG. , the battery pack / cell voltage 226 can be reduced from the time t r The first voltage V a 226a decreases to time t m The second voltage V b 226b, wherein the second voltage V b 226b can be at a predefined minimum voltage threshold V of the battery cell / battery pack min Inside.

[0045] Therefore, when the battery controller 50 uses the power capacity processing algorithm to determine the predicted battery power P predicted And when the nonlinear voltage variation of a single battery cell 68 is unknown, the battery controller 50 can rely on an optional safety mechanism (e.g., power limit) to avoid over-discharging that particular battery cell 68. In some cases, the battery controller 50 can reduce the predicted available battery power at a rate greater than a predefined threshold rate (e.g., at a rate greater than required for efficient battery 24 operation). This may also affect vehicle performance and / or customer experience. In response to detecting a nonlinear drop in the voltage of a single battery cell 68 early during continuous discharge, the battery controller 50 can begin to reduce the predicted available battery power P earlier. predicted To ensure smooth vehicle operation. Additionally or alternatively, such early detection may be beneficial for vehicles 10 operating in areas with cold climate conditions where high battery cell resistance may limit discharge power.

[0046] When a plurality of but not all of the battery cells 68 experience a non-linear voltage drop during a discharge operation, the battery controller 50 may detect an increase in apparent resistance from the battery 24 voltage measurements. However, if the battery controller 50 changes the predicted battery power P using a calculation method based on voltage and current measurements (e.g., such as a PCP algorithm), the battery controller 50 may detect an increase in apparent resistance from the battery 24 voltage measurements. predicted , then the predicted power reduction rate The predefined rate of decrease threshold may be exceeded in the case where the total number of battery cells 68 experiencing non-linear voltage drop is less than a threshold voltage drop (e.g., a non-linear voltage drop detectable by the battery controller 50). Therefore, in this case, early detection of a non-linear drop in battery cell 68 voltage is also desirable and may improve the operation of the vehicle 10 and the customer experience.

[0047] When the rate of decrease of the battery cell voltage is greater than a predefined threshold rate of decrease (e.g., the rate of decrease is non-linear), the apparent resistance of the battery cell may increase rapidly. During such an event, the rate of resistance increase may exceed the predefined threshold. The battery controller 50 may be configured to detect an impending over-voltage drop condition.

[0048] The battery controller 50 may be configured to determine and / or adjust the predicted available battery power P during a non-linear voltage drop event. predicted The battery controller 50 may also be configured to predict an increased likelihood of a non-linear decrease in the voltage of the battery cell / battery. Upon detecting an increased likelihood, the battery controller 50 may begin to reduce the predicted available battery power P predicted , so that the predicted power reduction rate Less than a predefined rate of decrease threshold, for example, less than a rate of decrease that results in activation of the power limit of the battery 24. During discharge operation, upon detecting this possibility, the battery controller 50 can be configured to reduce the power capacity prediction to prevent and / or reduce operation of the battery 24 under stress.

[0049] In one example, the battery controller 50 may continuously monitor the current demand and voltage response of some or all of the battery cells 68 during discharge. The battery controller 50 may also be configured to estimate the resistance of the battery cell 68 (or battery pack 24) and the rate of change of resistance of the battery cell 68 (or battery pack 24) based on the detected voltage and / or current values. In another example, the battery controller 50 may determine that the battery cell / battery pack voltage drop is nonlinear in response to two or more measurements taken consecutively (e.g., measurements with a time domain difference of dt=0.1 sec), resulting in a discharge current above a threshold current, and the voltage of the battery cell 68 drops to or approaches a predefined minimum voltage V min, the resistance of the battery cell 68 is above a predefined resistance threshold, and the resistance change rate is above a certain resistance rate threshold. In yet another example, in response to detecting a reduced battery power condition, the battery controller 50 may be configured to begin reducing the predicted available battery power P predicted , making the power change rate is smooth, that is, the rate of change of power Less than a predefined power rate threshold.

