Abstract:
A method and apparatus is provided for discharging an improperly charged cell. A circuit detects when an unsafe event occurs that disables the rechargeable battery. When such an event occurs, a discharge circuit discharges the cell to a level making it safer to dispose, thereby lowering the risk of explosion. A thermal circuit may also be provided that senses the temperature of the cell and discharges the cell based on the sensed temperature.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to protection circuits for rechargeable batteries, and more particularly to a protection circuit for discharging a rechargeable battery when a protection circuit detects a fault condition. 
     BACKGROUND OF THE INVENTION 
     Many portable electronic devices utilize a rechargeable battery to provide power. These devices include computers, cellular telephones, pagers, radios, and the like. While there are many types of rechargeable batteries used today, including nickel cadmium and nickel metal hydride, lithium ion batteries have become a popular choice. Lithium ion batteries are typically smaller and lighter than other rechargeable battery types while charge capacity is increased. 
     The charging of lithium ion batteries is conducted in a different manner than the charging of nickel type rechargeable batteries. Generally, nickel type rechargeable batteries are charged by applying a constant current until the cell reaches a predetermined voltage or temperature. A lithium ion cell, however, uses a different charging process. First, the lithium ion cell is supplied with a current until the cell&#39;s voltage rises above a threshold. Next, the battery charger is held at the threshold until the current of the cell decreases to a predetermined level. 
     Therefore, if a lithium ion battery is placed within a charger designed for a nickel rechargeable battery, the result may be damaging to the battery. For example, the voltage of the lithium ion battery may rise to a dangerous level or overheat. If the battery is overcharged a potential for an explosion of the battery exists. 
     Protection circuits have been developed to prevent such overcharging, but may result in a battery that is unstable and unusable when disposed. 
     SUMMARY OF THE INVENTION 
     The present invention is directed at providing an apparatus and method that discharges a rechargeable battery when a fault condition has been detected while charging the battery cell. More specifically, if a protection circuit on the battery detects a fault condition, the battery cell is discharged to a safe level. 
     According to one aspect of the invention, a protection circuit determines when a fault condition occurs during charging. Upon the fault condition, the cell is disabled from being charged further and a discharge circuit discharges the charge stored in the cell. 
     According to another aspect of the invention, the protection circuit determines when an improper charging condition exists. An improper charging condition may include a charging current or voltage potential being above a predetermined threshold. When the improper charging condition is detected, the charging of the cell is stopped, and the discharge circuit discharges the charge stored within the cell to a safe level. 
     According to another aspect of the invention, the protection circuit includes a temperature protection circuit that determines when the temperature of the battery is above a predetermined threshold. When the temperature rises above the predetermined temperature the charging of the cell is stopped, and the discharge circuit discharges the charge stored within the cell. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an overview schematic diagram of a final discharge protection circuit for a rechargeable battery; 
     FIG. 2 shows a schematic diagram of a final discharge protection circuit for a rechargeable battery; 
     FIG. 3 illustrates a schematic diagram of a final discharge protection circuit including an improper charging protection circuit; 
     FIG. 4 illustrates a schematic diagram of a final discharge protection circuit including a temperature protection circuit; 
     FIG. 5 illustrates an overview flow diagram of the operation a final discharge protection-system; 
     FIG. 6 illustrates a flow diagram for monitoring the current during a charging process; and 
     FIG. 7 illustrates a flow diagram for monitoring the temperature during a charging process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanied drawings, which form a part hereof, and which is shown by way of illustration, specific exemplary embodiments of which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” The term “connected” means a direct electrical connection between the items connected, without any intermediate devices. The term “coupled” means either a direct electrical connection between the items connected, or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function. The term “signal” means at least one current, voltage, or data signal. The term “battery” includes single cell batteries and multiple cell batteries. The term “cell” includes a single cell and multiple cells. Referring to the drawings, like numbers indicate like parts throughout the views. 
     FIG. 1 shows an overview schematic of a final discharge battery protection system. As shown in the figure, final discharge battery system  100  includes charge input circuit  110 , protection circuit  120 , cell  130 , and discharge circuit  140 . 
