Patent Publication Number: US-2022239150-A1

Title: Method and device for energy harvesting and charging rechargeable energy storage devices

Description:
FIELD OF THE INVENTION 
     The invention relates to a method and device for energy harvesting. More specifically, it relates to a method and device for charging rechargeable energy storage devices. 
     DESCRIPTION OF PRIOR ART 
     The use of voltage converters for extracting energy from an energy harvester and charging an energy storage device are well known in the art. For example in WO2018234485, an integrated circuit comprising a voltage converter is described for transferring energy from an energy harvester to a rechargeable storage device. The energy stored in the rechargeable storage device is then used as a power source for an application load. The application load can be coupled directly or indirectly to the storage device. An indirect coupling is for example established by placing an auxiliary voltage converter between the storage device and the application load and wherein the auxiliary voltage converter is configured for regulating a specific required voltage for the application load. 
     A variety of energy harvesters can be used as energy sources such as for example photovoltaic cells (PV), thermoelectric generators (TEG), piezoelectric energy generators and electromagnetic energy generators. The rechargeable storage device is for example a rechargeable battery such as Li-ion battery, a supercapacitor or a conventional capacitor. 
     One of the problems with the known energy harvesting systems is that when initially starting with a depleted rechargeable storage device, it takes a long time to initially charge the rechargeable storage device with energy from the energy harvester. As a consequence, it also takes a long time before the application load can receive power from the rechargeable storage device and start operating. Especially if the rechargeable storage device is a supercapacitor, being at zero Volt when fully de-charged, the charging time of the supercapacitor can be very long. But also charging rechargeable batteries to a required charging level for being ready supplying power to an application load during a sufficiently long time period can take a considerable long charging time. 
     A second problem is related to the variable conditions inherent to energy harvesting systems which result in situations where the energy harvester is not supplying continuously energy over a longer period of time, e.g. over time periods of multiple days. Depending on the type of energy harvester, energy harvesting can be interrupted over considerable long time intervals, e.g. time intervals of several hours, which degrades the reliability and long term functionality of the application load. Depending on the power consumption of the application load, this can result in an application load being stopped from operating. 
     For the second problem, back-up systems have been proposed wherein for example a primary battery is connected to the application load during the time intervals the energy harvester is not supplying energy. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and device for energy harvesting and charging a rechargeable storage device in an efficient way such that an application load coupled with the rechargeable storage device for receiving power can start operating more quickly, i.e. within a few minutes, even in situations where the rechargeable storage device is initially fully depleted. A further object is that the application load can continue to operate even under conditions wherein the energy harvester is interrupted over a longer period of time, for example interruptions of several hours or even several days. The object is also to maximize the extraction and use of energy from the energy harvester. 
     The present invention is defined in the appended independent claims. The dependent claims define advantageous embodiments. 
     According to a first aspect of the invention, a method for energy harvesting and supplying electrical power to an application load is provided. The application load to be powered with the energy harvested can be any type of application such as for example portable devices, sensors, external circuits, or wireless transmitters. 
     The method for energy harvesting according to the invention uses a voltage converter system for converting input power into output power and for charging at least a first and a second rechargeable storage device. Typically, the voltage converter system comprises one or more voltage converters. 
     The method according to the invention comprises steps of coupling a first power input path between an energy harvester and the voltage converter system for transferring input power from the energy harvester to the voltage converter system, monitoring a parameter V Batt1  and a parameter V Batt2  indicative of a charging level of respectively the first rechargeable storage device and the second rechargeable storage device and coupling the first rechargeable storage device to an application load such that the first rechargeable storage device when charged can supply power to the application load. 
     In embodiments, the parameters V Batt1  and V Batt2  correspond to a voltage of respectively the first and the second rechargeable storage device. In other embodiments, the parameters V Batt1  and V Batt2  correspond to respectively a first and a second accumulated charge acquired by for example charge counters counting accumulated charges during the charging process of the first and second rechargeable storage devices. 
     The method further comprises a step of coordinating charging of the first and the second rechargeable storage device by repetitively performing sub-steps of: 
       1   a ) coupling a first power output path between the voltage converter system and the first rechargeable storage device for transferring output power from the voltage converter system to the first rechargeable storage device,
   2   a ) operating the voltage converter system for charging the first rechargeable storage device with energy from the energy harvester until the parameter V Batt1  has reached an upper threshold value V Batt1-up , and wherein the charging of the first rechargeable storage device with energy from the harvester comprises transferring charges from the energy harvester to the first rechargeable storage device,
   3   a ) if V Batt1  has reached the upper threshold value V Batt1-up  and if V Batt2  is below an upper threshold value V Batt2-max  then
         i) decoupling the first power output path and coupling a second power output path between the voltage converter system and the second rechargeable storage device for transferring output power from the voltage converter system to the second rechargeable storage device, and   ii) operating the voltage converter system for charging the second rechargeable storage device with energy from the energy harvester, and wherein the charging of the second rechargeable storage device with energy from the harvester comprises transferring charges from the energy harvester to the second rechargeable storage device,
 
 4   a ) if during the charging of the second rechargeable storage device the parameter V Batt1  has subsequently decreased from the upper threshold value V Batt1-up  down to a lower threshold value V Batt1-low , with V Batt1-low &lt;V Batt1-up , then decoupling the second power output path and restart at step  1   a ).
       
     The method according to the invention further comprises a step of transferring energy from the second rechargeable storage device to the first rechargeable storage device if i) the parameter V Batt1  of the first rechargeable storage device has dropped below a critical threshold value V Batt1-SW , with V Batt1-SW &lt;V Batt1-low , and if ii) the parameter V Batt2  is equal or above a predefined threshold value V Batt2-low . The step of transferring energy comprises sub-steps of:  1   b ) decoupling the first power input path,  2   b ) coupling a second power input path between the second rechargeable storage device and the voltage converter system for transferring input power from the second rechargeable storage device to the voltage converter system, and  3   b ) operating the voltage converter system for charging the first rechargeable storage device with energy from the second rechargeable storage device until the parameter V Batt1  of the first rechargeable storage device has reached the upper threshold value V Batt1-up . 
     Advantageously, by charging a second rechargeable storage device during a de-charging phase of the first rechargeable storage device, i.e. during the decrease of V Batt1  from the upper threshold value V Batt1-up  down to the lower threshold value V Batt1-low , the application load can continue to operate during both the entire charging process of the second rechargeable storage device and during the entire process of repetitively re-charging the first rechargeable storage device. In this way, the use of the application load is not disturbing or interrupting the energy harvesting and the storage of energy in the second rechargeable storage device. 
     Advantageously, the second rechargeable storage device can be used to recharge the first storage device when the energy harvester is not operating. 
