Abstract:
A battery protection system is provided for a rechargeable battery. The system has a switch module that selectively interrupts battery current based on a control signal, a battery voltage sensor that senses battery voltage, a battery temperature sensor that generates a battery temperature signal, and a control module that generates said control signal based on said battery temperature signal and said battery voltage.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to battery systems, and more particularly to a battery charger for battery systems.  
       BACKGROUND OF THE INVENTION  
       [0002]     Rechargeable batteries, particularly nickel-metal hydride (NiMH) batteries, are useful in many types of applications. For example, the batteries may be used as a backup power supply for stationary applications such as cellular towers. The batteries provide backup power during a main grid outage. In such an application, it is desirable that the batteries are connected to a battery charging circuit that maintains the state of charge of the batteries. Under certain circumstances, charging NiMH batteries may cause the batteries to overheat, which may damage the batteries or other components.  
       SUMMARY OF THE INVENTION  
       [0003]     A battery protection system is provided for a rechargeable battery. The system has a switch module that selectively interrupts battery current based on a control signal, a battery voltage sensor that senses battery voltage, a battery temperature sensor that generates a battery temperature signal, and a control module that generates the control signal based on the battery temperature signal and the battery voltage.  
         [0004]     A battery protection circuit is also provided. The circuit has a switch module that selectively interrupts battery current based on a control signal. A first oscillator module generates a first signal having a first period and a temperature dependent oscillator module generates a second signal having a second period. A duty cycle generator receives the first signal and the second signal, and generates the control signal. The control signal is pulse-width modulated at the first period with a duty cycle established by the second period.  
         [0005]     A method for protecting a rechargeable battery is provided. The method includes monitoring a battery temperature and monitoring a battery voltage, and selectively interrupting current flow to the battery based on the battery temperature and the battery voltage.  
         [0006]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0008]      FIG. 1  is a block diagram and electrical schematic of a rechargeable power supply;  
         [0009]      FIG. 2A  is a functional block diagram of the power supply of  FIG. 1  being used as a primary power supply for a load;  
         [0010]      FIG. 2B  is a functional block diagram of the power supply of  FIG. 1  being recharged;  
         [0011]      FIG. 2C  is a functional block diagram of the power supply of  FIG. 1  being used as a backup power supply in combination with a primary power supply;  
         [0012]      FIG. 3  is a graph of operating voltage as a function of depth of discharge (DOD) at 25° C.;  
         [0013]      FIG. 4  is an electrical schematic of an exemplary switch module;  
         [0014]      FIG. 5  is a flowchart illustrating steps of a method for charging rechargeable batteries;  
         [0015]      FIG. 6  is a functional block diagram of a battery charger according to some implementations of the present invention;  
         [0016]      FIG. 7  is an electrical schematic of one implementation of the battery charger of  FIG. 6 ;  
         [0017]      FIG. 7A  depicts waveforms of the battery charger of  FIG. 7 ; and  
         [0018]      FIG. 8  is a perspective view of a rechargeable power supply. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the term module and/or device refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group), and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. For purposes of clarity, the same reference numerals will be used to identify similar elements.  
         [0020]     Referring now to  FIG. 1 , a block diagram and electrical schematic of a rechargeable power supply  2  is shown. The power supply  2  has a housing  4  containing a battery  6 . The battery  6  has a number of individual cells  8   a ,  8   b , . . . ,  8   n , collectively referred to as cells  8 . In some implementations, the cells  8  may be nickel-metal hydride (NiMH) cells, although other types of batteries may be used. A positive battery terminal  10  is connected to a power supply positive terminal  12  by a positive bus  14 . A switch module  16  selectively connects a negative battery terminal  18  to a negative bus  20 . The negative bus  20  is connected to a power supply negative terminal  22 . The positive and negative power supply terminals  12 ,  22  are preferably positioned along an end face of the housing  4 , thereby facilitating electrical interconnection of a plurality of the rechargeable power supplies  2 . An intermediate connection  24  connects the negative battery terminal  18  to the switch module  16 . A switch control module  26  generates a switch control signal  28  based on battery voltage and temperature as will be described below. A resettable circuit breaker or fuse (not shown) may also be placed in series with the battery  6 .  
