Abstract:
An electrical leakage detection circuit detects electrical leakage of a battery in an electric vehicle. A first transistor switch coupled between a first and second resistor is actuated for coupling the first resistor and the second resistor. A second transistor switch coupled between a third and fourth resistor is actuated for electrically coupling the third and fourth resistor. A controller generates control signals for actuation of the first transistor switch and the second transistor switch. A first voltage is measured across the traction battery cell string. A second voltage is measured across the second resistor. A third voltage is measured across the third resistor. The controller detects electrical leakage by measuring the first, second, and third voltages and applying them in equations utilizing these voltages with constants of the first, second, third, and fourth resistances.

Description:
BACKGROUND OF INVENTION 
     An embodiment relates to leakage measurement for batteries. 
     Electric vehicles utilize use one or more electric motors or what is known as traction motors for propulsion. One form of electric vehicle includes plug-in electric vehicle (PEV) which utilizes rechargeable battery packs that can be charged from an electric grid, and the electricity stored onboard drives or contributes to drive the wheels for propulsion. The rechargeable battery packs are often connected in series for supplying a high voltage energy supply to meet the vehicle&#39;s operating requirements. 
     The battery voltages are monitored for identifying the battery&#39;s state-of-charge, determining an end-of-life so the battery can be replaced prior to failure, and for determining whether a short exists in the high-voltage system or whether any small leakage currents exists for identifying unknown battery drain. 
     Current systems that monitor electrical leakage from the battery are either complex and/or costly, using numerous switches on each battery cell within a battery cell string to identify a leakage. Therefore, a system that provides the advantages of identifying current leakage utilizing a low-cost and less complex detection circuit is needed. 
     SUMMARY OF INVENTION 
     An advantage of an embodiment is the detection of leakage current in a battery for an electric vehicle. The system includes an electrical leakage detection circuit that introduces an ohmic breach to determine how much current flows into a resistance during different circuit configurations. Resistance dividers are implemented by an actuation of switches in both a positive branch domain and negative branch domain of the circuit. A voltage equality is introduced and if the measured leakage resistance is lower than certain predetermined thresholds, then an electrical leakage is detected. 
     In addition a leakage test is performed during the vehicle operation to determine whether electrical leakage is occurring from the batteries during vehicle operation. This operation utilizes an ohmic resistance of the positive branch of the electric vehicle and an ohmic resistance of the negative branch of the electric vehicle. 
     An embodiment includes an electrical leakage detection circuit detecting electrical leaks for a plurality of batteries forming a traction battery cell string in an electric vehicle. The electric vehicle includes a high voltage domain galvanically isolated from a low voltage domain. The leakage circuit includes a plurality of resistor elements including a first resistor, a second resistor, a third resistor, and a fourth resistor coupled between a positive terminal of the traction battery cell string (the positive branch) and a negative terminal of the traction battery cell string (the negative branch). A chassis ground is coupled between the second resistor and the third resistor. A first transistor switch coupled between the first resistor and the second resistor selectively is actuated for electrically coupling the first resistor and the second resistor. A second transistor switch coupled between the third resistor and the fourth resistor is selectively actuated for electrically coupling the third resistor and the fourth resistor. A controller generates control signals for actuation of the first transistor switch and the second transistor switch. The first transistor switch and the second transistor switch are actuated to selected positions. A first voltage is measured across the traction battery cell string. A second voltage is measured across the second resistor. A third voltage is measured across the third resistor. The controller detects electrical leakage as a function of the first voltage, at least two of the resistor elements, and at least one of the second voltage and the third voltage. For the leakage test, the controller defines a first threshold voltage which corresponds to a mild leakage level, and a second threshold voltage which corresponds to a severe leakage level. These first and second thresholds are applied to the second and third voltages, and in response to these thresholds being exceeded, appropriate vehicle actions are taken. 
