Patent Publication Number: US-11031803-B2

Title: Start-up apparatus for battery management circuit and battery management system having the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefits of U.S. provisional application Ser. No. 62/574,756, filed on Oct. 20, 2017, and Taiwan application serial no. 107109488, filed on Mar. 20, 2018. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification. 
    
    
     BACKGROUND 
     Technical Field 
     The disclosure relates to an electronic apparatus, and more particularly to a start-up apparatus for a battery management circuit and a battery management system. 
     Description of Related Art 
     Thanks to the development of technology, reusable rechargeable batteries have been widely used in various electronic devices, such as mobile phones, laptop computers, and electric vehicles, as the power source. However, rechargeable batteries can only store a limited amount of power. When the power of a rechargeable battery is depleted and turns the battery into a dead battery state, the internal circuits of the electronic device, especially the battery management circuit, will not be able to operate normally. The battery management circuit inside the electronic device is used to manage the power amount of the rechargeable battery. When the power amount of the rechargeable battery decreases, the battery management circuit may control a charging switch to allow an external power source (e.g., adapter or charging facility) to charge the rechargeable battery. The operating power of the conventional battery management circuit is supplied only by the rechargeable battery. When the rechargeable battery is in the dead battery state, the conventional battery management circuit cannot operate normally. Since the battery management circuit cannot operate normally, the external power source cannot charge the rechargeable battery. Thus, for the conventional technology, it is required to detach the rechargeable battery in the dead battery state from the electronic device and load the rechargeable battery in the dead battery state into an external special charging device. After the external special charging device finishes charging the rechargeable battery, the rechargeable battery then needs to be removed from the external special charging device to be put back to the electronic device. 
     In view of the above, it is required to provide a start-up apparatus to solve the problem that the electronic device cannot operate normally due to exhaustion of the power of the internal battery. 
     SUMMARY 
     The disclosure provides a start-up apparatus for a battery management circuit and a battery management system that enable the battery management circuit to operate normally when a battery module is in a dead battery state. 
     According to an embodiment of the disclosure, a start-up apparatus of a battery management circuit is provided. The start-up apparatus of the battery management circuit includes a transformer, a switch circuit, a control circuit, and a rectifier circuit. The transformer includes a primary winding, an auxiliary winding, and a secondary winding. A first terminal of the primary winding is coupled to a first external power path. A first terminal of the switch circuit is coupled to a second terminal of the primary winding, and a second terminal of the switch circuit is coupled to a second external power path. The control circuit is coupled to the auxiliary winding to receive power. The control circuit controls a conduction state of the switch circuit. The rectifier circuit is coupled to the secondary winding of the transformer. The rectifier circuit supplies power to the battery management circuit. 
     According to an embodiment of the disclosure, a battery management system is provided. The battery management system includes a battery module, a battery management circuit, and a start-up apparatus. The battery management circuit is coupled to the battery module to manage a power amount of the battery module. When the battery module has sufficient power, the battery module supplies power to the battery management circuit. The start-up apparatus is coupled to the battery management circuit. The start-up apparatus includes a transformer, a switch circuit, a control circuit, and a rectifier circuit. The transformer includes a primary winding, an auxiliary winding, and a secondary winding. A first terminal of the primary winding is coupled to a first external power path. A first terminal of the switch circuit is coupled to a second terminal of the primary winding, and a second terminal of the switch circuit is coupled to a second external power path. The control circuit is coupled to the auxiliary winding to receive power. The control circuit controls a conduction state of the switch circuit. The rectifier circuit is coupled to the secondary winding of the transformer. When the battery module does not have sufficient power to supply the battery management circuit, the rectifier circuit supplies power to the battery management circuit. 
     Based on the above, the battery management system described in the embodiments of the disclosure may supply power to the battery management circuit by the battery module and/or the start-up apparatus. Thus, when the battery module is in a dead battery state, the start-up apparatus may supply power to the battery management circuit to enable the battery management circuit to maintain normal operation. 
     To make the aforementioned and other features of the disclosure more comprehensible, several embodiments accompanied with drawings are described in detail as follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the disclosure and, together with the description, serve to explain the principles of the disclosure. 