[0050] The battery controller 50 may respond to the battery 24 voltage being lower than a predefined voltage threshold (i.e., V(t+Δt)<V(t)) and the apparent resistance of the battery cell being higher than a predefined resistance threshold (i.e., R>R critical ), the resistance increase rate is higher than a predefined resistivity threshold (i.e., ), the discharge current (for example, a negative value according to the battery current sign convention of the discharge current) is greater than a predefined current threshold (ie, I<I min In some cases, the battery controller 50 may determine that a reduced power condition exists in response to detecting all of the above conditions for two or more consecutive measurements during a continuous discharge operation.

[0051] The battery controller 50 can be based on the open circuit voltage V OCV , and then can be based on the current battery SOC and the terminal voltage V measured at both ends of the battery cell / battery pack t Or the following formula to determine the apparent resistance R of the battery cell

[0052]

[0053] The battery controller 50 may be configured to determine the resistance change rate based on the following equation (2):

[0054]

[0055] Figure 3 An exemplary process 300 for determining an estimated available battery power is shown. The process 300 may begin at operation 302, where the battery controller 50 determines whether the magnitude of the discharge current is greater than a predefined current threshold. If the charge current is less than the current threshold, the battery controller 50 may determine at operation 310 that the estimated available battery power is the smaller of the power values ​​based on the power limit table and the power capacity prediction algorithm.

[0056] In response to the magnitude of the discharge current being greater than the predefined current threshold, the battery controller 50 may proceed to operation 304. At operation 304, the battery controller 50 may determine whether the voltage drop or the rate of voltage change is greater than the predefined voltage threshold. If the rate of voltage change is less than the voltage threshold, the battery controller 50 may proceed to operation 310, where the battery controller determines the estimated available battery power based on the smaller of the power value of the power limit table and the power capacity prediction algorithm.

[0057] In response to the voltage change rate being greater than the voltage threshold, the battery controller 50 may proceed to operation 306. At operation 306, the battery controller 50 may determine whether the apparent cell / pack resistance is greater than a predefined resistance threshold. If the resistance is less than the resistance threshold, the battery controller 50 may proceed to operation 310. Additionally, in response to detecting that the resistance is greater than the resistance threshold, the battery controller 50 may determine whether the resistance change rate is greater than a predefined resistance change rate threshold at operation 308. If the resistance change rate is less than the corresponding change rate threshold, the battery controller 50 may proceed to operation 310.

[0058] In response to determining at operation 308 that the resistance change rate exceeds the corresponding change rate threshold, the battery controller 50 determines at operation 312 whether a second consecutive measurement is detected to have occurred. If not, at operation 314, the battery controller 50 may store the first measurement event in a data storage device and determine the estimated available battery power based on the smaller of the power value of the power limit table and the power capacity prediction algorithm.

[0059] If it is detected that a second consecutive measurement has occurred, then at operation 316, the battery controller 50 may determine an estimated available battery power based on the lesser of the power values ​​of the power limit table, the power capacity prediction algorithm, and the predefined operating minimum voltage limit (OMVL). In one example, the estimated available battery power based on the operating minimum voltage limit may be less than the estimated available battery power of the power limit table and / or the power capacity algorithm. Therefore, the battery controller 50 may reduce the estimated available battery power contributed by the traction battery 24 to meet the current power demand and may activate the engine to assist in providing the required propulsion power. The process 300 may then end. In some cases, the process 300 may be repeated in response to receiving one or more of the current and voltage values ​​or in response to receiving different parameter values.

[0060] FIG. 4A to FIG. 4D Graphs 400-A, 400-B, 400-C, and 400-D are shown, respectively, representing parameter values ​​of an exemplary battery cell 68 at room temperature during a predefined drive cycle. The correlation between can be further affected by the measured voltage value V and current value I of the exemplary battery cell 68. Graphs 400-A, 400-B, 400-C, and 400-D can illustrate the battery controller 50 responding to the resistance change rate in response to one or more conditions being met. 402 Filtering of resistance R404. In one example, the battery controller 50 may filter (select) resistance change rates corresponding to predefined thresholds in response to detecting that one or more other conditions are met. and one or more values ​​of resistance R.

