Fast-charging a supercapacitor from energy harvesters
Supercapacitors are an essential energy storage mechanism in self-powered systems. Their high-energy capacities combined with their ability to provide high-power output make them ideal for ultra-low power wireless sensor node systems. Supercapacitors, however, discharge significantly during periods of low-energy harvesting input.
Energy harvesting ICs used to charge supercapacitors suffer from low efficiency during the initial charging stages until the supercapacitor reaches a nominal voltage. This causes a long wait for the supercapacitor to charge up to usable levels each time the system comes back up from a deep-sleep state, significantly hindering the widespread adoption of supercapacitors. This article describes ways to speed-up charging of a supercapacitor by more than 20 times when compared to existing systems. The solutions presented in this article use a solar cell as the energy harvester. These solutions are equally applicable to other energy harvesting sources as well.A simple diode charger
Figure 1. Schematic for charging a supercapacitor using a diode
The simplest way to charge a supercapacitor from a solar cell is through a diode. The supercapacitor can charge up to the open-circuit voltage of the solar cell under the prevailing light conditions, taking into account losses due to the diode. Figure 1 shows how a supercapacitor can be charged with the help of a diode. An auxiliary over-voltage protection circuit is required in most systems to protect the supercapacitor and the ensuing load electronics.
The simplicity of this solution makes it attractive for use in low-cost solar accessories. However, this method has multiple drawbacks. Firstly, it only works with multi-cell solar cells where the open-circuit voltage of the solar cells is larger than the over-voltage setting of the supercapacitor or the required load voltage. Thermoelectric harvesters that output low voltages cannot use this method to charge storage elements.
Further, this circuit regulates the solar cell to one diode drop above the voltage on the storage medium. This means that as the voltage on the storage medium moves around, based on load conditions, the solar cell regulation point also moves. For storage cells with a wide discharge curve or supercapacitors whose voltage can move significantly depending on the load demands, this is not a good solution since the solar cell is regulated to a voltage far from its maximum power point. The auxiliary over-voltage protection circuit needed in most low-power electronic systems also consumes quiescent current, which can affect system efficiency during periods of low-light.
Figure 2. Measured waveform of a 120 mF supercapacitor being charged using a diode
A key advantage of the diode charger is the time it takes to charge supercapacitors from a completely discharged state. Figure 2 shows how a 120 mF supercapacitor is charged from a completely discharged state using a 3S solar cell with a short circuit current ISC = 1 mA and an open circuit voltage VOC = 2V. The pink trace corresponds to the solar cell output (VIN), while the blue trace is the voltage of the supercapacitor (VSUP). The supercapacitor takes around 205 seconds to charge from 0V to 1.8V. The difference in the voltage between VIN and VSUP is the drop across the diode. The time taken to charge the supercapacitor to a voltage of VX using the diode charger can be approximated to Equation 1:
For a 120 mF supercapacitor to charge up to 1.8V with 1 mA ISC, Equation 1 gives a time of 216 seconds, which is very close to the observed time. Even though the charge time is low for the diode charger, the disadvantages mentioned preclude this solution from being used in a wide variety of energy harvesting systems.