     A method for detecting electrical leakage for a plurality of batteries forming a traction battery cell string in an electric vehicle is disclosed. The electric vehicle includes a high voltage domain galvanically isolated from a low voltage domain. A leakage detection circuit detects electrical leakage from the traction battery cell string. The leakage detection circuit includes a first resistor, a second resistor, a third resistor, a fourth resistor coupled between a positive terminal of the battery cell string and the negative terminal of the battery cell string. A first transistor switch is coupled between the first resistor and the second resistor. A second transistor switch is coupled between the third resistor and the fourth resistor. The chassis ground is coupled between the second resistor and the third resistor. A controller generates control signals for actuation of the first transistor switch and the second transistor switch. The first transistor switch and the second transistor switch are actuated to selected positions. A first voltage is measured across the traction battery cell string. A second voltage is measured across the second resistor. A third voltage is measured across the third resistor. An electrical leakage is detected as a function of the first voltage, at least two of the resistor elements, and at least one of the second voltage and the third voltage. For the leakage test, the controller defines a first threshold voltage which corresponds to a mild leakage level, and a second threshold voltage which corresponds to a severe leakage level. These first and second thresholds are applied to the second and third voltages, and in response to these thresholds being exceeded, appropriate vehicle actions are taken. 
     Another embodiment, known as Circuit Check, includes actuating a first transistor switch coupled between a first resistor and a second resistor. A second transistor switch coupled between a third resistor and a fourth resistor is actuated. Voltages are measured across the second resistor, third resistor, and a voltage source. Circuit Check failure is detected in response to the sum of second and third resistor voltages not equal to a product of the voltage source and the constant derived by resistor values. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of a battery leakage detection circuit. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a leakage detection circuit for performing circuit check and for monitoring ongoing leakage detection during operation. The leakage detection circuit may be utilized on electrified vehicles such as full hybrid electric vehicles (FHEV), plug-in electric vehicles (PEV), and a battery electric vehicle (BEV). A traction battery cell string  12  is shown generally at  12  to propel the electrified vehicle. The term traction refers to the traction battery cell string being used to provide power for propulsion of the electric vehicle. The traction battery cell string  12  may include any number of batteries configured to one another for providing the necessary voltage for providing power to propel and operate the vehicle. The electric vehicle is split into a high voltage (HV) domain and a low voltage (LV) domain. The HV domain distributes electrical power to systems, subsystem, and components requiring voltages substantially greater than 12V, whereas the LV domain distributes power to systems, subsystems, and components requiring about 12 volts or less. It should be understood that the HV domain is required to be galvanically isolated from the LV domain. 
     The leakage detection circuit includes a plurality resistances coupled between a positive terminal and a negative terminal of the traction battery cell string  12 . The plurality of resistances include a first resistor  14 , a second resistor  16 , a third resistor  18 , and a fourth resistor  20 . 
     The first resistor  14  and the second resistor  16  are selectively coupled by a first transistor switch  22 . A first gate drive circuit  24  is coupled to the first transistor switch  22  for opening and closing the first transistor switch  22 . The first transistor switch  22  is preferably a N-Channel MOSFET transistor; however, any transistor switch that has similar functionality and operation may be used. The first gate drive circuit  24  receives inputs from a voltage switching power supply  26  and a microcontroller  28 . The first gate drive circuit  24  is coupled to chassis ground  30 . 
     The microcontroller  28  controls actuation of the first gate drive circuit  24  and also determines leakage detection based on measurement input data which will be described in detail later. 
     The switching power supply  26  may be integrated as part of the leakage detection circuit or may be utilized as a separate circuit that provides functionality to other circuits or systems. The switching power supply  26  receives a 12 volt input and generates a positive bias voltage output (+V bias ) and a negative bias voltage output (−V bias ). The switching power supply has a connection to the chassis ground  30 . 
     The third resistor  18  and the fourth resistor  20  are selectively coupled by a second transistor switch  32 . A second gate drive circuit  34  is coupled to the second transistor switch  32  for opening and closing the second transistor switch  32 . The second transistor switch  32  is preferably a P-Channel MOSFET; however, any transistor switch that has similar functionality and operation may be used. The second gate drive circuit  34  receives inputs from the voltage switching power supply  26  and the microcontroller  28 , and is coupled to a node between the third resistor  18  and a source terminal of a p-channel MOSFET. The microcontroller  28  controls actuation of the second gate drive circuit  34  for opening and closing the second transistor switch  32 . 