         FIG. 1  is a circuit block diagram of the battery management system according to an embodiment of the disclosure. 
         FIG. 2  is a circuit block diagram of the battery management circuit of  FIG. 1  according to an embodiment of the disclosure. 
         FIG. 3A  to  FIG. 3C  are circuit diagrams of the start-up apparatus of  FIG. 1  according to different embodiments of the disclosure. 
         FIG. 4  is a circuit block diagram of the battery management system according to another embodiment of the disclosure. 
         FIG. 5  is a circuit block diagram of the battery management system of  FIG. 4  according to an embodiment of the disclosure. 
         FIG. 6  is a circuit block diagram of the battery management system according to yet another embodiment of the disclosure. 
         FIG. 7  is a circuit diagram of the start-up apparatus of  FIG. 6  according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS 
     The term “coupled (or connected)” as used throughout this specification (including the claims) may refer to any direct or indirect connection means. For example, if a first device is described as being coupled (or connected) to a second device, it should be understood that the first device may be directly connected to the second device, or the first device may be indirectly connected to the second device via another device or connection means. Moreover, elements/components/steps with the same reference numerals represent identical or similar parts in the drawings and embodiments wherever possible. Elements/components/steps with the same names or reference numerals in different embodiments may serve as a reference for one another. 
       FIG. 1  is a circuit block diagram of a battery management system according to an embodiment of the disclosure. As shown in  FIG. 1 , the battery management system  100  includes a battery module  11 , a battery management circuit  12 , and a start-up apparatus  13 . The battery module  11  may be formed by connecting a plurality of batteries in series. For example, the battery module  11  may include a plurality of lithium batteries or lithium ion batteries. According to the design requirements, the battery module  11  may include conventional batteries or other batteries. The battery management circuit  12  is coupled to the battery module  11 . The battery module  11  may supply power to the battery management circuit  12  when the battery module  11  has sufficient power. The battery management circuit  12  may manage the power amount of the battery module  11 . When the power amount of the battery module  11  decreases, the battery management circuit  12  may control a charging switch (not shown) to enable an external power source  14  to charge the battery module  11 . According to the design requirements, the external power source  14  may include an adapter or other charger infrastructure. In some embodiments, the external power source  14  is detachable from the battery management system  100 . 
     The start-up apparatus  13  is coupled to the battery management circuit  12 . In practical applications, the start-up apparatus  13  may be coupled to the external power source  14 . In some embodiments, the external power source  14  is detachable from the start-up apparatus  13 . In the case where the external power source  14  is connected to a first external power path P 1  and a second external power path P 2 , when the battery module  11  does not have sufficient power to supply the battery management circuit  12 , the start-up apparatus  13  may use the external power source  14  to supply power to the battery management circuit  12 , so as to enable the battery management circuit  12  to maintain normal operation. 
       FIG. 2  is a circuit block diagram of the battery management circuit  12  of  FIG. 1  according to an embodiment of the disclosure. In the embodiment shown in  FIG. 2 , the battery management circuit  12  includes a battery management chip  121  and a voltage regulation circuit  122 . The battery management chip  121  is coupled to the battery module  11  to manage the power amount of the battery module  11 . The voltage regulation circuit  122  couples the battery module  11  and a rectifier circuit  134  of the start-up apparatus  13  so as to receive the power provided by the battery module  11  and/or receive the power provided by the rectifier circuit  134 . Under normal conditions, the battery module  11  can supply power to the voltage regulation circuit  122 . That is, when the battery module  11  has sufficient power, the voltage regulation circuit  122  supplies the power provided by the battery module  11  to the battery management chip  121 . When the battery module  11  does not have sufficient power, the voltage regulation circuit  122  may supply the power provided by the rectifier circuit  134  to the battery management chip  121 , so as to enable the battery management chip  121  to maintain normal operation. 
