Abstract:
A method to extend the functionality of a battery, the method comprising drawing power from the battery, and repetitively drawing a current pulse greater than the minimum conditioning current from the battery, thereby conditioning the battery.

Description:
FIELD OF THE INVENTION 
     The present invention relates to batteries, and more specifically, to extending the functionality of a battery. 
     BACKGROUND 
     Batteries are used for many functions, to power portable computers, provide backup power, and power all types of portable devices. However, batteries have a limited lifetime. After a period of use, most rechargeable batteries develop “voltage depression,” which results in the battery run-time decreasing after each recharge. 
     FIG. 1 illustrates a prior art voltage curve, for a new battery  110  and an old battery  120 . The turn-off voltage  130  is set, for example for a camcorder, at a level below the level of the fully charged battery. Thus, a new battery, as can be seen, takes an hour to reach the turn-off voltage  130 . However, an old battery  120  drops down more rapidly, to reach the turn-off voltage  130  after a mere 2.5 minutes. Thus, the old battery cannot be used to power devices, since the useable time is minimal. 
     The prior art to reduce the “memory” effect has been to deep discharge the batteries, typically at a current discharge rate well below the normal operating current level for a given application. Neither the battery run time or lifetime is enhanced by this. In addition, the standard practice of discharging a rechargeable battery down only to about 1.12 volts, which is considered the fully discharged level for new batteries, contributes directly to the battery “memory” phenomenon where older batteries have greatly reduced run time. 
     SUMMARY OF THE INVENTION 
     A method to extend the functionality of a battery, the method comprising drawing power from the battery, and repetitively drawing a current pulse greater than the minimum conditioning current from the battery, thereby conditioning the battery. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
     FIG. 1 is a voltage diagram of a prior art battery before use and after use. 
     FIG. 2 is a diagram showing the pulser coupled to a device. 
     FIG. 3A is a block diagram of one embodiment of the pulser. 
     FIG. 3B is a block diagram of an alternate embodiment of the pulser. 
     FIG. 4 is a more detailed block diagram of one embodiment of the control circuit of the pulser. 
     FIGS. 5A-E are voltage and current diagrams of one embodiment of the response of the pulser. 
     FIG. 6 is an exemplary voltage and current diagram of an actual pulse response. 
     FIG. 7 is a diagram of the minimum conditioning current versus battery voltage and load current, for a severely voltage depressed system. 
     FIG. 8 is a diagram of the minimum conditioning current versus battery voltage and load current, for a new system. 
    
    
     DETAILED DESCRIPTION 
     A method and apparatus for extending battery functionality is described. By defining a conditioning curve, which is a level of current needed to condition the battery, the system can successfully condition batteries to eliminate voltage depression. This extends the functionality of the battery significantly. 
     The battery functionality for a rechargeable battery includes battery runtime, e.g. the time a single charge lasts, and useable battery lifetime, e.g. the number of times the battery may be recharged and have a useful discharge period. For the remainder of this application, the term “conditioning” or “conditioner” will be used, and should be understood to refer to extending the useable battery lifetime and/or runtime. 
     Using the pulser, which will be described in more detail below, the battery has close to an ideal discharge cycle that eliminates voltage depression and enhances the lifetime and runtime of the battery. 
     The rejuvenation zone, the level of current needed to condition the battery and extend its functionality, modulates depending on the state of the battery. The state of the battery includes the percentage of remaining charge in the battery, as well as the battery age and type. The minimum conditioning current needed for increasing the functionality of the battery decreases as the voltage level decreases. If the current being drawn from the battery is higher than this minimum conditioning current, the battery functionality is extended. This is referred to in this specification as “conditioning zone.” 
     For many batteries the nominal current is a dose approximation of the optimum impedance matching point, where the battery voltage is reduced by 50%. For one embodiment, the power transfer is a function of the current density, which is cell size and battery type dependent. However, since current is difficult to measure, using the impedance matching point is an excellent, and easily measured, approximation. The actual minimum conditioning curve is battery age dependent. A typical curve for a voltage depressed battery is shown in FIG. 7, while a matching curve for a non-depressed battery is shown in FIG.  8 . 
     In one embodiment, low duty-cycle high current load pulses throughout the discharge cycle pull current drawn from the battery above this minimum conditioning current level. These short duration pulses prevent voltage depression from occurring, and increase the functionality of the battery. Furthermore, the pulses do not reduce the runtime of the battery, since they are low duty cycle. For one embodiment, the duty cycle is typically less than 0.01%. The low duty cycle reduces the chances of overheating, or otherwise damaging the battery. 
