Patent Publication Number: US-2015069960-A1

Title: Auxiliary Battery Charging Apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation-in-Part of U.S. patent application Ser. No. 13/429,991 filed Mar. 26, 2012, which claims priority to Japanese Patent Application No. 2011-077935 filed Mar. 31, 2011. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an auxiliary battery charging apparatus for charging an auxiliary battery that is installed in a vehicle in addition to a main battery. 
     BACKGROUND 
       FIG. 1  illustrates an existing auxiliary battery charging apparatus. 
     An auxiliary battery charging apparatus  40  in  FIG. 1  is installed in vehicles such as hybrid vehicles, electric vehicles, etc., and it comprises a main battery  41  and a DC/DC converter  42  which decreases the output voltage of the main battery  41  to charge an auxiliary battery  43 . 
     The main battery  41  supplies electricity to a motor/generator  44 . 
     The auxiliary battery  43  is a battery which supplies electricity to a control circuit for controlling the driving of the motor/generator  44  and to electronic equipment such as a car navigation system, and its power consumption is not low. Accordingly, an increase in the power consumption of the main battery  41  associated with the charging of the auxiliary battery  43  is a matter of concern. 
     The main battery  41  is configured to connect a plurality of rechargeable battery cells in series so that the output voltage can be increased; however, as much as possible, it is necessary to eliminate variations between the output voltages of the rechargeable battery cells in order to reduce the overall degradation. 
     Accordingly, to eliminate as much as possible the variation between the output voltages of the rechargeable battery cells of the main battery  41 , the existing auxiliary battery charging apparatus  40  in  FIG. 1  comprises: a cell-monitoring cell balance circuit  45  which equalizes the output voltages of the rechargeable battery cells; and an ECU  46  which controls the operation of the cell-monitoring cell balance circuit  45 . 
     As an example, as a technique to equalize the output voltages of rechargeable battery cells (hereinafter referred to as “cell balancing”), there is a so-called active-system cell balancing wherein the output voltages of the rechargeable battery cells are equalized by discharging or charging the rechargeable battery cells via a transformer (see, for example, Japanese Laid-open Patent Publication No. 2001-339865). 
     SUMMARY 
     An object of the present invention is to provide an auxiliary battery charging apparatus which is capable of decreasing the power consumption of a main battery associated with charging of an auxiliary battery installed in a vehicle in addition to the main battery. 
     An auxiliary battery charging apparatus in accordance with the present invention comprises: a main battery provided with first and second rechargeable battery cells connected to each other in series; an external generator; a transistor provided between the external generator and an auxiliary battery; an operational amplifier with a positive input terminal, a negative input terminal, and an output terminal, wherein an output voltage of the second rechargeable battery cell is input to the positive input terminal, a voltage applied to the transistor is input to the negative input terminal, and the output terminal is connected to a control input terminal of the transistor; a first switch provided between the external generator and the transistor; and a control circuit which turns on the first switch to supply a current from the external generator via the transistor to the auxiliary battery during charging of the auxiliary battery. 
     As a result, the auxiliary battery does not need to be charged with electricity supplied from the main battery, and hence power consumption of the main battery associated with the charging of the auxiliary battery may be decreased. 
     The auxiliary battery charging apparatus also comprises: a transformer provided with a first coil connected to the external generator, a second coil connected to the first rechargeable battery cell, and a third coil connected to the second rechargeable battery cell; and a second switch provided between the first coil and the external generator. The control circuit may be configured to turn on or off the second switch to electromagnetically couple the first to third coils to each other while equalizing output voltages of the first and second rechargeable battery cells. 
     As a result, the rechargeable battery cells of the main battery may be charged with electricity supplied from the external generator, and variations between the output voltages of the rechargeable battery cells of the main battery may be reduced. 
     The control circuit may be configured to, while the main battery is not being used, turn on or off the second switch to equalize the output voltages of the first and second rechargeable battery cells, and then turn on the first switch to charge the auxiliary battery. 
