Patent Publication Number: US-2021165044-A1

Title: Failure diagnosis method and management apparatus for energy storage device

Description:
TECHNICAL FIELD 
     The present invention relates to a failure diagnosis method for a system in which an energy storage apparatus for supplying electric power to an electrical load and another power supply, which is one of an energy storage apparatus and a charger, are connected in parallel, and a management apparatus for an energy storage device. 
     BACKGROUND ART 
     Patent Document 1 discloses an energy storage apparatus that includes a circuit breaker connected in series with an energy storage device, and opens the circuit breaker to protect the energy storage device from overcharge or overdischarge when the overcharge or overdischarge of the energy storage device is predicted. In this energy storage apparatus, when the circuit breaker is out of order, it may not be possible to protect the energy storage device from overcharge or overdischarge. Therefore, failure diagnosis of the circuit breaker has been performed. 
     Patent Document 2 describes a battery pack in which a first switch and a second switch are connected in parallel. In Patent Document 2, an open command signal is first transmitted to a first switch, and a close command signal is transmitted to a second switch, to acquire an open voltage VAD. Next, a close command signal is transmitted to the first switch, and an open command signal is transmitted to the second switch, to acquire an open voltage VAR The failure of the first switch is diagnosed from a voltage difference ΔV between those voltages. 
     Patent Document 3 discloses a battery pack including a plurality of switches connected in parallel to each other and a both-end voltage detection unit that outputs a both-end voltage detection signal corresponding to each of both-end voltages of the plurality of switches. In Patent Document 3, the plurality of switches are sequentially designated at different times, and open command signals are given, to determine a switch failure based on the both-end voltage detection signal when each of the open command signals is given. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: JP-A-2017-135834 
     Patent Document 2: International Publication No. 2016/103721 
     Patent Document 3: JP-A-2014-036556 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     The techniques disclosed in Patent Document 2 and Patent Document 3 are each on the assumption that only one energy storage apparatus is connected to an electrical load. According to the techniques described in these patent documents, when another power supply (energy storage apparatus or charger) of substantially the same voltage is connected in parallel to an energy storage apparatus that supplies electric power to the electrical load, the voltage does not fluctuate even when the circuit breaker is opened, and it may be impossible to determine the failure of the circuit breaker. 
     The present specification discloses a technique in which in a system where a first energy storage apparatus for supplying electric power to an electrical load and another power supply, which is one of a second energy storage apparatus and a charger, are connected in parallel, even when the voltage of the first energy storage apparatus and the voltage of another power supply are substantially the same, it is possible to more reliably diagnose the failure of a circuit breaker provided in the first energy storage apparatus. 
     Means for Solving the Problems 
     A failure diagnosis method for a system in which a first energy storage apparatus that supplies electric power to an electrical load and another power supply that is one of a second energy storage apparatus and a charger are connected in parallel, the first energy storage apparatus including an energy storage device provided in a current path connecting a positive external terminal and a negative external terminal of the first energy storage device, detection unit that is provided in the current path and detects at least one of a current value of a current flowing through the energy storage device and a direction of the current, a first circuit breaker that is provided in the current path on a side opposite to the detection unit with reference to the energy storage device, or between the energy storage device and the detection unit, a bypass path provided in parallel with a section of the current path, the section including the energy storage device and the first circuit breaker and not including the detection unit, and a second circuit breaker provided in the bypass path. The failure diagnosis method includes: a first detection step of detecting at least one of a current value and a direction by the detection unit in a state where the first circuit breaker is open and the second circuit breaker is closed during discharge of the first energy storage apparatus; and a determination step of determining the failure of the first circuit breaker based on a detection result of the first detection step. 
     ADVANTAGES OF THE INVENTION 
     Even when the voltage of the first energy storage apparatus and the voltage of another power supply are substantially the same, it is possible to more reliably diagnose the failure of the circuit breaker provided in the first energy storage apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an engine starting system according to a first embodiment (with a first relay closed and a second relay open). 
         FIG. 2  is a schematic diagram of the engine starting system (with the first relay open and the second relay closed). 
         FIG. 3  is a schematic diagram of the engine starting system (with the first relay and the second relay closed). 
         FIG. 4  is a flowchart of a failure diagnosis process for the first relay. 
