Abstract:
Method and apparatus for contact detection in battery packs are disclosed. A battery pack, comprising: at least a first battery cell and a second battery cell, the first battery cell having a first electrode and a second electrode, and the second battery cell having a third electrode and a fourth electrode; a power bar for coupling the second electrode of the first battery cell to the third electrode of the second battery cell; a communication wire for signal communication between the first battery cell and the second battery cell, wherein the first battery cell comprises a supervisor, which comprises a voltage detector to measure a first voltage between the first electrode and the second electrode of the first battery cell in a first state, and to measure a second voltage between the first electrode of the first battery cell and the communication wire in a second state; and a digital circuit to compute a voltage difference between the first voltage and the second voltage, and to indicate degraded contact of the power bar if the voltage difference is out of a predetermined threshold range. A battery cell and a method for monitoring a battery pack are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 14/565,574 filed on Dec. 10, 2014. The disclosure of the above application is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This disclosure generally relates to battery packs, and more specifically to method and apparatus for supervision in battery packs. 
       BACKGROUND OF THE INVENTION 
       [0003]    Battery packs formed with a number of series-connected battery cells are widely employed for power supply, especially in mobile appliances. As known, in (hybrid) electric vehicles, a battery pack is used to generate a high voltage to drive the motor. In such a battery pack, the number of battery cells are coupled in series by electrically conducting power bars, wherein each power bar can electrically couple a positive electrode of one battery cell to a negative electrode of an adjacent battery cell, e.g., by soldering or bolting. 
         [0004]    Each component on a current path in the battery pack is preferably designed to have a small resistance in order to reduce useless power dissipation, especially when a current flowing through the battery cells is large (e.g., several 100 A). Usually, the contact resistance between the power bars and the battery electrodes may be taken into account. Moreover, this contact resistance will rise if the contact gets corroded, loose, aging, etc. If the pack current is high, even a contact resistance of just 1 m would be unacceptable. For example, with a pack current of 100 A, it would give rise to a power dissipation of 10 W, which is undesirable. Furthermore, a high contact resistance would lead to a hotspot that can severely limit the lifetime of the battery cell involved. In case of severe corrosion, the temperature rise may even cause a fire or explosion. 
         [0005]    Accordingly, it is desirable for methods and systems for contact detection in battery packs. 
       SUMMARY OF THE INVENTION 
       [0006]    Method and apparatus for contact detection in battery packs are disclosed. 
         [0007]    In one embodiment, a battery pack is disclosed, comprising: at least a first battery cell and a second battery cell, the first battery cell having a first electrode and a second electrode, and the second battery cell having a third electrode and a fourth electrode; a power bar for coupling the second electrode of the first battery cell to the third electrode of the second battery cell; a communication wire for signal communication between the first battery cell and the second battery cell, wherein the first battery cell comprises a supervisor, which comprises a voltage detector to measure a first voltage between the first electrode and the second electrode of the first battery cell in a first state, and to measure a second voltage between the first electrode of the first battery cell and the communication wire in a second state; and a digital circuit to compute a voltage difference between the first voltage and the second voltage, and to indicate degraded contact of the power bar if the voltage difference is out of a predetermined threshold range. 
         [0008]    In one aspect, the supervisor further comprises a first switch, and the voltage detector has a first input terminal coupled to the first electrode of the first battery cell and a second input terminal coupled to the first switch, wherein in the first state, the first switch couples the second input terminal of the voltage detector to the second electrode of the first battery cell; and wherein in the second state, the first switch couples the second input terminal of the voltage detector to the communication wire. 
         [0009]    In one aspect, in the second state, the voltage detector measures the second voltage between the first electrode of the first battery cell and the communication wire when the second battery cell transmits on the communication wire at a voltage of the third electrode of the second battery cell. 
         [0010]    In one aspect, the second battery cell further comprises a second switch, wherein in the second state, the second switch couples the communication wire to the third electrode of the second battery cell. In one aspect, the first switch and the second switch are single-pole double-throw (SPDT) switches. In one aspect, the degraded contact comprises at least one of contact corrosion or loose-contact. 
