Abstract:
The present invention discloses a battery charging controller for achieving a balanced battery charge. The battery charging controller includes a voltage divider, a switch module and a balance circuit. A reference voltage generated by the voltage divide is used to determine which battery unit in a battery module has an insufficient voltage lower than the others, so that the balance circuit controls the switch module to allow a larger current to charge a lower-voltage battery than a higher-voltage battery, so as to result in substantially the same voltage for each fully charged battery of the battery module.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a battery charging controller and a battery module using the battery charging controller, and more particularly to a battery charging controller for balancing a battery charge and a battery module using the battery charging controller. 
       BACKGROUND OF THE INVENTION 
       [0002]    As portable electronic products are developed rapidly, the demand for rechargeable batteries becomes increasingly larger. Rechargeable batteries include the conventional Ni—Ca battery, Ni—H battery, and Li-ion battery as well as the recently developed Li-polymer battery. The voltage provided by different type of rechargeable battery varies, and the required operating voltages of the portable electronic products are different. Therefore, battery manufacturers usually connect several batteries in series to produce a battery module for providing a voltage capable of meeting the operating voltage requirement of the portable electronic products. 
         [0003]    When the electric power of batteries of a battery module is exhausted, it is necessary to recharge the battery by a battery charger for the next time of use. However, the battery capacity is different due to mismatch in manufacture and application. For example, a 7.4V lithium battery module is formed by connecting two pieces of 3.7V lithium batteries in series. For factory default setting, the electric power storage capacities of the two pieces of batteries are 80% and 70% respectively. Since the lithium batteries will be damaged if they are overcharged, therefore the lithium battery charger will stop charging the battery module as soon as any one of the lithium batteries therein is fully charged. Now, the electric power storage capacities of the two batteries are 100% (maximum level of a battery charge) and 90% respectively. The battery module can be used until the electric power storage capacity of any one of the batteries therein drops to 0% (minimum level of a battery discharge). Therefore, it is necessary recharge the battery module for a further use when the electric power storage capacities of the two batteries drop to 10% and 0% respectively. 
         [0004]    From the description of the aforementioned example, the electric power storage capacities of batteries in a battery module are different, and thus the actual use of electric power storage capacity of the battery module is determined by the battery with the lowest electric power storage capacity. In addition to the variation of factory default electric power storage capacity of each battery of a battery module, a battery also self-discharges electric power, when the battery is not in use. Since each battery self-discharges electric power at a different rate, therefore an unbalanced electric power storage capacity will result among the batteries, and the usable capacity of the battery module is reduced as the using time of the battery increases, and the efficiency of using battery module becomes lower, and the using time of the battery module from fully charged to empty becomes shorter. 
         [0005]    With reference to  FIG. 1  for a schematic circuit diagram of a digital battery balancing controller disclosed in a datasheet of the Intersil ISL9208 product, a digital battery balancing controller  10  comprises a battery balance microprocessor  5  and a plurality of transistor switches S 1 -S 7 . The transistor switches S 1 -S 7  are connected with a plurality of batteries BAT 1 -BAT 7  in parallel through a plurality of resistors R 1 -R 7  respectively. The voltage of the batteries BAT 1 -BAT 7  is converted into a digital signal through an analog-to-digital converter (A/D converter), and the battery balancing controller  5  determines a higher-voltage battery by a built-in algorithm according to the digital signal of the voltage of the batteries BAT 1 -BAT 7  and conducts the transistor switch connected in parallel to the higher-voltage battery, such that the charging current of each battery can be adjusted according to the voltage of each battery to achieve a balanced charging function. However, the voltage of each battery must be converted into a digital signal by an analog-to-digital converter before the digital battery balancing controller  5  processes, and the analog-to-digital converter increases the chip area  10  of the digital battery balancing controller  10  significantly and incurs a high cost. In addition, the digital battery balancing controller  5  is restricted by its design, such as the ISL9208 chip can support a battery module composed of 5 to 7 pieces of batteries only, and the scope of applicability is limited. 
       SUMMARY OF THE INVENTION 
       [0006]    In view of the high cost of the conventional digital battery balancing controller, the present invention adopts an analog battery charging controller to achieve a balanced charge of batteries. Since the determining circuit of the analog controller comes with an area smaller than that of the analog-to-digital converter, the cost is lower, and the controller can be applied to a battery module composed of any number of batteries to give a broad scope of applicability. 