         FIG. 5  is a flowchart of a failure diagnosis process for a first relay according to a second embodiment. 
         FIG. 6  is a schematic diagram of an engine starting system according to a third embodiment (with the first relay closed, the second relay open, and third to fifth relays closed). 
         FIG. 7  is a diagram showing the relationship between the combination of the opening and closing of each relay and the presence or absence and direction of the current. 
         FIG. 8  is a schematic diagram of an engine starting system according to another embodiment (with the first relay closed and the second relay open). 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Summary of the Present Embodiment 
     A failure diagnosis method disclosed by the present specification is a failure diagnosis method for a system in which a first energy storage apparatus that supplies electric power to an electrical load and another power supply that is one of a second energy storage apparatus and a charger are connected in parallel, the first energy storage apparatus including an energy storage device provided in a current path connecting a positive external terminal and a negative external terminal of the first energy storage apparatus, detection unit that is provided in the current path and detects at least one of a current value of a current flowing through the energy storage device and a direction of the current, a first circuit breaker that is provided in the current path on a side opposite to the detection unit with reference to the energy storage device, or between the energy storage device and the detection unit, a bypass path provided in parallel with a section of the current path, the section including the energy storage device and the first circuit breaker and not including the detection unit, and a second circuit breaker provided in the bypass path. The failure diagnosis method includes: a first detection step of detecting at least one of a current value and a direction with the detection unit in a state where the first circuit breaker is open and the second circuit breaker is closed during discharge of the first energy storage apparatus; and a determination step of determining the failure of the first circuit breaker based on a detection result of the first detection step. 
     The first energy storage apparatus includes the bypass path provided in parallel with the section of the current path, the section including the energy storage device and the first circuit breaker and not including the detection unit With this configuration, upon diagnosis of the failure of the first circuit breaker during discharge of the first energy storage apparatus, even when the voltage of the first energy storage apparatus and the voltage of another power supply are substantially the same, the result of the detection by the detection unit differs between a case where the first circuit breaker is not out of order (when the first circuit breaker is opened) and a case where the first circuit breaker is out of order (when the first circuit breaker is not opened). In other words, the detection result varies between the case where the first circuit breaker is not out of order and the case where the first circuit breaker is out of order. Therefore, when the voltage of the first energy storage apparatus and the voltage of another power supply are substantially the same as in the conventional case, the failure of the first circuit breaker can be diagnosed more reliably than when the voltage does not fluctuate. 
     The detection unit may detect a direction of the current. In the determination step, when the direction detected in the first detection step is a charge direction in which the first energy storage apparatus is charged, it may be determined that the first circuit breaker is normal, and in the other cases, it is determined that the first circuit breaker is out of order. 
     Upon detection of the failure of the first circuit breaker during discharge, even when the voltage of the first energy storage apparatus and the voltage of another power supply are substantially the same, the direction of the current is reversed between the case where the first circuit breaker is out of order and the case where the first circuit breaker is not out of order. In other words, the detection result varies between the case where the first circuit breaker is out of order and the case where the first circuit breaker is not out of order. Therefore, the failure can be determined more reliably than when the voltage does not fluctuate as in the conventional case. 
     When the first circuit breaker is out of order, the current value may be smaller in a case where the current value detected by the detection unit may be 0 A (ampere) or a case where the direction cannot be determined. In this case, the direction cannot be determined, but in the failure diagnosis method described above, the failure is determined in a case except for the case where the detected direction is the charge direction (in a case where the detected direction is a discharge direction or the direction cannot be determined), so that the failure can be determined even when the direction cannot be determined. 
     The detection unit may detect a current value of the current. The failure diagnosis method may further include a second detection step of detecting a current value with the detection unit in a state where the first circuit breaker is closed and the second circuit breaker is open during discharge of the first energy storage apparatus. In the determination step, the failure of the first circuit breaker may be determined based on the current value detected in the first detection step and the current value detected in the second detection step. 
     In the first energy storage apparatus provided with the bypass path described above, even when the voltage of the first energy storage apparatus and the voltage of another power supply are substantially the same, a result of comparison between a current value detected during discharge of the first energy storage apparatus and a current value detected after the control so as to open the first circuit breaker and close the second circuit breaker differs between the case where the first circuit breaker is out of order and the case where the first circuit breaker is not out of order. In other words, the comparison result varies between the case where the first circuit breaker is not out of order and the case where the first circuit breaker is out of order. Therefore, the failure of the first circuit breaker can be diagnosed more reliably than when the voltage does not fluctuate as in the conventional case. 