     Advantageously, when the energy harvester is not operating, by coupling a second power input path between the second rechargeable storage device and the voltage converter system, the voltage converter system is being used for transferring charges from the second to the first rechargeable storage device. Hence, the second rechargeable storage device can be a dedicated storage device operating at for example a different maximum voltage than the first rechargeable storage device and having a large energy storage capacity. The two storage devices can also be made of a different technology, the first rechargeable storage device can for example be a rechargeable battery such as a lithium ion battery, and the second rechargeable storage device can for example be a supercapacitor. In this way, if the energy harvester is interrupted over a long time period, the energy stored in the second rechargeable storage device can be transferred to the first rechargeable storage device and be used to continue power the application load. 
     Preferably, the second rechargeable storage device has an energy storage capacity that is more than five times, more preferably more than ten times, larger than the energy storage capacity of the first rechargeable storage device. 
     According to a second aspect of the invention an integrated circuit for energy harvesting is provided. The integrated circuit for energy harvesting comprising a voltage converter system that is suitable for converting input power into output power for charging at least two rechargeable storage devices, a first terminal connectable with an energy harvester, a second terminal connectable with a first rechargeable storage device, a third terminal connectable with a second rechargeable storage device, a controller for controlling the voltage converter system, a plurality of power input paths comprising at least a first power input path for transferring input power from the first terminal to the voltage converter system, a plurality of power output paths comprising at least a first power output path for transferring output power from the voltage converter system to the second terminal and a second power output path for transferring output power from the voltage converter system to the third terminal, a monitoring unit coupled with the controller and configured for monitoring a parameter V Batt1  and a parameter V Batt2  indicative of a charging level of respectively the first rechargeable storage device and the second rechargeable storage device when connected to respectively the second and third terminal. 
     In embodiments, the parameter V Batt1  and the parameter V Batt2  correspond for example to a voltage sensed at respectively the second and third terminal. 
     The integrated circuit for energy harvesting according to the invention is characterized in that the plurality of power input paths comprises a second power input path for transferring input power from the third terminal to the voltage converter system, and in that the voltage converter system comprises an input selection circuit for selecting a power input path from the plurality of power input paths so as to receive an input power via the power input path selected and an output selection circuit for selecting a power output path from the plurality of power output paths so as to output an output power via the power output path selected, and in that the controller is configured to form and to switch between a number of specific combinations of a power input and a power output path based on a comparison of the parameter V Batt1  with first predefined threshold values and/or a comparison of the parameter V Batt2  with second predefined threshold values. The specific combinations the controller can form and switch comprise: i) a first combination formed by selecting the first power input path and selecting the first power output path, ii) a second combination formed by selecting the first power input path and selecting the second power output path, iii) a third combination formed by selecting the second power input path and selecting the first power output path. 
     Selecting “a” power input path from the plurality of power input paths so as to receive an input power via the power input path selected has to be construed as selecting “one” power input path from the plurality of power input paths so as to only receive the input power via the power input path selected. Similarly, selecting “a” power output path from the plurality of power output paths so as to output an output power via the power output path selected has to be construed as selecting “one” power output path from the plurality of power output paths so as to output the output power only via the power output path selected. 
     Advantageously, by providing a second power input path for transferring input power from the third terminal to the voltage converter system, a second rechargeable storage device coupled to this third terminal can not only be charged with energy from the energy harvester for forming an energy reservoir, but can also be used as an alternative energy source for charging the first rechargeable storage device when the energy harvester is not operating. Indeed, as the third terminal is also coupled to the voltage converter system for suppling input power, the voltage converter system can be used to transfer charges from the second rechargeable storage device, being coupled to the third terminal, to the first rechargeable storage device, being coupled to the second terminal. In this way, even if the energy harvester is not operating, the first storage device can be continued to be charged such that the application load coupled to the first storage device can continue to operate. 
     In embodiments, the controller of the integrated circuit is further configured for: switching from the first combination to the second combination if the parameter V Batt1  becomes equal or larger than an upper threshold value V Batt1-up  and if the parameter V Batt2  is lower than an upper threshold value V Batt2 -max, switching from the second combination to the first combination if the parameter V Batt1  has decreased from the upper threshold value V Batt1-up  down to a lower threshold value V Batt1-low , with V Batt1-low  &lt;V Batt1-up , and switching from the first combination to the third combination if the parameter V Batt1  has decreased from the lower threshold value V Batt1-low  down to a critical threshold value V Batt1-SW , with V Batt1-SW  &lt;V Batt1-low , and if V Batt2  is above a lower threshold value V Batt2-low , with V Batt2-low  &lt;V Batt2-max . 
     In embodiments, the integrated circuit comprises a fourth terminal connectable with an auxiliary energy source such as a primary battery, and the plurality of power input paths comprises a third input path for transferring input power from the fourth terminal to the voltage converter system, and wherein the specific combinations of input/output paths comprise a fourth combination formed by selecting the third power input path and selecting the first power output path. 
     In further embodiments, the voltage converter system comprises a voltage converter for converting input power received via the selected power input path into output power outputted via the selected power output path, and wherein the voltage converter is one of the following: a boost voltage converter, a buck voltage converter or a buck-boost voltage converter. In other words, in these embodiments a single voltage converter is used in combination with the input and output selection circuits. 
    
    
     
       SHORT DESCRIPTION OF THE DRAWINGS 
       These and further aspects of the invention will be explained in greater detail by way of example and with reference to the accompanying drawings in which: 
         FIG. 1 a    schematically represents an example of an energy harvesting system according to the present disclosure, 
         FIG. 1 b    schematically represents a further example of an energy harvesting system according to the present disclosure, 
         FIG. 2  illustrates a charging process of a first and a second rechargeable storage device using the method according to the present invention, 
         FIG. 3  schematically shows an energy harvesting system comprising an integrated circuit according to the present invention, 
         FIG. 4 a    shows an example of a voltage converter system according to the invention comprising a buck/boost voltage converter, 
         FIG. 4 b    shows an example of a voltage converter system according to the invention comprising two buck/boost voltage converters, 
         FIG. 5  shows an example of a voltage converter system according to the invention comprising an input selection circuit for selecting between three power input paths and an output selection circuit for selecting between three power output paths, 
         FIG. 6 a    shows an embodiment of a voltage converter system comprising two voltage converters, 
         FIG. 6 b    shows an alternative embodiment of a voltage converter system comprising two voltage converters, 
         FIG. 7  shows an embodiment of voltage converter system comprising three voltage converters, 
         FIG. 8  shows an embodiment of voltage converter system comprising three voltage converters and comprising an input selection circuit for selecting between three power input paths and an output selection circuit for selecting between two power output paths. 
     
    
    
     The drawings of the figures are neither drawn to scale nor proportioned. Generally, identical components are denoted by the same reference numerals in the figures. 