     Charging circuit  110  has an input port arranged to receive a charging signal and an output port arranged to provide the charging signal to protection circuit  120 . Protection circuit  120  includes an input port CHG, output ports CHGDEL and ADCHG. Protection circuit  120  is coupled to charge input circuit  110 , cell  130  and discharge circuit  140 . Cell  130  has an input port coupled to the CHGDEL port of protection circuit  120 , and an output port DRAIN coupled to discharge circuit  140 . Discharge circuit  140  has an input port DCHG coupled to cell  130  and an input port ACT coupled to the ADCHG port of protection circuit  120 . 
     According to one embodiment of the invention, cell  130  is a lithium ion cell. The cell, however, may be any type of rechargeable battery cell. As will be appreciated, there are many types of rechargeable cells, each having their own charging characteristics. For example, according to other embodiments, cell  130  may be nickel cadmium or nickel metal hydride. 
     Charge input circuit  110  is arranged to receive a charging signal and provide the charging signal to the CHG input port of protection circuit  120 . Protection circuit  120  determines if a fault condition relating to the charging exists. Protection circuit  120  may include many different functions to detect fault conditions. The fault condition may include improper charging conditions, such as a current level or voltage potential being above a predetermined threshold, or a temperature within the battery pack being above a predetermined temperature. According to other embodiments, fault conditions may include other faults related to the charging of a cell. During normal operation (when a fault condition is not detected) the CHGDEL output port of protection circuit  120  couples the charging signal to cell  130 . During this time, the cell is charged. When a fault condition is detected, protection circuit  120  stops coupling the charging signal to cell  130 , and notifies discharge circuit  140  to discharge the stored charge on cell  130 . The notification signal is provided through the ADCH port of protection circuit  120  to the ACT input port of discharge circuit  140 . According to one embodiment of the invention, the notification signal is a logical high (“1”) that enables discharge circuit  140 . As will be appreciated in view of the present disclosure, the notification signal may be any signal directing discharge circuit  140  to discharge cell  130 . The cell  130  is discharged to a safe level when the notification signal actives discharge circuit  140 . The cell  130  is discharged over a period of time. Cell  130  may be fully discharged or discharged to a predetermined level based on the cell type. Discharge circuit  140  may be designed to discharge cell  130  at a predetermined rate. 
     FIG. 2 shows an overview schematic diagram of a final discharge protection circuit  200  for a rechargeable battery. According to one embodiment, final discharge protection circuit  200  includes an improper charging protection circuit  210 , and a detection circuit  230 . According to another embodiment, protection circuit  200  includes improper charging protection circuit  210 , detection circuit  230 , and thermal protection circuit  220 . 
     Detection circuit  230  includes input ports ICHG and ITHM and an output port coupled to cell  130  and discharge circuit  140 . ICHG is coupled to improper charging protection circuit  210  and ITHM is coupled to thermal protection circuit  220 . Improper charging protection circuit  210  includes an input port coupled to charge input circuit  110  and an output port coupled to detection circuit  230 . Thermal protection circuit  220  has an output port coupled to detection circuit  230 . 
     Improper charging protection circuit  210  senses when an improper charging condition exists. An improper charging condition may exist when the current level is higher than a predetermined level or when a voltage is above a predetermined threshold. The predetermined levels are based on the charging specifications of the type of cell being charged. 
     When an improper charging condition exists, improper charge protection circuit  210  provides a signal to the ICHG port of detection circuit  230  indicating an improper charging condition exists. Detection circuit  230  detects when an improper charging condition exists and activates discharge circuit  140 , discontinues the charging of cell  130 , and cell  130  is discharged to a safe level by discharge circuit  140 . 
