Abstract:
Charging at least one rechargeable battery includes regulating a voltage applied to the at least one rechargeable battery by coupling a circuit between an output of a rectifier circuit and the at least one rechargeable battery, limiting the charging current to be within a first predetermined range and when a predetermined battery charging voltage is reached, dropping the charging current to be within a second predetermined range.

Description:
BACKGROUND 
       [0001]    Household devices, for example, cordless vacuum cleaners, use embedded batteries and external AC/DC power supply/battery charger units. Typically, the input voltage to the battery charger is not regulated and is higher than the battery charging voltage. During charging of the rechargeable battery, the charging current in the battery charger declines as the battery charging voltage increases. However, at full charge, the battery charger continues to supply trickle charging current to the rechargeable battery. 
         [0002]    Many household devices use NiCd or NiMH batteries, which can handle trickle charging current up to about C/50 (50 hours charge rate), where a 1 C rate is a charge rate that corresponds to a charging current that would result in a particular rechargeable battery becoming charged in 1 hour. However, due to cost limitations, many battery chargers do not comply with this requirement and overcharge the batteries when left on continuous charge, thus shortening the battery life. This results in significantly less runtime of the household device after only a short period of service. 
       SUMMARY 
       [0003]    Disclosed is a battery charger device for charging at least one rechargeable battery, the battery charger device dropping the trickle charging current to be below a predetermined limit. 
         [0004]    In an aspect, a battery charger device to charge at least one rechargeable battery includes a voltage regulator circuit configured to receive a DC voltage and provide a substantially constant regulated voltage output at a predetermined charging current and at a trickle charging current, the trickle charging current to be below a first predetermined value, and a resistor coupled with an output of the voltage regulator circuit and the at least one rechargeable battery to limit the charging current to be below a first predetermined value. 
         [0005]    The following are embodiments within the scope of this aspect. 
         [0006]    The battery charger devices further includes an AC/DC adapter circuit to provide a DC voltage to the voltage regulator circuit. The AC/DC adapter circuit includes a transformer having a secondary circuit and a primary circuit, and a rectifier circuit coupled to the secondary circuit of the transformer, the rectifier circuit to provide the DC voltage. The voltage regulator circuit includes a transistor having a control terminal, a current source and a current sink terminal and a diode connected to the control terminal of the transistor, the diode having a voltage characteristic selected to limit the substantially constant regulated DC voltage to be below a third predetermined value. The second predetermined value is lower than the first predetermined value, that is reached at the end of the charging cycle. The transistor is a bipolar junction transistor and the diode is a Zener diode. The resistance value is related to an average difference between the value of the voltage across the at least one rechargeable battery and the substantially constant regulated DC voltage. 
         [0007]    The substantially constant regulated voltage output is related to a maximum continuous charge voltage across the at least one rechargeable battery. The battery charger device further includes a load connected to the at least one rechargeable battery. The load is a DC motor of a portable electric device. The at least one rechargeable battery further includes at least one or a plurality of NiCd or NiMH cells. 
         [0008]    In an additional aspect, a battery charger device to charge at least one rechargeable battery includes a transformer having a secondary circuit and a primary circuit, a rectifier circuit coupled to the secondary circuit of the transformer, the rectifier circuit to provide a DC voltage; a voltage regulator circuit coupled to the rectifier circuit to provide a substantially constant regulated DC voltage and a charging current to the at least one rechargeable battery, a value of the charging current related to the substantially constant regulated voltage output and a value of a voltage across the at least one rechargeable battery, and a resistor coupled between an output of the voltage regulator circuit and the at least one rechargeable battery, the resistor having a resistance value provided to limit the charging current to be below a first predetermined value. 
         [0009]    The following are embodiments within the scope of this aspect. 
         [0010]    The voltage regulator circuit further limits a trickle charge current to be below a second predetermined value. The second predetermined value is lower than the first predetermined value. The voltage regulator circuit comprises a transistor having a control terminal, a current source and a current sink terminal; and a diode connected to the control terminal of the transistor, the diode having a voltage characteristic selected to limit the substantially constant regulated DC voltage to be below a third predetermined value. The transistor is a bipolar junction transistor and the diode is a Zener diode. 
         [0011]    The resistance value is related to an average difference between the value of the voltage across the at least one rechargeable battery and the substantially constant regulated DC voltage. The substantially constant regulated voltage output is related to a maximum continuous charge voltage across the at least one rechargeable battery. The battery charger device further includes a load connected to the at least one rechargeable battery. The load is a DC motor of a portable electric device. The at least one rechargeable battery further includes at least one or a plurality of NiCd or NiMH cells. 