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     The present disclosure will be described in terms of specific embodiments, which are illustrative of the disclosure and not to be construed as limiting. It will be appreciated by persons skilled in the art that the present disclosure is not limited by what has been particularly shown and/or described and that alternatives or modified embodiments could be developed in the light of the overall teaching of this disclosure. The drawings described are only schematic and are non-limiting. 
     Use of the verb “to comprise”, as well as the respective conjugations, does not exclude the presence of elements other than those stated. 
     Furthermore, the terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the disclosure described herein are capable of operation in other sequences than described or illustrated herein. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiments is included in one or more embodiment of the present disclosure. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one ordinary skill in the art from this disclosure, in one or more embodiments. 
     According to a first aspect of the invention, a method for energy harvesting and supplying electrical power to an application load is provided. The method for energy harvesting makes use of a system for energy harvesting comprising a voltage converter system. 
     A voltage converter system has to be construed as a system for converting input power received from an energy source into output power for charging a storage device. The energy source, for example an energy harvester, is supplying input power at an input voltage V in  and the voltage converter system is outputting output power at an output voltage V out  corresponding to the voltage of the storage device. The input voltage V in  can be higher or lower than the output voltage V out . Typically, the voltage converter system comprises one or more voltage converters and detailed embodiments of various voltage converter systems will be further described below. An example of a voltage converter is a DC-DC boost converter, a DC-DC buck converter or a DC-DC buck/boost converter. Generally, the voltage converter system is part of an integrated circuit, generally named power management integrated circuit (PMIC). 
     In  FIG. 1 a   , an example of a system  100  for energy harvesting is schematically shown. The system comprises a power management integrated circuit (PMIC)  1  comprising a voltage converter system  20  and a controller  40  for controlling the voltage converter system. An energy harvester  70  is coupled to a first terminal  11 , which is an input terminal for receiving power from the energy harvester at an input voltage V in . The PMIC  1  comprises a second terminal  12 , being an output terminal that is coupled to a first rechargeable storage device  50 . In this example, the application load  90  is coupled to the first storage device  50  by a direct connection. In other embodiments, a voltage regulator can for example be placed between the first storage device and the application load to generate a required voltage for the application load that is different from the voltage of the first storage device. The PMIC shown on  FIG. 1 a    comprises a third terminal  13  being connected with a second rechargeable storage device  60 . 
     The first rechargeable storage device is for example a rechargeable battery, a capacitor or supercapacitor and similarly, the second rechargeable storage device can also be either a rechargeable battery, a capacitor or supercapacitor. 
     In  FIG. 1 b   , a further example of a system for energy harvesting is schematically shown wherein a first application load  90   a  is coupled to a first storage device  50  and wherein a second application load  90   b  is coupled to a second storage device  60 . The first application load is for example a low-power load, for instance for monitoring purposes, and the second application load is for example a higher power load, for instance for communication means or actuation means. 
     The method for energy harvesting according to the present invention comprises steps of coupling a first power input path between an energy harvester and the voltage converter system for transferring input power from the energy harvester to the voltage converter system  20 , monitoring a parameter V Batt1  and a parameter V Batt2  indicative of a charging level of respectively the first rechargeable storage device  50  and the second rechargeable storage device  60  and coupling the first rechargeable storage device to an application load  90  such that the first rechargeable storage device when charged can supply power to the application load. 
     In embodiments, the parameters V Batt1  and V Batt2  correspond to a voltage of respectively the first and the second rechargeable storage device obtained by a voltage measurement. In other embodiments, the parameters V Batt1  and V Batt2  correspond to an amount of charges acquired with a charge counter. 
     When the first storage device is sufficiently charged, as indicated by an upper threshold voltage V Batt1-up  being reached, the first rechargeable storage device  50  can be used as a power supply for the application load  90 . The voltage threshold V Batt1-up  does not necessarily correspond to a fully charged storage device but it can be a value indicating that the first storage device is sufficiently charged to start supplying power to the application load. 
     The method according to the invention further comprises a step of coordinating charging of the first and the second rechargeable storage device by repetitively performing sub-steps  1   a ) to  4   a ) outlined here below. 
     Sub-step  1   a ) corresponds to coupling a first power output path between the voltage converter system and the first rechargeable storage device for transferring output power from the voltage converter system to the first rechargeable storage device. 
     Sub-step  2   a ) corresponds to operating the voltage converter system for charging the first rechargeable storage device with energy from the energy harvester until the parameter V Batt1  has reached an upper threshold value V Batt1-up . Hence, during the charging of the first rechargeable storage device, charges are being transferred from the energy harvester to the first rechargeable storage device. 
     Sub-step  3   a ) is performed if the conditions are fulfilled that V Batt1  has reached the upper threshold value V Batt1-up  and that V Batt2  is below an upper threshold value V Batt2 -max. Sub-step  3   a ) corresponds to: i) decoupling the first power output path and coupling a second power output path between the voltage converter system  20  and the second rechargeable storage device  60  for transferring output power from the voltage converter system to the second rechargeable storage device, and ii) operating the voltage converter system for charging the second rechargeable storage device with energy from the energy harvester. Hence, during the charging of the second rechargeable storage device, charges are being transferred from the energy harvester to the second rechargeable storage device. 
     Sub-step  4   a ) corresponds to decoupling the second power output path and restart at step  1   a ) if during the charging of the second rechargeable storage device the parameter V Batt1  has subsequently decreased from the upper threshold value V Batt1-up  down to a lower threshold value V Batt1-low , with V Batt1-low  &lt;V Batt1-up . 
     Hence, by repetitively performing sub-steps  1   a ) to  4   a ) the second rechargeable storage device  60  is being charged while maintaining the first rechargeable storage device  50  charged between charging levels V Batt1-low  and V Batt1-up . 
     In  FIG. 2 , a process for charging the first and second rechargeable storage device according to the method of the present disclosure is illustrated. The variation of the parameters V Batt1  and V Batt2  are shown as function of time, illustrating the charging and de-charging of respectively the first and second rechargeable storage device. As illustrated on  FIG. 2 , by performing the above mentioned sub-steps  1   a ) to  4   a ), the first storage device  50  remains charged by keeping the parameter V Batt1  of the first storage device  50  between the lower threshold value V Batt1-low  and the upper threshold value V Batt1-up , and in parallel the second storage device  60  is being charged with energy from the energy harvester while the first storage device  50  is supplying power to the application load. 
     The application load continues to be operated while the second storage device  60  is being charged such that there is no interruption in the operation of the application load. This is schematically illustrated with the example shown on  FIG. 2  where the time period wherein the application load is on and off is respectively indicated by “APPL. ON” and “APPL. OFF”. When the application load in on and when the second rechargeable storage device is being charged, then the first storage device is de-charging as indicated by the parameter V Batt1  of the first storage device decreasing from the upper threshold value V Batt1-up  down to the lower threshold value V Batt1-low . The lower threshold value V Batt1-low  is typically a value selected such that the first storage device is still sufficiently charged to provide electrical power to the application load. The values selected for the lower and upper threshold values for the first storage device depend on the type of storage device used, e.g. a rechargeable battery or a capacitor a or supercapacitor. The upper threshold V Batt1-up  is not necessarily equal to the maximum allowable voltage value V Batt1-max  of the first rechargeable storage device, V Batt1-up  can for example be a value that is smaller than V Batt1-max . 