     The system may be provided with a third circuit breaker in at least one of a current path connecting the first energy storage apparatus and the electrical load, a current path connecting another power supply and the electrical load, and a current path inside another power supply. The detection unit may detect a direction of the current, and in the determination step. When the direction detected in the first detection step is a charge direction in which the first energy storage apparatus is charged, it may be determined that both the first circuit breaker and the third circuit breaker are normal, and in the other cases, it may be determined that at least one of the first circuit breaker and the third circuit breaker is out of order. 
     According to the failure diagnosis method described above, not only the first circuit breaker provided in the first energy storage apparatus but also the third circuit breaker provided outside the first energy storage apparatus can diagnose the failure of the first energy storage apparatus. 
     Another power supply may be the second energy storage apparatus. 
     According to the failure diagnosis method described above, even when the first energy storage apparatus for supplying electric power to the electrical load and the second energy storage apparatus are connected in parallel, the failure of the first circuit breaker can be diagnosed. 
     Another power supply may have a voltage higher than a voltage of the first energy storage apparatus, and the system may include a step-down unit that steps down a voltage applied by another power supply. 
     The voltage of another power supply may be higher than that of the first energy storage apparatus as in an example where the voltage of the first energy storage apparatus is 12 V, and the voltage of another power supply may be 48 V. In this case, a voltage applied by another power supply may be stepped down to substantially the same voltage as that of the first energy storage apparatus by the step-down unit provided in the system. According to the above failure diagnosis method, even when the voltage of another power supply is stepped down to substantially the same voltage as that of the first energy storage apparatus, it is possible to more reliably diagnose the failure of the first circuit breaker. 
     A resistor or a constant current source may be provided in the bypass path. 
     According to the failure diagnosis method described above, it is possible to prevent the energy storage device from being short-circuited when the second circuit breaker is closed. 
     A management apparatus for an energy storage device disclosed in the present specification includes: a detection unit that is provided in the current path to which the energy storage device is connected and detects at least one of a current value of a current flowing through the energy storage device and a direction of the current; a first circuit breaker that is provided in the current path on a side opposite to the detection unit with reference to the energy storage device, or between the energy storage device and the detection unit; a bypass path provided in parallel with a section of the current path, the section including the energy storage device and the first circuit breaker and not including the detection unit; a second circuit breaker provided in the bypass path; and a management unit. The management unit executes a first detection step of detecting at least one of the current value and the direction with the detection unit in a state where the first circuit breaker is open and the second circuit breaker is closed during discharge of the energy storage device, and a determination step of determining the failure of the first circuit breaker based on a detection result of the first detection step. 
     According to the management apparatus described above, in the system where the first energy storage apparatus for supplying electric power to the electrical load and another power supply, which is one of the second energy storage apparatus and the charger, are connected in parallel, it is possible to more reliably diagnose the failure of the first circuit breaker even when the voltage of the first energy storage apparatus and the voltage of another power supply are substantially the same. 
     The techniques disclosed in the present specification can be realized in various modes such as an apparatus, a method, a computer program for realizing the apparatus or the method, and a recording medium on which the computer program is recorded. 
     First Embodiment 
     A first embodiment will be described with reference to  FIGS. 1 to 4 . 
     (1) Configuration of Engine Starting System 
     An engine starting system  1  (an example of the system) of the first embodiment will be described with reference to  FIG. 1 . The engine starting system  1  starts an engine of a vehicle. The engine starting system  1  includes a starter  10  (an example of the electrical load) for rotating a crankshaft of the engine, a starting energy storage apparatus  11  (an example of the first energy storage apparatus) for supplying electric power to the starter  10 , and an auxiliary energy storage apparatus  12  (an example of the second energy storage apparatus and another power supply) for supplying electric power to auxiliary equipment (headlight, air conditioner, audio, etc.) mounted on the vehicle. The auxiliary energy storage apparatus  12  is connected in parallel with the starting energy storage apparatus  11 , and electric power can be supplied from the auxiliary energy storage apparatus  12  to the starter  10 . 