         [0021]     Referring now to  FIG. 2A , the power supply  2  is shown connected to a load  30 . The battery  6  of the power supply  2  provides current to the load  30  when the switch module  16  is closed. The switch control module  26  will open the switch module  16  via the switch control signal  28  if the battery temperature is above a predetermined threshold.  
         [0022]     Referring now to  FIG. 2B , the power supply  2  is shown connected to a charging device  32 . The charging device  32  provides power to recharge the battery  6  of the power supply  2 . The switch control module  26  monitors the voltage and the temperature of the battery  6 , and controls charging of the battery  6  in accordance with a method described later.  
         [0023]     Referring now to  FIG. 2C , a primary supply  34  is connected across the load  30  and the power supply  2 . The primary supply  34  provides power to power the load  30  and/or to charge the battery  6  in the power supply  2 . If the primary supply  34  stops providing current, such as may happen during a power outage, the power supply  2  continues providing current to the load  30  until the primary supply  34  is restored. The amount of time that the power supply  2  can provide power is determined by several factors including the capacity (amp-hours) of the battery  6 , the state of charge (SOC) of the battery  6  at the time the primary supply  34  failed, the temperature of the battery  6 , and the current drawn by the load  30 .  
         [0024]     Referring now to  FIG. 3 , a family of curves  36  of a NiMH battery  6  is shown. A y-axis  38  indicates battery voltage and an x-axis  40  indicates degree of discharge (DOD) of each battery  6 . The family of curves  36  provides an indication of the voltage vs DOD at 25 deg. C. It can be seen that the battery voltage increases rapidly as DOD decreases. A cutoff voltage, or V CUTOFF ,  42  is selected to provide an indication that the battery  6  is fully charged.  
         [0025]     Referring now to  FIG. 4 , an exemplary switch module  16  is shown. The switch module  16  may be implemented using any combination of electromechanical relays, transistors, electronic and integrated circuits, and/or any material having very high and very low states of resistance. The depicted switch module  16  includes NMOS transistors Q 1 , Q 2 , and Q 3 . A source of each transistor Q 1-3  is connected to the negative bus  20 . A drain of each transistor Q 1-3  is connected to the intermediate connection  24 . A gate of each transistor Q 1-3  is connected to the switch control signal  28 . The switch module  16  may also include an indicator, such as an LED  44  with a current limiting resistor  46 , to indicate whether the switch module is open or closed while the battery  6  is charging.  
         [0026]     Referring now to  FIG. 5 , steps of a method for generating a switch control signal  28  are shown. In the described embodiment, the switch control signal is a PWM signal having a duty cycle between 0 and 100%, however it is understood that other types of control signals and/or other duty cycles may be used to control the switch module  16 .  
         [0027]     The method begins in step  48 . In step  50 , control determines whether temperature T BATT  of the battery  6  is above a predetermined temperature threshold T H . If the determination made in step  46  returns a true result, then control proceeds to step  52  and opens the switch module  16  by setting the duty cycle (DC) to zero percent. In step  50 , if T BATT  is below T H  then control proceeds to decision block  54  and determines whether the voltage V BATT  of the battery  6  is less than the predetermined battery voltage threshold V CUTOFF . If the determination made in step  54  returns a true result, then control proceeds to step  56 . In step  56 , control closes the switch module  16  by setting the duty cycle to one hundred percent. Returning to step  54 , if V BATT  is equal to or greater than the predetermined battery voltage threshold V CUTOFF , then control proceeds to step  58  and establishes a first time period T 1 . Control then proceeds to step  60  and generates a second time period T 2 , which is temperature compensated by T BATT . The time period T 2  decreases as T BATT  increases. In step  62 , control generates a switch control signal  28  having a DC that is derived from T 1  and T 2 .  
         [0028]     Referring now to  FIG. 6 , a functional block diagram of a switch control module  26  is shown. A temperature sensor  64  indicates the battery temperature T BATT . A thermostat  66  compares T BATT  to T H  and provides a switch enable signal SW_EN to a driver  68 . When T BATT  is greater than T H , the thermostat  66  opens the switch module  16  by turning off the driver  68 .  