     The method according to the present disclosure is characterized in that the method comprises a further step of transferring energy from the second rechargeable storage device to the first rechargeable storage device if i) the parameter V Batt1  of the first rechargeable storage device has dropped below a critical threshold value V Batt1-SW , with V Batt1-SW  &lt;V Batt1-low , and if ii) the parameter V Batt2  is equal or above a predefined threshold value V Batt2-low . The predefined threshold value V Batt2-low  is a value indicating that the second rechargeable storage device is charged to a minimum charging level allowing to transfer charges from the second to the first rechargeable storage device. 
     The step of transferring energy from the second to the first rechargeable storage device comprises sub-steps of:  1   b ) decoupling the first power input path,  2   b ) coupling a second power input path between the second rechargeable storage device and the voltage converter system for transferring input power from the second rechargeable storage device to the voltage converter system, and  3   b ) operating the voltage converter system for charging the first rechargeable storage device with energy from the second rechargeable storage device until the parameter V Batt1  of the first rechargeable storage device has reached the upper threshold value V Batt1-up . 
     In embodiments, the transferring of energy from the second rechargeable storage device  60  to the first rechargeable storage device  50  comprises a further sub-step  4   b ) in case the parameter V Batt1  has reached the upper threshold value V Batt1-up . Sub-step  4   b ) corresponds to performing at least one or a combination of the following steps: i) decoupling the first power output path  32   a  and/or decoupling the second power input path  31   b , ii) coupling the first power input path  31   a  and coupling the second power output path  32   b , iii) stop operating the voltage converter system  20 . 
     In case in sub-step  4   b ), the step ii) of coupling the first power input path  31   a  and coupling the second power output path  32   b  is applied, then if the energy harvester is supplying power, the second rechargeable storage device will continue to be charged with energy from the energy harvester. 
     The transferring of energy from the second to the first storage device when the parameter V Batt1  of the first storage device  50  has dropped below the critical threshold level V Batt1-SW  is also illustrated on  FIG. 2 . Such a drop of the parameter V Batt1  typically happens when the energy harvester is off or when the application load is consuming more power than the power the energy harvester is supplying. For example, as illustrated on  FIG. 2 , when the energy harvester is switching from an on state indicated by “E.H. ON” to an off state indicated by “E.H. OFF”, nor the first nor the second storage device can be charged with energy from the energy harvester. As a consequence, as the application load is on and hence consuming power, the voltage of V Batt1  when decreasing to V Batt1-low  will continue to further decrease until V Batt1-SW  is reached. As illustrated on  FIG. 2 , when V Batt1  drops below V Batt1-SW , the first storage device is recharged by using energy from the second storage device and when the energy harvester becomes again operational and supplies power, the energy of the energy harvester is used again to maintain the first storage device charged and to continue charging the second storage device, as discussed above. 
     By operating the voltage converter system for transferring energy from the second to the first storage device, the voltages of the first and second storage devices can be independent from each other and the first and second storage device can also be made of different technologies. For example, the first rechargeable storage device can be a Li-ion battery operating between 3.6 V and 4.0 V while the second rechargeable storage device can be a supercapacitor chargeable up to a maximum voltage of 2.7 V. 
     Preferably, the second rechargeable storage device  60  has an energy storage capacity that is more than five times, preferably more than ten times, larger than the energy capacity of the first rechargeable storage device  50 . In this way, the second storage device is forming a large energy reservoir that is suitable to maintain the application load operational under conditions where the energy harvester is down for a longer timer period. By taking a first storage device with a smaller energy storage capacity, it will also take less time to charge the first storage device and start operating the application load. 
     The second parameter V Batt2  allows to determine if the second storage device is sufficiently charged for providing output power and this can for example be determined by comparing V Batt2  with a predefined threshold value V Batt2-low , wherein the second storage device is considered charged if V Batt2 ≥V Batt2-low . 
     The second storage device is then considered not sufficiently charged to supply an output power if V Batt2 &lt;V Batt2-low . 
     In embodiments, the step of charging the first  50  and the second  60  rechargeable storage device comprises a further sub-step  3   a ) iii) if the situation occurs wherein V Batt2  has reached the upper threshold value V Batt2-max . The sub-step  3   a ) iii) corresponds to performing at least one of the following: a) decoupling the second power output path  32   b  and/or decoupling the first power input path  31   a, b ) stop operating the voltage converter system  20 , c) coupling the first power input path  31   a  and coupling the first power output path  32   b.    
     In some embodiments, before performing the step discussed above of coordinating a charging of the first and the second rechargeable storage device by repetitively performing sub-steps  1   a ) to  4   a ), an initial step is performed of precharging the second rechargeable storage device  60  up to a predefined charging level. The precharging of the second rechargeable storage device comprises steps of: i) coupling the second power output path  32   b  between the voltage converter system  20  and the second rechargeable storage device  60  for transferring output power from the voltage converter system to the second rechargeable storage device  60 , and ii) operating the voltage converter system  20  for charging the second rechargeable storage device  60  with energy from the energy harvester  70  until the parameter V Batt2  has reached a predefined threshold value V Batt2-PC , with V Batt2-PC  V Batt2-low . In this way, it is ensured that when the first rechargeable storage device  50  is charged and the load enabled, there is at least a minimum amount of energy already stored into the second rechargeable storage device  60  in order to guarantee a given autonomy for the application load. For example, if the energy harvester would stop supplying energy shortly after the application load started operating, there is at least sufficient energy available in the second rechargeable storage device that can be transferred to the first rechargeable storage device if V Batt1  drops below the critical threshold level V Batt1-SW . 
     In embodiments, the method of the present invention comprises an additional step for the situation where the parameter V Batt1  has dropped below the critical threshold value V Batt1-SW  and where the second rechargeable storage device  60  is not charged and hence no charges can be transferred from the second to the first rechargeable storage device. In this situation, if the parameter V Batt1  has dropped below the critical threshold value V Batt1-SW  and if the second rechargeable storage device  60  is not charged, the method comprises a step of decoupling the first power input path and coupling a third power input path between an auxiliary energy source, such as for example a primary battery, and the voltage converter system for transferring input power from the auxiliary energy source to the voltage converter system  20 , and operating the voltage converter system  20  for charging the first rechargeable storage device  50  with energy from the primary battery or the alternative power source until the parameter V Batt1  of the first storage device  50  has reached the upper threshold value V Batt1-up . 