     The details of implementation of the voltage regulation circuit  122  may be determined according to the design requirements. For example, in the embodiment of  FIG. 2 , the voltage regulation circuit  122  includes a diode  1221 , a diode  1222 , and a voltage regulator  1223 . The input terminal of the voltage regulator  1223  is coupled to the cathode of the diode  1221  and the cathode of the diode  1222 . The anode of the diode  1221  is coupled to the battery module  11  to receive power. The anode of the diode  1222  is coupled to the rectifier circuit  134  of the start-up apparatus  13  to receive power. The voltage regulator  1223  may convert the voltage received by the input terminal thereof to an operating voltage required by the battery management chip  121  to be supplied to the battery management chip  121  via the output terminal of the voltage regulator  1223 . For example, assuming that the output voltage of the battery module  11  is 64V (for example, the battery module  11  may be formed by connecting 16 4V lithium batteries in series), the voltage regulator  1223  may convert 64V to 5V to be supplied to the battery management chip  121 . 
     Referring to  FIG. 1 , the start-up apparatus  13  includes a transformer  131 , a switch circuit  132 , a control circuit  133 , and the rectifier circuit  134 . The transformer  131  includes a primary winding Np, an auxiliary winding Naux, and a secondary winding Ns. The external power source  14  may transmit power to the start-up apparatus  13  by the first external power path P 1  and the second external power path P 2 . The first terminal of the primary winding Np is coupled to the first external power path P 1 . The first terminal of the switch circuit  132  is coupled to the second terminal of the primary winding Np, and the second terminal of the switch circuit  132  is coupled to the second external power path P 2 . 
     The control circuit  133  is coupled to the auxiliary winding Naux to receive power. That is, when the external power source  14  is electrically connected to the first external power path P 1  and the second external power path P 2 , the power provided by the external power source  14  may be supplied to the control circuit  133  via the transformer  131 . The control circuit  133  may control the conduction state of the switch circuit  132 . For example, the control circuit  133  may generate a pulse width modulation (PWM) signal to the switch circuit  132 . Under the control of the PWM signal, the switch circuit  132  may determine the conduction state between the second external power path P 2  and the primary winding Np. Therefore, when the external power source  14  is electrically connected to the first external power path P 1  and the second external power path P 2 , the power provided by the external power source  14  may be supplied to the rectifier circuit  134  via the transformer  131 . 
     The rectifier circuit  134  is coupled to the secondary winding Ns of the transformer  131 . The rectifier circuit  134  may convert the AC power of the secondary winding Ns to DC power to be supplied to the battery management circuit  12 . If the battery module  11  has sufficient power, the voltage regulation circuit  122  of the battery management circuit  12  may supply the power provided by the battery module  11  to the battery management chip  121  of the battery management circuit  12 . If the battery module  11  does not have sufficient power to supply the battery management circuit  12 , the rectifier circuit  134  may supply power to the battery management circuit  12 . That is, the voltage regulation circuit  122  supplies the power provided by the rectifier circuit  134  to the battery management chip  121 . 
     To sum up, when the battery module  11  does not have sufficient power to supply the battery management circuit  12 , the start-up apparatus  13  may be excited to supply the power of the external power source  14  to the battery management circuit  12 , so as to enable the battery management circuit  12  to resume normal operation. After the battery management circuit  12  resumes normal operation, the battery management circuit  12  may turn on the charging switch (not shown) to charge the battery module  11  with the power of the external power source  14 . Thus, the battery management circuit  12  may directly charge the battery module  11  that is in the dead battery state without detaching the battery module  11  for charging with an external special charging device. Therefore, compared with the related art, in which the battery needs to be detached for charging with an external special charging device and then the charged battery needs to be installed back to the electronic device, the battery management system  100  of this embodiment provides a convenient battery charging method. 
       FIG. 3A  to  FIG. 3C  are circuit diagrams of the start-up apparatus  13  of  FIG. 1  according to different embodiments of the disclosure. In the embodiment shown in  FIG. 3A , the switch circuit  132  includes a power transistor  1321  and a current limiting resistor  1322 . According to the design requirements, the power transistor  1321  may be a metal-oxide-semiconductor field-effect transistor (MOSFET), a bipolar junction transistor (BJT), or an insulated gate bipolar transistor (IGBT). In this embodiment, the power transistor  1321  is an N-channel metal oxide semiconductor field-effect transistor (N-MOSFET). The first terminal (for example, drain) of the power transistor  1321  is coupled to the second terminal of the primary winding Np. The control terminal (for example, gate) of the power transistor  1321  is coupled to the control circuit  133 . The first terminal of the current limiting resistor  1322  is coupled to the second terminal (for example, source) of the power transistor  1321 , and the second terminal of the current limiting resistor  1322  is coupled to the second external power path P 2 . The current limiting resistor  1322  may limit the current that flows through the power transistor  1321  to prevent damage to the power transistor  1321 . 