     In the present embodiment, it is assumed that the voltage of the starting energy storage apparatus  11  and the voltage of the auxiliary energy storage apparatus  12  are substantially the same. Specifically, it is assumed that the starter  10  has a load of 12 V, and both the starting energy storage apparatus  11  and the auxiliary energy storage apparatus  12  have a voltage of 12 V. The voltage of the starting energy storage apparatus  11  may be larger than that of the auxiliary energy storage apparatus  12 , and the voltage of the auxiliary energy storage apparatus  12  may be larger than that of the starting energy storage apparatus  11 . 
     (2) Electrical Configuration of Starting Energy Storage Apparatus 
     As shown in  FIG. 1 , the starting energy storage apparatus  11  includes a current path  15  connecting the positive external terminal  13  and the negative external terminal  14 , an assembled battery  16  provided in the current path  15 , and a battery management system  17  (an example of the management apparatus). 
     The assembled battery  16  has a plurality of energy storage devices  18  connected in series. Each energy storage device  18  is a rechargeable secondary battery, specifically, a lithium ion battery, for example. The plurality of energy storage devices  18  may be connected in parallel or may be connected in combination of series and parallel. 
     The BMS17 includes a detection unit  19  for detecting the direction of the current flowing through the energy storage device  18 , a first relay  20  for interrupting the current path  15 , a bypass path  21 , a second relay  23 , a resistor  24 , and a management unit  22 . 
     The detection unit  19  is provided in the current path  15 , detects the direction of the current flowing through the energy storage device  18 , and outputs the detected direction to the management unit  22 . 
     The first relay  20  is provided between the positive external terminal  13  and the assembled battery  16  in the current path  15 . The first relay  20  interrupts the current path  15  when overcharge or overdischarge of the energy storage device  18  is predicted. 
     The bypass path  21  is for diagnosing the failure of the first relay  20 . The bypass path  21  is provided in parallel with a section of the current path  15 , the section including the assembled battery  16  and the first relay  20  and not including the detection unit  19 . 
     The second relay  23  and the resistor  24  are provided in the bypass path  21 . The second relay  23  is of a normally open type, and is closed by the management unit  22  when the failure diagnosis of the first relay  20  is performed. The resistor  24  is for preventing the energy storage device  18  from being short-circuited when the second relay  23  is closed. 
     The management unit  22  is operated by electric power supplied from the assembled battery  16  and includes a central processing unit (CPU), a read-only memory (ROM), a random-access memory (RAM), and the like. The CPU executes a control program stored in the ROM to execute various processes such as protection of the energy storage device  18  and failure diagnosis of the first relay  20 , which will be described later. 
     The management unit  22  may be provided with an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or the like, in place of the CPU or in addition to the CPU. 
     (3) Protection of Energy Storage Device 
     The management unit  22  estimates the state of charge (SOC) of the energy storage device  18 , and when the estimated SOC is not less than a predetermined upper limit or not more than a predetermined lower limit, the first relay  20  is opened on the assumption that overcharge or overdischarge is predicted. As a result, the current path  15  is cut off, and the energy storage device  18  is protected from overcharge and overdischarge. 
     (4) Failure Diagnosis of First Relay 
     When the first relay  20  is out of order and is not opened, the energy storage device  18  cannot be protected from overcharge or overdischarge. Therefore, the management unit  22  performs the failure diagnosis of the first relay  20 . The failure diagnosis of the first relay  20  is performed when the starting energy storage apparatus  11  is discharged. More specifically, the failure diagnosis of the first relay  20  is performed when a current (hereinafter referred to as discharge current) in the discharge direction stably flows from the starting energy storage apparatus  11 . For example, when the engine of the vehicle is stopped, a discharge current stably flows from the starting energy storage apparatus  11 . Hence the failure diagnosis of the first relay  20  is performed, for example, when the engine of the vehicle is stopped. 
     The discharge current of the starting energy storage apparatus  11  will be described with reference to  FIG. 1 . The discharge current of the starting energy storage apparatus  11  flows along a path X shown in  FIG. 1 . In  FIG. 1 , the current value when the discharge current stably flows is Ibat 1 . When a discharge current flows through the starting energy storage apparatus  11 , the direction of the current detected by the detection unit  19  is the discharge direction. 