[0061] like Figure 4A As shown in FIG. 1 , the battery controller 50 can select multiple resistance change rates in response to the following two items: Value: reduce the battery cell / battery pack voltage (e.g., V(t+Δt)<V(t)) and the discharge current (e.g., a negative value according to the battery current sign convention of the discharge current) greater than a predefined current threshold (i.e., I<I min <0). like Figure 4B As shown in FIG. 1 , the battery controller 50 may respond to the apparent resistance R being greater than a predefined resistance threshold and the resistance change rate being The resistance change rate value is selected if both are greater than predefined resistivity thresholds.

[0062] refer to Figure 4C , the battery controller 50 may respond to the detection FIG. 4A to FIG. 4B The filter condition has been a predefined number of consecutive measurements and the resistance change rate is selected value and the corresponding resistance R value. Figure 4D The rate of change of resistance occurring with respect to time 406 is shown The value of 408 (i.e., the resistance change rate that satisfies the filtering conditions of graphs 400-A, 400-B, and 400-C) In some cases, in response to detecting that a resistance change rate occurs at time t0 410 At 408, the battery controller 50 may determine that the voltage decrease (e.g., the voltage change rate) is greater than the predefined voltage rate threshold at or approximately at time t0 410. Therefore, the battery controller 50 may determine that the voltage decrease (e.g., the voltage change rate) is greater than the predefined voltage rate threshold based on the resistance change rate generated by the filtering stages shown in graphs 400-A, 400-B, and 400-C. The value of 408 is used to determine the event of a non-linear voltage rate of change for a voltage value less than a predefined voltage threshold.

[0063] Although reference to at least FIG. 4A to FIG. 4DAn exemplary calculation is described, but the value selection is not limited thereto. Thus, the battery controller 50 may be configured to perform similar calculations for a plurality of battery cells 68, a battery cell module 74, and / or the entire traction battery 24. When determining the parameter value of a battery cell combination, the battery controller 50 may calculate the voltage V, the resistance R, and the resistance change rate. The thresholds are adjusted to correspond to the number of battery cells 68 being evaluated before comparing the values ​​of to their corresponding thresholds. The battery controller 50 may determine the adjusted thresholds using algebraic calculations and / or based on the internal connections (e.g., series, parallel, etc.) between the plurality of battery cells 68 being evaluated. For example, if the algorithm is applied to n battery cells connected in series with each other, the effective resistance may be calculated as Additionally or alternatively, the battery controller 50 may determine the voltage of the traction battery 24 based on equation (3), or:

[0064]

[0065] Figure 5A A graph 500-A of a battery cell voltage 502 versus a drive cycle time 504 is shown. The battery cell voltage V cell The change in 506 may correspond to a high current pulse, such as, but not limited to, a high power demand, etc. Under one or more conditions 514, 516, 518, and 520 during the exemplary drive cycle test, the battery cell voltage V cell The rate of decrease of may be greater than a predefined voltage decrease threshold. In one example, the battery controller 50 may determine a power capacity decrease event.

[0066] Since the battery cell voltage V cell The apparent resistance R of the battery cell and / or the rate of change of the resistance of the battery cell decreases over time. The value of increases accordingly, so the threshold signal value can be selected (for the apparent resistance R and the resistance change rate Both of these) allow for the prediction of a possible reduction in battery cell power capacity to be consistent with the power limits associated with the operation of the traction battery 24. In one example, in response to the battery cell voltage V cell Less than the predefined minimum voltage V min , the battery controller 50 may activate one or more power limits associated with the operation of the traction battery 24. Additionally or alternatively, the battery controller 50 may select the apparent resistance R and the resistance change rate The thresholds for both are set to prevent the corresponding power limits from being triggered.