         [0029]     When T BATT  is less than or equal to T H , the driver  68  produces a PWM signal having a duty cycle that is set by a duty cycle (DC) generator  70 . The duty cycle generator  70  produces a  100 % duty cycle signal when a voltage sensor  72  and a comparator  74  determine that the battery voltage V BATT  is less than the cutoff voltage V CUTOFF . When the voltage sensor  72  and the comparator  74  determine that the battery voltage V BATT  is greater than the cutoff voltage V CUTOFF , the duty cycle generator  70  produces a duty cycle signal based on periods T 1  and T 2 . The period T 1  is derived from an oscillator module  76  and period T 2  has a variable value generated by a temperature compensated oscillator  78 . The temperature compensated oscillator  78  is synchronized with the period T 1 . The period T 2  varies as a function of T BATT . In some implementations, the period T 2  of the temperature compensated oscillator  78  decreases as T BATT  increases.  
         [0030]     Referring now to  FIGS. 7 and 7 A, one exemplary circuit that implements the switch control module  26  of  FIG. 6  is described. The battery voltage V BATT  is taken directly from the positive battery terminal  10  and ground is connected to the negative battery terminal  18 . A power supply voltage V CC  is derived by passing V BATT  through a low-pass RC filter (not shown).  
         [0031]     The oscillator module  76  is implemented with an integrated circuit IC 1 . The integrated circuit IC 1  is a 24-stage frequency divider, such as an MC14521B. One terminal each of a resistor R 1,  a resistor R 2 , and a capacitor C 1,  are connected together. The other end of the resistor R 1  is connected to an IN 1  input of the integrated circuit IC 1 . The other end of the resistor R 2  is connected to an OUT 2  output of the integrated circuit IC 1 . An input IN 2  and an output OUT 1  of the integrated circuit IC 1  are connected to the other end of the capacitor C 1 . This configuration of the resistors R 1  and R 2 , the capacitor C 1 , and the integrated circuit IC 1 , produces a square wave at an output Q 22  of the integrated circuit IC 1 . The square wave is shown in  FIG. 7A  at V Q22 . The square wave has a fixed period T 1  that is established by the resistor R 2  and the capacitor C 1 . The pulse-width of the square wave V Q22  is ½*T 1 . The square wave is output through a capacitor C 2  to a node Nd 1 . The node Nd 1  is pulled up to V cc  through a resistor R 4 . The node Nd 1  is an input to a duty cycle generator  70  and has a waveform that is dependent on the outputs of the comparator  74 , a master/slave switch  80 , and the oscillator module  76 . The waveform of node Nd 1  will be described after the operations of the connected circuit blocks are described.  
         [0032]     The voltage sensor  72  includes a voltage divider formed of an upper resistor R 3  and a lower resistor R 4 . One end of the upper resistor R 3  is connect to V BATT . One end of the lower resistor R 4  is connected to ground. The other end of the upper resistor R 3  and the other end of the lower resistor R 4  are connected together to form node Nd 2 , which is the center tap of the voltage divider. The node Nd 2  provides a scaled battery voltage signal.  
         [0033]     The comparator  74  has a NAND gate NA 1  with an open collector output. The NAND gate NA 1  is configured as an inverter. An input to the inverter is a node Nd 3  located at a connection of one end of a resistor R 5  and a cathode of a 3-terminal voltage regulator Z 1 . Examples of devices that may be used to implement the voltage regulator Z 1  include a TL431/TL431A/TL431B series of programmable voltage references available from Linfinity Microelectronics, Inc. The other terminal of the resistor R 5  is connected to V CC . An anode of the voltage regulator Z 1  is connected to ground. A reference pin of the voltage regulator Z 1  is connected to the node Nd 2 . A voltage at the node Nd 3  is low when the voltage regulator Z 1  is conducting, which occurs when V BATT  is greater than or equal to V CUTOFF . A conduction point of the voltage regulator Z 1  is established by the resistors R 3  and R 4 , which should have resistances selected such that voltage at the node Nd 2  causes the voltage regulator Z 1  to conduct when V BATT  is greater than or equal to V CUTOFF . An output of the NAND gate NA 1  connects to a cathode of a diode D 1 . An anode of the diode D 1  connects to the node Nd 1 .  
