Abstract:
A battery management system (BMS) is disclosed. In one embodiment, the BMS includes i) a first switching circuit electrically connected to a battery cell or a battery pack and configured to provide a voltage of the battery cell or the battery pack, ii) a capacitor electrically connected to the first switching circuit and configured to store the voltage provided from the first switching circuit and iii) a second switching circuit electrically connected to the capacitor and configured to provide the voltage stored in the capacitor, wherein the second switching circuit has first and second output terminals. The BMS may further include a pull-down circuit electrically connected to the second switching circuit, and configured to reduce impedance at the first output terminal of the second switching circuit.

Description:
RELATED APPLICATION 
     This application claims priority to and the benefit of Provisional Patent Application No. 61/487,675 filed on May 18, 2011 in the U.S Patent and Trademark Office, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field 
     The described technology generally relates to a battery management system. 
     2. Description of the Related Technology 
     In general, vehicles using gasoline, diesel oil or LPG as fuel generate a large quantity of carbon dioxide emissions which pollute the atmosphere and induce global warming, thereby damaging the global environment. Accordingly, research has been actively conducted on hybrid electric vehicles (HEV) having low emissions or electric vehicles (EV) having zero emissions. 
     An HEV is powered by an electric motor, which uses electricity supplied from a main battery via a power conversion circuit as a driving source, as well as an internal combustion engine. Further, an HEV may be operated so as to improve the fuel efficiency of the vehicle to a high degree according to a driving situation. 
     The HEV motor is switched between a driving mode and a generation mode through the control of a motor control unit (MTCU) when the vehicle is braked or decelerated. At this time, the main battery is charged by the electric energy generated from a generator (or driving motor) under the control of a battery management system (BMS) connected to the MTCU. The current applied to the main battery from the generator may vary and even be discontinuous according to driving conditions. 
     To reach an appropriate degree of performances, the number of battery cells gradually increases, and the BMS must be capable of effectively managing the various battery cells. 
     SUMMARY 
     One inventive aspect is a battery management system (BMS), which can measure a battery voltage in a more accurate and stable manner. 
     Another aspect is a battery management system (BMS) including a first relay, a capacitor, a second relay, a pull-down circuit block, and a microcomputer unit. The first relay is connected to a battery cell or opposite ends of a battery pack and relays a voltage of the battery cell or the battery pack. The capacitor is connected to the first relay and stores the voltage relayed by the first relay. The second relay is connected to the capacitor and relays the voltage stored in the capacitor. The pull-down circuit block includes a resistor connected between output ports of the second relay and a switch. The microcomputer unit controls on/off of the first relay, the second relay and the pull-down circuit block, and the voltage stored in the capacitor is input to the microcomputer unit. 
     Another aspect is a battery management system (BMS), comprising: a first switching circuit electrically connected to a battery cell or a battery pack and configured to provide a voltage of the battery cell or the battery pack; a capacitor electrically connected to the first switching circuit and configured to store the voltage provided from the first switching circuit; a second switching circuit electrically connected to the capacitor and configured to provide the voltage stored in the capacitor, wherein the second switching circuit has first and second output terminals; and a pull-down circuit electrically connected to the second switching circuit, and configured to reduce impedance at the first output terminal of the second switching circuit. 
     The above system further comprises a controller configured to control on and off operations of the first and second switching circuits, and configured to receive the voltage that has been stored in the capacitor. In the above system, the pull-down circuit comprises a pull-down resistor, and wherein a first end of the pull-down resister is electrically connected to the first output terminal of the second switching circuit. In the above system, the pull-down resistor has a resistance value in the range from about 0.5 MOhm to about 1.5 MOhm. In the above system, the pull-down resistor has a resistance value of about 1 MOhm. 
     In the above system, the pull-down circuit further comprises a switch electrically connected to a second end of the pull-down resistor. In the above system, the switch comprises a transistor. In the above system, the transistor has i) a first electrode electrically connected to the second end of the pull-down resistor, ii) a second electrode electrically connected to the second output terminal of the second relay and iii) a control electrode electrically connected to the controller. 