         [0011]    In one aspect, the battery pack further comprises: a pack controller for controlling operation of the battery pack, wherein the at least the first battery cell and the second battery cell communicate with the pack controller through a daisy chain of communication wires, wherein the digital circuit is integrated in the supervisor of the first battery cell, or the digital circuit is integrated in the pack controller. 
         [0012]    In one aspect, the digital circuit comprises: an operational amplifier, an adder, or a subtractor to calculate the voltage difference between the first and second voltages; and a comparator to determine whether the voltage difference is in the predetermined threshold range. 
         [0013]    In one aspect, the first electrode and the third electrode are positive electrodes, and the second electrode and the fourth electrode are negative electrodes; or the first electrode and the third electrode are negative electrodes, and the second electrode and the fourth electrode are positive electrodes. In one aspect, the voltage detector comprises an Analog-to-Digital Converter (ADC). 
         [0014]    In another embodiment, a battery cell is provided, comprising: a first electrode and a second electrode; a power bar coupled to the second electrode of the battery cell for power transmission; a first communication wire for signal communication from/to the battery cell; a first switch; a voltage detector having a first input terminal coupled to the first electrode of the first battery cell and a second input terminal coupled to the first switch, wherein the first switch is configured to couple the second input terminal of the voltage detector to the second electrode of the battery cell in a first state, and to couple the second input terminal of the voltage detector to the first communication wire in a second state, such that the voltage detector measures a first voltage between the first electrode and the second electrode of the first battery cell in the first state, and measures a second voltage between the first electrode of the first battery cell and the communication wire in the second state; and a digital circuit to compute a voltage difference between the first voltage and the second voltage, and to indicate degraded contact of the power bar if the voltage difference is out of a predetermined threshold range. 
         [0015]    In one aspect, the battery cell further comprises: a second communication wire for signal communication from/to the battery cell; and a second switch to couple the second communication wire to the first electrode of the battery cell in the second state. In one aspect, the battery cell further comprises a transmitter/receiver coupled to the first communication wire, wherein the second switch couples the transmitter/receiver to the second communication wire in the first state. 
         [0016]    In one aspect, the first switch and the second switch are single-pole double-throw (SPDT) switches. In one aspect, the degraded contact comprises at least one of contact corrosion or loose-contact. 
         [0017]    In yet another embodiment, a method for monitoring a battery pack that comprises at least a first battery cell and a second battery cell is provided, the first battery cell having a first electrode and a second electrode, and the second battery cell having a third electrode and a fourth electrode, wherein the second electrode of the first battery cell is coupled to the third electrode of the second battery cell by a power bar, the method comprising: measuring a first voltage between the first electrode and the second electrode of the first battery cell in a first state; measuring a second voltage between the first electrode of the first battery cell and a communication wire in a second state, wherein the communication wire is configured for signal communication between the first battery cell and the second battery cell; computing a voltage difference between the first voltage and the second voltage; and indicating degraded contact of the power bar if the voltage difference is out of a predetermined threshold range. 
         [0018]    In one aspect, the method further comprises: measuring the second voltage between the first electrode of the first battery cell and the communication wire in the second state when the second battery cell transmits on the communication wire at a voltage of the third electrode of the second battery cell. 
         [0019]    In one aspect, the method further comprises: coupling the communication wire to the third electrode of the second battery cell in the second state; and measuring the second voltage between the first electrode of the first battery cell and the communication wire. 
         [0020]    In one aspect, the degraded contact comprises at least one of contact corrosion or loose-contact. In one aspect, the first electrode and the third electrode are positive electrodes, and the second electrode and the fourth electrode are negative electrodes; or the first electrode and the third electrode are negative electrodes, and the second electrode and the fourth electrode are positive electrodes. 