         [0007]    Therefore, the primary objective of the present invention is to achieve the foregoing advantages by providing a battery balanced charging controller, comprising a voltage divider, a switch module and a balance circuit for balancing the charge of a first battery and a second battery. A negative terminal of the first battery is electrically coupled to a positive terminal of the second battery to from a connecting point, wherein a positive terminal of the first battery forms a first terminal and a negative terminal of the second battery forms a second terminal respectively. The voltage divider is coupled between the first terminal and the second terminal for providing an upper reference potential level and a lower reference potential level, and the potential level of the upper reference is higher than the lower reference potential level. The switch module comprises a first switch and a second switch, and the first switch is coupled to the first terminal and the connecting point, and the second switch is coupled to the second terminal and the connecting point. The balance circuit is coupled to the connecting point, the voltage divider and the switch module, and the balance circuit determines whether to pass a balanced charging current through the first switch or the second switch according to the potential level of the connecting point, the upper reference potential level and the lower reference potential level. 
         [0008]    The present invention also provides a battery module, comprising (N+1) battery units and N battery balanced charging controllers, wherein N is an integer greater than zero. The (N+1) battery units are electrically connected in series, wherein the M th  battery balanced charging controller is coupled to a first terminal, a second terminal and a connecting point of both serially connected M th  battery unit and (M+1) th  battery units, and the M th  battery balanced charging controller determines the potential level of the M th  battery unit and the (M+1) th  battery units according to the first terminal, the second terminal and the connecting point. If the potential difference between the M th  battery unit and the (M+1) th  battery units is greater than a predetermined percentage, a balanced charging current is conducted to the battery unit with the lower potential level. 
         [0009]    The above and other objects, features and advantages of the present invention will become apparent from the following detailed description taken with the accompanying drawing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic circuit diagram of a conventional digital battery balancing controller; 
           [0011]      FIG. 2  is a schematic circuit diagram of a battery balanced charging controller in accordance with a preferred embodiment of the present invention; and 
           [0012]      FIG. 3  is a schematic circuit diagram of a built-in battery module of a battery balanced charging controller in accordance with the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0013]    With reference to  FIG. 2  for a schematic circuit diagram of a battery balanced charging controller in accordance with a preferred embodiment of the present invention, the battery balanced charging controller  100  comprises a voltage divider, a switch module and a balance circuit. The voltage divider comprises a first resistor R 1 , a second resistor R 2  and a third resistor R 3 , and the first resistor R 1  is coupled to a positive terminal of a first battery BAT 1 , and the third resistor R 3  is coupled to a negative terminal of a second battery BAT 2 . The second resistor R 2  is coupled to the first resistor R 1  for producing an upper reference potential level V+, and coupled to the third resistor R 3  for producing a lower reference potential level V−. The impedances of the first resistor R 1  and the third resistor R 3  are equal, and the impedance of the second resistor R 2  is smaller than the impedance of the first resistor, preferably R 1 :R 2 :R 3 =100:1:100. The switch module comprises a first switch Q 1  and a second switch Q 2 , and the first switch Q 1  is coupled to a positive terminal of the first battery BAT 1  and a connecting point of the first battery BAT 1  and the second battery, and the second switch Q 2  is coupled to a negative terminal of the second battery BAT 2  and the connecting point. To limit the magnitude of current passing through the first switch Q 1  or the second switch Q 2 , preferably the switch module is coupled to the connecting point through a resistor Rext to prevent the batteries from being damaged by the overcharge current or the battery balanced charging controller  100  from being over heated. The balance circuit, coupled between the voltage divider and the switch module, comprises a first comparator  110 , a second comparator  115 , a NAND gate  120  and an AND gate  125 . A non-inverting terminal of the first comparator  110  is coupled to the connecting point of the second resistor R 2  and the third resistor R 3 , and an inverting terminal of the first comparator  110  is coupled to the connecting point of the first battery BAT 1  and the second battery BAT 2 . An inverting terminal of the second comparator  115  is coupled to the connecting point of the second resistor R 2  and the first resistor R 1 , and a non-inverting terminal of the second comparator  115  is coupled to the connecting point of the first battery BAT 1  and the second battery BAT 2 . 
         [0014]    To assure the battery balanced charging controller  100  of the normal operation, the battery balanced charging controller  100  further comprises a processing unit  105  including an over-temperature protection unit, an under voltage lockout unit and a start detection circuit. The over-temperature protection unit detects the temperature of the battery balanced charging controller  100 . If the temperature exceeds an over-temperature protection point, the over-temperature protection unit will generate an over-temperature protection signal to prevent the battery balanced charging controller  100  from being overheated or damaged. The under voltage lockout unit is coupled to a positive terminal of the first battery BAT 1  and a negative terminal of the second battery BAT 2 . If the potential difference between both terminals is lower than a predetermined starting voltage, the under voltage lockout unit will generate an under voltage signal to prevent an abnormal operation of the battery balanced charging controller  100  due to an insufficient operating voltage. After the start detection circuit receives a start signal EA of a high potential level, the battery balanced charging controller  100  is started, and such arrangement assure the battery balanced charging controller  100  to operate after the first battery BAT 1  and the second battery BAT 2  enter into a charging state. If there no start signal EA (or a start signal EA with a low potential level) is received, then the battery balanced charging controller  100  is at an OFF state, and the battery balanced charging controller  100  almost consumes no power at all. Unless the battery is at a charging state, the battery balanced charging controller  100  does not consume any battery power and achieve a power-saving effect. 