     As shown in  FIG. 2 , in the failure diagnosis of the first relay  20 , the management unit  22  performs control so as to open the first relay  20  and close the second relay  23 . When the first relay  20  is not out of order (i.e., when the first relay  20  is open), as shown by a path Y in  FIG. 2 , a current Ibat 2  flows from the auxiliary energy storage apparatus  12  to the starting energy storage apparatus  11 . As shown by the path Y, the current Ibat 2  flows through the bypass path  21  of the starting energy storage apparatus  11 . The direction of the current Ibat 2  is opposite to that of the current Ibat 1 , and the direction of the current detected by the detection unit  19  is the charge direction. 
     In contrast, as shown in  FIG. 3 , when the first relay  20  is out of order and does not open, the discharge current Ibat 1  flows from the starting energy storage apparatus  11  to the starter  10  as shown by the path X. Further, as shown by the path Z, the discharge current Ibat 2  flows from the starting energy storage apparatus  11  to the bypass path  21 . Since the current Ibat 1  flows through the detection unit  19 , the direction of the current detected by the detection unit  19  becomes the discharge direction when the first relay  20  is out of order. Since the voltage of the starting energy storage apparatus  11  and the voltage of the auxiliary energy storage apparatus  12  are substantially the same, the current supplied by the auxiliary energy storage apparatus  12  does not appear in  FIG. 3 . 
     When the first relay  20  is not out of order, the direction of the current detected after the control so as to open the first relay  20  and close the second relay  23  is reversed. 
     Specifically, when the first relay  20  is not out of order, the detected direction is the charge direction, and when the first relay  20  is out of order, the detected direction is the discharge direction. Therefore, the failure of the first relay  20  can be determined by detecting the direction of the current. 
     When the first relay  20  is out of order, the current value detected by the detection unit  19  may be 0 A (ampere), or the current value may be so small that the direction cannot be determined. In that case, the direction cannot be determined. Therefore, the management unit  22  determines that the first relay  20  is out of order in a case except for the case where the detected direction is the charge direction (in a case where the detected direction is the discharge direction or the direction cannot be determined) 
     (5) Failure Diagnosis Process for First Relay 
     A failure diagnosis process for the first relay  20  executed by the management unit  22  will be described with reference to  FIG. 4 . The present process is performed, for example, when the engine of the vehicle is stopped. 
     In S 101 , the management unit  22  performs control so as to open the first relay  20  and close the second relay  23 . 
     In S 102 , the management unit  22  detects the direction of the current with the detection unit  19  (an example of the first detection step). 
     In S 103 , the management unit  22  determines whether the direction detected in S 102  is the charge direction or the other direction. In the case of the charge direction, it is determined that the first relay  20  is not out of order, and the present process ends. In the other cases, it is determined that the first relay  20  is out of order, and the process proceeds to S 104  (an example of the determination step). 
     In S 104 , the management unit  22  executes a predetermined error process. 
     (5) Effects of Embodiment 
     According to the failure diagnosis method of the first embodiment, the starting energy storage apparatus  11  includes a bypass path  21  provided in parallel with the section of the current path  15 , the section including the assembled battery  16  and the first relay  20  but not including the detection unit  19 . With this configuration, upon diagnosis of the failure of the first relay  20  during discharge of the starting energy storage apparatus  11 , even when the voltage of the starting energy storage apparatus  11  and the voltage of the auxiliary energy storage apparatus  12  are substantially the same, the result of the detection by the detection unit  19  differs between the case where the first relay  20  is not out of order (when the first relay  20  is opened) and the case where the first relay  20  is out of order (when the first relay  20  is not opened). In other words, the detection result varies between the case where the first relay  20  is not out of order and the case where the first relay  20  is out of order. Therefore, when the voltage of the starting energy storage apparatus  11  and the voltage of the auxiliary energy storage apparatus  12  are substantially the same as in the conventional case, the failure of the first relay  20  can be diagnosed more reliably than when the voltage does not fluctuate as in the conventional case. 
     According to the failure diagnosis method of the first embodiment, the failure of the first relay  20  is determined based on the direction of the current. Upon detection of the failure of the first relay  20  during discharge, even when the voltage of the starting energy storage apparatus  11  and the voltage of the auxiliary energy storage apparatus  12  are substantially the same, the direction of the current is reversed between the case where the first relay  20  is out of order and the case where the first relay  20  is not out of order. In other words, the detection result varies between the case where the first relay  20  is not out of order and the case where the first relay  20  is out of order. Therefore, the failure can be determined more reliably than when the voltage does not fluctuate as in the conventional case. 