     The protection circuit  200 , as illustrated in FIG. 2, may also include thermal protection circuit  220 , according to one embodiment of the invention. Thermal protection circuit  220  helps to ensure that the battery does not reach an unsafe temperature. Thermal protection circuit  220  determines when the battery exceeds a predetermined temperature. The predetermined temperature is based on the safe charging temperature specifications associated with the cell being charged. When the predetermined temperature is exceeded, temperature protection circuit  230  provides a signal to the ITHM input port of detection circuit  230  indicating a fault condition. Detection circuit  230  detects the fault condition and activates discharge circuit  140 , discontinues the charging of cell  130 , and cell  130  is discharged to a safe level by discharge circuit  140 . 
     FIG. 3 shows a schematic of a final discharge protection circuit  300  including an improper charging protection circuit. As shown in the figure, final discharge protection circuit  300  includes: resistor circuit R 1 , fuse circuit F 1 , sense amplifier  310 , transistor M 1 , switch SW 1 , and cell  320 . 
     Sense amplifier  310  has a non-inverting terminal coupled to node N 2 , and an inverting terminal coupled to node N 1 . Fuse F 1  is coupled between node N 2  and node N 1 . Switch SW 1  is coupled between node N 1  and node N 4 . Transistor M 1  has a gate coupled to the output of sense amplifier  310 , a source connected to node N 4 , and a drain coupled to resistor circuit R 1 . Resistor circuit R 1  is coupled between node N 1  and the drain of transistor M 1 . Cell  320  is coupled between node N 1  and node N 4 . A positive terminal is coupled to node N 2  and a negative terminal is coupled to node N 4 . The positive terminal and negative terminal receive a charging signal (not shown), which is applied to charge cell  320 . 
     Switch SW 1  and fuse F 1  form a battery protection network that isolates cell  320  from an improper charging condition, such as attempting to charge the cell with an improper charger. When an improper charging condition exists, a control signal (Ctl) shorts switch SW 1 . According to one embodiment of the invention, switch SW 1  is a crowbar switch that shorts on a crowbar event. Sense amplifier  310  detects when the fuse is blown open (open circuited) and activates a discharge of cell  320  through transistor M 1  and resistor circuit R 1 . Fuse F 1  is selected such that is blows open when an improper charging condition is detected. According to one embodiment, an improper charging condition is when a predetermined current is exceeded that is determined to be unsafe or improper. The discharge time of the cell depends on the stored charge within the battery as well as the sizing of resistor circuit R 1  and transistor M 1 . Typically, an improperly charged battery will be discharged within an eight-hour period. This is advantageous because the battery may be discarded in a safer state than when fully charged, or overcharged. 
     Although transistor M 1  is shown as an NMOS device, transistor M 1  may be an NPN transistor, a PNP transistor, a Bipolar device, a MOS device, a GaAsFET device, a JFET device, as well as one or more components that are arranged to provide the function of transistor M 1  in the above described example. 
     FIG. 4 shows a schematic of a final discharge protection circuit  400  according to another embodiment of the invention. The protection circuit  400  illustrated in FIG. 4 is substantially similar to the final discharge protection circuit  300  as illustrated in FIG.  3 . However, final discharge protection circuit  400  includes a thermal protection circuit. As shown in the figure, protection circuit  400  includes the components as illustrated in FIG. 3 with the addition of thermal protection circuit  410 , detection circuit  420 , and switch SW_ 2 . As shown in the figure, protection circuit  430  includes fuse F 1 , resistor circuit R 1 , switches SW 1  and SW 2 , sense amplifier  415 , thermal protection circuit  410 , detection circuit  420 , transistor M 1  and cell  320 . 
     Sense amplifier  415  has a non-inverting terminal coupled to node N 2  and an inverting terminal coupled to node N 1 . Fuse F 1  is coupled between node N 2  and node N 1 . Switch SW 1  is coupled between node N 1  and node N 4 . Switch SW 2  is coupled between node N 1  and cell  320 . Transistor M 1  has a gate coupled to an output port of detection circuit  420 , a source coupled to node N 4 , and a drain coupled to resistor circuit R 1 . Resistor circuit R 1  is coupled between node N 1  and the drain of transistor M 1 . Cell  320  is coupled between switch SW 2  and node N 4 . Detection circuit  420  has an input coupled to thermal protection circuit  410  and an input port coupled to the output of sense amplifier  310 . A positive terminal is coupled to node N 2  and a negative terminal is coupled to node N 4 . The positive terminal and negative terminal receive a charging signal (not shown), which is applied to charge cell  320 . 