     In the embodiment shown in  FIG. 3A , the control circuit  133  may include an auxiliary power circuit  1331  and a pulse width modulation (PWM) circuit  1332 . The auxiliary power circuit  1331  is coupled to the first terminal of the auxiliary winding Naux of the transformer  131 , so as to provide an auxiliary power to the PWM circuit  1332  by using the power of the auxiliary winding Naux. The PWM circuit  1332  is coupled to the auxiliary power circuit  1331  to receive the auxiliary power. The PWM circuit  1332  may generate a pulse signal to the control terminal of the switch circuit  132  to control the conduction state of the switch circuit  132 . That is, the PWM circuit  1332  may generate a pulse signal to the control terminal of the power transistor  1321  to control the conduction state of the power transistor  1321 . 
     In the embodiment of  FIG. 3A , the PWM circuit  1332  includes a first resistor R 1  and a first diode D 1 . The first terminal of the first resistor R 1  is coupled to the first external power path P 1 . The second terminal of the first resistor R 1  is coupled to the control terminal of the power transistor  1321  of the switch circuit  132 . The first terminal of the first diode D 1  is coupled to the second terminal of the first resistor R 1 , and the second terminal of the first diode D 1  is coupled to the source of the power transistor  1321  of the switch circuit  132 . In the embodiment of  FIG. 3A , the auxiliary power circuit  1331  includes a second resistor R 2 , a first capacitor C 1 , and a second diode D 2 . The first terminal of the second resistor R 2  is coupled to the first terminal of the auxiliary winding Naux. The second terminal of the auxiliary winding Naux is coupled to the second external power path P 2 . The first terminal of the first capacitor C 1  is coupled to the second terminal of the second resistor R 2 , and the second terminal of the first capacitor C 1  is coupled to the second terminal of the first resistor R 1 . The first terminal (for example, cathode) of the second diode D 2  is coupled to the first terminal of the auxiliary winding Naux, and the second terminal (for example, anode) of the second diode D 2  is coupled to the second external power path P 2 . 
     The power supply performed by the start-up apparatus  13  will be described below with reference to  FIG. 3A  as an example. In  FIG. 3A , the primary winding Np of the transformer  131  may acquire power via the first external power path P 1  to generate a current, and the auxiliary winding Naux may sense the current of the primary winding Np to generate an induced voltage, so as to generate an induced current at the auxiliary winding Naux. Therefore, the auxiliary winding Naux may supply power to the auxiliary power circuit  1331 . The auxiliary power circuit  1331  may use the power of the auxiliary winding Naux to provide the auxiliary power. After receiving the auxiliary power, the PWM circuit  1332  may generate a pulse signal to the switch circuit  132 , so as to control the conduction state of the power transistor  1321  of the switch circuit  132 . Under the control of the pulse signal, the power transistor  1321  may determine the conduction state between the second external power path P 2  and the primary winding Np. As the modulation frequency of the pulse signal increases, the switching operation of the power transistor  1321  becomes more frequent, which causes the magnetic flux of the primary winding Np of the transformer  131  to change. Therefore, when the external power source  14  is electrically connected to the first external power path P 1  and the second external power path P 2 , the power provided by the external power source  14  may be transmitted to the secondary winding Ns via the primary winding Np of the transformer  131  and cause an induced current to be generated at the secondary winding Ns. The induced current of the secondary winding Ns is then rectified and filtered by the rectifier circuit  134  to generate the output voltage Vo for supplying power to the battery management circuit  12  in  FIG. 1 . 