     The chassis ground  30  is also coupled between the second resistor  16  and the third resistor  18 . 
     The leakage detection circuit further includes a first differential amplifier  35  and a second differential amplifier  36 . The first differential amplifier  35  is coupled across the second resistor  16  and the second differential amplifier  36  is coupled across the third resistor  18 . The first differential amplifier  35  and the second differential amplifier are both coupled to chassis ground  30 . The differential amplifiers are designed to amplify the difference between two voltages measured and output a voltage representing the difference, where the output voltage is referenced to chassis ground. 
     The first differential amplifier  35  and the second differential amplifier  36  are each coupled to a first analog-to-digital (A/D) converter  38  that is also coupled to chassis ground  30 . The A/D converter  38  converts each of the voltage inputs from the first differential amplifier  35  and second differential amplifier  36  to a digital value that is representative of the amplitude of the each respective input signals. 
     The microcontroller  28  is coupled to the A/D converter  38  for communicating the sensed voltage readings to the microcontroller  28  which are used to detect an electrical conductivity leakage. 
     The microcontroller  28  is also coupled to a voltage divider  40  through a second A/D converter  42  and a serial peripheral interface (SPI) isolator  44  for obtaining an instantaneous voltage reading of the traction battery cell string  12 . Since the microcontroller is part of the LV domain, the SPI isolator provides a galvanic isolation between the HV domain and the microcontroller  28  and to allow data to be communicated between the HV domain and the microcontroller  28 . The microcontroller  28  also sets the clock frequency, the clock polarity and phase with respect to the data being transmitted to the SPI isolator  44 . The microcontroller  28  is further coupled to a warning output device  50  for enabling a warning that alerts the driver of the vehicle when a fault is present. The warning output device may include a visual display, audible warning, or haptic warning. 
     To determine whether the leakage detection circuit is operating properly, the following circuit check is performed each time the electric vehicle is started. Alternatively, the circuit check may be performed during vehicle operation. Upon startup of the vehicle, the microcontroller  28  communicates control signals to the first gate drive circuit  24  and the second gate drive circuit  34  for simultaneously actuating both the first transistor switch  22  and the second transistor switch  32  to a closed position. 
     The microcontroller then records the simultaneous readings of the voltages across the second resistor  16  and the third resistor  18 . In addition, an instantaneous reading of the voltage of the traction battery cell string  12 , hereinafter referred to as V1, is recorded by the microcontroller  28 . The voltage is determined using the voltage divider  40  whereby the second A/D converter  42  reads the instantaneous voltage V1 and reports this respective voltage to the microcontroller  28  via the SPI isolator  44 . 
     If the leakage detect circuit is operating properly, then the sum of the voltages of the second and third resistor voltage measurements will be equal to the instantaneous voltage reading V1 times a constant that is a function of a value of the resistors  14 ,  16 ,  18 ,  20 . This is represented by the following formula:
 
 V ( R 2)+ V ( R 3)= K×V 1  (1)
 
where V(R2) is the voltage reading of the second resistor, V(R3) is the voltage reading of the third resistor, V1 is the instantaneous voltage reading of the traction battery cell string at the time V(R2) and V(R3) are measured, and K is a resistance value constant set forth by a resistance value of each of the resistors and is represented by the following equation:
 
[ R 2+ R 3]/[ R 1+ R 2+ R 3+ R 4]  (2)
 
where R1 is the resistance value of the first resistor, R2 is the resistance value of the second resistor, R3 is the resistance value of the third resistor, and R4 is the resistance value of the fourth resistor.
 
     If the determination is made that equality in equation (1) holds true (i.e., substantially equal), then the determination is made that the leakage detection circuit is operating properly. The term substantially equal relates to the difference being less than a predetermined value. Preferably, the predetermined value is 5% or less. 
     If the determination is made that the equality in equation (1) does not hold true (i.e., greater than 5%), then the determination is made that the leakage detection circuit has failed. In this case a warning is provided to the driver. The warning could be an output device that generates an audible signal, a visual signal, or a haptic signal. In addition, a Diagnostic Trouble Code (DTC) is stored. 
