Abstract:
When a rechargeable battery of a life safety device reaches its end of life, the life safety device provides an audible signal indicating that replacement is required. When the device is disconnected from line power, the rechargeable battery still contains a significant amount of energy. The device automatically begins controlled discharge the battery until the stored energy has been reduced to a safe level for disposal of the life safety device and its battery.

Description:
BACKGROUND 
     The present invention relates to life safety devices, and in particular to hazard detectors such as carbon monoxide and smoke detectors that are typically mounted on walls and ceilings of buildings. 
     Life safety devices are used in residential and commercial buildings to provide warning to occupants of hazards such as fire or buildup of unsafe gases, such as carbon monoxide. Life safety devices may be powered by a battery assembly that includes a rechargeable battery. The life safety device is connected to a source of AC power, which provides electrical power for charging the rechargeable battery. 
     When the life safety device reaches the end of its useful life, it may generate a warning signal, such as a chirping sound indicating that it has entered a “trouble” (or “end of life”) mode. This signals the user that it is time to disconnect the life safety device from AC power and throw it away. Rechargeable batteries, such as rechargeable lithium ion batteries, can contain a substantial amount of energy. If not discharged before disposal, they can pose a risk of being damaged and causing a fire and other serious injury. In the case of a life safety device, a rechargeable battery cannot be discharged quickly by shorting the output, because the battery assembly includes a battery protection circuit that will prevent rapid discharge in order to protect the battery. In addition, quickly discharging the battery can cause excessive amounts of heat. 
     In the past, life safety devices have made use of a mechanical element, such as a switch, to apply a resistance across the battery terminals in order to discharge the battery prior to disposal. A disadvantage of using a switch to discharge the battery is that it relies on the user to actuate the switch. If the user does not actuate the switch, the life safety device and its battery may be disposed of in an unsafe condition. 
     SUMMARY 
     Life safety device provides an automatic battery discharge operation after detecting that the device has reached an end of life condition. Life safety device includes a controller that causes an audible warning signal to be generated when an end of life condition is detected. When a user disconnects the life safety device from a source of AC power used to charge the rechargeable battery, the controller initiates a discharge sequence in which the battery is slowly discharged until the stored energy in the battery has been reduced to a safe level. 
     In one embodiment, the discharge battery is performed by battery test electronics. A low duty cycle discharge control pulse is used to drain power from the battery in a controlled way that does not produce significant amounts of heat, but completes the discharge of the battery in a short period of time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a life safety device. 
         FIG. 2  is a flow chart illustrating operation of the life safety device of  FIG. 1  in an end of life mode. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a block diagram of life safety device  10 , which may be, for example, a smoke alarm, a carbon monoxide (CO) alarm, a combination smoke and CO alarm, or a similar device for providing warning to occupants of a residence or other building of a potentially life threatening condition. Life safety device  10  is typically mounted on a wall or ceiling, and is connected to a source of alternating current (AC) power. 
     As shown in  FIG. 1 , life safety device includes low voltage supply  12 , battery assembly  14  (which includes rechargeable battery  16 , battery charging circuit  18 , booster circuit  20 , and battery protection circuit  22 ), regulator electronics  24 , hazards detector  26 , microcontroller unit (MCU)  28 , sounder circuitry  30 , and battery test electronics  32 . 
     Low voltage supply  12  is connected to an AC mains input, as represented by line input L and neutral input N. Low voltage supply  12  converts AC input power to DC charging power, which is provided to the Charge In input of battery assembly  14  and regulator electronics  24 . Low voltage supply  12  also provides an AC_ON monitoring signal to MCU  28 , which indicates that low voltage supply  12  is receiving AC power from the AC mains input. 
     Battery  16  of battery assembly  14  is a long life rechargeable battery, such as a lithium ion rechargeable battery. Battery charging circuit  18  maintains charge on battery  16  using the charging power from low voltage supply  12 . Booster circuit  20  increases battery voltage Vbatt, which may range from about 2.2 to 4.2 volts, to output voltage Vout, which is used by regulator electronics  24  to provide regulated voltage to hazards detector  26  and MCU  28 . Vout may be, for example, a constant voltage of about 8.7 volts. 
     Battery protection circuit  22  provides protection to battery  16  against over-current and over-discharge conditions. Battery protection circuit  22  enters protection modes, in which battery  16  may be disconnected from other circuit components when the battery voltage Vbatt is too low (an over-discharge condition) or when the current being drawn from battery  16  exceeds a maximum current level (over-current protection). 
     Hazards detector  26  may be, for example, a photoelectric or ionization type smoke detector, a carbon monoxide detector, or a combination smoke and carbon monoxide detector. The output of hazards detector  26  is provided to MCU  28 . 
     MCU  28  coordinates and controls the operation of life safety device  10 . Based upon inputs received from hazards detector  26 , MCU  28  determines whether a condition exists that requires sounding an alarm to warn users of a potentially dangerous condition. If an alarm is required, MCU  28  provides control signals to sounder circuitry  30  to generate the appropriate alarm. In some cases, the alarm will be an audible signal generated continuously or in pulses. In other embodiments, sounder circuitry  30  may generate a verbal message (or a combination of an audible signal and a verbal message) to occupants in response to a command from MCU  28 . 