     In the above system, the controller is configured to provide i) a first control signal to the first switching circuit and ii) a second control signal to the second switching circuit and the control electrode of the transistor. In the above system, the second control signal is configured to substantially periodically turn on and turn off the transistor. In the above system, the controller is configured to change the value of the second control signal without substantially affecting the voltage at the first output terminal of the second switching circuit. In the above system, the second control signal comprises a plurality of square waves that are substantially periodically repeated. In the above system, each of the switching circuits comprises a relay. The above system further comprises an output buffer. In the above system, the output buffer comprises an operational amplifier. 
     Another aspect is a battery management system (BMS), comprising: a first relay electrically connected to a battery and configured to relay a voltage of the battery; a capacitor electrically connected to the first relay and configured to store the voltage relayed from the first relay; a second relay electrically connected to the capacitor and configured to relay the voltage stored in the capacitor; and a resistor having first and second ends, wherein the first end is electrically connected to an output terminal of the second relay. 
     The above system further comprises a switching transistor electrically connected to the resistor and configured to be alternately turned on and turned off based on a control signal applied thereto. In the above system, the control signal is a square wave signal comprising a plurality of square waves that are substantially periodically repeated. 
     Another system is a battery system, comprising: a battery; a capacitor receiving a voltage from the battery; a voltage relay circuit electrically connected to the capacitor and configured to relay the voltage stored in the capacitor, wherein the voltage relay circuit comprises an output terminal; a resistor having first and second ends, wherein the first end is electrically connected to the output terminal of the voltage relay circuit; a switch electrically connected to the second end of the resistor; and a controller configured to control the switch. 
     In the above system, the switch is configured to substantially prevent a voltage drop at the output terminal of the voltage relay circuit based on a control signal received from the controller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of a battery management system (BMS) according to an embodiment. 
         FIG. 2  is a graph illustrating output waveforms of a second relay in a case where only a pull-down resistor is installed between output ports of the second relay according to an embodiment. 
         FIG. 3  is a graph illustrating output signals of a second relay in a case where a pull-down resistor and a switch are installed between output ports of the second relay according to an embodiment. 
         FIG. 4  is a graph illustrating an example enlarged view of a waveform of a second control signal shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a diagram illustrating a configuration of a battery management system (BMS) according to an embodiment. 
     The BMS  100  shown in  FIG. 1  measures and monitors the overall voltage V of a battery pack to protect the system. 
     Referring to  FIG. 1 , the BMS  100  includes a first relay  110 , a capacitor C f , a second relay  120 , an output buffer  130 , a pull-down circuit block  140  and a microcomputer unit (MCU)  150 . The BMS  100  may further include a first distribution resistor R d1  and a second distribution resistor R d2  connected in series to each other. The first and second distribution resistors R d1  and R d2  divide the overall voltage V and convert the same into signal levels to be transmitted to the MCU  150 . In another embodiment, first and second switching circuits can be used instead of the first and second relays  110  and  120 . For the purpose of convenience of description, the first and second relays  110  and  120  will be used to explain the disclosed embodiments. 
     Although not shown, the BMS  100  may further include an ND converter that converts an analog signal to a digital signal and provides the converted digital signal to the MCU  150 . 
     The first relay  110  may be electrically connected to opposite ends of a battery cell or a battery pack. The battery pack may include a plurality of battery cells (not shown) connected in series to each other. Therefore, the first relay  110  may include a main relay (not shown) and a plurality of sub relays (not shown). The sub relays may be electrically connected to opposite ends of the battery cells. The main relay may be electrically connected to the sub relays. 
     The first relay  110  receives a first control signal C trl1  from the MCU  150  and performs a switching operation. If the first relay  110  receives a first control signal C trl1  from the MCU  150 , it becomes an active high state to then be turned on. Here, voltages of the battery cells are relayed to the main relay via the sub relays and then relayed to the capacitor C f  via the main relay. 
