Abstract:
A battery pack detection circuit that can detect cold or false welding, charging status, and discharging status is disclosed. The battery pack detection circuit comprises a driving circuit electrically connected to a switch unit outside the battery pack detection circuit; a voltage detection and comparison circuit electrically connected to a multi-cell battery pack having a plurality of battery cells outside the battery pack detection circuit, wherein the voltage detection and comparison circuit is configured to detect cell voltages across each of the battery cells under at least two circuit connection conditions and compare the differences in cell voltage with a predetermined value, wherein the differences in cell voltage are derived from a subtraction operation performed on the cell voltages measured under the at least two circuit connection conditions; and an interface and control unit configured to receive commands from a SMBUS and provide a signal to a detection load.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims Priority from U.S. Provisional Application No. 61/457,483 filed on Apr. 8, 2011, which is incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    Embodiments of the present invention relate to battery pack detection circuits, and more particularly, to circuits for detecting cold or false welding, charging status, and discharging status in a multi-cell battery pack including a plurality of serial-connected battery cells. 
         [0004]    2. Background 
         [0005]    Rechargeable batteries are widely used in many applications to supply power for a variety of electronic devices. For example, portable devices such as cell phones, personal digital assistants (PDAs), laptops, and power tools use rechargeable batteries as a power source. The rechargeable batteries may be nickel-cadmium (NiCd), nickel-metal hydride (NiMH), or lithium ion (LiIon). 
         [0006]    Large capacity rechargeable batteries generally comprise multiple cells connected in parallel to deliver the required current and in series to deliver the required voltage. The multiple cells are ordinarily packaged as a battery pack. In order to form the battery pack, multiple connections between positive and negative electrodes of the battery cells are required.  FIG. 1  is a cross-sectional view showing the connections between the positive and negative electrodes of the battery cells. Referring to  FIG. 1 , multiple battery cells  12  are arranged in a row in an electrically insulating case (not shown). Each battery cell  12  has a positive electrode  14  on the top and a negative electrode  16  on the bottom. A connecting metallic plate  18  made of copper is disposed on both the positive electrode  14  of a battery cell  12  and the negative electrode  16  of another battery cell  12  to form a lap joint. Two welds  19  are formed on the top surface of each connecting metallic plate  18  on the positive electrode  14  side of one battery cell and two welds  19  are formed on the metallic plate  18  on the negative electrode  16  side of the other battery cell. 
         [0007]    With this arrangement, there is a possibility of a cold welding condition or a false welding condition occurring at the welds  19 , which can result in opening of the current loop in the battery pack. The cold welding condition and the false welding condition are difficult to detect after the battery pack is assembled. In addition, a weld of good quality may be damaged by an electric shock during shipping or maintenance. Therefore, there is a need to provide a circuit to solve the above-mentioned problems. 
       SUMMARY 
       [0008]    A battery pack detection circuit comprises a driving circuit electrically connected to a switch unit outside the battery pack detection circuit and configured to enable or disable a switch in the switch unit; a voltage detection and comparison circuit electrically connected to a multi-cell battery pack having a plurality of battery cells outside the battery pack detection circuit and configured to detect cell voltages across each of the battery cells under at least two circuit connection conditions and configured to compare differences in cell voltage with a predetermined value, wherein the differences in cell voltage are derived from a subtraction operation performed on the cell voltages measured under the at least two circuit connection conditions; and an interface and control unit connecting a system management bus (SMBUS) outside the battery pack detection circuit and a detection load outside the battery pack detection circuit, wherein the interface and control unit is configured to receive commands from the SMBUS and provide a signal to the detection load. 
         [0009]    In regards to the operation of the battery pack detection circuit, the method comprises the steps of: detecting and recording cell voltages across each battery cell in the multi-cell battery pack as V 1 A to VkA when the switch unit and the switch in the detection load are disabled; enabling the switch in the detection load; detecting and recording cell voltages across each battery cell in the multi-cell battery pack as V 1 B to VkB when a switch in the switch unit is disabled and at least one switch in the detection load is enabled; performing a subtraction operation to obtain voltage values V 1 C to VkC, wherein V 1 C=V 1 A−V 1 B, and VkC=VkA−VkB; comparing each of the voltage values to a predetermined value; and setting a fault flag if one of the voltage values is greater than the predetermined value. 