     As a result, it is possible to further stabilize a voltage that is applied to the auxiliary battery while the auxiliary battery is being charged. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an existing auxiliary battery charging apparatus. 
         FIG. 2  is a diagram illustrating an auxiliary battery charging apparatus in accordance with an embodiment of the present invention. 
         FIG. 3  is a diagram illustrating an example of a cell monitoring cell-balance circuit in accordance with the present embodiment. 
         FIG. 4  is a diagram schematically illustrating an example of a timing chart of the timing of turning on or off each switch. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIG. 2  is a diagram illustrating an auxiliary battery charging apparatus in accordance with an embodiment of the present invention. In  FIG. 2 , like parts are indicated by like numerals used for the existing auxiliary battery charging apparatus  40  in  FIG. 1 . 
     An auxiliary battery charging apparatus  1  in  FIG. 2  comprises: a main battery  41  configured with a plurality of rechargeable battery cells connected to each other in series; an external generator  2 ; a cell monitoring cell-balance circuit  3  for equalizing output voltages of the rechargeable battery cells of the main battery  41 ; a charging circuit  4  for charging an auxiliary battery  43  (e.g., a lead storage battery or the like) with electricity supplied from the external generator  2  by using an output voltage (or reference voltage) of some of the rechargeable battery cells of the main battery  41 ; and an ECU (Electronic Control Unit)  5  (or control circuit) for controlling operations of the cell monitoring cell-balance circuit  3  and the charging circuit  4 . The auxiliary battery charging apparatus  1  in accordance with the present embodiment is installed in vehicles such as hybrid vehicles, electric vehicles, or fork lift tracks. The reference voltage used to charge the auxiliary battery  43  is approximately as high as the voltage of the auxiliary battery  43  in a fully charged condition (e.g., 12V). 
     As described above, the auxiliary battery charging apparatus  1  in accordance with the present embodiment charges the auxiliary battery  43  with electricity supplied from the external generator  2 , and, unlike the existing auxiliary battery charging apparatus  40  in  FIG. 1 , the auxiliary battery charging apparatus  1  does not charge the auxiliary battery  43  with electricity supplied from the main battery  41 , so that power consumption of the main battery  41  associated with the charging of the auxiliary battery  43  may be decreased. 
     In addition, the auxiliary battery charging apparatus  1  in accordance with the present embodiment comprises the cell monitoring cell-balance circuit  3  and thus can reduce the variations between the output voltages of the rechargeable battery cells of the main battery  41 . 
       FIG. 3  is a diagram illustrating an example of the cell monitoring cell-balance circuit  3  and an example of the charging circuit  4 . In the main battery  41 , n number of modules  7  (i.e., modules  7 - 1  to  7 -n) configured with three cells  6  (e.g., rechargeable battery cells such as lithium ion rechargeable battery cells or nickel hydride rechargeable battery cells) are connected to each other in series, wherein the three cells  6  are connected to each other in series. The number of cells  6  forming one module  7  is not limited to three. 
     The cell monitoring cell-balance circuit  3  in  FIG. 3  comprises: a transformer  10  provided with a primary coil  8  connected to the external generator  2  and n number of secondary coils  9  (i.e., secondary coils  9 - 1  to  9 -n) each connected to one of the modules  7  in parallel; and a switch  11  (i.e., a second switch) provided between the external generator  2  and the primary coil  8 . The ratio between the number of turns of the primary coil  8  and the number of turns of all of the secondary coils  9  is 1:1. 
     The switch  11  is configured with, for example, a switching element such as a MOSFET. The switch  11  is turned on or off by a control signal S 1  output from the ECU  5 . The duty cycle of the control signal S 1  is, for example, 50%. 