         [0012]    In an additional aspect, a battery charger device to charge at least one rechargeable battery includes a rectifier circuit to provide a DC voltage, circuitry coupled to the rectifier circuit to receive the DC voltage and provide to the at least one rechargeable battery a substantially constant regulated DC voltage at a predetermined value of a charging current, the predetermined value related to a value of a voltage across the at least one rechargeable battery and a predetermined value of a trickle charging current, the predetermined value to be below a predetermined limit. In some embodiments, the circuitry is implemented by a switch-mode AC/DC adapter circuit. 
         [0013]    One or more aspects may provide one or more advantages. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a circuit schematic of a household device battery charger. 
           [0015]      FIG. 2  is a circuit schematic of an alternative household device battery charger implemented using a switch-mode AC/DC adapter circuit. 
           [0016]      FIGS. 3A-C  are a series of plots that illustrate exemplary battery charging voltage, battery charging current, and trickle charging current behaviors, respectively, for a 7.2 V NiCd rechargeable battery using a conventional charger circuit. 
           [0017]      FIGS. 4A-B  are a series of plots that illustrate exemplary battery charging voltage and battery charging current behaviors using an exemplary embodiment of the household device battery charger and a conventional charger circuit respectively. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Referring to  FIG. 1 , an exemplary household device charger or rechargeable battery charger device  10  to charge at least one rechargeable battery  12  that has at least one rechargeable cell is shown. The rechargeable battery  12  is typically used in household or portable electronic devices such as, for example, wireless vacuum cleaners, cordless telephones and rechargeable flashlights. In some embodiments, the rechargeable battery  12  includes NiCd or NiMH cells having cadmium or hydrogen-absorbing alloy for the anode and nickel for the cathode. Although  FIG. 1  shows a single rechargeable battery  12 , the rechargeable battery charger device  10  may be adapted to receive and charge two or more rechargeable batteries at the same time. Further, the rechargeable battery charger device  10  may charge different battery types including, for example, cylindrical batteries or prismatic batteries. 
         [0019]    The rechargeable battery charger device  10  is electrically coupled to an external AC power source  14 , such as a source providing power at a rating of 85V-265V and 50 Hz-60 Hz. As shown, the AC power source  14  is coupled to a transformer  16  in the rechargeable battery device  10 . In some embodiments, the transformer  16  is a current-limited linear transformer having a primary winding  16   a  and a secondary winding  16   b.    
         [0020]    The rechargeable battery charger device  10  includes a rectifier circuit, e.g., a full wave rectifier circuit  18  coupled to the secondary winding  16   b  of the transformer  16 . A stepped-down AC voltage is induced at the secondary winding  16   b  from the AC power source  14  coupled to the primary winding  16   a  of the transformer  16 . 
         [0021]    The full wave rectifier circuit  18  converts the stepped-down AC voltage to a low DC voltage (e.g., 1.5-14V) that is at a level suitable for charging the rechargeable battery  12 , e.g., DC voltages at levels of approximately between 1.0-1.5 V for each NiCD or NiMH cell described above. Other types of cells may have different voltage levels. 
         [0022]    A filter capacitor  20  is coupled across the output of the full wave rectifier circuit  18 . An open circuit voltage across such a filter capacitor  20  would be about 50% higher than the voltage across the rechargeable battery  12 . As the battery charging voltage increases, the voltage difference between the filter capacitor  20  and the rechargeable battery  12  decreases, while supplying nominal charging current. 
         [0023]    The full wave rectifier circuit  18  is coupled to a voltage regulator circuit  22  which, in turn, is connected to a current limiting resistor  24 . The voltage regulator circuit  22  includes a resistor  26 , a transistor, e.g., bi-polar junction transistor  28 , having a current sink terminal, e.g., collector  28   a , a control terminal, e.g., base  28   b , and a current source terminal, e.g., emitter  28   c , and a diode, e.g. Zener diode  30 , coupled in reverse bias between the control terminal  28   b  of the transistor  28  and a ground terminal  32 . Alternatively, the transistor  28  is a field effect transistor. Alternatively, the voltage regulator circuit  22  is implemented by an integrated voltage regulator chip, such as, for example, a three-terminal adjustable linear voltage regulator chip (LM 317). 