     FIG. 2 is a diagram showing the battery pulser coupled to a device. The battery  210  is coupled to the device  230 . The pulser  220  is coupled between the battery  210  and the device  230 . For one embodiment, the pulser  220  is only coupled between the battery  210  and the device  230  for a short time, to condition the battery  210 . After the battery functionality has been increased—for one embodiment one full charge cycle—the pulser  220  may be removed, until the battery&#39;s voltage depression again makes the use of the pulser  220  necessary. For another embodiment, the pulser  220  may be kept permanently between the battery  210  and the device  230 . 
     For one embodiment, the battery  210  may be any type of battery. For example, the battery may be a nickel based battery, such as nickel cadmium or nickel metal hydride. For another embodiment, the battery may be a lead-acid battery. For yet another embodiment, the battery may be a lithium ion battery. 
     In an exemplary application, the pulser  220  is contained in a housing that is interposed between a Nickel Cadmium battery pack  210  and a device  230  such as a camcorder. The pulser  220  can be considered as an attachment to the battery pack  210  that maintains the battery  210  in optimal condition. 
     The output of the pulser  220  may be disconnected by a switch, such that the current switching induced voltage spikes do not reach the device  230 , for one embodiment. For another embodiment, for a device that is insensitive to voltage variations, this switch may be eliminated, and the voltage may be directly passed to the device  230 . 
     FIG. 3A is a block diagram of one embodiment of the pulser. The pulser shown is designed to be used with a device that is insensitive to voltage variations. For example, this may be the case for power tools that have an electric motor as a load. This design may also be used for systems in which the battery is not in use during the conditioning process. For example, the conditioning system may be implemented in a battery storage system, in which the battery is stored awaiting use. 
     FIG. 3B is a block diagram of one embodiment of the pulser. The pulser  220  includes a control circuit  400  coupled between the positive and negative poles of the battery. The control circuit  400 , for one embodiment, has two outputs  360 ,  390 . The first output  360  controls a first switch  350 , while the second output  390  controls a second switch  340 . The first switch  350  couples the power from the battery to a device output  300  to which a device  230  may be coupled. Thus, when the first output  360  is asserted, switch  350  connects the battery  210  and the device  230 . When the first control circuit is deasserted, switch  350  disconnects the battery  210  from the device  230 . During this time, capacitor  330  powers the device  230 . 
     The second switch  340  couples the a controlled current from the output of battery  210  to ground  340 . Thus, when switch  340  is on, the battery output goes to ground  345 . 
     The control circuit  400  is designed to periodically draw a large controlled current from the battery  210 . The level of the current is designed to be greater than the minimum conditioning current, which will be described below. For one embodiment, the current drawn during the pulse is sufficiently large to reduce the voltage from battery  210  to one half the normally drawn voltage. Thus, if the battery  210  normally provides 6 volts, during the current pulse the voltage provided by battery  210  is reduced to 3 volts. 
     For one embodiment, these current pulses have a short duty cycle, such that capacitor  330  can provide power to the output  300  during the time when the battery  210  cannot provide a stable voltage. FIG. 5 below illustrates in more detail the respective current and voltages, seen by the battery  210 , the control circuit  400 , and the output  300 , i.e. a device coupled to the pulser  220 . 
     For one embodiment, for circuits which are insensitive to voltage variations, such as power tools, the battery disconnect switch  350  may be eliminated. 
     FIG. 4 is a more detailed block diagram of one embodiment of the control circuit  400 . The control circuit  400  includes a voltage regulator  410 , to set the FET switch  340  gate on-state voltage level, which determines the current pulse level. The voltage regulator  410  has as an input the output of the battery. The output of linear voltage regulator  410  is coupled to driver  440 . Driver  440 , for one embodiment, uses complimentary P-channel and N-channel FETs (CMOS) in its output. When driver  440  is asserted, the internal P-channel FET is on, and connects the output of the voltage regulator  445  to the gate of switch  340 . The P-channel FET has a sufficiently low ON resistance to insure that there is virtually zero voltage drop across it, thus guaranteeing that the linear regulator  410  output voltage  445  is accurately impressed on the gate of switch  340 . 
     The battery output is coupled to inverter  450 . 
     The control circuit  400  further includes a pulse generator  430 . The pulse generator  430  is responsible for generating the current pulse, as well as a blocking pulse, as will be described below. Pulse generators  430  are known in the art. 
     The output of pulse generator  430  is input to a logical OR  460 , a logical AND  470 , and a delay  420 . The logical OR  460  drives signal  360 , while the logical AND  460  drives signal  390 . As described above, signal  360  controls switch one, while signal  390  controls switch two. 
