the variable power energy harvesting system of the invention may be embodied in several alternative configurations for efficiently harvesting energy during alternatively low and high power input conditions , such as a solar system for example , which may operate under both low and high insolation conditions wherein available input power may vary by orders of magnitude . in a solar powered system , for example , under low insolation conditions , such as cloudy outdoor , conditions or indoors , solar panel power output is greatly reduced . it is often nevertheless desirable to harvest the small amount of available energy . the harvested energy may be used to run a low - power system or may be stored in batteries or other storage elements . in low power battery operated systems , this harvested energy can be enough to eliminate drain from standby power , extending battery life . this can facilitate continual operation without the frequent need for additional external charging . it is also often desirable to have the capability to maximize energy harvesting under high insolation conditions with the same system . this can require multiple modes of operation to get the most power from a solar panel , when the available power can change by several orders of magnitude , such as when moving a portable solar powered system from within a building having artificial lighting out into direct sunlight . due to these and other challenges and potential problems with the current state of the art , improved methods , apparatus , and systems for energy harvesting would be useful and advantageous . initially referring primarily to fig1 , an exemplary embodiment of a variable power energy harvesting system 100 has a control loop 102 , which includes control logic 104 . the system uses comparators 106 to assess the available harvested voltage , e . g ., v panel in relation to preselected high and low levels . a boost converter 110 with the low - power hysteretic control loop 102 based on harvested voltage v panel is used to regulate the power harvesting apparatus , in this example solar panel array 112 , at its mppt ( maximum power point tracking ) voltage . the hysteretic control loop 102 may be run as the only control , or may be used in conjunction with additional control ( s ) when the available harvested power is sufficient to power additional control circuitry . for example , the system may include the capability to detect the condition that power is being delivered to a load above a threshold level , and then engage a more sophisticated mppt regulation control . the power required for the operation of the mppt regulation is preferably small relative to the available harvested power . optionally , a temperature sensor may be provided for monitoring operating temperature . operating temperature may be used to adjust the harvested voltage based on temperature - induced effects on system performance . now referring primarily to fig2 , an example of an embodiment of such a variable power energy harvesting system 200 is shown . it has been found that actively monitoring output power v batt enables the system to choose the optimum harvested voltage v panel to maximize power output , e . g ., v batt , realized from the solar panel array 112 . a suitable current sensor 202 is used to track the output v batt . in the event the harvested voltage v panel is less than output voltage v batt , as determined by the sensor 202 , the control logic 104 may be used to select the optimal output voltage level v batt with a view toward maximizing power harvested ( v panel ) from the solar panel 112 . the current sensor 202 and mppt ( digital to analog converter ) dac 204 provide the functionality needed by the control loop 102 to improve the energy harvesting efficiency of the system 200 based on the conditions experienced by the energy source , e . g . solar panel 112 , that affect the voltage available ( e . g ., v panel ) for use by the system 200 . in the case of low - power applications for which it is particularly desirable to monitor the load , such as portable electronics powered by a battery , the system 200 may be configured to briefly wake up to check the status of the load , e . g ., v batt , and determine whether conditions allow the system to continue charging . this wake up is preferably operated at a relatively low duty cycle , so as to not dramatically change the power delivered to the load v batt . alternative views of an exemplary embodiment of a variable power energy harvesting system are shown in fig3 , depicting a schematic diagram of the system 300 , and fig4 , illustrating the operation of the system 300 . in this example , a dc / dc synchronous switching li - ion battery charger system 300 includes fully integrated power switches , internal compensation 302 , and full fault protection 304 . the system 300 utilizes temperature - independent photovoltaic maximum power point tracking ( mppt ) circuitry 306 to provide power output v batt from energy harvesting apparatus 312 during full charge constant - current ( cc ) mode . a preferred switching frequency of 1 mhz enables the use of small filter components , resulting in system size and cost advantages . in a presently preferred embodiment , in full - charge mode the duty cycle is controlled by the mppt regulator 306 . once termination voltage is reached , the regulator operates in voltage mode . when the regulator is disabled ( en is low ), the system draws less than 10ua quiescent current . now referring primarily to fig4 , an example of the operation of the variable power energy harvesting system in the context of a solar battery charger is shown . when the output voltage , such as battery charging voltage , is at a low threshold , e . g ., below 3 . 