     The first relay  110  may include an electromechanical relay or a semiconductor solid state relay (SSR) (e.g., a photo-MOS relay), but not limited thereto. 
     The electromechanical relay may include an inductor and an electromechanical switch. The electromechanical relay may be driven such that the electromechanical switch is magnetized by a current flowing in the inductor to perform a mechanical switching operation. 
     The photo-MOS relay may be an electronic switch that performs a switching operation in response to light. 
     The photo-MOS relay may include a light emitting diode (LED), a photo-sensor diode, and a metal oxide semiconductor field effect transistor (MOSFET). When a voltage is applied to the LED, light is generated, and the generated light is transferred to the photo-sensor diode. The photo-sensor diode exposed to the light turns on the photo-MOS relay as the voltage is applied between a source and a gate of the MOSFET. 
     The capacitor C f  may be a flying capacitor. The opposite ends of the capacitor C f  may be electrically connected to the first relay  110 . For example, the opposite ends of the capacitor C f  may be electrically connected to the main relay of the first relay  110 . When the first relay  110  is turned on, the capacitor C f  may receive and store the voltage of the battery pack through the first relay  110 . 
     The second relay  120  may be electrically connected to the opposite ends of the capacitor C f . The second relay  120  may receive a second control signal C trl2  from the MCU  150  and may perform a switching operation. If the second relay  120  receives the second control signal C trl2  from the MCU  150 , it becomes an active high state to then be turned on. Here, the second relay  120  may relay the voltage stored in the capacitor C f  to the MCU  150  through the output buffer  130 . 
     The second relay  120  may include an electromechanical relay or a semiconductor solid state relay (SSR) (e.g., a photo-MOS relay), but not limited thereto. 
     The electromechanical relay may include an inductor and an electromechanical switch. The electromechanical relay may be driven such that the electromechanical switch is magnetized by a current flowing in the inductor to perform a mechanical switching operation. 
     The photo-MOS relay may be an electronic switch that performs a switching operation in response to light. 
     The photo-MOS relay may include a light emitting diode (LED), a photo-sensor diode, and a metal oxide semiconductor field effect transistor (MOSFET). When a voltage is applied to the LED, light is generated, and the generated light is transferred to the photo-sensor diode. The photo-sensor diode exposed to the light turns on the photo-MOS relay as the voltage is applied between a source and a gate of the MOSFET. 
     The output buffer  130  may be electrically connected between an output port of the second relay  120  and an input port of the MCU  150 . The output buffer  130  may include an operational amplifier (OP-AMP). A non-inverting terminal (+) of the OP-AMP  130  may be electrically connected to a first output V po  of the second relay  120 . An output port of the OP-AMP  130  may be electrically connected to form a negative feedback loop together with an inverting port (−) of the OP-AMP  130  for stabilizing of the output and may be electrically connected to the MCU  150 . 
     The pull-down circuit block  140  may be electrically connected between output ports V po , V no  of the second relay  120 . The pull-down circuit block  140  receives the second control signal C trl2  from the MCU  150  and reduces impedance at the output ports V po , V no  of the second relay  120 . 
     The pull-down circuit block  140  may include a pull-down resistor R pd  and a switch T connected in series to each other between output ports V po , V no  of the second relay  120 . 
     The pull-down resistor R pd  reduces impedance at the output ports V po , V no  of the second relay  120  thereby preventing malfunction of the output buffer  130 . A first terminal of the pull-down resistor R pd  may be electrically connected to the first output port V po  of the second relay  120 . The pull-down resistor R pd  may have a resistance value of about 0.5 MOhm to about 1.5 MOhm. The pull-down resistor R pd  may have a resistance value of about 1 MOhm. The resistance value of about 0.5 MOhm to about 1.5 MOhm may provide an optimum balance between suppression of high impedance and transferability of high frequency signal. For example, if the pull-down resistor R pd  is higher than or equal to about 0.5 MOhm and the second relay  120  operates in a high level, the amount of current input to the pull-down resistor R pd  may sufficiently suppress impedance. As another example, if the resistance value of the pull-down resistor R pd  is equal to lower than about 1.5 MOhm, high frequency signal may be sufficiently well transferred to the output buffer  130 . However, depending on the embodiment, the pull-down resistor R pd  may have a resistance value less than about 0.5 MOhm or greater than about 1 MOhm. 
