Abstract:
The present invention teaches methods and systems for detecting internal battery abnormalities during charging and discharging states. The embodiments of the invention includes a circuit for determining charging and/or discharging state of the battery, a circuit for sampling the battery voltage at sequential time points, a circuit for measuring the decline of the voltage, a counter for counting T dec  the time while the voltage is in decline, a circuit for measuring the rate of the decrease of the voltage, a circuit for producing an indicator for internal abnormality if one or more of the following conditions is met: (a) the battery is in the charging state and T dec  exceeds a predetermined time; (b) the battery is in the charging state and the decrease of the voltage exceeds a predetermined voltage; and (c) the battery is in the discharging state and the rate of the decrease of the voltage exceeds a predetermined decline rate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Chinese Patent Application No. 200710166052.6, filed Oct. 30, 2007. 
     FIELD OF THE INVENTION 
     The present invention relates to methods and related devices for battery protection. 
     BACKGROUND 
     The lithium-ion battery has many advantages, such as its small size, high energy density, high cycle life, low self-discharge rate and no memory effect. Increasingly it has been used broadly as a major power device for mobile phones, notebook computers, digital cameras, electric cars and other products. Lithium-ion battery, however, is susceptible to damage and may catch fire if used improperly such as in case of overcharging or short circuits. Protective devices have been known for many years for detecting and preventing battery damage caused by such external factors, e.g., short circuits or overcharging. However, even in the course of normal use, lithium-ion batteries may develop side reactions in its internal electrochemical reactions, especially in the process of charging and discharging the battery. Such abnormal internal conditions may seriously affect the battery performance and its life. They also may produce large amounts of gas and cause the battery internal pressure to increase rapidly, leading to explosions and fires. It is thus desirable to use a protective circuit to monitor the charging and discharging of lithium-ion batteries to prevent the development of the abnormal internal conditions. 
     SUMMARY 
     The present invention teaches methods and devices for detecting a battery&#39;s internal abnormality during charging or discharging of the battery. One embodiment of the invention is a method for detecting a battery&#39;s internal abnormality during the charging state of the battery. It comprises sampling the battery voltage for a period of time, determining whether the voltage is in decline, determining the time period of continuous voltage decline, determining the amplitude of the voltage decline, and producing an indicator for internal abnormality if the time period of continuous voltage decline exceeds a predetermined time or the amplitude of the voltage decline exceeds a predetermined voltage threshold. 
     Another embodiment is a method for detecting a battery&#39;s internal abnormality during the discharging state. It comprises sampling the battery voltage for a period of time, determining whether the voltage is in decline, measuring the rate of the voltage decline, determining whether the rate of the voltage decline exceeds a predetermined decline rate, tracking T rate , which is the time while the rate of the voltage decline exceeds a predetermined decline rate, producing an indicator for internal abnormality if T rate  exceeds a preset threshold. 
     Yet another embodiment is a system for preventing battery damage by internal abnormalities during charging or discharging. It comprises a circuit for determining whether the battery is in charging or discharging status, a circuit for sampling the battery voltage at sequential time points, a circuit for measuring the decline of the voltage, a first counter for counting T dec , which is the time while the voltage is in decline, a circuit for measuring the rate of the decrease of the voltage, and a circuit for producing an indicator for internal abnormality if one or more of the following conditions are met: (a) the battery is in the charging state and T dec  exceeds a predetermined time; (b) the battery is in the charging state and the decrease of the voltage exceeds a predetermined voltage threshold; and (c) the battery is in the discharging state and the rate of the decrease of the voltage exceeds a predetermined decline rate. 
     A group of embodiments further include a charging/discharging circuit and a switch that turns off the charging/discharging circuit in response to the indicator of internal abnormality described above. Other variations, embodiments and features of the present invention will become evident from the following detailed description, drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram for a prior art battery protection switching circuit; 
         FIG. 2  is a flowchart showing an embodiment of the present invention; 
         FIG. 3  shows the functional modules of an embodiment of the present invention; 
         FIGS. 4-6  illustrate embodiments of the invention shown in  FIG. 3 , namely,  FIG. 4  depicts an implementation of the Status Module  301 ,  FIG. 5  depicts an implementation of the Voltage Module  302  and part of the Logic Module  303 , and  FIG. 6  depicts an implementation of the main part of the Logic Module  303  and the Drive Module  304 ; 
         FIG. 7  shows another embodiment of the invention; and 
         FIG. 8  shows a flow chart implementation of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. 