     When the first relay  20  is out of order, the current value may be smaller in a case where the current value detected by the detection unit  19  may be 0 A (ampere) or a case where the direction cannot be determined. In this case, the direction cannot be determined, but in the failure diagnosis method of the first embodiment, the failure is determined in a case except for the case where the detected direction is the charge direction (in a case where the detected direction is the discharge direction or the direction cannot be determined), so that the failure can be determined even when the direction cannot be determined. 
     According to the failure diagnosis method of the first embodiment, the resistor  24  is provided in the bypass path  21 , so that it is possible to prevent the energy storage device  18  from being short-circuited when the second relay  23  is closed. 
     According to the BMS17 of the first embodiment, in the engine starting system  1  in which the starting energy storage apparatus  11  and the auxiliary energy storage apparatus  12  are connected in parallel, even when the voltage of the starting energy storage apparatus  11  and the voltage of the auxiliary energy storage apparatus  12  are substantially the same, it is possible to more reliably diagnose the failure of the first relay  20 . 
     Second Embodiment 
     A second embodiment will be described with reference to  FIGS. 2, 3 , and  5 . The engine starting system according to the second embodiment includes a current sensor as the detection unit  19  and diagnoses the failure of the first relay  20  from a current value measured by the current sensor. Similarly to the first embodiment, in the second embodiment as well, the failure diagnosis of the first relay  20  is performed when the discharge current stably flows from the starting energy storage apparatus  11 . 
     As shown in  FIG. 2 , in the failure diagnosis of the first relay  20 , the management unit  22  performs control so as to open the first relay  20  and close the second relay  23  when the discharge current stably flows from the starting energy storage apparatus  11 . 
     As shown in  FIG. 2 , when the first relay  20  is not out of order, the current Ibat 2  flows from the auxiliary energy storage apparatus  12  to the path Y. When the first relay  20  is not out of order, the current value (i.e., the current Ibat 1  shown in  FIG. 1 ) at the time of the discharge current stably flowing from the starting energy storage apparatus  11  does not match a current value (ibat 2 ) detected after the control so as to open the first relay  20  and close the second relay  23 . 
     In contrast, as shown in  FIG. 3 , when the first relay  20  is out of order and does not open, the current Ibat 1  flows from the starting energy storage apparatus  11  to the path X. Since the detection unit  19  is on the path X, the current value Ibat 1  is detected by the detection unit  19  when the first relay  20  is out of order. Thus, when the first relay  20  is out of order, the current value (ibat 1 ) at the time of the discharge current stably flowing from the starting energy storage apparatus  11  matches the current value (ibat 1 ) detected after the control so as to open the first relay  20  and close the second relay  23 . 
     Therefore, the failure of the first relay  20  can be diagnosed by determining whether or not the current value detected after the control so as to open the first relay  20  and close the second relay  23  matches the current value (ibat 1 ) detected when the discharge current stably flows from the starting energy storage apparatus  11 . 
     (1) Failure Diagnosis Process for First Relay 
     A failure diagnosis process for the first relay  20  according to the second embodiment will be described with reference to  FIG. 5 . 
     In S 201 , the management unit  22  detects a current value with the detection unit  19  (an example of the second detection step). 
     In S 202 , the management unit  22  performs control so as to open the first relay  20  and close the second relay  23 . 
     In S 203 , the management unit  22  detects a current value with the detection unit  19  (an example of the first detection step). 
     In S 204 , the management unit  22  determines whether or not the current value detected in S 201  and the current value detected in S 203  match. When the current values do not match, it is determined that the first relay  20  is not out of order, and the present process ends. When the current values match, it is determined that the first relay  20  is out of order, and the process proceeds to S 205  (an example of the determination step). 
     In S 205 , the management unit  22  executes a predetermined error process. 