[0067] Figure 6 The method for predicting the available battery power P is shown. predicted (Hereinafter, P pred) and may be performed by the battery controller 50, as shown in at least Figure 1C The process 600 may begin at operation 602, where the battery controller 50 receives one or more values ​​of discharge current, discharge voltage, and estimated SOC. The parameter values ​​of current, voltage, SOC, etc. may correspond to parameter values ​​of a single battery cell, a combination of battery cells, or an entire battery pack. At operation 604, the battery controller 50 may determine the predicted available battery power P by selecting the smaller of the power values ​​obtained using the power limit table and the power capacity prediction algorithm. pred .

[0068] The battery controller 50 may receive the required power value P at operation 606. req At operation 608, the battery controller 50 may determine the required power value P req Is it greater than the estimated available battery power P pred In response to determining the required power value P req Less than the predicted available battery power P pred At operation 610, the battery controller 50 may use the traction battery to meet the current power demand P req At operation 612, in response to the required power value P req Greater than the predicted available battery power P pred , the battery controller 50 can use a combination of power provided by the traction battery and the engine to meet the current power demand P req Process 600 may then end. In some cases, process 600 may be repeated in response to receiving one or more values ​​of discharge current, discharge voltage, and estimated SOC for the battery cell / battery pack.

[0069] Figure 7 An exemplary process 700 for estimating available battery power based on the rate of change of resistance of a battery cell / battery pack is shown. Process 700 may begin at operation 702, where the battery controller 50 receives one or more values ​​of a discharge current, a discharge voltage, and an estimated SOC. The parameter values ​​of current, voltage, SOC, etc. may correspond to parameter values ​​of a single battery cell, a combination of battery cells, or an entire battery pack. At operation 704, the battery controller 50 may determine a predicted available battery power P by selecting the smaller of the power values ​​obtained using the voltage drop analysis, the power limit table, and the power capacity prediction algorithm. pred .

[0070] The battery controller 50 may receive the required power value P at operation 706. req At operation 708, the battery controller 50 may determine the required power value P req Is it greater than the predicted available battery power Ppred In response to determining the required power value P req Less than the predicted available battery power P pred At operation 710, the battery controller 50 may use the traction battery to meet the current power demand P req At operation 712, the battery controller 50 may use the power generated by the traction battery and the engine in response to the required power value P req Greater than the predicted available battery power P pred The combination of electricity provided to meet the current electricity demand P req Process 700 may then end. In some cases, process 700 may be repeated in response to receiving one or more values ​​of discharge current, discharge voltage, and estimated SOC for the battery cell / battery pack.

[0071] FIG. 8A to FIG. 8B Exemplary graphs 800 -A and 800 -B are shown, respectively, for predicting available battery power. Fig. 8A An exemplary graph 800-A is shown for initiating a reduction in predicted available power by the battery controller 50. The graph 800-A may include changes in predicted battery pack power 806 with respect to time 808, and may also include changes in voltage 812 at or approximately the same time as the changes in predicted battery pack power. Power curves 804 and 802 may each indicate a battery power P predicted by the power limit table. pack_long_PLT and power capacity processing algorithm power P pack_shrt_PCP Although the reference Fig. 8A The power curves 804 and 802 described may indicate changes in power prediction based on PLT and PCP, respectively, but the description is not limited thereto. In some cases, the power curve 804 may indicate a long-term power prediction based on a power limit table, while the power curve 802 may indicate a short-term power prediction based on a combination of a power limit table and a PCP algorithm.

[0072] Voltage curve 810 may indicate changes in cell / battery voltage at or about the same time as changes in predicted battery pack power. In some examples, graph 800-C may correspond to one or more portions of graph 200-C and / or graphs 200-A and 200-B. Power curve 804 may, for example, represent a portion of power curve 206, power curve 802 may represent a portion of power curve 210, and voltage curve 810 may represent a portion of voltage curve 217.

