An Architectural Charge Management Interface for Energy-Harvesting Systems

Energy-harvesting devices eliminate batteries, instead collecting their operating energy from environmental sources. A device stores energy into a capacitor, drawing energy to perform tasks and powering off to recharge when the energy is exhausted. State-of-the-art charge management systems for these devices aim to avoid power failure during task execution by reasoning about task energy cost. We identify that the innate equivalent series resistance (ESR) in energy storage capacitors breaks energy-based systems' guarantees; running high current load on a high-ESR capacitor causes a substantial voltage drop that rebounds once the load is removed. This voltage drop is disregarded by systems that only reason about energy. If the drop lowers the voltage below the system's operating threshold, however, the device powers off while stored energy remains. Though ESR is well understood in hardware design, this is the first work to argue that software for batteryless devices must also be aware of ESR.

This work presents Culpeo, a hardware/software mechanism and architectural interface to relay the effect of ESR in the power system to software. We develop static and dynamic implementations of Culpeo and demonstrate on real batteryless devices that considering ESR restores correctness guarantees broken by energy-only charge management. We then demonstrate how to integrate Culpeo's safe voltage into state-of-the-art schedulers, restoring task deadline guarantees for applications with predictable energy harvesting. Finally, we propose an onchip Culpeo hardware implementation that allows for runtime monitoring of the effects of ESR to respond to changes in harvestable power.