     In the embodiment of  FIG. 3A , the rectifier circuit  134  may include a diode  1341  and a capacitor  1342 . The anode of the diode  1341  is coupled to the first terminal of the secondary winding Ns of the transformer  131 . The cathode of the diode  1341  generates an output voltage Vo to the battery management circuit  12 . The first terminal of the capacitor  1342  is coupled to the cathode of the diode  1341 . The negative terminal of the capacitor  1342  and the second terminal of the secondary winding Ns are coupled to a reference voltage (for example, ground voltage GND). In addition, the output voltage Vo of the rectifier circuit  134  may be adjusted according to the ratio of the number of turns of the primary winding Np to the number of turns of the secondary winding Ns of the transformer  131 . 
       FIG. 3B  is a circuit diagram of the start-up apparatus  13  of  FIG. 1  according to another embodiment of the disclosure. The transformer  131 , the switch circuit  132 , the control circuit  133 , and the rectifier circuit  134  shown in  FIG. 3B  may be implemented by referring to the descriptions of  FIG. 3A  and therefore details thereof will not be repeated hereinafter. In the embodiment shown in  FIG. 3B , the control circuit  133  of the start-up apparatus  13  is provided with an auxiliary power circuit  1331  and a PWM circuit  1332 B. The auxiliary power circuit  1331  and the PWM circuit  1332 B shown in  FIG. 3B  may be implemented by referring to the descriptions of the auxiliary power circuit  1331  and the PWM circuit  1332  shown in  FIG. 3A  and therefore details thereof will not be repeated hereinafter. Unlike the PWM circuit  1332  of  FIG. 3A , the PWM circuit  1332 B of  FIG. 3B  further includes a Zener diode ZD. The first terminal (for example, cathode) of the Zener diode ZD is coupled to the second terminal of the first resistor R 1 , and the second terminal (for example, anode) of the Zener diode ZD is coupled to the second external power path P 2 . The Zener diode ZD helps to maintain a stable bias voltage between the gate and the source of the power transistor  1321 , so as to protect the power transistor  1321  from a surge current or static electricity. The power supply performed by the start-up apparatus  13  in  FIG. 3B  may be understood by referring to the descriptions of  FIG. 3A  and therefore details thereof will not be repeated hereinafter. 
       FIG. 3C  is a circuit diagram of the start-up apparatus  13  of  FIG. 1  according to yet another embodiment of the disclosure. The transformer  131 , the switch circuit  132 , the control circuit  133 , and the rectifier circuit  134  shown in  FIG. 3C  may be implemented by referring to the descriptions of  FIG. 3A  and therefore details thereof will not be repeated hereinafter. In the embodiment shown in  FIG. 3C , the control circuit  133  of the start-up apparatus  13  is provided with an auxiliary power circuit  1331 C and a PWM circuit  1332 . The auxiliary power circuit  1331 C and the PWM circuit  1332  shown in  FIG. 3C  may be implemented by referring to the descriptions of the auxiliary power circuit  1331  and the PWM circuit  1332  shown in  FIG. 3A  and therefore details thereof will not be repeated hereinafter. Unlike the auxiliary power circuit  1331  of  FIG. 3A , the auxiliary power circuit  1331 C of  FIG. 3C  further includes a second capacitor C 2  and a third resistor R 3 . As shown in  FIG. 3C , the first terminal of the second capacitor C 2  is coupled to the second terminal (for example, anode) of the second diode D 2 , and the second terminal of the second capacitor C 2  is coupled to the second external power path P 2 . The first terminal of the third resistor R 3  is coupled to the second terminal of the second diode D 2 , and the second terminal of the third resistor R 3  is coupled to the second external power path P 2 . In the auxiliary power circuit  1331 C of  FIG. 3C , the second capacitor C 2  and the third resistor R 3  may provide a longer discharge path for the power transistor  1321 . The power supply performed by the start-up apparatus  13  in  FIG. 3C  may be understood by referring to the descriptions of  FIG. 3A  and therefore details thereof will not be repeated hereinafter. 