     The protection circuit  400  includes a thermal protection circuit  410  to protect the battery from reaching unsafe temperature levels and an improper charging circuit for protecting the cell from an improper charging condition, such as attempting to charge the cell with an improper charger. When an improper charging condition exists, switch SW 1  is shorted. According to one embodiment of the invention, switch SW 1  is a crowbar switch that shorts on a crowbar event. Sense amplifier  415  detects when the fuse is blown open (open circuited) and provides an activation signal to detection circuit  420  that activates a discharge of cell  320  through transistor M 1  and resistor circuit R 1 . Fuse F 1  is selected such that is blows open when an improper charging condition is detected. According to one embodiment, an improper charging condition is when a predetermined current is exceeded that is determined to be unsafe or improper. 
     Detection circuit  420  determines when either the sense amplifier  415  detects when the fuse F 1  is blown or when the battery exceeds a predetermined temperature. According to one embodiment of the invention, detection circuit  420  may be as simple as an OR circuit. Thermal protection circuit  410  determines when a predetermined temperature is exceeded at the cell and is arranged to provide a signal to detection circuit  420  indicating the predetermined temperature has been exceeded. When detection circuit  420  detects an improper temperature, switch SW 2  is opened preventing any further charging of cell  320 . Detection circuit  420  also activates a discharge of cell  320  through transistor M 1  and resistor circuit R 1 . The discharge time of the cell depends on the stored charge within the battery as well as the sizing of resistor circuit R 1  and transistor M 1 . Typically, an improperly charged battery will be discharged within an eight-hour period. This is advantageous because the battery may be discarded in a safer state then when fully charged, or overcharged. 
     FIG. 5 illustrates a logical flow for a final discharge protection system. After a start block, the logical flow moves to a block  510 , at which point the logic monitors the charger connection to the cell. Moving to a decision block  520 , a determination is made as to whether a charger is connected to the cell. When a charger is not connected to the cell, the logic returns to block  510  to continue monitoring for a charger connection. When a charger is connected to the cell, the logic moves to a block  530 , where the charging is monitored (See FIGS. 6 and 7 and related discussion). Transitioning to a decision block  540 , a determination is made if a fault condition exists. A fault condition is determined based on the characteristics and properties of the cell being charged. During normal operation (when no fault condition has been detected), the logic flows to a decision block  550  that determines if charging of the cell is complete. If the charging is complete, the logic flows to an end block and terminates. If the charging is not complete, the logical flow returns to block  530  to continue monitoring charges of the cell. When a fault condition has been detected, the logical flow moves to a block  560 , at which point the charging of the cell is stopped. Moving to a block  570 , the cell is discharged to a safe level and the battery is disposed (block  580 ). The logical flow then ends. 
     FIG. 6 illustrates a flow diagram for monitoring the current during a charging process. After a start block, the logical flow moves to a block  610  that monitors the current being applied to the cell. Decision block  620  determines if the. current is above a predetermined threshold. The predetermined threshold is set based on the properties and characteristics of the battery cell being charged. When the current is not above the threshold the logical flow returns to block  610  at which point the current continues to be monitored. When the current is above the threshold, the logical flow moves to a block  630  that sets an error condition indicating that a fault condition exists. The logical flow then ends. 
     FIG. 7 illustrates a flow diagram for monitoring the temperature during a charging process. After a start block, the logical flow moves to a block  710  that monitors the temperature of the battery. Decision block  720  determines if the temperature of the cell is above a predetermined threshold. The predetermined threshold is set based on the properties and characteristics of the battery cell being charged. When the temperature is not above the threshold the logical flow returns to block  710  at which point the temperature of the cell continues to be monitored. When the temperature of the cell is above the threshold, the logical flow moves to a block  730  that sets a fault condition indicating that the temperature of the cell has exceeded a predetermined temperature. The logical flow then ends. 
     The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.