     Examples of primary batteries are alkaline batteries or zinc-carbon batteries. Advantageously, as the voltage converter is used to transfer the charges from the primary battery to the first rechargeable storage device, the voltage of the primary battery does not need to be the same as the maximum voltage of the first storage device. The primary battery can for example be a AAA cell having a typical voltage level of 1.5 V while the first rechargeable storage device can be a rechargeable Li-Ion battery chargeable up to a typical voltage of 3.7 V. 
     In embodiments, the voltage converter system  20  comprises one or more voltage converters. A voltage converter is for example a DC-DC buck/boost voltage converter configured for operating in a buck mode if V in  &gt;V out  and for operating in a boost mode if V in &lt;V out , with V in  and V out  being respectively the input and output voltage of the voltage converter. 
     In embodiments, the voltage converter system  20  comprises a voltage converter configured for converting input power received via the coupled power input path  31   a ,  31   b ,  31   c  into output power outputted via the coupled power output path  32   a ,  32   b ,  32   c , and wherein said voltage converter is one of the following: a boost voltage converter, a buck voltage converter or a buck-boost voltage converter. In other words, in these embodiments a single voltage converter is charging the first and second storage device and is also transferring energy from the second to the first storage device under the conditions as discussed above. 
     According to a second aspect of the invention, an integrated circuit for energy harvesting is provided and an example of a system for energy harvesting  100  comprising such an integrated circuit  1  is shown on  FIG. 3 . With such a system for energy harvesting, comprising the integrated circuit according to the invention, the method of energy harvesting discussed above comprising steps of charging a first and a second rechargeable storage device and steps of transferring charges from the second to the first rechargeable storage device, can be applied in an automated and controlled way. 
     The integrated circuit for energy harvesting according to the invention has to be construed as a microchip comprising integrated circuits and a number of input and output pins, also named terminals. The microchip can have for example between 16 and 32 terminals. Generally, the microchip has a compact packaging resulting in a square or rectangular footprint with sides having a length between 1 and 5 mm. 
     As illustrated on  FIG. 3 , the integrated circuit  1  for energy harvesting comprises a first terminal  11  connectable with an energy harvester  70 , a second terminal  12  connectable with a first rechargeable storage device  50  and a third terminal  13  connectable with a second rechargeable storage device  60 . The integrated circuit further comprises a voltage converter system  20  suitable for converting input power into output power for charging at least two storage devices, a controller  40  for controlling the voltage converter system  20  and a monitoring unit  45  coupled with the controller  40  and configured for monitoring a parameter V Batt1  and a parameter V Batt2 . The parameters V Batt1  and V Batt2  are indicative of a charging level of respectively the first rechargeable storage device and the second rechargeable storage device when connected to respectively the second  12  and the third terminal  13 . 
     In embodiments, the parameter V Batt1  and the parameter V Batt2  correspond to a voltage sensed at respectively the second  12  and the third  13  terminal. In other embodiments, the parameters V Batt1  and V Batt2  correspond to an amount of charge counted by a charge counter during the charging process of respectively the first and second rechargeable storage devices. 
     As shown on  FIG. 3 , the integrated circuit  1  comprises a plurality of power input paths  31   a , 31   b  for transferring input power from an energy source to the voltage converter system and a plurality of power output paths  32   a , 32   b  for transferring output power from the voltage converter system to the output terminals of the integrated circuit. These input and output power paths have to be construed as electrical conductors. However, the voltage converter system, when in operation, only uses one power path to receive input power and one output path to output the power. Therefore, the voltage converter system  20  comprises an input selection circuit  31  for selecting one power input path from the plurality of power input paths so as to receive an input power via the power input path selected. The voltage converter further comprises an output selection circuit  32  for selecting one power output path from the plurality of power output paths so as to output an output power via the output path selected. 
     The integrated circuit  1  comprises at least a first power input path  31   a  configured for transferring input power from the first terminal  11  to the voltage converter system  20 , and a second power input path  31   b  for transferring input power from the third terminal  13  to the voltage converter system  20 . The integrated circuit further comprises at least a first power output path  32   a  for transferring output power from the voltage converter system  20  to the second terminal  12 , and a second power output path  32   b  for transferring output power from the voltage converter system to the third terminal  13 . In this way, when a second storage device is connected to the third terminal and when for example the energy harvester is not operating, the voltage converter system can transfer charges from the second rechargeable storage device to the first rechargeable storage device. 
     The controller  40  is configured to form and switch between a number of specific combinations of power input and power output paths based on a comparison of the parameter V Batt1  with first predefined thresholds values and/or a comparison of the parameter V Batt2  with second predefined threshold values. The specific combinations the controller can form are: i) a first combination formed by selecting the first power input path  31   a  and selecting the first power output path  32   a , ii) a second combination formed by selecting the first power input path  31   a , and selecting the second power output path  32   b  and iii) a third combination formed by selecting the second power input path  31   b  and selecting the first power output path  32   a . As will be further discussed in more detail below, the controller not only can form one of these three specific input/output combinations but can also switch from one specific combination to another specific combination based on the comparison of the parameter V Batt1  and/or parameter V Batt2  with the predefined threshold values. The first predefined threshold values comprise for example the threshold values V Batt1-SW , V Batt1-low  and V Batt1-up  and the second predefined threshold values comprise for example the threshold values V Batt2-max  and V Batt2-low  discussed above. 
     The first and second combination of selected power input/output paths as defined above correspond to a combination wherein the voltage converter system is transferring power from the energy harvester to respectively the first and second storage device. The third combination corresponds to the voltage converter system transferring power from the second to the first storage device. By configuring upper and lower thresholds levels for V Batt1 , the method of energy harvesting according to the present invention can be implemented by switching between the combinations of input/output paths defined depending on the parameters V Batt1  and/or V Batt2 . Indeed, the method of energy harvesting discussed above comprises these steps of switching between the first and second combination of power input/output paths to keep V Batt1  between the threshold values V Batt1-low  and V Batt1-up  and at the same time charge the second storage device. The method according to the invention discussed above also comprises a step of switching to the third combination of power input/output paths wherein energy is transferred from the second to the first storage device if V Batt1  drops below the critical threshold value V Batt1-SW . The condition of switching to the third combination of power input/output paths is only performed if the second storage device is charged which is determined, as discussed above, by comparing V Batt2  with a threshold value. 
     The controller is performing the switching between the combinations of selected power/input paths based on the conditions and threshold values of the parameters V Batt1  and V Batt2  as discussed above. In other words, the controller is configured for switching from the first combination to the second combination if the parameter V Batt1  becomes equal or larger than an upper threshold value V Batt1-up  and if the parameter V Batt2  is lower than an upper threshold value V Batt2-max , switching from the second combination to the first combination if the parameter V Batt1  has decreased from the upper threshold value V Batt1-up  down to a lower threshold value V Batt1-low , with V Batt1-low &lt;V Batt1-up , switching from the first combination to the third combination if the parameter V Batt1  has decreased from the lower threshold value V Batt1-low  down to a critical threshold value V Batt1-SW , with V Batt1-SW &lt;V Batt1-low , and if V Batt2  is above a lower threshold value V Batt2-low , with V Batt2-low &lt;V Batt2-max . 