       FIG. 4  is a circuit block diagram of the battery management system according to another embodiment of the disclosure. As shown in  FIG. 4 , the battery management system  400  includes a battery module  11 , a battery management circuit  12 , and a start-up apparatus  13 A. The battery module  11 , the battery management circuit  12 , and the start-up apparatus  13 A shown in  FIG. 4  may be implemented by referring to the descriptions of the battery module  11 , the battery management circuit  12 , and the start-up apparatus  13  shown in  FIG. 1  and therefore details thereof will not be repeated hereinafter. Unlike the start-up apparatus  13  of  FIG. 1 , the start-up apparatus  13 A of  FIG. 4  further includes a trigger circuit  135 . The trigger circuit  135  is coupled to the control circuit  133  and the battery management circuit  12 . The trigger circuit  135  determines whether to disable the control circuit  133  according to a control command of the battery management circuit  12 . 
     For example,  FIG. 5  is a circuit diagram of the start-up apparatus  13 A of  FIG. 4  according to an embodiment of the disclosure. As shown in  FIG. 5 , the start-up apparatus  13 A includes a transformer  131 , a switch circuit  132 , a control circuit  133 , a rectifier circuit  134 , and a trigger circuit  135 . The control circuit  133  is provided with an auxiliary power circuit  1331  and a PWM circuit  1332 . The transformer  131 , the switch circuit  132 , the control circuit  133 , the auxiliary power circuit  1331 , the PWM circuit  1332 , and the rectifier circuit  134  shown in  FIG. 5  may be implemented by referring to the descriptions of  FIG. 3A  and therefore details thereof will not be repeated hereinafter. The trigger circuit  135  is coupled to the PWM circuit  1332 . 
     In the embodiment of  FIG. 5 , the trigger circuit  135  may include a photocoupler. For example, a light emitting part  1351  of the photocoupler may be a light emitting diode, and a light receiving part  1352  of the photocoupler may be a phototransistor. The first terminal of the light emitting part  1351  of the photocoupler is coupled to the battery management circuit  12 . The second terminal of the light emitting part  1351  of the photocoupler is coupled to a reference voltage (for example, ground voltage). The first terminal of the light receiving part  1352  of the photocoupler is coupled to the PWM circuit  1332  of the control circuit  133 , and the second terminal of the light receiving part  1352  is coupled to the second external power path P 2 . When the battery module  11  does not have sufficient power to supply the battery management circuit  12 , the battery management circuit  12  does not operate normally. At this time, the light receiving part  1352  (phototransistor) of the photocoupler is turned off. The operation performed by the start-up apparatus  13 A shown in  FIG. 5  when the light receiving part  1352  (phototransistor) is turned off may be understood by referring to the descriptions of the start-up apparatus  13  shown in  FIG. 3A  and therefore details thereof will not be repeated hereinafter. Thus, when the battery module  11  does not have sufficient power to supply the battery management circuit  12 , the start-up apparatus  13 A may provide the power of the external power source  14  to the battery management circuit  12 , so as to enable the battery management circuit  12  to resume normal operation. After the battery management circuit  12  resumes normal operation, the battery management circuit  12  may turn on the charging switch (not shown) for the external power source  14  to charge the battery module  11 . Therefore, the battery module  11  is not in the dead battery state and may supply power to the battery management circuit  12 . 
     In the case where the battery module  11  has sufficient power to supply the battery management circuit  12 , the battery management circuit  12  may send a control command to the trigger circuit  135 , so as to disable the PWM circuit  1332  of the control circuit  133 . For example, the battery management circuit  12  may provide a driving current to enable the light emitting part  1351  (light emitting diode) of the photocoupler to emit light, which turns on the light receiving part  1352  (phototransistor) of the photocoupler. When the light receiving part  1352  (phototransistor) is turned on, the gate of the power transistor  1321  of the switch circuit  132  is kept at a low potential. That is, the power transistor  1321  is kept in the off state. Therefore, in the case where the battery module  11  has sufficient power to supply the battery management circuit  12 , the start-up apparatus  13 A may stop supplying power to the battery management circuit  12  under the control of the battery management circuit  12 . 