     After the vehicle is started and an initial circuit check is conducted, the electrical leakage circuit can perform ongoing leakage resistance testing during the operation of the vehicle. This is considered an ohmic leakage resistance test. This ohmic leakage resistance test utilizes the ohmic resistance of the electric vehicle (i.e., resistance of the circuit from the chassis ground to the positive terminal of the battery, and the resistance of the circuit from the chassis ground to the negative terminal of the battery, respectively). It is understood that the traction battery cell string  12  is galvanically isolated from the vehicle chassis. R leakHi    46  as shown in  FIG. 1  is the resistance between the positive-most (+) terminal of the traction battery cell string  12  and the chassis ground  30 , which represents an ohmic resistance to the positive branch of the high voltage (HV) side of the vehicle. The term positive-most terminal is defined herein as the last positive terminal of the traction battery cell string for which the wiring harness is coupled to for distributing power to the vehicle. R leakLo    48  as shown in  FIG. 1  is the resistance between the negative-most (−) terminal of the traction battery cell string  12  and the chassis ground  30 , which represents an ohmic resistance to the negative branch of the high voltage (HV) side of the vehicle. The term negative-most terminal is defined herein as the last negative terminal of the traction battery cell string for which the wiring harness is coupled to prior to distributing power to the vehicle. It should be understood that both R leakHi    46  and R leakLo    48  may be undesirable and may or may not exist. For example, in a typical vehicle circuit with no faults, the ohmic value may be 5 Megohms or higher for both R leakHi    46  and R leakLo    48 . If there is a fault condition, then the amount of ohmic resistance, for instance, may be 200 kilohms or lower for either R leakHi    46  or R leakLo    48 . When the ohmic resistance is a larger number, then it is determined that the amount of undesirable leakage is low, which represents a non-failure condition. When the ohmic resistance is a smaller number, then it is determined that more leakage is present than expected, which indicates a potential faulty condition. These undesirable resistances can be caused by faults such as pinched wires or contamination, (e.g. salt water) which creates an undesirable ohmic path from a high voltage node to the chassis ground  30 . Ordinarily, just one of these undesirable resistances R leakHi    46  or R leakLo    48  would exist, corresponding to a single point failure. For both R leakHi    46  and R leakLo    48  to simultaneously measure below for instance, a measurement of 200 kilohms would constitute a double failure in Failure Mode and Effect Analysis (FMEA) terms. Although this scenario is rare, such a fault condition is possible. Therefore, equations (3) and (4), discussed are set up under the assumption of detecting a Single Point failure (i.e., one of either R leakHi    46  or R leakLo    48  measuring lower than for example 200 kilohms but not both simultaneously). 
     A double point failure will be discussed as follows, which means leakages R leakHi    46  and R leakLo    48  are simultaneously lower than a respective ohmic threshold (e.g., 200 kilohms). In a worst case scenario, R leakHi    46  and R leakLo    48  are substantially matching (i.e., about the same ohmic value) and are both, for example, 200 kilohms or below. This is a worst case is because this combination puts the smallest voltage for detection across the second resistor  16  (V(R2)) or third resistor  18  (V(R3)). Even for the worst case, it is still possible to notice that the measured resistance is much lower in comparison to the typical vehicle ohmic value. For example, R leakHi    46  would typically measure 5 Megohms in an unfaulted vehicle condition. However, if R leakHi    46  and R leakLo    48  are simultaneously 200 kilohms or below and matching, then the circuit will provide a reading of (R3+R4) ohms for R leakLo  and (R1+R2) ohms for R leakHi . This will not be accurate, since typical values will be for instance, 500 kohms for R1 and R4, and 5 kohms for R2 and R3. As a result, a R leakHi  in this instance of 200 kohms would read about 500 kohms, which is incorrect. One method of handling a double fault situation as described herein is that a mild leakage threshold can be set in ohms slightly higher than (R3+R4). Therefore, a leakage would be indicated in this use case. Now, since this is a double fault which will tend to discharge the traction battery and thereby lead to the particular pack to be brought back in for replacement, it is also reasonable to set the mild leakage boundary at the 500 ohms per volt value per FMVSS305, which will be on the order of half of the (R3+R4) value. 