     The output of delay  420  is the second input into logical OR  460  and logical AND  470 . The output of logical OR  460  drives inverter  450 , while the output of local AND  470  drives driver  440 . Thus, when both the delay  420  and pulse generator  430  are on, signal  390  is asserted (one). When either the pulse generator  430  or the delay  420  is on, signal  360  is asserted (zero). Thus, signal  360  starts earlier, by the delay, and ends later by the delay, than signal  390 . FIGS. 5A-E clarify these signal relationships. 
     As stated previously, the linear voltage regulator  410  in FIG. 4 is used to set the current pulse level. The drain/source current of switch  340  is primarily determined by the transconductance of the FET and the gate to source voltage. By varying the driver voltage  445 , the gate voltage of the FET is varied, and the drain/source current of switch  340  will vary in proportion to the gate voltage. 
     For one embodiment, for circuits which are insensitive to voltage variations, such as power tools, the delay logic, the AND logic, the OR logic, and the driver  450  may be eliminated. Then, the pulses generated by pulse generator  430  may be directly coupled to the output, without isolating the device. In that case, the device sees the voltage spike at the end of the current pulse, as well as the lowered voltage. If the device is not damaged by such variations in voltage, the circuit may be substantially simplified. 
     FIGS. 5A-E are voltage and current diagrams of one embodiment of the response of the pulser. FIGS. 5A-E illustrate a single current pulse, and the various responses to the current pulse. Typically, the pulse frequency is between 100 pulses per second and 1 pulse per minute. 
     FIG. 5B illustrates the battery current being drawn from the battery during the pulse. Note that the pulse has a slew rate—the slope of the pulse as it rises and falls—and is not perfectly rectangular. The slew rate effects the overshoot  520  that is shown in the battery voltage, FIG.  5 A. 
     For one embodiment, the pulse lasts approximately 25 μs. For one embodiment, the pulse may be between 1 μs and 500 μs. Note that other pulse widths may be used. Typically, pulse widths of over 500 μs cause voltage droop and internal heating in FET  340 , which may raise the junction temperature above the safe limit. Typically, pulse widths under 5 μs require such a high slew rate that the overshoot  520  becomes too large. Thus, generally, the pulse rate is between 5 μs and 500 μs. Note that the pulse width controls the amount of power transferred into the battery. The amount of power needed for conditioning depends on the battery type and the cell size. 
     FIG. 5A illustrates the battery voltage. As can be seen, the battery voltage is significantly reduced during the current pulse. For one embodiment, by decreasing the slew rate of the battery current  530 , the voltage spike  520  is reduced. 
     The current switch drive  550 , FIG. 5C, corresponds to signal  390 , which indicates when battery current  530  starts to rise, and starts to fall. Current switch drive  390  pulls the current pulse from the battery. 
     The battery disconnect switch drive  560 , FIG. 5D corresponds to signal  360 . As can be seen, the signal  560  starts prior to the current switch drive  550 , and ends after the current switch drive  550 . While the battery disconnect switch drive  560  is active (e.g. low), the battery is disconnected from a device coupled to the pulser. For one embodiment, the delay before and the delay after the current switch drive  550  is identical, and determined by the delay set by delay unit  420 . 
     Load device voltage  570 , in FIG. 5E, is the voltage seen by a device coupled to the pulser. As can be see, when battery disconnect switch drive  560  is active, the battery is disconnected from the device, and the load device voltage  570  starts to slowly drop. The capacitor, resisting the change in voltage, maintains the voltage, and thus the voltage level sinks slowly. When the battery disconnect switch drive  560  is turned off, effectively reconnecting the battery and the device, the voltage level increases to the previous level. 
     Note that the battery voltage spike  520  is not seen by the load device voltage  570 , because the battery disconnect switch drive  560  disconnects the battery from the device, during the spike  520 . For one embodiment, there may be a very small spike. 
     For one embodiment, the capacitor is sized such that the voltage droop  580  is minimized. For one embodiment, the voltage droop  580  is less than 1%. Thus, the device is not affected by the current pulse. 
     For one embodiment, for circuits which are insensitive to voltage variations, such as power tools, the battery may remain coupled to the device, and the voltage levels seen by the device would be battery voltage  510 , including spikes  520 . 
     FIG. 6 is an exemplary voltage and current diagram of an actual pulse response. The voltage and current levels indicated are exemplary. As can be seen, the current increases for 4 μs to 20 Amperes. The current  650  may overshoot  660  slightly, which has no negative effect. 
     The battery voltage  610  decreases correspondingly, due to the internal resistance of the battery. For one embodiment, the current  650  is increased to drop battery voltage  610  to half its previous value. By decreasing the voltage  610  to half its previous value—3 volts from 6 volts in this example—there is an impedance match between the battery and the pulser. 