     In embodiments, the monitoring unit  45  comprises a signal comparator for comparing the parameters V Batt1  and V Batt2  with predefined threshold values. As mentioned above, the parameters V Batt1  and V Batt2  correspond for example to a voltage resulting from a voltage measurement, an amount of charge resulting from a charge counter or a detection of any other quantity that is representative for a charging status of a rechargeable energy storage device. The signal comparator can either be an analogue signal comparator or a digital signal comparator, known in the art. For embodiments wherein a digital signal comparator is used, the generally analogue signals V Batt1  and V Batt2  are first digitized using an ADC (analog to digital converter). The predefined threshold values can be values locally memorized by the controller, or the predefined threshold values can be generated by a reference voltage generator, or a voltage configurator external to the PMIC can be used and threshold values can be transmitted through a configuration terminal or connector. 
     The voltage converter system  20  comprises one or more voltage converters and a voltage converter is for example a boost voltage converter, a buck voltage converter or a buck-boost voltage converter. In  FIG. 4 a    and  FIG. 5  examples of embodiments of a voltage converter system  20  are shown comprising a single voltage converter for converting input power received via the selected power input path into output power outputted via the selected power output path. In  FIG. 4 b   ,  FIG. 6 a    and  FIG. 6 b    examples of a voltage converter system  20  comprising two voltage converters are shown and in  FIG. 7  and  FIG. 8 , examples of voltage converter systems comprising three voltage converters are shown. These various embodiments of voltage converter systems  20  will be further discussed in more detail here below. 
     The use of the term “controller” has to be construed in the broadest sense as being an electronic digital circuit generally comprising combinatory logic. The controller controlling the voltage converter system is configured for controlling for example switches of one or more voltage converters and for controlling the switches of the input and output selection circuit. 
     An embodiment of a voltage converter system comprising a single voltage converter for converting input power, received via the selected power input path, into output power outputted via the selected power output path, is schematically illustrated on  FIG. 4 a   , and  FIG. 5 . The single voltage converter shown makes use of an inductor  25  which is generally located outside the integrated circuit and which can be coupled to the integrated circuit with for example two dedicated terminals  14 ,  15  or by any other suitable coupling means. The operation of the voltage converter and the selection of the input and output paths will be further discussed. 
     In the embodiment shown on  FIG. 4 a   , the input selection circuit  31  comprises a first input switch SW 1 -IN for enabling and disabling a current flow in the first power input path  31   a , a second input switch SW 2 -IN for enabling and disabling a current flow in the second power input path  31   b , a first output switch SW 1 -OUT for enabling and disabling a current flow in the first power output path  32   a  and a second output switch SW 2 -OUT for enabling and disabling a current flow in the second power output path  32   b.    
     Remark that when a specific input power path is selected it implies by definition that the other remaining input paths are de-selected. Hence selecting “a” power input path from the plurality of power input paths so as to receive an input power via the power input path selected has to be construed as selecting “one” power input path from the plurality of power input paths. This is a consequence of the fact that the voltage converter system can only receive one power source as input channel and hence only select one power input path at a time. The same is true for the power output paths, if a specific output path is selected, it implies by definition that the other remaining output paths are de-selected as only one output path can be selected at a time. Hence, selecting “a” power output path from the plurality of power output paths so as to output an output power via the power output path selected has to be construed as selecting “one” power output path from the plurality of power output paths. On the other hand, it is possible to de-select all power input paths and/or de-select all power output paths, for example to stop a transfer of power. 
     To charge the first rechargeable storage device  50  with energy from the energy harvester  70 , the first input path  31   a  and the first output path  32   a  are to be selected and hence the other input and output paths are to be de-selected and remain de-selected during the charging of the first storage device  50 . The second input path  31   b  and the second output path  32   b  are for example de-selected by opening respectively switches SW 2 -IN and SW 2 -OUT. These switches are shown on  FIG. 4   a.    
     To charge the second rechargeable storage device  60  with energy from the energy harvester, the first input path  31   a  and the second output path  32   b  are to be selected and the other input and output paths are to be de-selected and remain de-selected during the charging of the second rechargeable storage device. The second input path  31   b  and the first output path  32   a  are for example de-selected by opening respectively switches SW 2 -IN and SW 1 -OUT. 
     To charge the first rechargeable storage device  50  with energy from the second rechargeable storage device  60 , the second input path  31   b  and the first output path  32   a  are to be selected and the other input and output paths are to be de-selected and maintained de-selected during the charge transfer from the second to the first rechargeable storage device. The first input path  31   a  and the second output path  32   b  are for example de-selected by opening respectively switches SW 1 -IN and SW 2 -OUT. 
     In a preferred embodiment, the voltage converter  20  is a DC-DC buck/boost voltage converter as illustrated on  FIG. 4 a    that is capable of operating in either a boost mode or a buck mode. When the voltage converter input voltage is smaller than the voltage converter output voltage, the buck/boost voltage converter will operate in a boost mode. On the other hand, the buck/boost voltage converter will operate in a buck mode if the input voltage is higher than the output voltage. For example, when the first input path  31   a  and the first output path  32   a  are selected the input and output voltages for determining the operation mode correspond to respectively the voltage at the first terminal  11  and the voltage at the second terminal  12 . 
     To operate the buck/boost voltage converter shown on  FIG. 4 a    in a boost mode for charging the first storage device with energy from the energy harvester, the switch SW 1 -IN is maintained closed and the switch  27   a  remains open during the charging of the first storage device. The boost mode starts with a magnetic energy charging phase of the inductor  25  wherein the switch  27   b  is closed and the switch SW 1 -OUT is open, followed by a magnetic energy de-charging phase wherein the switch  27   b  is opened and the switch SW 1 -OUT is closed. As known in the art, by repetitively controlling the switches  27   b  and SW 1 -OUT, power is transferred in a boost mode from the first terminal  11 , i.e. where the energy harvester is connected, to the second terminal  12 , where the first rechargeable storage device is connected. 
     To operate the buck/boost voltage converter shown on  FIG. 4 a    in a buck mode for charging the first storage device, the switch SW 1 -OUT is maintained closed and the switch  27   b  remains open during the charging of the first storage device. The buck mode starts with a magnetic energy charging phase of the inductor  25  wherein the switch  27   a  is open and the switch SW 1 -IN is closed, followed by a magnetic energy de-charging phase wherein the switch  27   a  is closed and the switch SW 1 -IN is opened. As known in the art, by repetitively controlling the switches  27   a  and SW 1 -IN, power is transferred in a buck mode from the first terminal  11 , i.e. where the energy harvester is connected, to the second terminal  12 , where the first rechargeable storage device is connected. 