     (2) Effects of the Embodiment 
     In the starting energy storage apparatus  11  provided with the bypass path  21 , even when the voltage of the starting energy storage apparatus  11  and the voltage of auxiliary energy storage apparatus  12  are substantially the same, a result of comparison between a current value detected during discharge of the starting energy storage apparatus  11  and a current value detected after the control so as to open the first relay  20  and close the second relay  23  differs between a case where the first relay  20  is out of order and a case where the first relay  20  is not out of order. In other words, the comparison result varies between the case where the first relay  20  is not out of order and the case where the first relay  20  is out of order. Therefore, the failure of the first relay  20  can be diagnosed more reliably than when the voltage does not fluctuate as in the conventional case. 
     Third Embodiment 
     A third embodiment will be described with reference to  FIGS. 6 to 7 . As shown in  FIG. 6 , the engine starting system  2  according to the third embodiment includes a third relay  30 , a fourth relay  31 , and a fifth relay  32  in addition to the configuration of the first embodiment. Each of the third to fifth relays is an example of the third circuit breaker. 
     The third relay  30  is provided in a current path  33  connecting the starting energy storage apparatus  11  and the starter  10 . The fourth relay  31  is provided in a current path  34  connecting the auxiliary energy storage apparatus  12  and the starter  10 . The third relay  30  and the fourth relay  31  are opened and closed by an ECU of the vehicle. 
     The fifth relay  32  is provided in a current path  35  to which the energy storage device  18  is connected in the auxiliary energy storage apparatus  12 . The fifth relay  32  is opened and closed by the management unit  22  (not shown) provided in the auxiliary energy storage apparatus  12 . 
     The detection unit  19  according to the third embodiment can detect both the current value and the direction of the current. 
     The management unit  22  according to the third embodiment performs a failure diagnosis similar to that of the first embodiment to determine whether the first relay  20  and the third to fifth relays are all normal or at least one of these relays is out of order. 
     A specific description will be given with reference to  FIG. 7 . When the first relay  20  is opened at the time of performing failure diagnosis, the first relay  20  is normal. Therefore, the first relay  20  is normal (◯) when being open and abnormal (x) when being closed. 
     On the contrary, the third to fifth relays are normal when being closed, so that the third to fifth relays are normal (◯) when being closed and abnormal (x) when being open. In  FIG. 7 , when the relays  4  and  5  are “◯,” it means that both the fourth relay  31  and the fifth relay  32  are closed (normal), and when “x,” it means that at least one of the fourth relay  31  and the fifth relay  32  is open (abnormal). 
     As shown in  FIG. 7 , in a case where the same failure diagnosis as in the first embodiment is performed, when both the first relay  20  and the third to fifth relays are normal (◯), a current value larger than 0 amperes is detected by the detection unit  19  (i.e., the current is present), and the direction of the current detected by the detection unit  19  is the charge direction. In contrast, when at least one of these relays is abnormal (x), no current flows to the detection unit  19  (i.e., the current is absent), or the direction of the detected current is the discharge direction. 
     Therefore, the management unit  22  according to the third embodiment determines whether the first relay  20  and the third to fifth relays are all normal or at least one of these relays is out of order from the presence or absence of the current flowing through the detection unit  19  and the direction of the current. 
     According to the failure diagnosis method according to the third embodiment, not only the first relay  20  provided in the starting energy storage apparatus  11  but also the third to fifth relays provided outside the starting energy storage apparatus  11  can diagnose the failure of the starting energy storage apparatus  11 . 
     Other Embodiments 
     The techniques disclosed in the present specification are not limited to the embodiments described with reference to the above description and drawings, and the following embodiments, for example, are also the technical scope disclosed in the present specification. 
     (1) In the above embodiment, the second energy storage apparatus (auxiliary energy storage apparatus  12 ) has been described as an example of another power supply, but another power supply may be an external detachable charger for charging the first energy storage apparatus. For example, the external charger may be connected to a cigar socket of the vehicle, and the starting energy storage apparatus  11  may be charged by the connected charger. In this case as well, the failure of the first relay  20  in the starting energy storage apparatus  11  can be diagnosed similarly to the case where the second energy storage apparatus is connected. 
     (2) In the above embodiment, the auxiliary energy storage apparatus  12  has been described as the second energy storage apparatus, but the second energy storage apparatus may be a backup energy storage apparatus that supplies electric power to the electrical load (starter  10 ) in place of the starting energy storage apparatus  11  when the voltage of the starting energy storage apparatus  11  drops. 