     The battery voltage  610  undershoots  620  slightly when the current pulse is first started. For one embodiment, the undershot  620  is a result primarily of internal battery capacitance, and other factors. 
     At the end of the pulse, there is a large battery voltage overshoot  630 . The amplitude of the battery overshoot  630  may be controlled by altering the slew rate at which the current turns off. If the slew rate is decreased (e.g. slope  640  is made gentler, the amplitude of the voltage overshoot  630  may be decreased. This may be useful for devices that are very sensitive to voltage variations. Note, however that for one embodiment the battery voltage  610 , at the time of the overshoot  630 , is isolated from any device coupled to the pulser, as was described above. 
     FIG. 7 is a diagram of the minimum conditioning current curves versus battery voltage and load current, for a highly voltage depressed battery system. Note that this Figure does not illustrate the pulses described above. It shows the relationship of the minimum conditioning currents  730 ,  735  with respect to battery voltage  710 . As can be seen, when the battery is fully charged, the minimum conditioning currents  730 ,  735  are quite high, a large multiple of the standard battery load current  720 . Therefore, the current pulses must be large, compared to the standard load current. 
     The minimum DC current conditioning curve  730  illustrates the conditioning current needed, if a steady current were being pulled from the battery. The pulsed current conditioning curve  735  illustrates the current levels needed for conditioning when the current is pulled from the battery using current pulses, as described above. As can be seen, the DC conditioning curve  730  requires higher currents than the pulsed conditioning curve  735 . This is the result of the slew rate effect. Rapidly increasing and decreasing currents have a larger effect than a steady current. Thus, because the conditioning current is pulsed, rather than pulled as a steady DC current, a lower current level, and therefore less power, needs to be pulled from the battery to effect conditioning. 
     Compare this to FIG. 8, illustrating a similar battery&#39;s rejuvenation curve, if the battery does not suffer from voltage depression. As can be seen, the minimum DC current conditioning curve  830  and minimum pulsed conditioning curve  835  retain their relationship to each other. However, both are considerably higher in a new battery than in a voltage depressed battery. 
     Thus, a prior art device that is unaware of the conditioning curve, may accidentally hit the conditioning current level, for a severely voltage depressed battery. However, as the battery becomes rejuvenated or conditioned, the conditioning curve moves up, requiring higher and higher currents. Thus, without awareness of the battery condition-dependence of the conditioning curves, prior art devices could not consistently condition batteries. They may ride the conditioning curve, hitting it occasionally as the battery becomes more voltage depressed. However, the battery is never fully rejuvenated using this method. 
     The system described conditions the battery, by using short, repeated, current pulses. For one embodiment, the current pulses are periodic, e.g. every 10 seconds. For another embodiment, the current pulses may occur at irregular intervals. However, repeated pulses are used, above the minimum conditioning current level. For one embodiment, the current pulses are designed to reduce the battery voltage, during the duration of the pulse, to half its normal value. This provides an impedance matching, which has been shown to be most effective for the batteries tested. During the current pulse, while the battery voltage is reduced, for one embodiment, the battery voltage is disconnected from any device coupled to the battery and pulser. A capacitor or similar system provides power for a device coupled to the battery during this time. For one embodiment, the battery is disconnected a slight time interval before, and disconnected a small time interval after the current pulse. This prevents the voltage spike that occurs at the end of the current pulse from affecting the device. 
     Note that although the above description states that the battery may be in use during this conditioning process, it need not be in use. This conditioning may be done when the battery &amp; pulser are not coupled to any device. In that case, for one embodiment, the simplified system, described above as FIG. 3A may be used, and the disconnection logic may be eliminated. 
     For one embodiment, the battery may be conditioned 100%, at which point the battery behaves like a new battery. However, in some cases, the battery may be partially damaged, or otherwise unable to be fully conditioned. For example, if a battery has certain irreparable damage, some portion of the battery may not be conditionable. However, certain types of damage may be repaired using the above conditioning technique. For example, damaged cells may be restored, since the high current pulses have the effect of reducing internal shorts, and in some cases eliminating them. 
     Thus, a typical conditioning may maintain the battery at 95% effectiveness, for example. For one embodiment, the typical conditioning results vary by battery type, as well as the type of use that was made of the battery. For example, a camcorder battery may have a lower level of conditioning if the battery has been left discharged for an extended time, or if the battery has been stored in an excessively hot location, leading to battery damage. 
     Note that the voltages and current levels provided in FIGS. 5,  6 ,  7 , and  8  are exemplary, based on the expected results derived from experiments using 1000 mA/hour rechargeable NiCad batteries. It is to be understood that other batteries would have similar, but not identical curves. 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.