[0073] The battery controller 50 may respond to the detection of the power capacity processing algorithm power P pack_shrt_PCP 802 is less than the battery power P predicted by the power limit table pack_long_PLT 804 and start to reduce the time t r814 Reduce the predicted power P predicted For example, the battery controller 50 may set the predicted available battery power to the power value P', or the smaller of the values ​​of the power curves 802 and 804. The battery cell / battery pack voltage 810 may be m 816 reaches a predefined minimum voltage threshold, so that at time t r 814 and at time t m 816, a time 817 has passed. The battery controller 50 may calculate the predicted available battery power P predicted From at time t r The first predicted power 818a of 814 decreases to time t m The difference d 815 between the first predicted power 818a and the second predicted power 818b may indicate P predicted The predicted rate of power reduction can be determined based on the difference 815 about the elapsed time 817.

[0074] Figure 8B A graph 800-B is shown for determining a long-term power prediction based on the minimum of the power limit table value, the PCP algorithm value, and the resistance change rate. Graph 800-B may include a change in the predicted battery pack power 824 with respect to time 826, and also includes a state of the operating minimum voltage signal 830 at or approximately the same time as the change in the predicted battery pack power 824. Power curves 822 and 820 may respectively indicate the battery power P predicted by the power limit table. pack_long_PLT and power capacity processing algorithm power P pack_shrt_PCP In one example, at time t OMVL Before detecting a change in the state of the operating minimum voltage signal 828 at 832, the battery controller 50 may calculate the predicted available battery power P predicted Set to correspond to the short-term forecast power P pack_shrt_PCP 820.

[0075] In response to the time t OMVL 832 detects a change in the state of the operating minimum voltage signal 828 (e.g., from invalid to valid), the battery controller 50 may calculate the predicted available battery power P predicted Set to the battery power P corresponding to the power limit table prediction pack_long_PLT 822 and power capacity processing algorithm Power P pack_shrt_PCPIn some examples, the state change of the operating minimum voltage signal 828 from invalid to valid can correspond to one or more predefined operating conditions, such as, but not limited to, the battery voltage is less than a predefined voltage threshold (i.e., V(t+Δt)<V(t)), the battery cell / battery pack apparent resistance is higher than a predefined resistance threshold (i.e., R>R critical ), the resistance increase rate is higher than a predefined resistivity threshold (i.e., ), the discharge current (for example, a negative value according to the battery current sign convention of the discharge current) is greater than a predefined current threshold (ie, I<I min <0). In some other cases, the state change of the operating minimum voltage signal 828 from invalid to valid can be in response to detecting that all of the above conditions have occurred for two or more consecutive measurements during the continuous discharge operation. The battery controller 50 can be configured to determine the predicted available battery power P based on one or more of the voltage measurement of the entire battery pack, the voltage measurement of each individual battery cell in the battery pack, and the voltage measurement of the battery cell module or selected battery cell. predicted .

[0076] The battery controller 50 may continue to calculate the predicted available battery power P before detecting a change in state of the signal 828 from valid to invalid. predicted Set to the battery power P corresponding to the power limit table prediction pack_long_PLT 822 and power capacity processing algorithm Power P pack_shrt_PCP 820, including when the lesser of P pack_long_PLT The predicted value of 822 becomes greater than that according to P pack_shrt_PCP The operating minimum voltage signal 828 may be changed between prediction strategies at time t m 834 changes the state from valid to invalid. In some cases, the state change of the operating minimum voltage signal 828 from valid to invalid can correspond to the battery cell / battery pack discharge voltage being less than the threshold minimum battery cell / battery pack voltage V min At time t OMVL 832 and t m The elapsed time 836 between 834 can be greater than Fig. 8A The elapsed time is 817.

[0077] Predicted available battery power P predicted From the first time t OMVL The first predicted power 818a of 832 is reduced to time t m The second predicted power 818b of 834. Since the elapsed time 836 is greater than the elapsed time 817, Figure 8B The predicted rate of change of power in (e.g., determined based on the slope of line 842) may be less than Fig. 8A The predicted rate of change of power in In some cases, the battery controller 50 may use the smaller of the power predictions from the PLT and the PCP during normal operation (such as during operation where non-linear voltage rate of change is not continuously occurring).

