# On the Impact of Mobility on Battery-Less RF Energy Harvesting System Performance
## 1 Introduction  
Battery-free sensors gradually realize the vision of perpetual and maintenance-free sensing applications [1]. The energy harvesting sensors extract energy from the environment and store it in supercapacitors, which can operate forever without maintenance due to unlimited charge-discharge cycles [2]. As the amount of harvested energy depends on the environment, the sensor node can no longer be expected to operate continuously, but instead, will operate intermittently whenever energy becomes available, for example when the node moves into a high energy area.  
Small supercapacitors are attractive for mobile systems because they charge and boot the battery-less sensor node in a matter of seconds whenever energy becomes available. This choice, however, prevents the sensor node from performing energy-intensive operations, such as communicating with a remote base station. Large capacitors, on the other hand, may take such a long time to charge, that the node may leave a high energy area long before the capacitor gets sufficient charge to start-up the node, let alone perform any useful task. Furthermore, as the node leaves the high energy area, the partial charge in the large supercapacitor dissipates rapidly without being used. Surprisingly, the design trade-offs in selecting an optimal storage capacity and the impactof mobility on the charging performance of intermittently-powered battery-less systems have not been described in the literature.  
The supercapacitors differ from traditional batteries by low energy density and high self-discharge current: Once fully charged, the supercapacitors discharge at a much higher rate than a traditional battery. Prior work focused on supercapacitor characterization [3], modelling and supercapacitor-aware task scheduling [4]. The benefits of small capacitors for mobile applications and the capacitor-based energy harvesting sensor platform were proposed by [5]. Recently, [PERSON] et al. [1] developed a federated energy storage concept, where each peripheral, such as radio, sensor or a microcontroller, is assigned a dedicated capacitor for increased reliability so that faulty or misconfigured peripherals do not affect the shared energy storage and thus the operation of the entire system. The paper does not, however, address the impact of mobility on charging and the application performance of battery-less sensors.  
In this paper, we propose a novel dual capacitor energy storage platform that dynamically selects an appropriate energy storage based on the amount of ambient energy. The dynamic switching is not trivial since it requires tracking and predicting the amount of available energy in the environment. We show that the hybrid approach allows meeting the otherwise contradictory requirements by avoiding the design trade-offs present in traditional single-capacitor designs. We define a new performance metric, which we call usable harvested energy, as the amount of energy directly available to applications and show that it can be a fraction of total energy harvested by the node due to supercapacitor self-discharge current and residual charge. Finally, we evaluate the proposed platform through extensive simulations and a hardware prototype based on the PowerCast RF energy harvesting development kit [6]. To the best of our knowledge, this is the first work to investigate the impact of mobility on battery-less wireless sensors and to propose a working solution based on a novel energy prediction algorithm.  
The rest of the paper is structured as follows. Sections 2 and 3 contain a system model and describe the proposed approach including a hardware platform and a learning algorithm. Sections 4 and 5 contain the results of simulation and prototype measurements, respectively. Section 6 reviews related work on supercapacitor-based energy harvesting sensor systems, and finally, Section 7 concludes the paper.