     During the course of operation of life safety device  10 , MCU  28  will periodically perform a battery test using battery test electronics  32 . At the appropriate time, MCU  28  provides a battery test pulse BAT_TEST to battery test electronics  32 , which causes battery test electronics  32  to turn on and draw current from the Vbatt output of battery assembly  14 . Battery test electronics  32  provides test output BAT_VOLT to MCU  28  that represents the measured battery voltage while the discharge is taking place. During this normal battery test operation, the battery test pulse BAT_TEST is very short (typically 100 microseconds). The duration of the battery test pulse is selected to be just long enough to make sure that a steady state condition is reached. The battery voltage is measured, and the test is then terminated so that battery  16  is allowed to recover from the discharge. 
     MCU  28  includes an internal timer for determining when life safety device  10  has reached its normal end of life. At that time, MCU  28  initiates an end of life mode which notifies the user that device  10  should be replaced, and automatically controls discharging of battery  16  so that a disposal of life safety device  10 , battery  16  will not pose a safety hazard due to the charge remaining in battery  16 . In performing the automatic discharging function, MCU  28  makes use of battery test electronics  32  to perform discharging of battery  16  at a low duty cycle that will not cause overheating and will not result in battery protection circuitry  22  preventing discharging of battery  16 , while still performing the discharging in a rapid manner. 
       FIG. 2  is a flow diagram illustrating the operation of MCU  28  as it transitions from standby mode  40  to end of life (EOL) mode  50 . As part of standby mode  40 , MCU  28  checks to determine whether the end of life (EOL) limit has been reached (step  60 ). If the EOL limit has not been reached, MCU  28  continues normal operation mode  40 . If the EOL limit has been reached, MCU  28  enters the end of life (EOL) mode ( 50 ). At the beginning of at the end of life mode  50 , MCU  28  causes sounder circuitry  30  to produce a distinctive signal that indicates to the user that device  10  needs replacement (step  62 ). In one embodiment, this unique signal is a chirping sound generated every  30  seconds. 
     MCU  28  checks the status of the AC_ON signal from low voltage supply  32  (step  64 ). As long as the AC_ON signal indicates that device  10  is still connected to AC power, MCU  28  returns to standby mode  40 . Each time step  60  is reached, steps  62  and  64  will be repeated. In other words, MCU  28  continues to monitor the output of hazards detector  26 , and will cause an alarm to be generated through sounder circuitry  30  if a hazardous condition is detected. Sounder circuitry  30  will continue to generate the chirping sound while MCU  28  continues to monitor the output of hazards detector  26 . 
     When device  10  is unplugged from the AC source, the AC_ON signal changes state to indicate that AC power is not present. MCU  28  then turns off all functions of device  10 , including the chirping output of sounder circuitry  30  and the monitoring of hazards detector  26  (step  66 ). The only function remaining is a battery discharge function performed by MCU  28  in conjunction with battery test electronics  32 . 
     MCU  28  turns battery test electronics  32  on and off with the BAT_TEST signal according to a low duty cycle will produce controlled discharging of battery  16  (step  68 ). MCU  28  continues to check the AC_ON signal to see whether device  10  has been reconnected to AC power (step  70 ). During this discharge function, no user interface (such as through sounder circuitry  30 ) is active. 
     The duty cycle the BAT_TEST signal from MCU  28  during the controlled discharge function is low enough so that significant heating does not result from turning the battery test electronics on to discharge battery  16 . At the same time, the duty cycle is selected so that the controlled discharging of battery  16  occurs relatively quickly in preparation for disposal of device  10 . The duty cycle is less than 50% and preferably less than about 25%. In one embodiment, the duty cycle of the BAT_TEST signal during controlled discharge operation is one (1) millisecond ON followed by nine (9) milliseconds OFF. Thus discharging takes place only during one-tenth of each 10 millisecond cycle. The ON time of one millisecond used during the controlled discharge operation is significantly longer than the ON time used during normal battery test (which is typically on the order of 10 microseconds). 
     MCU  28  continues to monitor the AC_ON signal in case either AC mains power from the power utility has been temporarily interrupted, or the user has disconnected device  10  temporarily from AC power, but then reconnected it (step  70 ). If the AC-ON signal indicates that device  10  has been reconnected (or AC power has been restored), MCU  28  returns to standby mode  40  and then reenters EOL mode  50  at step  60 . The process shown in  FIG. 2  will then be repeated, with MCU  28  causing sounder circuitry  30  to once again alert the user that end of life has been reached, and that device  10  should be replaced. 
     If device  10  is not reconnected to AC power, MCU  28  continues to operate as long as battery assembly  14  can provide enough power to operate MCU  28  and battery test electronics  32  (step  72 ). If adequate power to continue operation is present, further discharging is needed, and MCU  28  returns to step  68 . Once power is no longer adequate to run MCU  28  and battery test electronics  32 , the discharge process is complete. Device  10  is dead and can be disposed of with household refuse (step  74 ). 
     The automatic controlled discharge function associated with end of life mode  50  will only occur when MCU  28  has determined that device  10  has reached the end of life. Controlled discharge only begins after device  10  has been disconnected from AC power. If device  10  is reconnected to AC power, the controlled discharge function will not recommence until device  10  is again disconnected from AC power. 
     If device  10  is disconnected from AC power prior to a determination that device  10  has reached its end of life, standby mode  40  will continue with device  10  using the power from battery  16 . EOL mode  50  will not be entered, because the EOL limit has not been reached (step  60 ). For example, if AC power from a power utility is temporarily lost, device  10  will continue to operate on power from battery  16 . When AC power is established, battery  16  will be recharged by low voltage supply  12 . 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.