         [0021]    With this invention, the (onset of) degradation (e.g., corrosion or loosening) of a contact can be detected by the cell supervisor, without the need to add external components or wires to the battery pack. By measuring a first voltage between a first electrode and a second electrode of a battery cell in a first state, measuring a second voltage between the first electrode and a communication wire in a second state, and then monitoring the voltage difference between the first voltage and the second voltage, an increase of the contact resistance can be detected for each individual power bar in the battery pack. By reusing the communication wire to detect the second voltage, extra components or wires can be reduced. All required additional circuitry can be easily integrated into the cell supervisor. Such detection is beneficial to improve the performance of the battery pack, extend the battery lifetime between recharges, and avoid potential damage to the battery pack. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Further features and advantages will become apparent from the following and more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings, in which like referenced numerals generally refer to the same parts or elements throughout the drawings, and in which: 
           [0023]      FIG. 1  is a top view of an exemplary battery pack, according to an embodiment of the invention; 
           [0024]      FIG. 2  schematically depicts a block diagram of a battery pack with integrated supervisors, according to an embodiment of the invention; 
           [0025]      FIG. 3A  is a block diagram of a battery pack with integrated supervisors, according to an embodiment of the invention; 
           [0026]      FIG. 3B  is a block diagram of a battery pack with integrated supervisors, according to another embodiment of the invention; and 
           [0027]      FIG. 4  is a block diagram of a battery pack with integrated supervisors, according to yet another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The detailed description set forth below in connection with the accompany drawings is intended as a description of exemplary embodiments of the present invention and is not intended to represent the only exemplary embodiments in which the present invention can be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the exemplary embodiments of the specification. It will be apparent to those skilled in the art that the exemplary embodiments of the specification may be practiced without these specific details. In some instances, well known structures and devices are shown in block diagram form in order to avoid obscuring the exemplary embodiments presented herein. 
         [0029]    The term “example” or “exemplary” as used throughout this application is only by way of illustration, and not limitation. Further, it will be understood that when an element is referred to as being “connected to” or “coupled to” another element, it can be directly connected or coupled to the other element or intervening elements may be present. For reference numerals with letter character designations such as “ 202   a ” or “ 202   b ”, the letter character designations may differentiate two like parts or elements present in the same figure. Such letter character designations may be omitted when it is intended that a reference numeral to encompass all parts having the same reference numeral in all Figures. 
         [0030]      FIG. 1  is a top view of an exemplary battery pack  100  comprised of a plurality of battery cells  110 . For clarity, the battery pack  100  is illustrated with 8 battery cells  110 . However, it should be understood that, a battery pack according this invention can have more or less battery cells. The battery cells  110  are coupled in series by electrically conducting power bars  102 , each of which connects a positive electrode of one battery cell to a negative electrode of an adjacent battery cell. Such power bars  102  can be electrically connected to the cell electrodes by soldered or bolted contacts  104 , etc. 
         [0031]      FIG. 2  schematically depicts a block diagram of a battery pack  200  with integrated supervisors  212 , according to an embodiment of the invention. The battery pack  200  includes a plurality of battery cells  210  serially connected by power bars  202  (e.g.,  202   a ,  202   b ). Each battery cell  210  has a positive electrode A and a negative electrode B. The individual ends of the power bars  202  may be electrically coupled to the respective cell electrodes by soldered or bolted contacts, or the like. 
         [0032]      FIG. 2  also shows an optional conversion resistor, Rconv, connected in series with the battery cells  210 . An optional current monitor  220  can detect a voltage drop across the resistor Rconv, which effectively reflects the pack current flowing through the serially-connected battery cells  210  and thus may be used for purpose of supervision. The battery pack  200  may further include a pack controller  230  to control the operation of the battery pack  200  based on detected measurements and/or input commands. The battery cells  210  (as well as the current monitor  220 , if applicable) may communicate with the pack controller  230  through a daisy chain of communication wires  206 . In particular, a signal intended to be communicated between any battery cell  210  and the pack controller  230  may pass through other battery cells  210  (or other components, if any) located therebetween. The communication on the communication wire  206  can take place in the current domain or voltage domain. 