     When the first storage device  50  is charged, i.e. the parameter value V Batt1-up  is reached, the second storage device  60  connected to the third terminal  13  of the integrated circuit can start to be charged with energy from the energy harvester. Therefore, the first output path  32   a  is de-selected by opening switch SW 1 -OUT and by maintaining this switch open during the charging of the second rechargeable storage device. 
     For charging the second rechargeable storage device with energy from the energy harvester, depending on the input and output voltages of the voltage converter, the voltage converter will also operate in a buck or a boost mode. For operating in a boost mode, the switch SW 1 -IN is maintained closed and the switch  27   a  remains open. Similarly as discussed above, the boost mode starts with a magnetic energy charging phase of the inductor wherein the switch  27   b  is closed and the switch SW 2 -OUT is open, followed by a magnetic energy de-charging phase wherein the switch  27   b  is opened and the switch SW 2 -OUT is closed. This cycle of magnetically charging and de-charging the inductor is cyclically repeated. 
     To charge the second rechargeable storage device in a buck mode with energy from the energy harvester, the switch SW 2 -OUT is maintained closed and the switch  27   b  remains open. The buck mode starts with a charging phase of the inductor  25  wherein the switch  27   a  is open and the switch SW 1 -IN is closed, followed by a de-charging phase wherein the switch  27   a  is closed and the switch SW 1 -IN is opened. This cycle of magnetically charging and de-charging the inductor is cyclically repeated. 
     The various switches shown on  FIG. 4 a   , i.e. switches with references  27   a ,  27   b , SW 1 -IN, SW 2 -IN, SW 1 -OUT and SW 2 -OUT, have to be construed as electronic switches configured for opening or closing an electrical conducting path or conductor. These switches are for example analogue electronic switches known in the art. These switches make use of for example MOS transistors. With the exemplary embodiment shown on  FIG. 4 a   , the number of electronic switches of the integrated circuit is limited as some of these switches are not only used as the standard switches needed for operating the DC/DC voltage converter but are also forming the switches for the input and output path selection circuit. 
     As discussed above, the energy converter system  20  is not limited to a specific number of voltage converters. In  FIG. 4 b    an example of voltage converter system  20  is shown comprising two buck/boost voltage converters and wherein the voltage converter system comprises a plurality of switches with references  27   a ,  27   b ,  28   a ,  28   b , SW 1 -IN, SW 2 -IN, SW 1 -OUT and SW 2 -OUT. The switches  27   a ,  27   b , SW 1 -IN, SW 1 -OUT and SW 2 -OUT are used for operating a first buck/boost voltage converter for transferring power from the energy harvester to either a first rechargeable storage device at a voltage V Batt1  or to a second rechargeable storage device at a voltage V Batt2 . The switches  28   a ,  28   b , SW 2 -IN, SW 1 -OUT are used for operating a second buck/boost voltage converter for transferring power from a second rechargeable storage device at a voltage V Batt2  to a first rechargeable storage device at a voltage V Batt2 . Remark that the switches SW 1 -IN, SW 2 -IN, SW 1 -OUT and SW 2 -OUT are not only used for the nominal operation of the buck/boost voltage converters but these switches are also forming part of an input selection circuit  31  and an output selection circuit  32  for selecting an input power path and an output power path as schematically illustrated on  FIG. 4 b   . In this example, each of the buck/boost voltage converters makes use of a dedicated inductor  25 ,  26 . 
     In some embodiments, in addition to the nominal power switches for operating the one or more voltage converters of the voltage converter system  20 , additional dedicated switches are used for forming the input and/or output selection circuit. A number of embodiments, as shown on  FIG. 6 a    to  FIG. 8 , will be further discussed. 
     In  FIG. 6 a    an example of an embodiment of a voltage converter system  20  is shown comprising, besides the input  31  an output  32  selection circuits, two voltage converters  21   a  and  21   b . As further illustrated on  FIG. 6 a   , there are in this example two power input paths  31   a  and  31   b  and two power output paths  32   a  and  32   b . The first voltage converter  21   a  is used for converting input power received via the first power input path  31   a  into output power outputted via the first  32   a  or via the second  32   b  power output path, depending on what power output path is selected by the output selection circuit  32 . The second voltage converter  21   b  is used for converting input power received via the second power input path  31   b  into output power outputted via the first power output path  32   a . The two voltage converters can make use of one or two inductors (not shown on  FIG. 6 a   ). The two voltage converters  21   a ,  21   b  do not necessarily have to be of the same type, for example, the first voltage converter  21   a  can be a buck-boost voltage converter and the second voltage converter  21   b  can be a buck voltage converter or a boost voltage converter. 
     In  FIG. 6 b   , an alternative embodiment is shown that can perform the same functionalities as the embodiment shown on  FIG. 6 a   , but wherein the output selection circuit  32  makes use of three switches SW 1 -OUT, SW 2 -OUT and SW 3 -OUT instead of two output switches. 
     In  FIG. 7 , an example of an embodiment of a voltage converter system  20  comprising three voltage converters  21   a ,  21   b ,  21   c  is shown. The first voltage converter  21   a  is converting input power received via the first power input path  31   a  into output power outputted via the first power output path  32   a . The second voltage converter  21   b  is converting input power received via the second power input path  31   b  into output power outputted via the first power output path  32   a . Finally, the third voltage converter  21   c  is converting input power received via the first power input path  31   a  into output power outputted to the second power output path  32   b . The three voltage converters can make use of one, two or three inductors (not shown on  FIG. 7 ). 
     In  FIG. 8 , an example is shown where the input selection circuit  31  can select from three power input paths  31   a ,  31   b  and  31   c . The voltage converter system  20  shown comprises three voltage converters  21   a ,  21   b  and  21   d . The functionality of the first  21   a  and second  21   b  voltage converter in this example is the same as for the example shown on  FIG. 6 a    and discussed above. The third voltage converter  21   d  is used for converting input power received via the third power input path  31   c  into output power outputted via the first  32   a  power output path. 
     For the integrated circuit for energy harvesting according to the invention, the person skilled in the art can specify other embodiments of voltage converter systems  20  than the ones described above and shown on  FIG. 4 a    to  FIG. 8 . What the embodiments of voltage converter systems according to the invention have in common is that they comprise an input selection circuit  31  for selecting a power input path from a plurality of power input paths so as to receive an input power via the power input path selected and an output selection circuit  32  for selecting a power output path from the plurality of power output paths so as to output an output power via the power output path selected. Remark that, as discussed above, in some embodiments the switches used to select a power input path or to select a power output path correspond to the power switches of the DC/DC voltage converter used for the nominal operation of the DC/DC voltage converter. In this way, the total number of switches needed for the voltage converter system can be limited. 