       FIG. 1  is a prior art circuit for detecting over-current charging or discharging problems and for shutting down the charging or discharging circuit. The circuit consists of the Control IC  106 , the charging MOS switch  105  and discharging MOS switch  104 . The control IC monitors the battery voltage and circuit current. The Control IC&#39;s charge control terminal  102  is connected to the charging MOS switch  105 , while its discharging control terminal  103  is connected to the discharging MOS switch  104 . Upon detecting a charging over-current condition, terminal  102  output turns from high to low, shutting down switch  105 . In a discharging over-current situation, terminal  103  goes from high to low and shut down switch  104 . The charging/discharging status in  FIG. 1  is reflected in the voltage difference between V M  (the voltage at the over-current detection terminal  101 ) and V B  (the voltage of the battery anode). If V M  is less than V B , the battery is being charged; otherwise, the battery is being discharged. The voltages can be measured or continuously monitored with various known methods. 
     The present invention discloses a method for monitoring internal abnormalities of a battery. When the battery is being charged, if the battery voltage shows a sudden and significant decrease, or the battery voltage decreases slowly but consistently for an extended period of time, it is an indication of an internal abnormality. For the latter indicator, to prevent false alarms, the present invention does not count it as an internal abnormality if after a period of voltage decline, the voltage starts to increase for a certain period of time. For detecting internal abnormalities when the battery is being discharged, if the rate of the voltage reduction is unusually fast, it is also an indication of a battery internal problem. Upon detecting internal abnormalities, the present invention turns off the charging or discharging circuit to prevent battery damage or fire hazards. 
     The embodiment of the present invention may include several basic functional modules. As shown in  FIG. 2 , functional module  201  determines the charging/discharging status of the battery and module  202  measures the battery voltage. Module  203  determines whether the battery has any internal abnormality. When such abnormality is detected, module  204  switches off the charging or discharging circuit. 
       FIG. 3  is another embodiment of the present invention. It shows the structural modules of an implementation of the invention. Status Module  301  detects the charge/discharge status, i.e., whether the battery is being charged or discharged. The Voltage Module  303  measures battery voltage and its changes. The Logic Module  303  determines whether there is an internal abnormality based on the measurement data. The Drive Module  304  switches off the charging or discharging circuit in response to signals from Logic Module  303 . 
     The modules in the above-described system can be implemented by various embodiments. For example, the Status Module  301  may be implemented by the circuit shown in  FIG. 1  for measuring and comparing V M  and V B  to determine whether the battery is in the discharge or charge state. The Voltage Module  302  may be implemented using pulse or continuous sampling method to obtain digital readings of the battery voltage. The Logic Module  303  may be designed in a way that it outputs an internal abnormality signal under the following three conditions: 
     (1) If the battery is being charged and the battery voltage decreases by an amplitude larger than a predetermined threshold value. The threshold amplitude change is 175-225 mV (e.g., 200 mV). 
     (2) If the battery is being charged and the battery voltage continues to decrease for a time period longer than a predetermined time threshold. The threshold time is 8-23 seconds (e.g., 15 seconds). 
     (3) If the battery is in the discharge state and the rate of decrease of the battery voltage is greater than a predetermined threshold. The threshold rate is 250-350 mV/sec (e.g., 300 mV/second). The minimal time period for the abnormal voltage decline rate is 250-750 μs (e.g., 500 μs). 
     The Drive Module  304  may be implemented using a circuit similar to what is shown in  FIG. 1 , where the charging or discharging circuit may be switched off by controlling the switches  105  or  104 , respectively. 
       FIGS. 4-6  illustrate certain embodiments of the present invention.  FIG. 4  shows a circuit implementation of Status Module  301 , using a comparator  401  and inverter  402 . The comparator  401  compares the voltages V M  and V B , as shown in  FIG. 1 . The comparator&#39;s output signal EN turns high when V M &lt;V B , i.e., when the battery is the charging state. A discharging state signal, EN 2 , is obtained through an inverter  402 . When the battery is in the discharging state, i.e., when V M &gt;V B , the signal EN turns low and EN 2  turns high. The status signals EN and EN 2  are used to enable detection circuits for internal abnormalities occurred in the charging and discharging state, respectively, as shown in  FIG. 6 . 