         [0033]    According to an embodiment of this invention, one or more of the battery cells  210  may have an integrated supervisor  212  for at least contact detection for the battery cell  210 . Such a supervisor  212  may be integrated within the battery cell  210 . Alternatively, the supervisor  212  can be built into the battery pack  200  and external to the associated battery cell  210 , preferably as close as possible to the battery cell being monitored. 
         [0034]    According to an embodiment of this invention, the battery cell  210  supplies voltages VDD and VSS for the associated supervisor  212 , where the DC level of the communication wire  206  can be set based on (e.g., equal to) the VDD of the transmitting supervisor  212 . During a normal condition, the power bar  202  between each pair of battery cells  210  has the same (or substantially the same) potential as the connected cell electrodes, and thus can act as the signal ground (VSS) for the corresponding communication wire  206 . For example, the potential on the communication line  206   a  is approximately equal to the potential on the electrode A of the battery cell  210 - 1 , or the potential on the electrode A of the battery cell  210 - 1  plus a cell voltage of the battery cell  210 - 2 , depending on which supervisor  212  is transmitting on the communication line  206   a.    
         [0035]    However, if a contact between the power bar  202  and a cell electrode is degraded (e.g., corroded, loose, or otherwise compromised), a contact resistance between the power bar  202  and the cell electrode will increase and cause an increased voltage drop. Accordingly, the potential downstream in the current path from the degraded contact will decrease as compared to that in a normal situation. For example, the potential on the communication line  206   a  will decrease when the battery cell  210 - 2  is transmitting on the communication line  206   a  to the battery cell  210 - 1 , as compared with normal condition. Therefore, it is possible for the supervisor  212  to perform contact supervision by monitoring potentials on the electrodes and the communication lines  206 . Moreover, the communication line  206   a  may be selectively connected to the electrode B of the battery cell  210 - 2 , so that the supervisor  212  in the battery cell  210 - 1  can measure a potential on the electrode B of the battery cell  210 - 2 . 
         [0036]    In particular, the supervisor  212  associated with the battery cell  210 - 1  is configured to measure a first voltage across the battery cell  210 - 1  (e.g., a voltage between the two electrodes A and B of the battery cell  210 - 1 ) and a second voltage between the electrode B of the battery cell  210 - 1  and the communication line  206   a . By monitoring a voltage difference between the first voltage and the second voltage, an increase of the contact resistance can be measured for power bar  202   a . For example, the supervisor  210  or the pack controller  230  may compute a voltage difference between the first voltage and the second voltage for a certain battery cell  210 , and compare the voltage difference to a threshold value (or a threshold range), so as to ascertain whether a contact of an associated power bar is compromised (e.g., corroded, loose, or the like), as further described in details below. 
         [0037]      FIG. 3A  is a block diagram of a battery pack with integrated supervisors  212 , according to an embodiment of this invention. By way of illustration, two battery cells  210 - 1  and  210 - 2  in the battery pack are shown, and they are connected to each other with a power bar  202   b  at nodes C and D. The battery cells  210 - 1  and  210 - 2  have supervisors  212 - 1  and  212 - 2  integrated therein, respectively. Note that the segment AC or BD represents the electrode of the respective battery cell  210 . Other battery cells in the battery pack can be configured similarly. 
         [0038]    The supervisor  212  each can include a digital circuit  326  for performing analysis on detected measurements or controlling operation of the battery cell  210  based on the measurements and/or commands from the pack controller  230  (see  FIG. 2 ). A transmitter/receiver (TRx)  328  may also be provided for signal communication via the communication wires  206 . For example, the TRx  328  may receive information from the digital circuit  326  and communicate it to other battery cells  210  and/or to the pack controller  230  via the communication wires  206 . Also, the TRx  328  may receive a signal from other battery cells  210  and/or the pack controller  230  via the communication wires  206  and communicate it to the digital circuit  326 . 
         [0039]    Optionally, the supervisor  212  may further include a temperature sensor  324  for continuously monitoring the temperature within the battery cell  210 , a pressure sensor (not shown), as well as other sensors for measuring other parameters of the battery cell  210 . The digital circuit  326  may receive and analyze the various measurements to determine the state of health of the battery cell over time, and to control the operation of the battery cell  210  based on the measurements or commands from the pack controller  230 . 