     (3) In the above embodiment, the case has been described as an example where the voltage of the first energy storage apparatus (starting energy storage apparatus  11  in the first embodiment) and the voltage of another power supply (in the first embodiment, the auxiliary energy storage apparatus  12  is provided) are substantially the same. In contrast, the voltage of another power supply may be higher than the voltage of the first energy storage apparatus. The voltage of the auxiliary energy storage apparatus  12  may be higher than that of the starting energy storage apparatus  11  as in an example where the voltage of the starting energy storage apparatus  11  may be 12 V, and the voltage of the auxiliary energy storage apparatus  12  may be 24 V, 48 V, or higher. In this case, the voltage applied by the auxiliary energy storage apparatus  12  may be stepped down to substantially the same voltage as that of the starting energy storage apparatus  11  by a step-down unit (e.g., DC-to-DC converter) provided in the engine starting system. According to the failure diagnosis method described in the above embodiment, even when the voltage of the auxiliary energy storage apparatus  12  is stepped down to substantially the same voltage as the starting energy storage apparatus  11 , it is possible to more reliably diagnose the failure of the first relay  20 . 
     (4) In the embodiment described above, the case has been described as an example where the bypass path  21  is provided with the resistor  24 , but a constant current source (e.g., constant current diode) may be provided instead of the resistor  24 . 
     (5) In the third embodiment, the case has been described as an example where the engine starting system  2  has three relays of the third to fifth relays as the third circuit breakers, but the engine starting system  2  may have only one or two of these relays. 
     (6) In the above embodiment, the starter  10  has been described as an example of the electrical load, but the electrical load is not limited to the starter  10 , and any equipment may be used so long as it consumes electric power. Although the engine starting system has been described as an example of the system in the above embodiment, the system may be any system so long as the first energy storage apparatus for supplying electric power to the electrical load and another power supply are connected in parallel. 
     (7) In the second embodiment, the case has been described as an example where the bypass path  21  is provided in parallel with the section of the current path  15  of the starting energy storage apparatus  11 , the section including the assembled battery  16  and the first relay  20  and not including the detection unit  19 . In contrast, as in the engine starting system  3  shown in  FIG. 8 , the bypass path  21  may be provided in parallel with a section including the assembled battery  16 , the first relay  20 , and the detection unit  19  (current sensor). In this case, when the first relay  20  is normal, the first relay  20  is opened at the time of failure diagnosis, so that the current value measured by the current sensor is 0 A (ampere). In contrast, when the first relay  20  is out of order, the first relay  20  is not opened, so that the current value measured by the current sensor is larger than 0 A. Therefore, the failure of the first relay  20  can be determined based on whether or not the current value measured by the current sensor is 0 A. 
     (8) In the third embodiment, the case has been described as an example where the detection unit  19  can detect both the current value and the direction of the current, but only the direction of the current may be detected. Whether the first relay  20  and the third to fifth relays are all normal or at least one of these relays is out of order may be determined only from the direction of the current. Specifically, when the direction of the current is the charge direction, it may be determined that all the relays are normal, and when the direction of the current is not the charge direction (in a case where the detected direction is the discharge direction or the direction cannot be determined), it may be determined that at least one of the relays is out of order 
     (9) Although the lithium ion battery has been described as an example of the energy storage device  18  in the above embodiment, the energy storage device  18  may be a capacitor accompanied by an electrochemical reaction. 
     DESCRIPTION OF REFERENCE SIGNS 
       1 : engine starting system (example of system) 
       2 : engine starting system (example of system) 
       10 : starter (example of electrical load) 
       11 : starting energy storage apparatus (example of first energy storage apparatus) 
       12 : auxiliary energy storage apparatus (example of second energy storage apparatus and another power supply) 
       13 : positive external terminal 
       14 : negative external terminal 
       15 : current path 
       17 : battery management system (example of management apparatus) 
       18 : energy storage device 
       19 : detection unit 
       20 : first relay (example of first circuit breaker) 
       21 : bypass path 
       22 : management unit 
       23 : second relay (example of second circuit breaker) 
       24 : resistor 
       30 : third relay (example of third circuit breaker) 
       31 : fourth relay (example of fourth circuit breaker) 
       32 : fifth relay (example of fifth circuit breaker) 
       33 : current path 
       34 : current path 
       35 : current path