Abstract:
A battery charger with improved overcharge protection mechanism and method of charging a battery with such charger. In one embodiment, the charger includes: (1) a voltage sensor for sensing a voltage of a battery that the battery charger is charging and (2) a controller, coupled to the voltage sensor, that adjusts a charge mode of the battery charger when samples of the voltage taken over a predetermined period of time are within a predetermined range thereby to prevent overcharging of the battery.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to battery chargers and, more specifically, to a battery charger with improved overcharge protection mechanism and method of charging a battery with such charger. 
     BACKGROUND OF THE INVENTION 
     Cordless telephones have become a staple in today&#39;s homes. They offer nearly the portability of a cellular telephone, but cost no more to use than a wireline telephone. 
     Cordless telephones consist of a base and a handset, each containing a wireless transceiver. The base plugs into a wireline telephone jack and derives its power from a wall outlet. The handset is battery-powered. The base often provides a cradle for the handset to allow the handset to be conveniently stored and recharged. 
     The number of cordless telephones is and has been rapidly increasing due to user demands for higher mobility and broader general utility. This has resulted in a higher demand for improved performance from the rechargeable batteries to allow the telephones to be operationally available for a greater percentage of the time. Conventional battery charging and monitoring systems are focused on simplicity of operation, regardless of any degenerative impact that the systems may have on the batteries themselves. For example, a conventional battery charging and monitoring system may provide a fixed charging current during the charging period. This fixed charging current is typically maximized to assure that the batteries are charged quickly. Alternatively, a battery may be charged only by a trickle current, which may not damage the battery, but requires an unacceptably long period of time to charge a fully discharged battery. 
     In spite of these problems, an even more critical problem exists. Batteries quick-charged for faster availability are often at risk of being overcharged. Overcharging a battery usually results in partial, or even permanent, damage to the battery. In addition, overcharging results in a waste of electricity. 
     To address this problem, prior art battery chargers attempted to detect when the battery was fully charged so they could prevent overcharging and damaging the battery. Designers of such battery chargers noticed that batteries tend to exhibit a small, but detectible, drop in their terminal voltage when they reach a full charge. 
     Unfortunately, not all batteries exhibit a detectible voltage drop. In fact, some batteries exhibit no drop whatsoever. To complicate matters, battery characteristics change over time. Batteries that may have exhibited detectible drops at one time, now no longer may. These facts kept prior art battery chargers from being able to detect full charges reliably for all batteries that they may be called upon to charge. 
     Accordingly, what is needed in the art is a battery charger, and related method, that can more reliably determine when a battery is fully charged. In addition, what is needed in the art is a cordless telephone set that incorporates the charger or the method. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, the present invention provides a battery charger with improved overcharge protection mechanism and method of charging a battery with such charger. In one embodiment, the charger includes: (1) a voltage sensor for sensing a voltage of a battery that the battery charger is charging and (2) a controller, coupled to the voltage sensor, that adjusts (or terminates) a charge mode (such as a quick charge mode) of the battery charger when samples of the voltage taken over a predetermined period of time are within a predetermined range thereby to prevent overcharging of the battery. 
     The present invention rests on the recognition that batteries do not always exhibit a dramatic voltage drop when they reach full charge. Instead, they sometimes simply maintain a relatively constant voltage. Prior art battery chargers that quick-charge until they detect the voltage drop continue to quick-charge after the battery has been fully charged, wasting power, generating heat and potentially harming the battery. The present invention instead recognizes the substantial constancy of the voltage as indicating a full charge and adjusts (or completely terminates) the charge mode in response. The present invention enjoys substantial utility in avoiding the waste of power, generation of heat and risk of battery damage that the prior art realizes. 
     In one embodiment of the present invention, the predetermined period of time is at least 10 minutes. In an embodiment to be illustrated and described, the predetermined period of time is 30 minutes. Six samples taken at five-minute-long periodic intervals are filtered, smoothed or compared to a range of allowable reference voltages to determine whether the sensed voltage is constant enough to recognize that the battery has been fully charged and terminate quick-charging as a result. 
     In one embodiment of the present invention, the controller enters a trickle charge mode of the battery charger when the samples taken over the predetermined period of time remain within the predetermined range. Those skilled in the art are familiar is with quick charge and trickle charge modes for battery chargers. The present invention makes advantageous use of such known modes and improves substantially upon the conditions under which such modes are selected. Of course, later-discovered quick charge and trickle charge modes fall within the broad scope of the present invention. 