         [0040]    According to an embodiment of this invention, each of the supervisors  212  may include a voltage detector  312 , for measuring a voltage difference between two input terminals of the voltage detector  312 . For better understanding, the following description may be directed to a certain battery cell (e.g., battery cell  210 - 1 ), but can be applied to other similarly configured battery cells. As shown in  FIG. 3A , a first input terminal of the voltage detector  312 - 1  may be coupled to the electrode A of the battery cell  210 - 1 , and a second input terminal of the voltage detector  312 - 1  may be coupled to a first switch S 1 . In a first state, the first switch S 1  may couple the second input terminal of the voltage detector  312 - 1  to the electrode B of the battery cell  210 - 1 , such that the voltage detector  312 - 1  measures a first voltage between the electrodes A and B of the battery cell  210 - 1 . Typically, the first voltage represents a cell voltage of the battery cell  210 - 1 . 
         [0041]    In a second state, the first switch S 1  may couple the second input terminal of the voltage detector  312 - 1  to the communication wire  206 - b , such that the voltage detector  312 - 1  measures a second voltage between the electrode A of the battery cell  210 - 1  and the communication wire  206   b . In an embodiment, the first switch S 1  may be a single-pole double-throw (SPDT) switch, so as to selectively connect the second input terminal of the voltage detector  312  to the electrode B or the communication wire  206 . 
         [0042]    The communication on the communication wire  206   b  can take place in the current domain or voltage domain. If the communication is in the current domain (the transmitting potential, e.g., VDD, remains unchanged), the receiving side acts as virtual ground for the associated voltage detector  312 - 1 , which is capable of performing a DC measurement between the electrode A of the battery cell  210 - 1  and the communication line  206   b  directly. On the other hand, if the communication is in the voltage domain, the voltage measurement may take place when the TRx  328 - 2  is transmitting on the communication line  206   b  at a potential of the electrode A of the battery cell  210 - 2 . In particular, when the TRx  328 - 2  transmits at a voltage of the electrode A of the battery cell  210 - 2  (e.g., supply potential VDD), the potential on the communication wire  206   b  is the same as the electrode A of the battery cell  210 - 2 . Consequently, the second voltage between the electrode A of the battery cell  210 - 1  and the communication wire  206   b  reflects a voltage between the electrode A of the battery cell  210 - 1  and the electrode A of the battery cell  210 - 2 . 
         [0043]    In addition, filtering or timing may be employed to separate a DC voltage from the communication signal on the communication line  206 . Even no information needs to be communicated on the communication line  206   b , the digital circuit  326 - 2  may instruct the Trx  328 - 2  to transmit dummy signal on the communication line  206   b  based on scheduled timing, for purpose of voltage measurement. As each supervisor  212  (in particular, the digital circuit  326 ) knows what communication is ongoing on the corresponding communication line  206 , the digital circuit  326  can control the voltage detector  312  to perform voltage measurement at appropriate times. 
         [0044]    The voltage detector  312  may pass the measured first and second voltages to the digital circuit  326 , which may calculate a voltage difference between the first and second voltages. Alternatively, the digital circuit  326  may pass the measured first and second voltages to the pack controller  230  (e.g., via the TRx  328  and the communication wires  206 ), which may calculate a voltage difference between the first and second voltages. Such voltage difference between the first voltage and the second voltage represents a voltage drop on the power bar  202 , and may reflect a contact condition between the power bar  202  and the cell electrodes. Consequently, the digital circuit  326  or the pack controller  230  may compare the computed voltage difference to a predetermined threshold (or threshold range), so as to ascertain whether a contact of an associated power bar is compromised (e.g., corroded, loose, or the like). The supervisor  210  or the pack controller  230  may indicate degraded contact of the power bar  202   b  if the voltage difference is out of the predetermined threshold range (e.g., the voltage difference is higher than a threshold). 