     During this test, the first transistor switch  22  is closed for a first predetermined period of time (e.g., 2 seconds) while the second transistor switch  32  remains open. The current flows from the top of V1, flows through R1, transistor switch Q1, and R2, into the chassis ground node and then through RleakLo to V_BOT. During the first predetermined period of time, the microcontroller  28  records voltage measurements for the second resistor (VR2 High ) and the instantaneous voltage of the traction battery cell string (V1 High ). 
     Thereafter, the first transistor switch  22  is opened and the second transistor switch  32  is closed for a second predetermined period of time (e.g., 2 seconds). The current starts from the top of V1, flows through R leakHi , into the chassis ground node, and then through R3, transistor switch Q2, R4, and then to V_BOT. During the second predetermined period of time, the microcontroller  28  records voltage measurements for the second resistor (VR3 Low ) and the instantaneous voltage of the traction battery cell string (V1 Low ). The value of R leakLo  can be calculated by knowing (VR2 High ) and V1 High  and the resistance values of the first resistor  14  and the second resistor  16 . Similarly, R leakHi  can be calculated by knowing (VR3 Low ) and V1 Low  and the value of the third resistor  18  and the fourth resistor  20 . 
     For R leakLo , the equality equation for determining if leakage is occurring is represented by the following formula: 
                       [       V   ⁢           ⁢   1       [       R   ⁢           ⁢   1     +     R   ⁢           ⁢   2     +     R   RleakLo       ]       ]     ×   R   ⁢           ⁢   2     =       V   ⁡     (     R   ⁢           ⁢   2     )       .             (   3   )               
where V1 is the measured instantaneous voltage of the traction battery cell string, R1 is the resistance value of the first resistor, R2 is the resistance value of the second resistor, V(R2) is the measured voltage across the second resistor, and R leakLo  is the ohmic resistance from the negative branch to the LV domain chassis.
 
     Given each of the known and measured parameters in eq. (3), the value of R leakLo  can be calculated. R leakLo  is then compared to a first predetermined threshold and a second predetermined threshold. If the value of R leakLo  is greater that the first predetermined threshold, then a determination is made that no fault is present in the vehicle system. If the value of R leakLo  is between the first and second predetermined thresholds, then a fault determined to be present in the vehicle system. This fault requires that driver be notified by a warning indicator or similar alerting the driver that the vehicle electrical system should be serviced in the near future. If the value of R leakLo  is less than the second predetermined threshold, then servicing of the vehicle is required before the vehicle can be further operated. Under such circumstances, a flag may be set which prevents the vehicle from being started after the vehicle is turned off. 
     For R leakHi , the equality equation for determining when leakage is occurring is represented by the following formula: 
                       [       V   ⁢           ⁢   1       [       R   ⁢           ⁢   3     +     R   ⁢           ⁢   4     +     R   leakHi       ]       ]     ×   R   ⁢           ⁢   3     =       V   ⁡     (     R   ⁢           ⁢   3     )       .             (   4   )               
where V1 is the measured instantaneous voltage of the traction battery cell string, R3 is the resistance value of the third resistor, R4 is the resistance value of the fourth resistor, V(R3) is the measured voltage across the third resistor, and R leakHi  is the ohmic resistance from the positive branch to the LV domain.
 
     Given each of the known and measured parameters in eq. (4), the value of R leakHi  can be calculated. R leakHi  is then compared to the first predetermined threshold and the second predetermined threshold. If the value of R leakHi  is greater that the first predetermined threshold, then a determination is made that no fault is present in the vehicle system. If the value of R leakHi  is between the first and second predetermined thresholds, then a fault determined to be present in the vehicle system. This fault requires that driver be notified by a warning indicator or similar alerting the driver that the vehicle electrical system should be serviced in the near future. If the value of R leakHi  is less than the second predetermined threshold, then servicing of the vehicle is required before the vehicle can be further operated. Under such circumstances, a flag may be set which prevents the vehicle from being started after the vehicle is turned off. 
     While certain embodiments of the present invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.