         [0015]    The NAND gate  120  is coupled to the processing unit  105  and the first comparator  110  for controlling a switching of the first switch Q 1  according to signals outputted from the processing unit  105  and the first comparator  110 . To prevent the potential level of a signal outputted by the NAND gate  120  being too low and damaging the first switch Q 1 , the NAND gate  120  can be connected to an operating voltage Vr 1  higher than the voltage at the negative terminal of the second battery BAT 2  in order to prevent an output of a signal with a too-low potential level. The AND gate  125  is coupled to the processing unit  105  and the second comparator  115  for controlling the switching of second switch Q 2  according to signals outputted from the processing unit  105  and the second comparator  115 . To prevent the potential level of a signal outputted by the AND gate  125  being too high and damaging the second switch Q 2 , the AND gate  125  can be connected to an operating voltage Vr 2  lower than the voltage at a positive terminal of the first battery BAT 1  in order to prevent an output of a too-high potential level. To prevent the first switch Q 1  and the second switch Q 2  from damaging due to the first switch Q 1  and the second switch Q 2  simultaneously being conducted to pass through over currents, the battery balanced charging controller  100  further comprises a time delay unit coupled to the balance circuit and the switch module, such that one of the first switch Q 1  and the second switch is conducted after a predetermined time interval from the other of the first switch Q 1  and the second switch being cutoff, so as to avoid the issue of both switches being conducted at the same time. 
         [0016]    The operation of the battery balanced charging controller  100  is described as follows. If the start signal EA is of a high potential level, indicating that the first battery BAT 1  and the second battery BAT 2  of the battery module enter into a charging state, the processing unit  105  will determine whether an abnormal condition exists or not, such as a voltage difference between the first terminal of the battery module (which is the positive terminal of the first battery) and the second terminal of the battery module (which is the negative terminal of the second battery) is lower than a predetermined starting voltage, or the battery balanced charging controller  100  is under an over-temperature condition. If the potential difference and temperature are normal, then the processing unit  105  will issue a high potential level signal to start operating the battery balanced charging controller  100 , or else the processing unit  105  will issue a low potential level signal to stop operating the battery balanced charging controller  100 . 
         [0017]    Assumed that both of the first battery BAT 1  and second battery BAT 2  are lithium batteries, and the rated voltage is 3.7V, and the fully charged voltage is 4.2V, and the voltages of the first battery BAT 1  and the second battery BAT 2  are 2.8V and 2.5V before being charged, and the battery module is plugged into a charging socket, the voltage supplied by the battery module will be 5.3V (=2.8V+2.5V), and thus the upper reference potential level V+ is 5.3V*(R 2 +R 3 )/(R 1 +R 2 +R 3 )□2.663V, and the lower reference potential level V− is 5.3V*R 3 /(R 1 +R 2 +R 3 )□2.639V, where R 1 :R 2 :R 3 =100:1:100. Now, a voltage signal DET at the connecting point of the first battery BAT 1  and the second battery BAT 2  is 2.5V, and the first comparator  110  outputs a first comparison signal with a high potential level and the second comparator  115  outputs a second comparison signal with a low potential level. Without any abnormal condition, the processing unit  105  also outputs a high potential level signal, such that the AND gate  125  outputs a low potential level signal to turn off the second switch Q 2  (which is a n-type MOSFET in this embodiment), and the NAND gate  120  outputs a low potential level signal to turn on the first switch Q 1  (which is a p-type MOSFET in this embodiment) for passing a balanced charging current through the first switch Q 1 . The second battery BAT 2  having a lower voltage receives a charging current greater than the charging current received by the first battery BAT 1  having a higher voltage by an additional balanced charging current, so that the battery voltages of the two batteries can become closer during the charging process. In the example above, if the potential level DET of the connecting point is lower than the lower reference potential level V−, the balanced charging current will pass through the first switch Q 1 , and if potential level DET of the connecting point is higher than the upper reference potential level V+, indicating that the battery voltage of the first battery is lower, the balanced charging current will pass through the second switch Q 2 . 