0 volts in this example , the system 300 enters a pre - charge state and applies a small , programmable charge current to safely charge the battery to a level for which full charge current can be applied . once the full charge mode has been initiated , the system 300 attempts to maximize available charge current to the battery by adjusting its duty cycle to regulate its input voltage to the mpp voltage of the energy harvesting apparatus , in this example photovoltaic cells . if sufficient current is available from the pv cell to exceed the safe 1c charge rate of the battery , then the programmable 1c current limit function will take priority over the mpp control function and the pv cell voltage will rise above the mpp value . when the battery voltage has increased sufficiently to warrant entering a maintenance mode ( constant voltage ), the pwm control loop forces a constant voltage across the battery . once in constant voltage mode , current is monitored to determine when the battery is fully charged . this regulation voltage as well as the 1c charging current may be set to change based on battery temperature . for example , in a preferred embodiment , there are four temperature ranges which may be set independently , for example , 0 - 10 ° c ., 10 - 45 ° c ., 45 - 50 ° c . and 50 - 60 ° c . the 0 ° c . and 60 ° c . thresholds stop charging and have 10 degrees of hysteresis . the intermediate points have 1 degree of hysteresis . a thermal shutdown is also provided . in the event the temperature of the system exceeds 170 ° c . ( in typical implementations ), the sw outputs tri - state in order to protect the system 300 from damage . the nflt and all other protection circuitry remains active to inform the system 300 of the failure mode . once the system 300 cools sufficiently , e . g ., to 160 ° c . ( typical ), the system 300 attempts to restart . if the system 300 reaches 170 ° c ., the shutdown / restart sequence repeats . an internal current limit is preferably maintained . the current through the inductor is sensed on a cycle by cycle basis and in the event a selected current limit is reached , the cycle is abbreviated . current limit is always active when the regulator is enabled . an under - voltage lockout feature is also preferably provided . in this example , the system 300 is held in the “ off ” state until the harvested voltage v panel reaches a selected threshold , 3 . 6v , for example . there is preferably a 200 mv hysteresis on this input , which requires the input to fall below 3 . 4v before the system 300 disables . a battery over - voltage protection circuit designed to shutdown the charging profile if the battery voltage is greater than the termination voltage is also preferably provided . the termination voltage may preferably be changed based on user programming , so the protection threshold is set to 2 % above the termination voltage . shutting down the charging profile puts the system in a fault condition . a variable power energy harvesting system 500 is depicted in fig5 . in this exemplary preferred embodiment , a buck converter configuration is shown . in this system 500 , the design is configured to anticipate the operational condition that the harvested voltage v panel may be greater than the required output voltage v batt . when the available harvested power is too low to power any active control circuitry , the pass device ( sw 1 ) in the buck converter 510 switches to the “ on ” state . this does not allow mppt to function , but can nevertheless provide the highest output power because the system power overhead can be almost zero . as an alternative , this can also be done with a parallel switch to the main pass device . similar to boost mode , the device can briefly wake up to check load status and available harvested power , e . g ., v panel . again , this should be at a low enough duty cycle to not dramatically change the power delivered to the load . a second mode of operation in a buck configuration is a low power hysteretic control based on the harvested voltage ( v panel ). this requires some power overhead , so the available power should be sufficient to run the required circuitry . similar to the hysteretic control in the boost mode , this can be to a fixed voltage , a changing voltage based on temperature , or an active control that monitors the output power to regulate the energy harvesting apparatus , e . g ., panel 512 , to its mppt voltage . finally , in the event the available power is sufficient , a more sophisticated control may be activated to allow for better mppt regulation . the efficiency gains from better mppt regulation should be significant enough to overcome the additional power overhead in running the additional control . alternative views of an exemplary embodiment of a variable power energy harvesting system having a buck converter are shown in fig6 . in preferred embodiments , a “ power good ” ( pg ) pin is used to indicate a fault condition or inability to charge . the output is an open - drain type . when en is low , the system 500 is nominally active . assuming no fault conditions , the pg pin is open . an external resistor r connected between the pg pin and an external i / o rail pulls the pg output up to the rail voltage , indicating that charging is underway . in the event a fault occurs , the pg pin is pulled to ground . the three events which can trigger a pg fault indication are preferably , input under - voltage , output over - voltage , and thermal shutdown . in preferred embodiments , the boost configuration of the system 500 is a dc / dc synchronous switching boost converter with fully integrated power switches , internal compensation , and full fault protection . a temperature - independent photovoltaic maximum power point tracking ( mppt ) system 500 thus embodied endeavors to maximize output current to the load , making it advantageous as a supply for battery charging applications . a switching frequency of 2 mhz is preferably chosen to enable the use of small external components for portable applications . examples of the operation of the system 500 are described for two typical scenarios . in one example , an intermediate charger circuit may be used between the system 500 and a battery or other storage element . the terminal voltage is set high . when the system starts up and ramps the output voltage above the pg threshold , the pg flag is set . until the load is capable of sinking the full amount of current available from the boost converter , the output rises to the light load regulation value of 5 . 0v . once sufficient load is applied to the system , the load itself determines the output voltage of the converter . in this case , the mpp tracking function adjusts the harvested input voltage of the system in order to maximize the output current ( and thus output power ) into the load . in another example , the system may be used to directly charge a li - ion battery , with the terminal vterm set low . insolation of the pv panel allows immediate charging of the battery . the mpp tracking function works to deliver the maximum possible charge current to the battery until the termination voltage of 4 . 