     The switch  11  is turned on and off while the output voltages of the modules  7 - 1  to  7 -n are being equalized (i.e., during cell balancing). As a result, an alternating current flows through the primary coil  8 , thereby electromagnetically connecting the primary coil  8  to the secondary coils  9 - 1  to  9 -n. In this case, if, for example, the voltage of the secondary coil  9 - 1  is higher than the output voltage of the module  7 - 1 , a current flows from the secondary coil  9 - 1  to the module  7 - 1  to charge the module  7 - 1 . Then, when the output voltages of the modules  7 - 1  to  7 -n are equalized, e.g., when the output voltages of the modules  7 - 1  to  7 -n each become a lower limit threshold Vmin or higher and an upper limit threshold Vmax or lower, the switch  11  is turned off. As a result, the output voltages of the modules  7 - 1  to  7 -n may be equalized. 
     The charging circuit  4  in  FIG. 3  comprises an operational amplifier  12 , a MOSFET  13 , and a switch  14  (i.e., the first switch). The switch  14  is configured with, for example, a switching element such as a relay or MOSFET and is turned on or off by a control signal S 2  output from the ECU  5 . 
     That is, the MOSFET  13  is provided between the external generator  2  and the auxiliary battery  43 . The switch  14  is provided between the external generator  2  and the MOSFET  13 . The output voltage of the module  7 -n is input to the positive input terminal of the operational amplifier  12  as a reference voltage, a voltage applied to the MOSFET  13  is input to the negative input terminal of the operational amplifier  12 , and the output terminal of the operational amplifier  12  is connected to the gate terminal (the control input terminal) of the MOSFET  13 . An imaginary short-circuit of the positive terminal and the negative terminal of the operational amplifier  12  causes a voltage that is essentially as high as the reference voltage (e.g., 12V) to be applied to the MOSFET  13 . The gate terminal of the MOSFET  13  has applied to it a voltage that is lower than the voltage applied to the MOSFET  13  by about 1V, thereby turning on the MOSFET  13 . In this way, the MOSFET  13  is provided between the external generator  2  and the auxiliary battery  43  to form a regulator through the negative feedback of the output voltage of the operational amplifier  12  via the MOSFET  13 , and hence a voltage that is essentially as high as the output voltage of the module  7 -n may be applied to the auxiliary battery  43  via the operational amplifier  12  and the MOSFET  13 . Accordingly, even if the external generator  2  is composed of a solar-power generator or the like and the output voltage of the external generator  2  is thus unstable, a relatively stable voltage may be applied to the auxiliary battery  43  all the time. 
     While the auxiliary battery  43  is being charged, the switch  14  is turned on to electrically connect the external generator  2  to the MOSFET  13 , and hence a current flows from the external generator  2  via the MOSFET  13  to the auxiliary battery  43 , with the result that the auxiliary battery  43  is charged. The output voltage of the external generator  2  is on the order of the reference voltage or higher. 
       FIG. 4  is a diagram schematically illustrating an example of a timing chart of the timing of turning on or off the switches  11  and  14 . Assume that, while an ignition signal IG output from an upper ECU that controls the entire vehicle is at a high level, i.e., while the main battery  41  is being used by the motor/generator  44  (e.g., while the vehicle is running), the control signals S 1  and S 2  are at a low level and the switches  11  and  14  are OFF. 
     First, the ECU  5  turns on or off the switch  11  using the control signal S 1  when the ignition signal IG is switched from the high level to the low level, i.e., when the use of the main battery  41  is stopped (e.g., while the vehicle is being parked). In this way, upon the turning on or off of the switch  11  via the control signal S 1 , electricity supplied from the external generator  2  causes an alternating current to flow through the primary coil  8 , and hence the primary coil  8  and the secondary coils  9 - 1  to  9 -n are electromagnetically connected to each other. In this case, when, for example, the output voltage of the module  7 - 1  is higher than the voltage applied to the secondary coil  9 - 1 , a current flows from the module  7 - 1  to the secondary coil  9 - 1 , and hence the module  7 - 1  is discharged. When the output voltage of the module  7 - 2  is lower than the voltage applied to the secondary coil  9 - 2 , a current flows from the secondary coil  9 - 2  to the module  7 - 2 , and hence the module  7 - 2  is charged. In this way, through the charging and discharging of the modules  7 - 1  to  7 -n via the transformer  10 , the output voltage of each of the modules  7 - 1  to  7 -n gradually approaches the average value of the output voltages of the modules  7 - 1  to  7 -n. As a result, the output voltages of the modules  7 - 1  to  7 -n may be equalized. In this case, in addition, electricity supplied from the external generator  2  increases the average value of the output voltages of the modules  7 - 1  to  7 -n. In this case, the ECU  5  monitors the output voltages of the modules  7 - 1  to  7 -n. 