     The switch T may be electrically connected between a second terminal of the pull-down resistor R pd  and a second output port V no  of the second relay  120 . The switch T may be a transistor. The transistor T may include a first electrode, a second electrode and a control electrode. The first electrode of the transistor T may be electrically connected to the second terminal of the pull-down resistor R pd . The second electrode of the transistor T may be electrically connected to the second output port V no  of the second relay  120 . The control electrode of the transistor T may receive the second control signal C trl2  from the MCU  150 . 
     The MCU  150  may apply the first control signal C trl1  to the first relay  110  and the second control signal C trl2  to the second relay  120  and the switch T to control the first relay  110 , the second relay  120  and the pull-down circuit block  140  to be turned on/off, respectively. 
     Waveforms of the first control signal C trl1  and the second control signal C trl2  may be complementary with respect to each other. For example, if the value of the first control signal C trl1  is in a high level, the value of the second control signal C trl2  may be in a low level. In addition, if the value of the first control signal C trl1  is in a low level, the value of the second control signal C trl2  may be in a high level. Hereinafter, the operation, function and effect of the BMS  100  will be described. 
     First, the MCU  150  may output the first control signal C trl1  having a high level value to the first relay  110 . Here, the second control signal C trl2  output from the MCU  150  may have a low level value, and the second relay  120  and the pull-down circuit block  140  may be turned off. Therefore, the first relay  110  receives the first control signal C trl1  and becomes in an active high state to then be turned on. Here, the voltage of the opposite ends of the battery pack is divided by the distribution resistors R d1  and R d2  and then transferred to the capacitor C f  via the first relay  110 . 
     Next, the MCU  150  may output the second control signal C trl2  having a high level value to the first relay  110 . Here, the first control signal C trl1  output from the MCU  150  may have a low level value, and the first relay  110  may be turned off. Therefore, the second relay  120  and the switch T may become in an active high state to then be turned on. Here, the second relay  120  may output the voltage stored in the capacitor C f . 
     The BMS  100  may include an circuit element such as an inverter (not shown), connected to the MCU  150 , which generates electro-magnetic wave that causes electrical or electromagnetic noise in the BMS  100 , particularly, at the output of the second relay  120 . For example, the output ports V po , V no  of the second relay  12  generally become high impedance state due to the electrical or electromagnetic noise. The high impedance at the output ports V po , V no  of the second relay  12  may subsequently cause malfunction of the OP-AMP  130 , since the OP-AMP  130  may generate an output to the MCU  150  even when there is no output from the second relay  12 . 
     In order to prevent the malfunction of the OP-AMP  130 , the BMS  100  may include the pull-down resistor R pd  installed between the output ports V po , V no  of the second relay  120 . The pull-down resistor R pd  reduces the impedance at the output ports V po , V no  of the second relay  120 , thereby allowing the OP-AMP  130  to generate a normal output. However, in a case where only the pull-down resistor R pd  is connected between the output ports V po , V no  of the second relay  120 , a voltage drop may be generated at the output ports V po , V no  of the second relay  120 . Therefore, the pull-down resistor R pd  and the switch T connected in series to each other may be installed between the output ports V po , V no  of the second relay  120 . The switch T and the second relay  120  may be operated by the second control signal C trl2  output from the MCU  150 . In one embodiment, the second control signal C trl2  alternately turns on and turns off both the switch T and the second relay  120 . In one embodiment, the second control signal C trl2  has a very short period, for example, about 200 ms. In another embodiment, the period of the second control signal C trl2  is in the range from about 150 ms to about 250 ms. In still another embodiment, the period of the second control signal C trl2  is about 200 ms. Therefore, even if a voltage drop at the outputs V po , V no  of the second relay  120  occurs due to the pull-down resistor R pd , since the period of the second control signal C trl2  is very short, the final output of the second relay  120  can be substantially uniform. 