0v is reached . at this point , the device automatically transitions to an accurate voltage regulation mode to safely maintain a full charge on the battery . the current through the inductor is sensed on a cycle by cycle basis and if current limit is reached , the cycle is abbreviated . current limit is always active when the boost converter is enabled . if the temperature of the system exceeds a selected threshold , such as 150 ° c ., for example , the sw outputs tri - state in order to protect the system from damage . the pg and all other protection circuitry remain active to inform the system of the failure mode . once the system cools to a lower threshold , e . g ., 140 ° c ., the system attempts to restart . in the event the system again reaches 150 ° c ., the shutdown / restart sequence repeats . the pg output is pulled low to signal the existence of a fault condition . the system 500 preferably also has an output over - voltage protection circuit 504 which prevents the system 500 from reaching a dangerously high voltage under sudden light load conditions . the typical over - voltage detection threshold is 102 % of the terminal voltage value . in the event of such a condition , the pg output is pulled low to signal a fault condition . input under - voltage protection 504 is also preferably provided . the system 500 monitors its input voltage and does not permit switching to occur when the input voltage drops below a selected threshold , e . g ., 250 mv . switching resumes automatically once the input voltage is above a higher selected threshold , e . g ., 275 mv . in addition , the pg output is pulled low to signal a fault condition . as shown in fig7 , the variable power energy harvesting system may be implemented in a buck - boost configuration 600 , that is , a configuration in which the harvested voltage v panel may be greater or less than the load voltage v batt . this can be done with all or part of the features of the boost and buck configurations shown and described herein working in parallel with a control mechanism to select which function should be active under given conditions . the buck - boost system 600 is a dc / dc synchronous switching charge controller which utilizes a temperature - independent photovoltaic maximum power point tracking ( mppt ) circuit in efforts to optimize power from energy harvesting apparatus such as a solar panel . the system 600 controls fet devices in buck , boost , and buck / boost configurations in order to support a wide range of system power levels and output voltages . a preferred 100 khz switching frequency results in low system quiescent current levels . the system 600 includes integrated battery charge controls for li - ion , nimh and lead acid batteries . in full - charge mode the duty cycle is controlled by the mppt regulator . fig8 illustrates an implementation of a variable power energy harvesting system 700 demonstrating a buck configuration similar to that shown and described with reference to fig1 - 4 , with the addition of a parallel charge pump to work when the harvested voltage v panel is below the output v batt . this charge pump is controlled to run when the harvested voltage v panel is within a certain voltage range . any of the above configurations can be combined with a traditional mppt component . the system may be operated with the traditional mppt solution in a standby low power state while one of the above configurations is active , and then begin to run when the available power is sufficient to run the traditional solution . an additional alternative feature of a variable power energy harvesting system 800 is shown in fig9 . this embodiment illustrates a configuration in which , under ultra - low power conditions , the energy harvesting apparatus , such as an array of pv panels , can be reconfigured to reduce losses . a pv panel is configured such that the cells may be recombined in series and / or parallel combinations in order to set the mppt voltage of the panel to approximate the voltage of the load . in this case , the load may be connected directly to the panel output or a simple linear circuit may be used to connect the load . the reconfiguring of the stack may be controlled by polling the conditions or as part of a control loop to maximize energy harvesting . alternatively , or additionally , a single capacitor or array of capacitors may be connected to all or some portion ( s ) of the energy harvesting apparatus such as a solar panel stack . once these capacitors receive a level of charge from the energy harvesting stack , e . g ., solar cells , this charge may be combined together or transferred to the power control circuitry for output . these capacitors are preferably controlled such that the voltage on the capacitors is held close to the mpp voltage of the energy harvesting apparatus . in the event of a low energy harvesting level , e . g ., some of the solar panel is blocked so that it is not producing sufficient power , the capacitors are used to provide substitute power in the interim until a higher energy harvesting level is achieved . many variations are possible within the scope of the invention . in preferred embodiments , the apparatus of the invention preferably includes circuitry adapted to provide the capability to regulate various levels of power produced by associated energy harvesting apparatus . for purposes of clarity , detailed descriptions of functions , components , and systems familiar to those skilled in the applicable arts are not included . the methods and apparatus of the invention provide one or more advantages including but not limited to , improved energy harvesting efficiency and / or improved operating ranges for energy harvesting systems . while the invention has been described with reference to certain illustrative embodiments , those described herein are not intended to be construed in a limiting sense . for example , variations or combinations of functions and / or materials in the embodiments shown and described may be used in particular cases without departure from the invention . various modifications and combinations of the illustrative embodiments as well as other advantages and embodiments of the invention will be apparent to persons skilled in the arts upon reference to the drawings , description , and claims .