         [0045]    Similarly, the voltage detector  312 - 2  in the battery cell  210 - 2  may monitor the contact condition of the power bar  202   a  by measuring a first voltage between the electrodes A and B of the battery cell  210 - 2 , measuring a second voltage between the electrode A of the battery cell  210 - 2  and the communication wire  206   a  when the TRx  328 - 2  receives signal communication on the communication wire  206   a  from a next battery cell, and determining whether a voltage difference between the first and second voltages is out of the predetermined threshold range. In the same way, it is possible to monitor all the contacts between the power bars  202  and the associated cell electrodes in the battery pack. 
         [0046]    As shown by dashed lines in  FIG. 3A , the first switch S 1  can be controlled by the digital circuit  326 . Consequently, all the first switches S 1  in the battery cells  210  can be controlled synchronously/individually, so that the voltage detector  312  in each battery cell  210  measures either a first voltage between the electrodes A and B of the battery cell  210 , or a second voltage between the electrode A of the battery cell  210  and the corresponding communication wires  206  when the battery cell  210  receives signal communication on the communication wire  206 . For example, in a first state, each first switch S 1  may couple the second input terminal of the voltage detector  312  to the electrode B of the battery cell  210 , such that the voltage detector  312  measures a cell voltage of the battery cell  210 . In a second state, the first switch S 1  may couple the second input terminal of the voltage detector  312  to the communication wire  206 , such that the voltage detector  312  measures a second voltage between the electrode A of the battery cell  210  and the communication wire  206  when the battery cell  210  receives signal communication on the communication wire  206 . Consequently, the pack controller  230  or the digital circuit  326  may control the first switch S 1  to periodically, intermittently, or randomly measure the second voltage between the electrode A of the battery cell  210  and the communication wire  206 . 
         [0047]    Although  FIG. 3A  shows that the voltage detector  312  is always coupled to the positive electrode A of the battery cell  210 , the polarity of each battery cell  210  may be reversed, as shown in  FIG. 3B . In a first state, the first switch S 1  may couple the second input terminal of the voltage detector  312 - 1  to the electrode B of the battery cell  210 - 1 , such that the voltage detector  312 - 1  measures a first voltage between the electrodes A and B of the battery cell  210 - 1 . In a second state, the first switch S 1  may couple the second input terminal of the voltage detector  312 - 1  to the communication wire  206   b , such that the voltage detector  312 - 1  measures a second voltage between the electrode A of the battery cell  210 - 1  and the communication wire  206   b , when the TRx  328 - 2  transmits at a voltage of the electrode A of the battery cell  210 - 2  (e.g., ground potential VSS). In particular, when the TRx  328 - 2  transmits at a voltage of the electrode A of the battery cell  210 - 2  (e.g., ground potential VSS), the potential on the communication wire  206   b  is the same as the electrode A of the next battery cell  210 - 2 . Accordingly, the supervisor  210  or the pack controller  230  may indicate degraded contact of the power bar  202   b  if the voltage difference between the first and second voltages is out of the predetermined threshold range (e.g., the voltage difference is higher than a threshold). 
         [0048]      FIG. 4  is a block diagram of a battery pack with integrated supervisors  212 , according to yet another embodiment of this invention.  FIG. 4  is similar to  FIG. 3A , except that the supervisor  212  further includes a second switch S 2  coupled to the communication wire  206 . In a first state, the first switch S 1  may couple the second input terminal of the voltage detector  312 - 1  to the electrode B of the battery cell  210 - 1 , such that the voltage detector  312 - 1  measures a first voltage between the electrodes A and B of the battery cell  210 - 1 . In the first state, the second switch S 2  (and/or S 2 ′ in battery cell  210 - 2 ) may be open, or may couple the corresponding communication wire  206  to the TRx  328  for signal communication. 