       FIG. 5  implements the Voltage Module  302  and part of the logic module. It starts with an oscillator  501  which provides a high-frequency clock signal, Clk-in, to a frequency divider  502 . The frequency divider  502  outputs a square wave clock signal Clk_out to the sampling circuit  504 . The battery voltage V battery  is fed to the sampling circuit  504  through an RC filer  503 . The sampling circuit has an Analog-to-Digital converter which outputs digitized battery voltage measurements to the buffer  505 . The buffer  505  feeds the voltage samples at the clock point X (i.e., V x ) and X+1 (i.e., V x+1 ) to the comparator  506 . 
     Comparators  506  and  507 , together with Logic  508  and Delay Timer  509 , form a delay circuit that keeps track whether the battery voltage goes up temporarily in a voltage decline to prevent false alarm. When V x+1  is lower than V x , the output of the comparator  506  goes to high. The logic unit  508  then signals to the buffer  505  to keep V x  as the reference voltage (V ref ). All the voltage samples (V actual ) is compared to V ref  through the comparator  507 . When V actual  is higher than V ref , the output signal of comparator  507 , ctrl, goes to low, which triggers the logic unit  508  to command the delay timer  509  to start counting the “delay,” an indicator of the time period when the battery voltage increases. When the delay reaches a preset threshold (3 seconds, for example), the logic unit  508  tells the buffer to store the V actual  as the new V ref  and sends out the first reset signal, reset 1 . If the delay is shorter than the threshold time, the logic unit  508  resets the delay timer  509  through a second reset signal, reset 2 . 
       FIG. 6  shows an embodiment of the Logic Module  303 . During the charging state, the internal abnormality is detected by the voltage amplitude drop detector, which consists of a subtractor  601  and a comparator  602 . The subtractor  601  outputs a signal ΔV which is the difference between the amplitude of the reference voltage (V ref ) and the actual voltage (V actual ). The comparator  602  compares ΔV to a preset threshold for ΔV and its output goes to high if ΔV is greater than the threshold. The output of the comparator  602  goes to an AND gate  607  through an inverter. 
     Both  601  and  602  are controlled by the charging status indicator EN, which is provided by the charging/discharging status circuit shown in  FIG. 4 . Only when EN is high (i.e., when V M  is smaller V B  as shown in  FIG. 4 ) are  601  and  602  enabled, which ensures that they only work during the battery&#39;s charging state. Circuits  601  and  602  are also controlled by the “ctrl” signal from the comparator  507  of  FIG. 5 . When ctrl turns high, it means that it is the initiation point for the abnormality detection period and the subtractor  601  and the comparator  602  are thus enabled. 
     The ctrl and EN signals also initiate the abnormality counter  603 . When the abnormal time duration reaches a preset threshold value, the output of  603  turns high, which is connected to an AND gate  607  via an inverter. The abnormality timer  603  may be reset by the reset 1  signal from  FIG. 5 , which indicates the return of the battery to normal status and the counting of the abnormality time period is restarted. 
     During the battery&#39;s discharging state, the internal abnormality may be detected by the circuit consisting of a voltage decline rate detector (dV/dt Detector)  604 , comparator  605  and abnormality timer  606 . The dV/dt Detector  604  may include a differentiator that measures the rate of voltage decline (i.e., −dV/dt). The output of  604  is compared with a preset threshold dV/dt value through the comparator  605 . The output of  605  turns high when the rate of voltage decline is greater than a preset threshold value, which in turn starts the abnormality timer  606 . When the abnormality lasts longer than a threshold time, the output of the abnormality timer  606  goes to low. The output of the abnormality timer  606  also goes to the AND gate  607 . The discharging abnormality circuits  604 ,  605 , and  606  are controlled by the discharging status indicator EN 2  from  FIG. 4 . 
     The AND gate  607  thus receive three abnormality signals as its inputs. If any one of the three abnormality signals is low, the output of the AND gate  607  (i.e., Co-drive) goes to low, which signals an internal abnormality of the battery. 
     The Drive Module  304  in  FIG. 3  is implemented with the circuit shown in a dotted rectangle in  FIG. 6 . The internal abnormality signal Co_drive is combined with the external abnormality signal Co as the inputs for a NAND gate. The output of the NAND gate becomes the final drive signal (Co_final) through an inverter. Co_final becomes low if either Co_drive or Co becomes low, which shuts down the charging or discharging gate switch to protect the battery. 