     Next, when the output voltages of the modules  7 - 1  to  7 -n are equalized, the ECU  5  turns off the switch  11  to terminate the charging of the modules  7 - 1  to  7 -n. After this, when the ECU  5  determines that the auxiliary battery  43  needs to be charged, it, for example, monitors the voltage of the auxiliary battery  43 , and when the ECU  5  determines that the voltage of the auxiliary battery  43  is equal to a threshold Vth1 or lower, it turns on the switch  14  to charge the auxiliary battery  43 . In this case, as an example, the auxiliary battery  43  may be charged until it is fully charged or it may be charged until a little before it is fully charged. 
     To prevent the output voltage of the external generator  2  from exceeding the pressure-withstanding capacity of the operational amplifier  12  or the MOSFET  13  while the auxiliary battery  43  is being charged, the ECU  5  may be configured to turn off the switch  14  to disconnect the external generator  2  from the auxiliary battery  43  when the output voltage of the external generator  2  becomes a threshold Vth2 or higher. 
     To prevent the output voltage of the external generator  2  from exceeding the pressure-withstanding capacity of the operational amplifier  12  or the MOSFET  13  while the auxiliary battery  43  is being charged, a protective circuit composed of a plurality of serially connected diodes or the like may be provided between the switch  14  and the connection point at which the MOSFET  13  and the negative terminal of the operational amplifier  12  are connected to each other. 
     As described above, the auxiliary battery charging apparatus  1  in accordance with the present embodiment comprises the regulator configured with: the MOSFET  13  provided between the external generator  2  and the auxiliary battery  43 ; and the operational amplifier  12 , wherein the output voltage of the module  7 -n is input to the positive input terminal of the operational amplifier  12 , and the output voltage of the operational amplifier  12  returns to the operational amplifier  12  through the negative feedback via the MOSFET  13 . As a result, a voltage that is essentially as high as the output voltage of the module  7 -n may be applied to the auxiliary battery  43 , so that the auxiliary battery  43  can be charged by turning on the switch  14  provided between the external generator  2  and the MOSFET  13  to supply a current from the external generator  2  to the auxiliary battery  43 . Accordingly, the auxiliary battery  43  does not need to be charged with electricity supplied from the main battery  41 , and hence the power consumption of the main battery  41  associated with the charging of the auxiliary battery  43  may be decreased. 
     The auxiliary battery charging apparatus  1  in accordance with the present embodiment is configured to charge the auxiliary battery  43  after cell balancing, and hence a voltage which has been further stabilized may be applied to the auxiliary battery  43 . 
     In the auxiliary battery charging apparatus  1  in accordance with the present embodiment, only a voltage (i.e., the reference voltage), not electric power, is supplied from the module  7 -n to the operational amplifier  12 , and hence the power consumption of the main battery  41  associated with the charging of the auxiliary battery  43  may be decreased. 
     The auxiliary battery charging apparatus  1  in accordance with the present embodiment does not need to be provided with the DC/DC converter  42  in  FIG. 1 , and hence the circuit scale may be reduced accordingly. 
     In the aforementioned embodiment, the auxiliary battery  43  is charged after cell balancing; however, the auxiliary battery  43  may instead be charged after the ECU  5  determines that the main battery  41  has been fully charged. 
     In the aforementioned embodiment, a bipolar transistor may be used instead of the MOSFET  13 . 
     In the embodiment described above, the cell monitoring cell-balance circuit  3  may be omitted. 
     In accordance with the present invention, it is possible to decrease the power consumption of a main battery associated with charging of an auxiliary battery that is installed in a vehicle in addition to the main battery.