         [0024]    A value of the output voltage of the voltage regulator circuit  22  is a function of the maximum continuous charging voltage that is permissible across the rechargeable battery  12 . For example, in order to operate a typical, portable vacuum cleaner, a 7.2 V battery having six rechargeable NiCd cells in series is required. Accordingly, the maximum charging voltage across the rechargeable battery  12  and thus the value of the output voltage across the terminals of the voltage regulator circuit  22  is selected to be higher than 7.2 V (e.g., 8.4 V). 
         [0025]    The voltage drop between the control  28   b  and current source  28   c  terminals for a bipolar junction transistor (transistor  28 ) is typically around 0.6 V. Thus, in embodiments in which the maximum charging voltage is 8.4 V, a Zener diode having a reverse breakdown voltage characteristic at 9 V (0.6V plus 8.4V) is provided as the diode  30  to compensate for the voltage drop. 
         [0026]    The resistance value of the current limiting resistor  24  is related to the desired charging current, the average difference between the charging voltage across the rechargeable battery  12  and the value of the output voltage of the voltage regulator circuit  22 . As described above, in one example, the value of the output voltage of the voltage regulator circuit  22  is selected to be 8.4 V (the maximum charging voltage across the rechargeable battery  12 ) Accordingly, if the average battery charging voltage is 7.8 V, and the desired charging current is 150 mA, the resistance value of the current limiting resistor  24  is around (8.4 V-7.8 V)/0.15 A˜4 Ohms. 
         [0027]    The desired charging current for the rechargeable battery charger device  10  is selected based on the time available for charging. For example, in some embodiments, the charging current to be used in the rechargeable battery charger device  10  is selected to be an average of C/10 (10 hours charge rate). 
         [0028]    The resistance value of the current limiting resistor  24  in the rechargeable battery charger device  10  is also selected to limit the trickle charging current to be below a predetermined limit. Thus, when the voltage across the rechargeable battery  12  approaches the value of the output voltage across the terminals of the voltage regulator circuit  22 , e.g., 8.4 V (1.4 V/rechargeable cell), the charging current drops to a trickle charge value that is below the predetermined limit. 
         [0029]    For example, after the first 10 hours of charge (e.g., a full charge cycle at an average of C/10 charge rate), when the voltage across the rechargeable battery  12  approaches 8.4 V, the charging current supplied by the rechargeable battery charger device  10  automatically drops to C/50 or lower. As a result, the rechargeable battery  12  is maintained in a healthy state regardless of the usage or charging pattern. 
         [0030]    The rechargeable battery  12  is connected to a load, e.g., load motor  36  of a typical household device, e.g., a portable vacuum cleaner through a switch  34 . As shown, a fuse  38  is typically included in the rechargeable battery charger device  10  to protect the rechargeable battery  12  and load motor  36  from load currents greater than a predetermined rated value (e.g., ˜20 A). 
         [0031]    In some embodiments, the transformer  16  and the full wave rectifier circuit  18  are disposed within an AC power source adapter (not shown), rather than within the rechargeable battery charger device  10  ( FIG. 1 ). Accordingly, the AC/DC adapter would receive an AC voltage and provide a DC voltage. In other embodiments the transformer  16  is disposed in an AC power source adapter (not shown), and the full wave rectifier circuit  18  is part of the rechargeable battery charger device, similar to that shown in  FIG. 1 . 
         [0032]    Referring now to  FIG. 2 , an alternative arrangement for the rechargeable battery charger device, is a switch-mode AC/DC adapter circuit  40  as shown. This arrangement  40  includes a full wave rectifier  41  on the primary side of the transformer  42  and has the voltage regulator implemented using an integrated controller device. As shown, at least one rechargeable battery  12  is connected across the output terminals of the switch-mode AC/DC adapter circuit  40 . As described above, the switch-mode AC/DC adapter circuit  40  receives and charges more than one rechargeable battery at the same time. In addition, the switch-mode AC/DC adapter circuit  40  may charge different battery types including, for example, cylindrical batteries or prismatic batteries. 
         [0033]    The rechargeable battery  12  is connected to a load, e.g., load motor  36  of a vacuum cleaner, through switch  34  and fuse  38 . 
         [0034]    The switch-mode AC/DC adapter circuit  40  includes the transformer  42  having a primary winding  42   a  and a secondary winding  42   b . Accordingly, the switch-mode AC/DC adapter circuit  40  has a primary side  40   a  and a secondary side  40   b.    