         [0018]    Since the present invention divides a voltage by using a resistor, a reference potential level is generated according to the ratio of the impedances of the resistors to determine whether or not it is necessary to perform a balanced charging operation by the balanced charging current. If the voltage difference between the first battery BAT 1  and the first battery BAT 2  is lower than a predetermined percentage, the battery balanced charging controller  100  will turn off the first switch Q 1  and the second switch Q 2 . In the aforementioned example, the predetermined percentage is 1%. 
         [0019]    During the processing of charging the first battery BAT 1  and the second battery BAT 2 , if the start signal EA to be changed into a low potential level or an abnormal condition occurs (such as the balanced charging current passes through the switch module or other factors causing the battery balanced charging controller  100  to have a temperature higher than a predetermined over-temperature protection point or the operating voltage of the battery balanced charging controller  100  is lower than a predetermined starting voltage), the processing unit  105  will output a low potential level signal to stop the operation of the battery balanced charging controller until the foregoing condition is removed. Since a noise of the circuit may causes a potential level DET of the connecting point varying to be lower than the lower reference potential level V− and higher than the upper reference potential level V+, both first switch Q 1  and second switch Q 2  will be turned on at the same time. Therefore the battery balanced charging controller  100  can install a time delay unit comprising a first time delay device  130  and a second time delay device  135  to avoid the issue of both of the first switch Q 1  and the second switch Q 2  being conducted at the same time. If the NAND gate  120  outputs a low potential level signal to turn on the first switch Q 1 , while driving the AND gate  125  to output a low potential level signal to turn off the second switch Q 2 , the AND gate  125  will be able to output a high potential level signal to turn on the second switch Q 2  until the first delay device  130  receives a signal of high potential level output by the NAND gate  120  and outputs a signal of high potential level after a predetermined delay time from receiving. Similarly, if the AND gate  125  outputs a high potential level signal to turn on the second switch Q 2 , while driving the NAND gate  120  to output a high potential level signal to turn off the first switch Q 1 , the NAND gate  120  will be able to output a low potential level signal to turn on the first switch Q 1  until the second time delay device  135  receives a signal of low potential level output by the AND gate  125  and outputs a signal of low potential level after a predetermined delay time from receiving. With the foregoing arrangement, we can assure that one of the first switch Q 1  and the second switch is conducted after a predetermined time interval from the other of the first switch Q 1  and the second switch being cutoff, so as to avoid the issue of both switches being conducted at the same time 
         [0020]    The battery balanced charging controller  100  of the invention not only provides a battery balanced charging effect for two serially connected batteries, also applies the battery balanced charging effect to three or more serially connected battery balanced charging. If battery module includes (N+1) serially connected battery units, N battery balanced charging controllers will be used, so that any two connected battery units has a battery balanced charging controller for the balanced charging 
         [0021]    With reference to  FIG. 3  for a built-in battery module of a battery balanced charging controller in accordance with the present invention, a battery module composed of three battery units is used for illustrating the invention. The first battery balanced charging controller  100   a  is coupled to a positive terminal of a first battery unit BAT 1 , a negative terminal of a second battery unit BAT 2  and a connecting point of the two battery units BAT 1 , BAT 2 . The second battery balanced charging controller  100   b  is coupled to a positive terminal of the second battery unit BAT 2 , a negative terminal of a third battery unit BAT 3  and a connecting point of the two battery units BAT 2 , BAT 3 . While the battery module is being charged, a start signal EA is at a high potential level to start the first battery balanced charging controller  100   a  and the second battery balanced charging controller  100   b.  The first battery balanced charging controller  100   a  compares the voltages of the first battery unit BAT 1  and the second battery unit BAT 2 . If a higher-potential battery unit has a potential level greater than a lower-potential battery unit by a predetermined percentage, a switch connected to the higher-potential battery unit in parallel will be turned on, so as to conduct a balanced charging current to the lower-potential battery unit. In the meantime, the second battery balanced charging controller  100   b  also compares the voltages of the second battery unit BAT 2  and the third battery unit BAT 3  to pass the balanced charging current to a lower-potential battery unit having a voltage when the voltage difference between the second battery unit BAT 2  and the third battery unit BAT 3  is lower than a predetermined percentage. Therefore, the battery module with the built-in battery balanced charging controller in accordance with the present invention can assure that each battery unit in the battery module is fully charged and the battery module has better efficiency and longer using time than those of a general battery module. 
         [0022]    As described above, the present invention completely fulfills the three requirements on patent application: innovation, advancement and industrial usability. In the aforementioned texts the present invention has been disclosed by means of preferred embodiments thereof; however, those skilled in the art can appreciate that these embodiments are simply for the illustration of the present invention, but not to be interpreted as for limiting the scope of the present invention. It is noted that all effectively equivalent changes or modifications on these embodiments should be deemed as encompassed by the scope of the present invention. Therefore, the scope of the present invention to be legally protected should be delineated by the subsequent claims.