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Supercapacitors offer an attractive energy storage solution for lifetime &#x201C;fit and forget&#x201D; photovoltaic (PV) energy harvesting powered wireless sensor nodes for Internet of Things (IoT) applications. Whilst their low storage capacity is not an issue for sub-mW PV applications, energy loss in the charge redistribution process is a concern. Currently, there is no effective method to estimate the storage of the supercapacitor in IoT applications for optimal performance with sub-mW input. The existing energy-based method requires supercapacitor model parameters to be obtained and the initial charge state to be determined, consequently it is not suitable for practical applications. This paper defines a charge-based method, which can directly evaluate supercapacitor's storage with straightforward calculations. Time constant analysis and experimental tests demonstrate that with the newly proposed method, the manufacturer-specified tiny leakage current, although measured long after postcharge (e.g., 72 h), can be directly used, making the storage estimation for a supercapacitor in IoT applications as simple as that for an ordinary capacitor. In addition, the demonstrated tiny leakage current at the required energy storage for a sub-mW PV-powered IoT application enables a supercapacitor alone to be employed as the storage mechanism, thus achieving lifetime battery-replacement-free, self-powered IoT nodes.

Charge-Based Supercapacitor Storage Estimation for Indoor Sub-mW Photovoltaic Energy Harvesting Powered Wireless Sensor Nodes