     In one embodiment of the present invention, the controller alternatively terminates the quick charge mode when the voltage drops by at least a predetermined amount between successive ones of the samples. As stated above, some batteries do not exhibit a voltage drop upon attaining full charge; others do. Accordingly, in one embodiment to be illustrated and described, the battery charger looks for either voltage constancy or drop, taking either as an indicator of full charge. 
     In one embodiment of the present invention, the controller alternatively terminates the quick charge mode upon expiration of a predetermined maximum charge time. This watchdog function ensures that the quick charge mode does not continue indefinitely. In an embodiment to be illustrated and described, the predetermined maximum charge time is about four hours. 
     In one embodiment of the present invention, the controller takes the samples at periodic intervals. Of course, the controller may take samples aperiodically. The present invention does not require a certain number of samples, a particular period of time or periodicity in sampling. 
     In one embodiment of the present invention, the battery charger is part of a cordless telephone base and the battery is part of a cordless telephone handset. Those skilled in the pertinent art will realize, however, that the present invention will find wide-ranging use in a variety of applications that employ rechargeable batteries. 
     The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a cordless telephone set that can incorporate a battery charger constructed according to the principles of the present invention; 
     FIG. 2 illustrates a flow diagram of charge states that the charger of FIG. 1 can attain and the conditions under which the charger changes charge states; and 
     FIG. 3 illustrates a graphical representation of battery voltage as a function of time as a battery is charged according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring initially to FIG. 1, illustrated is a cordless telephone set  100  that can incorporate a battery charger  150  constructed according to the principles of the present invention. The telephone set  100  comprises a base  110  and a handset  120 . 
     The base  110  includes an RF transceiver  112  and an antenna  114 . The base  110  further includes a cradle  116  for holding the handset  120  when not in use. The handset  120  includes its own RF transceiver  122  and antenna  124 . The handset  120  further includes a battery  126  for storing the electrical current needed to operate the telephone  100 . The handset  120  still further includes a controller  128 , constructed according to the principles of the present invention, for use with the charger  150  and battery  126 . 
     The charger  150  in the base  110  includes contacts  118  for providing recharging current from the charger  150  to the battery  126  in the handset  120  when the handset  120  is cradled in the base  110 . In addition, the controller  128  is employed to control the state in which the charger  150  provides the recharging current to the battery  126 , in addition to determining when the battery  126  has been fully charged. 
     Turning now jointly to FIG.  2  and FIG. 3, illustrated are a flow diagram  200  of charge states that the charger  150  of FIG.  1  can attain and the conditions under which the charger  150  changes charge states (FIG. 2) and a graphical representation  300  of battery voltage as a function of time as a battery  126  is charged according to the principles of the present invention (FIG.  3 ). The flow diagram  200  begins in a start step  202 . The start step  202  corresponds to a starting point  310 , arbitrarily established at zero minutes, in FIG. 3. A voltage sensor samples the voltage in the battery  126  being charged in a step  204 . As can be seen in FIG. 3, the voltage of the battery  126  can range between 0.0 and 5.0 volts. 
     FIG. 3 illustrates both first and second battery voltage curves  320 ,  330 . The first battery voltage curve  320  represents a first example in which the voltage of the battery  126  exhibits a small but detectible drop in voltage  340  (for example, 30 millivolts) when the battery  126  has reached full charge. The second battery voltage curve  330  represents a second example in which the voltage of the battery  126  does not exhibit a detectible small drop in voltage  340  when the battery  126  reaches full charge. 
     Turning back to FIG. 2, an internal counter (not illustrated), which may be a counter that uses an existing 8 kHz digital cordless telephone sampling clock, is read in a step  206 . The counter reading is used to determine whether the battery  126  has been quick charging for longer than a predetermined period in a step  208 . 
     If the battery  126  is determined to have been quick charging for the maximum charging time (for example, four hours), the quick charging process ends in a step  290 , and trickle charging is reinstated, to prevent possible damage to the battery  126  due to overcharging. The approximate four hour maximum corresponds to the 240+ minutes illustrated along the x-axis of FIG.  3 . If the battery  126  has not exceeded the maximum charging time, the controller  128  must then determine by which charge state the battery  126  is being charged in a step  210 . Although the maximum charging time is illustrated as approximately four hours, the present invention is broad enough to encompass any maximum charging time. 
     In a step  212 , the battery  126  is trickle charged. For example, each time the telephone handset  120  is placed back into the cradle  116  for charging, the controller  128  initially sets the charger  150  to charge the battery  126  with a trickle charge. FIG. 3 designates this first charge state as “1× Charge State.” 