       FIG. 7  is another embodiment of the invention. The dotted lines show the modules  301 - 304  illustrated in  FIG. 3 . The Status Module  301  is implemented similarly as in  FIG. 4 . The Voltage Module  301  contains a capacitor  701  and an NMOS  702  for measuring the battery voltage V battery  at time X and X+1. Module  301  also contains a comparator  703  for detecting voltage decline. When V M  is smaller than V B , EN is high and EN 2  is low. NMOS  702  is off, which enables the capacitor  701  to maintain the voltage at X+1. Thus, the comparator&#39;s inputs are the voltages at X (Vx) and X+1 (Vx+1) and its output is Ares 1 . When the voltage decrease, i.e., Vx is greater than Vx+1, Ares 1  goes to high which signals the start of the battery abnormality detection period. 
     The Logic Module  303  has four major components. The first component of module  303  is a differential comparator  704  which determines whether the voltage drop ΔV is larger than a threshold ΔV ref . The output signal of  704  (Bres 1 ) turns high when the voltage drop is greater than a preset threshold which causes Co-drive of the Logic  707  to become low, indicating internal abnormality. 
     Module  303  also contains a differentiator, which consists of comparator  705 , capacitor  706 , and resistors R 1  and R 2 . The differentiator is controlled by the discharge signal EN and measures the rate of the voltage decline, dV/dt, and compares it with a reference value through comparator  706 . The output of  706  (Cres 1 ) goes to high when dV/dt is larger than the threshold value, which turns Co-drive of Logic  707  to low, signaling battery abnormality. 
     NOR gate  708 , NMOS  709 , PMOS  710  and capacitor  711  form a delay circuit. The delay threshold is determined by the discharge time of capacitor  711 . The delay circuit determines whether the abnormality lasts longer than the threshold time, which is the delay time. If such is the case, the oscillator OSC starts to work and the timer starts to count. The signals Ares 1  and Cres 1  are inputted into the Logic  707  to indicate whether the threshold time should be for the charging or discharging state. 
     Module  304  is similar to that in  FIG. 6 , where the internal abnormality signal Co_drive is combined with the external abnormality signal Co to trigger the shutting down of the charge or discharge circuit. 
       FIG. 8  shows a flow chart that depicts the logic flow of an embodiment of the present invention. The battery voltage  801  is sampled by an analog-to-digital sampler  802 , the latter outputting a digital actual voltage measurement V actual . V actual  is compared with a reference voltage, V ref , through a comparator  806 . If the V actual  is not greater than V ref , the device seeks the charging/discharging status ( 809 ). If the device is in the charging state, the rate of voltage decline (dV/dt) is measured and compared to a threshold value ( 810 ). If dV/dt is not greater than a preset threshold, the A/D sampler  802  continues with the voltage sampling. If dV/dt is greater than the reference threshold, the charging abnormality timer is enabled ( 811 ). If the abnormality time counted by the timer is shorter than a threshold value, the A/D sampler continues. If the time of the abnormality reaches the threshold value, the charging circuit is shut down ( 812 ). 
     When V actual  is greater than V ref  ( 807 ) and the device is in the discharging state, the discharging abnormality timer is enabled ( 813 ). When the discharging abnormality time reaches a preset value ( 814 ), the discharging circuit is shut down. If the discharging abnormality time is shorter than the preset value, the system examines whether the voltage drop is greater than a threshold value ( 815 ). If the voltage drop is indeed greater than the threshold, the discharging circuit is shutdown; otherwise, the A/D sampler will continue its sampling process. 
     Blocks  807 ,  808 ,  816 ,  817 , and  818  together implement the function that resets the abnormality timer if the voltage decline is reversed to a period of voltage increase. When V actual  is greater than V ref , which means that a voltage drop is reversed to voltage increase at least temporarily, the delay timer  816  is enabled. If the voltage increase continues for a period of time, as indicated by the count of the delay timer, which exceeds a preset threshold ( 817 ), the abnormality timer resets and the last V actual  becomes V ref  ( 818 ). However, if the delay time does not reaches its threshold when V actual  again becomes less than V ref , the delay timer is reset and the counting for the voltage decline time continues ( 808 ). 
     Although the invention has been described in detail with reference to several embodiments, additional variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.