       FIG. 6  is a circuit block diagram of the battery management system according to yet another embodiment of the disclosure. As shown in  FIG. 6 , the battery management system  600  includes a battery module  11 , a battery management circuit  12 , and a start-up apparatus  13 B. The battery module  11 , the battery management circuit  12 , and the start-up apparatus  13 B shown in  FIG. 6  may be implemented by referring to the descriptions of the battery module  11 , the battery management circuit  12 , and the start-up apparatus  13  shown in  FIG. 1  and therefore details thereof will not be repeated hereinafter. Unlike the start-up apparatus  13  of  FIG. 1 , the start-up apparatus  13 B of  FIG. 6  further includes an excitation circuit  136 . The first terminal of the excitation circuit  136  is coupled to the first terminal of the primary winding Np of the transformer  131 , and the second terminal of the excitation circuit  136  is coupled to the second terminal of the primary winding Np. The excitation circuit  136  may increase an excitation current generated by the primary winding Np of the transformer  131 , so as to increase the power conversion efficiency of the transformer  131  from the primary winding Np to the secondary winding Ns. 
     When the battery module  11  does not have sufficient power to supply the battery management circuit  12 , the start-up apparatus  13 B may accelerate the excitation by the excitation circuit  136  to supply the power of the external power source  14  to the battery management circuit  12 , so as to enable the battery management circuit  12  to resume normal operation. After the battery management circuit  12  resumes normal operation, the battery management circuit  12  may turn on the charging switch (not shown) to charge the battery module  11  with the power of the external power source  14 . Thus, the battery management circuit  12  may directly charge the battery module  11  that is in the dead battery state without detaching the battery module  11  for charging with an external special charging device. Therefore, compared with the related art, in which the battery needs to be detached for charging with an external special charging device and then the charged battery needs to be installed back to the electronic device, the battery management system  600  of this embodiment provides a convenient battery charging method. 
     For example,  FIG. 7  is a circuit diagram of the start-up apparatus  13 B of  FIG. 6  according to an embodiment of the disclosure. As shown in  FIG. 7 , the start-up apparatus  13 B includes a transformer  131 , a switch circuit  132 , a control circuit  133 , a rectifier circuit  134 , and an excitation circuit  136 . The transformer  131 , the switch circuit  132 , the control circuit  133 , and the rectifier circuit  134  shown in  FIG. 7  may be implemented by referring to the descriptions of  FIG. 3A  and therefore details thereof will not be repeated hereinafter. In the embodiment shown in  FIG. 7 , the excitation circuit  136  includes a resistor  1361 , a capacitor  1362 , and a diode  1363 . The first terminal of the resistor  1361  and the first terminal of the capacitor  1362  are both coupled to the first terminal of the primary winding Np of the transformer  131 . The first terminal (for example, cathode) of the diode  1363  is coupled to the second terminal of the resistor  1361  and the second terminal of the capacitor  1362 , and the second terminal (for example, anode) of the diode  1363  is coupled to the second terminal of the primary winding Np of the transformer  131 . 
     The resistor  1361 , the capacitor  1362 , and the diode  1363  in the excitation circuit  136  may increase the current path of the primary winding Np of the transformer  131  to enhance the excitation current of the primary winding Np. Therefore, the power conversion efficiency between the primary winding Np and the secondary winding Ns of the transformer  131  is improved. The operations of the transformer  131 , the switch circuit  132 , the control circuit  133 , and the rectifier circuit  134  shown in  FIG. 7  may be understood by referring to the descriptions of  FIG. 3A  and therefore details thereof will not be repeated hereinafter. 
     In conclusion, the battery management system described in the embodiments of the disclosure may supply power to the battery management circuit by the battery module and may also supply external power to the battery management circuit by the start-up apparatus. Thus, when the battery module is in the dead battery state, the start-up apparatus may serve as the power source of the battery management circuit to enable the battery management circuit to maintain/resume normal operation. Therefore, even if the battery module is unable to supply power to the battery management circuit, the battery management system can still charge the battery module that is in the dead battery state without detaching the battery module. In addition, disposing the excitation circuit in the start-up apparatus effectively improves the power conversion efficiency of the transformer and further enhances the power supply efficiency of the start-up apparatus. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the disclosure covers modifications and variations provided that they fall within the scope of the following claims and their equivalents.