         [0010]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The objectives and advantages of the present invention are illustrated with the following description and upon reference to the accompanying drawings in which: 
           [0012]      FIG. 1  is a cross-sectional view showing the connections between the positive and negative electrodes of the battery cells; 
           [0013]      FIG. 2  is a block diagram of a battery system in accordance with an embodiment of the present disclosure; 
           [0014]      FIG. 3  is a block diagram of a battery system in accordance with another embodiment of the present disclosure; 
           [0015]      FIG. 4  is a block diagram of a battery system in accordance with another embodiment of the present disclosure; and 
           [0016]      FIG. 5  is a block diagram of a battery system in accordance with yet another embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The disclosure relates to a battery pack detection circuit for a multi-cell battery pack.  FIG. 2  is a block diagram of a battery system  20  in accordance with an exemplary embodiment. The battery system  20  includes a multi-cell battery pack  22 , a battery pack detection circuit  24 , a detection load  26 , and a switch unit  28 . 
         [0018]    Referring to  FIG. 2 , a plurality of rechargeable battery cells  222 ,  224 , and  226  are connected in series to form the multi-cell battery pack  22 . A positive terminal and a negative terminal on the battery pack  22  are coupled to battery terminals  212  and  214  that are configured to provide current to operate a system load (not shown), such as a portable electronic device, and are configured to receive charge current from a power source (not shown), such as an AC/DC adapter. 
         [0019]    The switch unit  28  of the battery system  20  includes two field effect transistors (FETs) M C  and M D  coupled in series between the battery pack  22  and the battery terminal  212 . 
         [0020]    The detection load  26  is connected in parallel with the battery pack  22 . In one embodiment of the present disclosure, the detection load  26  comprises a resistor R 1  and a switch S 1  that are connected in series. When receiving an input signal EN, the switch S 1  turns on and a current I 1  is drawn from the battery pack  22 . 
         [0021]    Referring to  FIG. 2 , the battery pack detection circuit  24  comprises a driving circuit  242 , a voltage detection/comparison circuit  244 , and an interface and control unit  246 . The driving circuit  242  is configured to provide driving signals and disable signals to the switch unit  28 . The voltage detection/comparison circuit  244  is configured to detect the cell voltage across each of the battery cells  222  to  226  and then compare them to a reference voltage. The interface and control unit  246  is configured to receive commands from the SMBUS  29  and provide an enable signal EN to the detection load  26 . 
         [0022]    In operation, the voltage detection/comparison circuit  244  detects and records the cell voltage across each of the battery cells  222  to  226  as V 1 A, V 2 A, and V 3 A when the switch unit  28  and the switch S 1  are disabled. Subsequently, the switch unit  28  remains off and the switch S 1  turns on after receiving the enable signal EN from the interface and control unit  246 . Next, the voltage detection/comparison circuit  244  detects and records the cell voltage across each of the battery cells  222  to  226  as V 1 B, V 2 B, and V 3 B. 
         [0023]    The voltage detection/comparison circuit  244  performs a subtraction operation to obtain voltages V 1 C, V 2 C, and V 3 C, wherein voltages V 1 C=V 1 A-V 1 B, V 2 C=V 2 A-V 2 B, and V 3 C=V 3 A-V 3 B. After obtaining the voltages V 1 C, V 2 C, and V 3 C, the voltage detection/comparison circuit  244  compares each of them to a reference voltage VR to obtain a comparison value. 
         [0024]    In one embodiment of the present disclosure, a cold welding condition occurs at the connection on one of the cells  222 ,  224 , and  226 . Therefore the voltages V 1 C, V 2 C, and V 3 C are greater than the reference voltage VR due to a high internal impedance of the battery pack  22 . The voltage detection/comparison circuit  244  provides the comparison result to the driving circuit  242 . Based on the comparison result, a fault flag can be set and the switch  28  remains disabled. The voltage detection/comparison circuit  244  also provides the comparison result to a host (not shown) over the SMBUS  29 . 
         [0025]    Referring to  FIG. 3 , a current sense resistor R 2  is added between the negative terminal of the battery pack  22  and ground. The battery pack detection circuit  24 ′ further comprises a current detection circuit  248  to monitor the charging current to the battery pack  22  by means of the current sense resistor R 2 . Because the charging current varies over time, the voltage detection/comparison circuit  244  can compare the voltages V 1 C, V 2 C, and V 3 C with a threshold value in proportion to the charging current value to obtain a more precise comparison result. 
         [0026]    When the battery pack  22  is used for long periods of time, the internal impedance of each of the battery cells  222  to  226  may vary. In one embodiment of the present disclosure, a storage element (not shown) is added to the battery pack detection circuit  24  to record the cell voltage across each of the battery cells  222  to  226  at different times. Therefore, if the cell voltage across one of the battery cells  222  to  226  exceeds a high threshold value, an alarm signal can be issued to the host over the SMBUS  29 . In one embodiment of the present disclosure, a fault flag can be set when the cell voltage across one of the battery cells  222  to  226  exceeds the high threshold value. 