[0078] Additionally or alternatively, in response to detecting, for example, at time t OMVL 832 detects a possible non-linear voltage drop condition, the battery controller 50 may determine a new minimum discharge power P that the traction battery 24 can provide based on a predefined operating minimum voltage limit (OMVL). new In one example, the voltage V of the battery 24 based on the operating minimum voltage limit pack It can make the n series-connected battery cells inside the traction battery, V pack_OMVL =ΣV cell_OMVL The new minimum discharge power P new can be smaller than the corresponding predicted power value determined according to either the PLT or PCP algorithm, that is, P new <P pack_shrt_PCP and P pack_long_PLT Because the battery controller 50 predicts the available battery power P predicted Set to the minimum of the predicted power, for example, in P pack_long_PLT , P pack_long_PLT , and P new The minimum predicted power is selected among the prediction strategies, so when P new When the strategy returns the minimum predicted power (i.e., P new <P pack_shrt_PCP <P pack_long_PLT ), the battery controller 50 can be based on the new minimum discharge power P new strategy to set the predicted available battery power P predicted .

[0079] The battery controller 50 may respond to, for example, the time t OMVL 832 detects that the operation minimum voltage limit signal 828 is valid, and the battery controller 50 can new And start to reduce the predicted available power P predicted Because the new algorithm is more efficient than the power limit (e.g., at time t r 814) earlier (e.g., at time t OMVL 832) detects a potential non-linear voltage drop (predicted by the PCP / PLT algorithm currently in use), so the rate of power reduction under the new algorithm may be less than the rate of reduction that occurs under the PCP or PLT algorithms. FIG. 8A to FIG. 8BThe values ​​of the cell / battery voltage 810 relative to the predefined minimum cell / battery voltage under PCP and PLT power prediction are shown for one of the cases where two consecutive measurements occur with a nonlinear voltage change rate. If the battery controller 50 detects a nonlinear voltage change rate (e.g., at time t OMVL 832) If power reduction measures are applied, the power reduction rate can be less than the power reduction rate determined using the PCP algorithm. Therefore, using the proposed new algorithm can enable the battery controller 50 to improve power management during continuous discharge operation.

[0080] The process, method or algorithm disclosed herein can be delivered to or implemented by a processing device, a controller or a computer (which can include any existing programmable electronic control unit or a dedicated electronic control unit). Similarly, the process, method or algorithm can be stored as data and instructions in many forms that can be executed by a controller or a computer, including (but not limited to) information permanently stored on non-writable storage media such as ROM devices and information variably stored on writable storage media such as floppy disks, tapes, CDs, RAM devices and other magnetic and optical media. The process, method or algorithm can also be implemented as a software executable object. Optionally, the process, method or algorithm can 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.

[0081] The words used in the specification are descriptive words, rather than restrictive words, and it should be understood that various changes can be made without departing from the spirit and scope of the present disclosure. As previously mentioned, the features of various embodiments can be combined to form other embodiments of the present invention that may not be clearly described or shown. Although various embodiments may have been described as providing advantages or being superior to other embodiments or prior art implementations in terms of one or more desired characteristics, it is recognized by those skilled in the art that one or more features or characteristics can be sacrificed to achieve the desired overall system properties depending on the specific application and implementation. These properties may include but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. Therefore, in terms of one or more characteristics, the embodiments described as being less desirable than other embodiments or prior art implementations are not outside the scope of the present disclosure and may be desired for a specific application.

[0082] According to the present invention, a system for a vehicle is provided, the system having: an engine and an electric machine, each configured to propel the vehicle; and a controller configured to reduce power to the electric machine and activate the engine in response to a resistance of a traction battery and a change in the resistance being greater than a corresponding threshold during battery discharge.

[0083] According to an embodiment, the resistance and the resistance change are greater than the respective threshold values ​​for at least a predetermined period of time.

[0084] According to an embodiment, the resistance and the resistance change are based on a measured discharge current and a measured discharge voltage.

[0085] According to an embodiment, the resistance change is due to a decrease in the measured discharge voltage.