       FIG. 2  is a graph illustrating output waveforms of a second relay  120  in a case where only a pull-down resistor R pd  is installed between the output ports V po , V no  of the second relay  120  according to an embodiment. 
     In  FIG. 2 , the horizontal axis indicates time t[s], and the vertical axis indicates output voltage V of the second relay  120 . Specifically, the graph labeled {circle around ( 1 )} indicates a signal output from the second relay  120  in a case where only the pull-down resistor R pd  of 1 MOhm is installed between output ports of the second relay  120 . The graph labeled {circle around ( 2 )} indicates the first control signal C trl1 , and the graph labeled {circle around ( 3 )} indicates the second control signal C trl2 . 
     In the time period up until time {circle around (a)}, the first control signal C trl1  has a high level value and the second control signal C trl2  has a low level value as shown in  FIG. 2 . Here, the first relay  110  is turned on to then relay the voltage V of the battery pack to the capacitor C f  while the second relay  120  is turned off. In the time period after time {circle around (b)}, the first control signal C trl1  has a low level value and the second control signal C trl2  has a high level value. Here, the first relay  110  is turned off and the second relay  120  is turned on to then output the voltage stored in the capacitor C f . The output buffer  130  may transmit the voltage output from the second relay  120  to the MCU  150 . In a section between {circle around (a)} and {circle around (b)}, value levels of the first control signal C trl1  and the second control signal C trl2  are changed. In one embodiment, the value levels of the first control signal C trl1  and the second control signal C trl2  are not immediately changed between high and low values. Rather, the first control signal C trl1  and the second control signal C trl2  temporarily have substantially the same value in the time period between {circle around (a)} and {circle around (b)} and the value levels thereof are then changed to low. 
     In the case where only the pull-down resistor R pd  of 1 MOhm is installed between the output ports V po , V no  of the second relay  120 , the impedance at the output ports V po , V no  of the second relay  120  can be reduced. However, a voltage drop may occur to the output ports V po , V no  of the second relay  120  by the pull-down resistor R pd . 
       FIG. 3  is a graph illustrating output signals of a second relay  120  in a case where a pull-down resistor R pd  and a switch T are installed between the output ports V po , V no  of the second relay  120  according to an embodiment.  FIG. 4  is a graph illustrating an example enlarged view of a waveform of a second control signal shown in  FIG. 3 . 
     If the value of the first control signal C trl1  is changed from a high level to a low level and the value of the second control signal C trl2  is changed from a low level to a high level, the first relay  110  is turned off and the second relay  120  is turned on. In one embodiment, the MCU  150  repeatedly outputs signals of high and low level. In this embodiment, the second relay  120  and the switch T are alternately turned on and off based on the repeated control signals. Waveforms of the second control signal C trl2  may be a square wave signal, as shown in  FIG. 4 , but not limited thereto. In one embodiment, the value of the second control signal C trl2  is changed without substantially affecting the output of the second relay  120 . In another embodiment, variation of the value of the second control signal C trl2  controls the output signals of the second relay  120  to minutely oscillate, for example, as shown in  FIG. 3  (see the graph labeled {circle around ( 3 )}). In this embodiment, the negative feedback loop of the output buffer  130  makes the output signal of the second relay  120  more stable. 
     If the second relay  120  and the switch T are turned on, the pull-down resistor R pd  is connected between the outputs V po , V no  of the second relay  120 , the impedance at the output ports V po , V no  of the second relay  120  can be reduced. Furthermore, since the switch T is substantially periodically turned off, the output voltage of the second relay  120  is maintained at a substantially constant level, as shown in  FIG. 3  (see the graph labeled {circle around ( 3 )}), without a voltage drop. 
     According to at least one of the disclosed embodiments, a battery voltage can be measured in a more accurate and stable manner by suppressing high impedance at the output of a relay circuit (or a switch) while preventing a voltage drop. 
     While embodiments have been shown and described with reference to the accompanying drawings, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the claims.