     In a step  214 , the battery  126  is allowed to trickle charge for five minutes. After the five minutes expires, the process advances to a second charge state (a step  216 ), and then returns to the step  204 . FIG. 3 designates this second charge state as “2× Charge State.” 
     After the battery voltage is again read in the step  204  and the counter is read in the step  206 , it is concluded that the battery  126  has not been quick charging past the four hours in the step  208 . Specifically, because quick charging has not started, the counter will not have reached the maximum charging time. In the step  210 , since the charger  150  has been incremented by one charge state, the process moves to a step  220 . Then, it is determined whether the voltage of the battery  126  is below a minimum operating voltage for the telephone  100  in a step  222 . Although the minimum operating voltage of the telephone  100  is 3.6 volts in the illustrated embodiment, the present invention is broad enough to encompass any minimum operating voltage for whatever device into which the battery  126  is placed. 
     If the voltage is determined to be less than 3.6 volts, the battery  126  is considered not fit for quick charging and must continue to be trickle charged. In this case, the process advances to a step  228  before returning to the step  220 . The charger  150  remains in the second charge state until the battery voltage is greater than the minimum operating voltage. Thus, the second charge state is used to determine if the battery  126  is capable of obtaining the capacity necessary to operate the telephone  100 . 
     In FIG. 3, both the first and second battery voltage curves  320 ,  330  are illustrated as being greater than 3.6 volts. As a result, the process continues to a step  224 . Although in the illustrated embodiment the full charge of the battery  126  is 4.25 volts, the present invention is broad enough to encompass any full charge voltage. If the battery voltage is determined to be greater than 4.25 volts, the battery  126  is found to be fully charged; thus, quick charging of the battery  126  is not required. The method then advances to the step  290  and trickle charging is reinstated. 
     However, if the battery  126  is not found to be greater than 4.25 volts, the controller  128  sets the charger  150  to quick charge the battery  126  in a step  226 . In addition, in the step  226 , the internal quick charge counter described above is started, and a maximum reference voltage (V MAX ) is established using the current voltage of the battery  126 . V MAX  will be used later to help determine the charge held by the battery  126 . 
     In FIG. 3, the first and second battery voltage curves  320 ,  330  are not greater than 4.25 volts. Thus, the process continues with the step  214 , then to the step  216 , and then to the step  204 . 
     After the battery voltage is again read in the step  204  and the counter is read in the step  206 , it is again determined that the battery  126  has not been quick charging past the four hours in the step  208 , since quick charging only began five minutes earlier. In the step  210 , the charger  150  is now determined to be in the third charge state, so the process moves to a step  230 . FIG. 3 designates the third charge state as “3× Charge State.” 
     Then, it is again determined whether the voltage of the battery  126  is fully charged in a step  232 . If the battery  126  is determined to be fully charged, quick charging of the battery  126  ceases at the step  290  and trickle charging is reinstated. However, if the battery voltage is not greater than 4.25 volts, it is determined whether voltage in the battery  126  is rising in a step  234 . 
     Specifically, the current battery voltage read at the beginning of this third charge state is compared to the V MAX  established during the second charge state. If the current battery voltage is not greater than the V MAX , thus the battery  126  is not charging, a problem with the battery  126  is assumed and quick charging is halted in a step  236 . At this point, the process is returned to the step  212  for more trickle charging. In addition, the quick charge counter is halted in the step  236 , since quick charging ceases once the process is returned to the first charge state. 
     In FIG. 3, both the first and second battery voltage curves  320 ,  330  are illustrated as rising during the third charge state. Thus, because voltage in the battery  126  is rising, V MAX  is set to the current battery voltage in a step  238 . It is then determined whether the process is in a sixth charge state in a step  240 . If the charger is determined to be in the sixth charge state, the process continues to the step  214 , and then to the step  216 , before returning to the step  204 . If the charger is not determined to be in the sixth charge state, the process moves to a step  242 , and then to a step  244 , before returning to the step  232 . Referring back to FIG. 3, because the first and second battery voltage curves  320 ,  330  are still in the third charge state, the process would follow the latter branch. 
     Once back in the step  232 , the charger is now in the fourth charge state, which FIG. 3 designates as “4× Charge State.” However, during the fourth charge state, the process follows the same path as the third charge state. Although now in the fourth charge state, when the process reaches the step  240 , the answer will again be NO and the process will again go to the step  242  and the step  244  before returning to follow the same path again. 