         [0034]     A waveform at the node Nd 1  will now be described. A common pole of the master/slave switch  80  is connected to the node Nd 1 . If the switch  80  is in the master position M and closing a path to ground, then the switch  80  will hold the node Nd 1  at ground. If the switch  80  is in the slave position S and V BATT  is less than V CUTOFF , then the diode D 1  will be forward biased by the comparator  74  and the voltage of the node Nd 1  will be held low. If the switch  80  is in the slave position and V BATT  is greater than or equal to V CUTOFF , then the comparator  74  will prevent the diode D 1  from conducting. The voltage of the node Nd 1  will then pulse low with each falling edge from the output Q 22  of the integrated circuit IC 1 . The low pulses have a period T 1  as shown in  FIG. 7A  at V Nd1 .  
         [0035]     The node Nd 1  is an input to the duty cycle generator  70 . The duty cycle generator  70  is formed from a set-reset (SR) flip-flop  82  fabricated of NAND gates NA 2  and NA 3 . An output of the NAND gate NA 2  is connected to an input of the NAND gate NA 3 . An output of the NAND gate NA 3  is connected to an input of the NAND gate NA 2 . A remaining input of the NAND gate NA 2  operates as the set input of the SR flip-flop and is connected to the node Nd 1 . A remaining input of the NAND gate NA 3  operates as the reset input of the SR flip-flop and is connected to the node Nd 3 . An output of the duty cycle generator  70  is taken from the output of NA 2 . The output voltage of the duty cycle generator  70  is high after the node Nd 1  is pulsed low, and low after the node Nd 3  is pulsed low. The output of the duty cycle generator  70  will remain high when either the switch  80  or the comparator  74  holds the node Nd 1  low.  
         [0036]     The temperature compensated oscillator  78  is formed around an integrated circuit IC 2 , which is a 24-stage frequency divider such as an MC14521B. An output of a NAND gate NA 4  provides a node Nd 4 . A waveform at the node Nd 4  is a logical complement of the waveform at the node Nd 1  as is shown in  FIG. 7A  at V Nd4 . A Reset input of the integrated circuit IC 2  is connected to the node Nd 4 . As is shown in  FIG. 7A  at V Q20 , an output Q 20  of the integrated circuit IC 2  is low while the Reset input of the integrated circuit IC 2  is held high by the node Nd 4 .  
         [0037]     One end each of a resistor R 6 , a thermistor R T1 , and a capacitor C 3 , are connected together. A remaining end of the resistor R 6  is connected to an In 1  input of the integrated circuit IC 2 . A remaining end of the capacitor C 3  is connected to an output Out 1  and to an input In 2  of the integrated circuit IC 2 . A remaining end of the thermistor R T1  is connected to an output Out 2  of the integrated circuit IC 2 . The resistance of thermistor R T1  decreases as its temperature increases. The thermistor R T1  is preferably positioned in proximity to a battery  6  (not shown) that is connected to the switch module  16 . An optocoupler  84  selectively couples a resistor R 7  in parallel with the thermistor R T1 . The optocoupler  84  has an internal LED with an anode pulled up to V CC  by a resistor R 8 . A cathode of the LED is connected to an output of the thermostat  66 .  