         [0049]    In a second state, the first switch S 1  may couple the second input terminal of the voltage detector  312 - 1  to the communication wire  206   b . Meanwhile, the second switch S 2 ′ couples the communication wire  206   b  to the electrode A of the battery cell  210 - 2 , such that the voltage detector  312 - 1  measures a second voltage between the electrode A of the battery cell  210 - 1  and the electrode A of the next battery cell  210 - 2 . In an embodiment, the second switch S 2  may be a single-pole double-throw (SPDT) switch, so as to selectively connect the communication wire  206  to the electrode A or the TRx  328 . As described above, the digital circuit  326  or the pack controller  230  may compute a voltage difference between the first and second measured voltages, and compare the computed voltage difference to a predetermined threshold (or threshold range), so as to ascertain whether a contact of an associated power bar  202   b  is compromised (e.g., corroded, loose, or the like). The supervisor  210  or the pack controller  230  may indicate degraded contact of the power bar  202  if the voltage difference is out of the predetermined threshold range (e.g., the voltage difference is higher than a threshold). 
         [0050]    As shown by dashed lines in  FIG. 4 , the first switch S 1  and second switch S 2  can be controlled by the digital circuit  326 . Consequently, the first switch S 1  and second switch S 2  in one battery cell  210  can be controlled synchronously, so that the voltage detector  312  measures either a first voltage between the electrodes A and B of the battery cell  210 , or a second voltage between the electrode A of the battery cell  210  and the electrode A of the next battery cell  210 . For example, in a first state, the first switch S 1  may couple the second input terminal of the voltage detector  312  to the electrode B of the battery cell  210 , and the voltage detector  312  measures a cell voltage of the battery cell  210 . Also, each second switch S 2  may couple the communication wire  206  to the TRx  328 , which may communicate between the battery cells  210  via the communication wires  206 . In a second state, the first switch S 1  may couple the second input terminal of the voltage detector  312  to the communication wire  206 , and a second switch S 2  couples the communication wire  206  to the electrode A of the battery cell  210 , such that the voltage detector  312  measures a second voltage between the electrode A of the battery cell  210  and the electrode A of the next battery cell  210 . In this situation, the TRx  328  suspends the communication between the battery cells  210 . Consequently, the pack controller  230  or the digital circuit  326  may control the first switch S 1  and second switch S 2  to periodically, intermittently, or randomly measure the second voltage between the electrode A of the battery cell  210  and the electrode A of the next battery cell  210 . 
         [0051]    With the supervisor  212  being located within the battery cell  210 , the battery cell  210  supplies voltages VDD and VSS for the supervisor  212  from the respective electrodes A and B. In a normal condition, the digital circuit  326 - 1  will detect a zero (or small) voltage difference between the first and second voltages. Otherwise, if a contact C (or D) between the power bar  202   b  and the associated cell electrode is degraded and thus has an increased resistance, there will be an increased voltage drop on the degraded contact, causing the potential on the electrode A of the battery cell  210 - 2  to decrease. Consequently, the digital circuit  326 - 1  will detect a voltage difference change between the first and second voltages. If the detected voltage difference (e.g., the absolute value thereof) is out of a predetermined threshold range (e.g., exceeding a predetermined threshold), the digital circuit  326 - 1  may assert a signal to indicate degraded contact of the power bar  202   b . For example, digital circuit  326 - 1  may send an alert signal to the TRx  328 - 1 , which in turn transmits the alert signal to the pack controller  230 . 
         [0052]    Similarly, the voltage detector  312 - 2  in the battery cell  210 - 2  may monitor the contact condition of the power bar  202   a . In the same way, it is possible to monitor all the contacts between the power bars  202  and the associated cell electrodes in the battery pack. Although  FIG. 4  shows that the voltage detector  312  is always coupled to the negative electrode A of the battery cell  210 , the polarity of each battery cell  210  may be reversed. The operations are similar to that as described above with respect to  FIG. 4 , and will not be repeated here. 