     The charger will now be in the fifth charge state, which FIG. 3 designates as “5× Charge State,” but will again be returned to the step  232  to follow the same path. After returning this time, the charger is in the 6th charge state, which FIG. 3 designates as “6× Charge State,” when it reaches the step  240 . Because the charger is now in the sixth charge state, the process continues to the step  214 , and then to the step  216 , before returning to the step  204 , as discussed above. 
     After the battery voltage is again read in the step  204  and the counter is read in the step  206 , it must again be determined that the battery  126  has not been quick charging past the four hours in the step  208 . Referring to the first and second battery voltage curves  320 ,  330  in FIG. 3, the process would only be at the 30 minute point, far short of the four hour maximum charging time. In the step  210 , the charger  150  is now determined to be in the seventh charge state, so the process moves to a step  270 . FIG. 3 designates the seventh charge state as “7× Charge State. 
     Then, it is again determined whether the battery voltage is continuing to rise in a step  272 . This is again determined by comparing the current battery voltage with the last recorded V MAX . If the battery voltage is not found to be rising, it must be determined whether the battery  126  is fully charged in a step  274 . However, if the battery voltage is determined to be rising, the V MAX  is set to the current battery voltage in a step  276 . The new V MAX  is then used to determine whether the battery  126  is fully charged in a step  278 . 
     In the step  274 , it is determined whether the battery  126  has reached full charge by determining if the current battery voltage has dropped below V MAX  by a preset value. The preset amount may be 0.3 volts, however the present invention is not limited to any particular value. As previously described, once a battery has reached full charge, the voltage of the battery will often demonstrate a slight, but detectable, drop from peak voltage reached during charging. Unfortunately, not all batteries demonstrate such a drop in voltage, rendering sole use of this method inadequate. If the battery voltage has dropped below the V MAX  by the preset amount, the battery  126  is found by the voltage drop method to be fully charged, and the process continues to the step  290 . 
     However, if the battery voltage does not detectably drop after reaching full charge, the process advances to the step  278  in which it is determined whether V MAX  a remains relatively constant for predetermined periods of time. If, in the step  278 , the battery  126  is determined to be fully charged, the process moves to the step  290 , since no further quick charging is required. If V MAX  is not constant for the preset periods of time, the process advances to the step  228 , to the step  204 , to the step  210  and back to the step  270 . Then, the above-described process for the seventh charge state is repeated until the battery  126  is fully charged. 
     For example, the charger  150  may determine that the battery  126  is fully charged when its voltage remains constant for six five-minute periods. In this example, and referring back to the second battery voltage curve  330 , the voltage begins to level off at a first period  350 . Five minutes later, the voltage remains relatively constant at a second period  352 . The voltage continues to remain relatively constant through third  354 , fourth  356 , fifth  358  and sixth  360  periods, each at five-minute intervals. Since the second battery voltage curve  330  does not demonstrate the measurable drop in voltage  340 , the prior method discussed above could not determine whether the battery  126  is fully charged. However, because the voltage remained relatively constant for the predetermined periods, a controller  128  employing this second method would determine that the battery  126  had reached full charge and cease its quick charging. Whether the voltage remains relatively constant can be determined by filtering, quantizing, averaging or truncating the current battery voltage. Of course, the present invention is not limited to a particular manner for determining whether the battery voltage has remained relatively constant for the preset period. By having this second method to cease quick charging when the battery  126  is fully charged, the charger  150  avoids the damage and waste associated with overcharging the battery  126  mentioned above. 
     Although in this example the first period of constant voltage occurs at approximately 115 minutes of quick charging, the present invention is not so limited. Thus, when the voltage of a charging battery becomes relatively constant can occur at any time during the charging process, limited only by the original charge left in the battery  126  when charging commences. In addition, the present invention is not limited to periods spaced at five-minute intervals. Accordingly, a charger  150  having a controller  128  employed to detect a fully charged battery  126  constructed according to the principles of the present invention can detect a constant voltage over any given time period. 
     Therefore, according to a controller  128  incorporating the present invention, the battery  126  is determined to be fully charged by one of two methods. First, if the battery  126  demonstrates a drop in voltage  340  when it reaches full charge, a method designed to detect this drop in voltage  340  is employed. However, if the battery  126 , even when fully charged, does not demonstrate such a drop in voltage  340 , the battery  126  may continue to be quick charged for the full four hours, or whatever maximum time quick charging is permitted. In this case, a second method designed to detect whether the peak battery voltage has become relatively constant is employed to determine when the battery  126  is fully charged. 
     Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.