     In an embodiment according to the present invention, the controller  40  is configured for performing a step A) of selecting the first input path  31   a  and repetitively performing the following sub-steps A 1 ) to A 4 ): A 1 ) selecting the first output path  32   a , A 2 ) operating the voltage converter  20  for converting input power received via the first power input path  31   a  into output power outputted via the first power output path, A 3 ) if V Batt1  becomes equal or larger than an upper threshold value V Batt1-up  then de-selecting the first output path  32   a , and A 4 ) if V Batt2  is lower than an upper threshold value V Batt2-max  then
     i) selecting the second output path  32   b,      ii) operating the voltage converter system  20  for converting input power received via the first power input path  31   a  into output power outputted via the second power output path  32   b , and   iii) de-selecting the second output path  32   b  and restart with sub-step A 1 ) if   

     V Batt1  has subsequently decreased from the upper threshold value V Batt1-up  down to a lower threshold value V Batt1-low , with V Batt1-low  &lt;V Batt1-up . 
     In this way, by performing the above outlined step A) and its sub-steps A 1 ) to A 4 ), the controller  40  is maintaining the parameter V Batt1  between the threshold values V Batt1-low  and V Batt1-up . 
     In embodiments, the controller  40  is further configured for performing a step B) if V Batt1  has decreased from the lower threshold value V Batt1-low  down to a critical threshold value V Batt1-SW , with V Batt1-SW  &lt;V Batt1-low , and if V Batt2  is above a lower threshold value V Batt2-low , with V Batt2-low  &lt;V Batt2 -max. The step B) is composed of the following sub-steps: B 1 ) selecting the second input path  31   b , B 2 ) selecting the first output path  32   a , and B 3 ) operating the voltage converter  20  for converting input power received via the second power input path  31   b  into output power outputted via the first power output path  32   a.    
     In this way, by transferring charges from the second to the first rechargeable storage device, the controller avoids the first rechargeable storage device from being fully de-charged when the energy harvester is not operating. At the same time the application load coupled to the first rechargeable storage device can continue to operate even if the energy harvester has stopped operating. 
     In an embodiment according to the present invention, the controller is configured, when performing sub-step A 4 ) discussed above, for additionally performing a step iv) if V Batt2  has reached the upper threshold value V Batt2 _max. The additional step step iv) comprises performing one of the following steps: i) de-selecting the second power output path  32   b  and/or de-selecting the first power input path  31   a , ii) stop operating the voltage converter system  20 , iii) selecting the first input path  31   a  and selecting the first output path  32   a.    
     In this way, by performing this additional step A 4 ) iv), there are no longer charges being transferred to the second rechargeable storage device connected to the third terminal of the integrated circuit. This avoids the second rechargeable storage device from being overcharged. 
     In embodiments, when in step A 4 ) iv) the option iii) of selecting the first input path  31   a  and selecting the first output path  32   a , is applied, then the energy harvester is again continuing charging the first rechargeable storage device with energy from the energy harvester. If the threshold V Batt1-up  is not a maximum charging level for the first rechargeable storage device and if a maximum charging level V Batt1-max  exists with V Batt1-max &gt;V Batt1-up , then in this situation where the second rechargeable storage device is fully charged, the first rechargeable storage device can be continued to be charged to the maximum charging level of V Batt1-max . 
     In an embodiment, when performing step B mentioned above, the controller is configured to additionally perform a sub-step B 4 ), namely if the voltage V Batt1  becomes equal or larger than the upper threshold value V Batt1-up  then performing at least one of the following steps: i) de-selecting the first power output path  32   a  and/or de-selecting the second power input path  31   b , ii) selecting the first input path  31   a  and selecting the second output path  32   b , iii) stop operating the voltage converter  20 . In embodiments, when the step ii) is applied, then the energy harvester will, when operational, continue to charge the second rechargeable storage device. 
     In this way, by performing the additional sub-step B 4  there are no longer charges being transferred from the second rechargeable storage device, connected to the third terminal, to the first rechargeable storage device, connected to the second terminal. This avoids the first rechargeable storage device from being overcharged. Following the charging of the first rechargeable storage device up to the upper threshold value V Batt1-up  with charges from the second rechargeable storage device, the value of V Batt1  will start decreasing again if the application load is consuming power. If the energy harvester is still not operating or still not sufficiently providing power, even after selecting the first power input path and the first power output path, the parameter V Batt1  will continue to drop and drop again below V Batt1-SW . When V Batt1  drops below V Batt1-SW  then power will again be transferred from the second to the first rechargeable storage device. On the other hand, if the energy harvester has become operational and provides more power than power consumed by the application load, then when V Batt1  has decreased down to V Batt1-low , power is transferred from the energy harvester to the first rechargeable storage device and V Batt1  is increasing again until V Batt1  has reached the upper threshold V Batt1-up . 
     In embodiments, the monitoring unit  45  is configured to monitor a parameter V H  that indicates if the energy harvester connected to the first terminal is operational or not. In embodiments, this parameter could be a voltage measured at the first input terminal. Based on this parameter V H , the controller can decide when to re-select the first power input path for receiving power from the energy harvester and charging the first rechargeable storage device with charges from the energy harvester instead of charging the first rechargeable storage device with charges from the second rechargeable storage device. 
     The integrated circuit  1  for energy harvesting according to the present invention is not limited to the number of power input paths and the number of power output paths. In  FIG. 5 , an integrated circuit  1  with a voltage converter is shown having an input selection circuit  31  configured to select between three power input paths  31   a ,  31   b  and  31   c  and having an output selection circuit  32  configured to select between three power output paths  32   a ,  32   b  and  32   c . In this example, the input selection circuit  31  and the output selection circuit  32  have respectively additional switches SW 3 -IN and SW 3 -OUT. 
     In further embodiments, a third power input path  31   c  is used for transferring input power from a fourth terminal to the voltage converter system  20 . The fourth terminal is connectable with an auxiliary energy source, such as for example a primary battery. In these embodiments, if the parameter V Batt1  has decreased from the lower threshold value V Batt1-low  down to the critical threshold value V Batt1-SW  and if V Batt2  is below the lower threshold value V Batt2-low  then, the controller  40  is configured for performing steps of C 1 ) selecting the third power input path  31   c , C 2 ) selecting the first power output path  32   a  and C 3 ) operating the voltage converter  20  for converting input power received via the third power input path  31   c  into output power outputted via the first power output path  32   a.    
     In this way, if for example the energy harvester is not operating and if the second rechargeable storage device is not charged, an auxiliary power source coupled to the fourth terminal can be used to charge the first rechargeable storage device. 
     In a particular embodiment, a third power output path  32   c  is coupling the voltage converter system with a fifth terminal connectable with for example an auxiliary rechargeable storage device being at a voltage V AUX . In this way, when the first and the second rechargeable storage device are fully charged, the third rechargeable storage device can be charged.