         [0035]    The primary side  40   a  of the switch-mode AC/DC adapter circuit  40  includes a full wave rectifier circuit  42  coupled to an AC power source  14  and a capacitor  44  coupled across the output terminals of the full wave rectifier circuit  42 . A switching transistor  46  is also connected in series with the primary winding  42   a  of the transformer  42 . In some embodiments, the switching transistor  46  is implemented using a metal-oxide-semiconductor field-effect transistor (MOSFET). 
         [0036]    A primary controller  50  controls the switching cycle of the switching transistor  46 . Accordingly, the primary controller  50  turns the switching transistor  46  on and off with a feedback-controlled duty cycle. 
         [0037]    The secondary side  40   b  of the switch-mode AC/DC adapter circuit  40  includes a rectifier diode  52  connected in series with the secondary winding  42 B of the transformer  42  and a filter capacitor  54  connected to the anode terminal of the rectifier diode  52 . The terminals of the filter capacitor  54  are further coupled to a voltage divider circuit  56  having resistors  56   a  and  56   b.    
         [0038]    A secondary side controller  58  is coupled across resistor  56   b  of the voltage divider circuit  56 . In some embodiments, the secondary side controller  58  has control loops to perform both current and voltage regulation. The secondary side controller  58  is implemented by an integrated controller chip, such as, for example, TSM1052 from STMicroelectronics, Geneva, Switzerland. The secondary side controller  58  further transmits the feedback signals, V out  and I out  to the primary controller  50  through an isolated optical transmission path  60 . 
         [0039]      FIGS. 3A-C  illustrate exemplary battery charging voltage  62 , battery charging current  64 , and trickle charging current  66  behaviors, respectively, for a 7.2 V NiCd rechargeable battery using a conventional charger circuit (not shown). 
         [0040]    As shown in  FIG. 3B , although the battery charging current  64  drops a little, the trickle charging current  66  remains significant even after the rechargeable battery is fully charged (approximately 10 hours). As shown in  FIG. 3C , the instantaneous trickle charging current  66  after 48 hours of charge fluctuates so that the peak value is around 330 mA. As described above, for a NiCd battery of 7.2 V, the recommended trickle charge limit is around 30 mA, e.g., C/50 or 6×1500/50 at a one hour charge rate. Consequently, overcharging the rechargeable battery  12  continuously in this manner shortens the life of the rechargeable battery  12  and reduces the runtime of the household device. 
         [0041]      FIGS. 4A-B  illustrate exemplary battery charging voltage  68  (voltage across the at least one rechargeable battery  12  in  FIG. 1 ) and battery charging current  70  behaviors, respectively, for six 1500 mAh NiCd cells in series subjected to over 24 hours of charge at a constant voltage of 8.4 V and current limited up to 500 mA using a rechargeable battery charger of the type shown in either of  FIG. 1  or  FIG. 2 . Exemplary battery charging voltage  72 , and the battery charging current  74  behaviors, respectively, for the six 1500 mAh NiCd cells in series subjected to the over 24 hours of charge at a constant voltage of 8.4 V using a conventional rechargeable battery charger circuit are also shown. At a maximum charging current of about 500 mA, the NiCD batteries that were initially substantially entirely depleted, would become fully charged in approximately 10 hours. 
         [0042]    When the battery charging current, e.g., exemplary battery charging currents  70 ,  74 , is applied to the rechargeable battery  12 , the battery charging voltage, e.g., exemplary battery charging voltages  68 ,  72 , at the output terminals of the rechargeable battery charger device  10  increases and reaches an average voltage level of 8.2-8.6 V (8.4V plus/minus 0.2V). Thereafter, the battery charging voltage  68  corresponding to embodiments of the rechargeable battery charger device  10 , e.g., circuit shown in  FIG. 1 , is maintained at a constant voltage level. For comparison, the battery charging voltage  72  corresponding to a conventional rechargeable battery charger circuit is also shown in  FIG. 4A . As shown, the conventional rechargeable battery charger circuit depicts uneven output voltage. 
         [0043]    After full charge is achieved, the rechargeable battery charger device  10  causes the fluctuations, e.g., spikes appearing in the exemplary battery charging current  70 , to average to below a very low predetermined trickle charge value, e.g., lower than C/50. For example, an average value of the battery charging current  70  drops to be below 30 mA, i.e., IC=1500 mAh; C/50=30 mAh. Subsequently, the battery charging current  70  continues to remain at the very low predetermined trickle charge value until the rechargeable battery  12  is removed from the rechargeable battery charger device  10 . 
       OTHER EMBODIMENTS 
       [0044]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.