         [0053]    Referring back to  FIGS. 3A, 3B and 4 , in an embodiment, the voltage detector  312  may be implemented with an Analog-to-Digital Converter (ADC) to sense the voltage (drop) between the two input terminals. The ADC may convert the measured voltage to a digital value and send it to the digital circuit  326  for processing. An advantage of digitizing is that the measured “contact” voltage can be divided by the pack current, so the contact resistance can be measured to a desired accuracy. In some applications, the ADC  312  may be time-multiplexed to alternately convert the first voltage or the second voltage. In a further embodiment, the digital circuit  326  may comprise an operational amplifier, an adder, a subtractor or a software routine to calculate a voltage difference between the first and second voltages. Moreover, the digital circuit  326  may comprise a comparator or a software routine to determine whether the voltage difference is within a predetermined threshold range (e.g., to compare the voltage difference with a predetermined threshold). In optional embodiments, the digital circuit  326  may be embodied in the supervisor of the battery cell  210 , or may be embodied in the pack controller  230 . 
         [0054]    The threshold range (or threshold value) may be programmable or configurable, to optimize the system for different applications or usage scenarios. Such threshold range (or threshold value) may be determined based on the normal range (or normal value) and an appropriate margin. By example and without limitation, suppose a normal contact has a typical resistance of 0.1 mOhm and the pack current has a peak value of 200 A, resulting a peak contact voltage of 20 mV which is in the “normal” range. An appropriate threshold value for the comparator might be set as 40 mV, which would allow a contact resistance that has been doubled. The margins depend on the specific application and the overall constraints imposed on the battery pack. 
         [0055]    The information detected by the voltage detector  312 , among other measurements monitored by the supervisor  212 , may be transmitted to the pack controller  230  through TRx  328  and the daisy chain of communication wires  206 . As an option, either an OK/NOK signal indicating that the voltage across a power bar contact is below/above a threshold may be transmitted to the pack controller  230 . Alternatively, the supervisor  212  may send the measured voltages to the pack controller  230 , which then makes a determination as to whether the power bar contact is operating normally. In addition, the transmitted signal may include an identifier corresponding to the contact(s) being detected, such that the degraded contact(s) can be located accurately. 
         [0056]    Any appropriate countermeasures may be taken if a degraded contact (e.g., due to corrosion, loose contact, etc.) is detected. For example, the pack controller  230  may warn the user (e.g., a car driver) that the battery pack should be checked or repaired. In addition or alternatively, the pack controller  230  may automatically cut off the battery pack upon detection of dramatic increase in the contact resistance either during charging or discharging, to protect the battery cells/pack from damage. Because the pack controller  230  can identify where the degraded contact is located, a user or technician can make correction very quickly and precisely. For example, a loose bolt may be fastened or a corroded connector may be replaced, before the battery degrades catastrophically. 
         [0057]    The embodiments described above provide a variety of advantages. The degradation (such as corrosion or loosening) of a contact can be detected by the cell supervisor. By measuring a first voltage between a first electrode and a second electrode of a battery cell in a first state, measuring a second voltage between the first electrode and a communication wire in a second state, and then monitoring the voltage difference between the first voltage and the second voltage, an increase of the contact resistance can be detected for each individual power bar in the battery pack. By reusing the communication wire to detect the second voltage, extra components or wires can be reduced. All required additional circuitry can be easily integrated into the cell supervisor. Such detection is beneficial to improve the performance of the battery pack, extend the battery lifetime between recharges, and avoid potential damage to the battery pack. In addition, the contact detection can be combined with other measurements, such as voltage, temperature, pressure, etc., without the need to add external components or wiring. 
         [0058]    Aspects of the present invention are believed to be applicable to a variety of different types of devices, systems and arrangements involving batteries and/or battery control, including those involving automotive applications. While the present invention is not necessarily so limited, various aspects of the invention may be appreciated through a discussion of examples using this context. 
         [0059]    Although the various exemplary embodiments have been described in detail with particular reference to certain exemplary aspects thereof, it should be understood that the invention is capable of other embodiments and its details are capable of modifications in various obvious respects. As is readily apparent to those skilled in the art, variations and modifications can be affected while remaining within the spirit and scope of the invention. Accordingly, the foregoing disclosure, description, and figures are for illustrative purposes only and do not in any way limit the invention, which is defined only by the claims.