[0086] According to an embodiment, the resistance is based on one of a measured battery voltage and a measured battery current, and the resistance change is based on one of a change in the measured battery voltage and a change in the measured battery current.

[0087] According to an embodiment, the controller is further configured to reduce the power to a predefined value.

[0088] According to the present invention, a vehicle method includes: reducing power from a traction battery to an electric machine, by a vehicle controller, and activating an engine in response to a resistance of the traction battery and a change in the resistance being greater than respective thresholds during battery discharge, wherein each of the electric machine and the engine is configured to propel the vehicle.

[0089] According to an embodiment, the resistance and the resistance change are greater than the respective threshold values ​​for at least a predetermined period of time.

[0090] According to an embodiment, the resistance and the resistance change are based on a measured discharge current and a measured discharge voltage.

[0091] According to an embodiment, the resistance change is due to a decrease in the measured discharge voltage.

[0092] According to an embodiment, the resistance is based on one of a measured battery voltage and a measured battery current, and the resistance change is based on one of a change in the measured battery voltage and a change in the measured current.

[0093] According to an embodiment, the power is reduced to a predefined value.

[0094] According to the present invention, a vehicle system is provided, which has: an engine and a motor, each of which is configured to propel the vehicle; and a controller, which is configured to reduce the battery power provided to the motor and activate the engine in response to the magnitude of the battery discharge current being greater than a current threshold and the rate of decrease of the battery discharge voltage being greater than a rate of change threshold.

[0095] According to an embodiment, the magnitude is greater than the current threshold and the rate is greater than the rate of change threshold for at least a predetermined period of time.

[0096] According to an embodiment, the controller is further configured to reduce the power to a predefined value.

Claims

1. A system for a vehicle, comprising: an engine and an electric motor, each configured to propel the vehicle; and A controller is configured to reduce power to the electric machine and activate the engine in response to a resistance of a traction battery and a change in the resistance being greater than respective thresholds during battery discharge.

2. The system of claim 1, wherein the resistance and the change in resistance are greater than the respective thresholds for at least a predetermined period of time.

3. The system of claim 1, wherein the resistance and the change in resistance are based on a measured discharge current and a measured discharge voltage.

4. The system of claim 3, wherein the change in the resistance is due to a decrease in the measured discharge voltage.

5. The system of claim 1, wherein the resistance is based on one of a measured battery voltage and a measured battery current, and the change in the resistance is based on one of a change in the measured battery voltage and a change in the measured battery current.

6. The system of claim 1, wherein the controller is further configured to reduce the power to a predefined value.

7. A method for a vehicle, comprising: Responsive to a resistance of a traction battery and a change in the resistance being greater than respective thresholds during battery discharge, by a vehicle controller, power from the traction battery to the electric machine is reduced and an engine is activated, wherein each of the electric machine and the engine is configured to propel the vehicle.

8. The method of claim 7, wherein the resistance and the change in resistance are greater than the respective thresholds for at least a predetermined period of time.

9. The method of claim 7, wherein the resistance and the change in resistance are based on a measured discharge current and a measured discharge voltage.

10. The method of claim 9, wherein the change in the resistance is due to a decrease in the measured discharge voltage.

11. The method of claim 7, wherein the resistance is based on one of a measured battery voltage and a measured battery current, and the change in the resistance is based on one of a change in the measured battery voltage and a change in the measured current.

12. The method of claim 7, wherein the power is reduced to a predefined value.

13. A system for a vehicle, comprising: an engine and an electric motor, each configured to propel the vehicle; and A controller is configured to reduce the battery power provided to the motor and activate the engine in response to the magnitude of the battery discharge current being greater than a current threshold and the rate of decrease of the battery discharge voltage being greater than a rate of change threshold.

14. The system of claim 13, wherein the magnitude is greater than the current threshold and the rate is greater than the rate of change threshold for at least a predetermined period of time.

15. The system of claim 13, wherein the controller is further configured to reduce the power to a predefined value.

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