         [0027]      FIG. 4  is a block diagram of a battery system  40  in accordance with another embodiment of the present disclosure. Referring to  FIG. 4 , in which like elements of  FIG. 2  are shown having like reference designations, the battery system  40  includes a multi-cell battery pack  22 ′, a battery pack detection circuit  42 , a switch element  44 , and a fuse element  46 . 
         [0028]    Referring to  FIG. 4 , a positive terminal and a negative terminal on the battery pack  22 ′ are coupled to battery terminals  212 ′ and  214 ′ that are configured to receive charging current from a power source (not shown), such as an AC/DC adapter. Because overcharging the battery pack  22 ′ can lead to explosion, flame or other hazardous situations, the battery pack detection circuit  42  is designed to prevent over-voltage charging of the battery pack  22 ′. The battery pack detection circuit  42  is configured to detect the cell voltage across each of the battery cells  222 ′ to  226 ′. If one of the detected cell voltages exceeds a predetermined threshold for a predetermined period of time, the battery pack detection circuit  42  will output a signal OVP as an over-voltage protection signal to the switch element  44 . After receiving the signal OVP, the switch element  44  turns on and the fuse element  46  is opened, cutting off the charging current from the power source at the terminal  212 ′. 
         [0029]    However, if one of the wires W 1 , W 2 , W 3 , and W 4  between the battery pack  22 ′ and the battery pack detection circuit  42  fails to be connected, the battery pack detection circuit  42  cannot execute the OVP check function since the detected cell voltages are lower than the predetermined threshold in this condition. In such case, the battery pack  22 ′ is at risk of damage or explosion due to the excessively high voltage. To solve this problem, the battery pack detection circuit  42  is provided. 
         [0030]    Referring to  FIG. 4 , the battery pack detection circuit  42  comprises a control circuit  422  and a detection/comparison unit  424 . The control circuit  422  is configured to control the status of the switch element  44 . The detection/comparison unit  424  is configured to detect the voltage at the positive terminal of the battery pack  22 ′ and the cell voltage across each of the battery cells  222 ′ to  226 ′. 
         [0031]    In operation, when the detection/comparison unit  424  detects that the positive terminal of the battery pack  22 ′ and the battery pack detection circuit  42  are connected, the battery system  40  enters a detection mode. While in the detection mode, the battery pack detection circuit  42  performs a power on reset (POR) function and the comparison voltage of the detection/comparison unit  424  is reduced to a level that is below the lower limit voltage of the battery cell, such as 3V. The duration of time for which the battery system  40  is in the detection mode is fixed and constant. When the detection mode duration has passed, the battery system  40  changes modes from the detection mode to a normal mode, and the switch element  44  remains disabled if all wires W 1 , W 2 , W 3 , and W 4  between the battery pack  22 ′ and the battery pack detection circuit  42  are connected. 
         [0032]    However, if the detection/comparison unit  424  detects that the cell voltage across any one of the battery cells  222 ′ to  226 ′ is below the lower limit of the battery cell in the detection mode, the detection/comparison unit  424  will output a signal to the control circuit  422 . After the battery system  40  enters the normal mode, the control circuit  422  outputs the signal OVP to the switch element  44 , so the fuse element  46  is opened. In this manner, an open connection between the battery pack  22 ′ and the battery pack detection circuit  42  can be detected. 
         [0033]      FIG. 5  is a block diagram of a battery system  50  in accordance with yet another embodiment of the present disclosure. Referring to  FIG. 5 , a latch circuit  426  is added between the detection/comparison unit  424 ′ and the control circuit  422 ′. The fuse element  46  is replaced with a switch element  52 , such as an FET in this example. In one embodiment of the present disclosure, the latch circuit  426  is used to latch a signal from the detection/comparison unit  424 ′ when the detection/comparison unit  424 ′ detects that the cell voltage across any one of the battery cells  222 ′ to  226 ′ is below the lower limit of the battery cell in the detection mode. Therefore, the signal OVP remains at HIGH after the above condition occurs, and the switch element  52  remains disabled in the detection mode and the normal mode. 
         [0034]    In another embodiment of the present disclosure, the latch circuit  426  is used to latch a signal from the detection/comparison unit  424 ′ when the detection/comparison unit  424 ′ detects that the cell voltage across any one of the battery cells  222 ′ to  226 ′ exceeds an upper limit of the battery cell (i.e., 4.4V) in the normal mode. In this condition, the signal OVP is issued, and the switch element  52  is switched off to prevent the battery pack  22 ′ from overcharging. 
         [0035]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, many of the processes discussed above can be implemented in different methodologies and replaced by other processes, or a combination thereof. 
         [0036]    Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.