         [0038]     The output Q 20  of integrated circuit IC 2  generates a pulse train having a period T 2  and a pulsewidth of ½*T 2  as is shown in  FIG. 7A  at V Q20 . The period T 2  decreases as the temperature of R T1  increases. The period of T 2  also decreases when R 7  is switched in parallel with R T1 . The output Q 20  of the integrated circuit IC 2  is connected to a capacitor C 4  An opposite end of the capacitor C 4  is connected to the node Nd 3 , which is the reset input of the duty cycle generator  70 . Each time the output Q 20  of the integrated circuit IC 2  transitions from high to low, a low-going pulse appears at the node Nd 3  as is shown in  FIG. 7A  at V Nd3 . Each low-going pulse causes the output of the duty cycle generator  70  to go low. When the node N d1  is carrying the pulses initiated by the output Q 22  of the integrated circuit IC 1 , the output signal from the duty cycle generator  70  is a PWM signal. The PWM signal has a period T 1  established by the oscillator module  76  and a duty cycle established by the temperature compensated oscillator  78 . An output signal of the duty cycle generator  70  operating in such a situation appears in  FIG. 7A  at V Nd5 . The output signal may be used as the control signal  28  as described later.  
         [0039]     The output signal from the duty cycle generator  70  is input to the driver  68 . The driver  68  has a transistor Q 4 . A resistor R 9  is connected between a base and emitter of the transistor Q 4 . A resistor R 10  is in series with the base of the transistor Q 4 . One end of a resistor R 14  is connected to the collector of the transistor Q 4 . The other end of the resistor R 14  provides the switch control signal  28  and is pulled down to ground through a resistor R 15 . The duty cycle of the switch control signal  28  is zero percent when the thermostat  66  turns off the transistor Q 4 . The duty cycle of the switch control signal is greater than zero percent when the transistor Q 4  is amplifying the signal from the node Nd 5 .  
         [0040]     The thermostat  66  has a temperature controller  86 . An example of a device suitable for use as the temperature controller  86  is an Analog Devices part number TMP01FS. A resistor R 11  is connected between a VREF output and a SETLOW input of the temperature controller  86 . A resistor R 12  is connected between the SETLOW input and a SETHI input of the temperature controller  86 . A resistor R 13  is connected between the SETHI input of the temperature controller  86  and ground. The temperature controller  86  has an output UNDER that is connected to the driver  68 . The output UNDER of the temperature controller  86  turns the transistor Q 4  on when the battery temperature T BATT  is below a predetermined threshold. The thermostat  66  is therefore preferably positioned proximate the battery  6  that is connected to the switch module  16 . The junction of the resistors R 11  and R 12  provides a voltage corresponding to a predetermined low battery temperature T L  threshold. The junction between the resistors R 12  and R 13  provides a voltage corresponding to a predetermined high battery temperature T H  threshold.  
         [0041]     The OVER and UNDER outputs of the temperature controller  86  are active low. The output UNDER is low when the battery temperature T BATT  is less than the predetermined high battery temperature T H . The output OVER is low when the battery temperature T BATT  is greater than the predetermined low battery temperature T L .  
         [0042]     The output OVER is connected to the cathode of the LED in the optoisolator  84 . The LED is turned on when the output OVER goes low, thereby selectively connecting resistor R 7  in parallel with R T1 . The thermostat  66  thereby provides a mechanism for selecting a range of period T 2  from two period ranges. The output UNDER drives the base of Q 4  and turns Q 4  off when the battery temperature T BATT  is greater than the high battery temperature T H . In the depicted embodiment, the high battery temperature T H  is selected to be 50 deg C. and the low battery temperature TL is selected to be 40 deg C. Other values may be used as needed to prevent the battery  6  from overheating while it is being charged.  
         [0043]     Referring now to  FIG. 8 , an exterior perspective view of a rechargeable power supply  2  is shown. The power supply  2  has a battery  6  with a number of prismatic cells  8   a - e . A housing  4  contains the battery  6 , a switch module  16 , and a control module  26 . In one of many embodiments, the master/slave switch  80  is accessible at from an exterior of the housing  4 . The power supply positive terminal  12  and the negative terminal  22  are positioned on the housing  4 .  
         [0044]     A plurality of the power supplies  2  may be coupled in series with one power supply  2  having the switch  80  set to the master position, and the remaining power supplies  2  having switches  80  set to slave. Such a series configuration of power supplies  2  allows the switch control module  26  of the power supply  2  set to master to control the charging and discharging of the batteries  6  in the remaining power supplies  2  that have switches  80  set to the slave position.  
         [0045]     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.