SPIE Proceedings Publications Maintenance-free, super-capacitor based WSN power supply Vidrascu G. Mihai 1) , Paul Svasta 2) , Marian Vladescu 3) 1) “Politehnica” University of Bucharest, Bucharest, Romania 2) Center for Technological Electronics and Interconnection Techniques (UPB-CETTI), “Politehnica” University of Bucharest, Romania 3) Optoelectronics Research Centre (UPB-CCO), “Politehnica” University of Bucharest, Romania ABSTRACT Super-capacitors are highly reliable devices, outlasting any existing battery in operating conditions, cycle number and ruggedness. They are perfect candidates for energy storage in remote wireless sensor network nodes. Other applications [1] use a combination of rechargeable battery as the main storage device and a low capacity EDLC as energy buffer. This paper describes the design and the results from a module consisting of a single large capacitor, a MPPC charger and a boost converter. Unlike other devices, this prototype uses one storage device (a single EDLC), it does not involve software components [2] and it can operate from a single solar cell. Keywords: WSN, power supply, solar cell, boost converter, super-capacitor. 1. INTRODUCTION Maintenance-free wireless sensor nodes are designed to operate for a long time in inaccessible areas or in extreme weather conditions. This paper focuses on the power storage device, which currently limits the device life time. A rechargeable battery has a limited number of charge-discharge cycles (up to several thousands, at extremely low discharge depth) and the capacity drops with every cycle. Deep discharge also reduces the battery’s capacity. Additionally, batteries have poor performances at low temperatures, and the good ones are far more expensive than the standard types. Super-capacitors have no problem with low temperatures (the leakage current is greatly reduced in cold weather) or deep discharge, and can maintain their current capability and low internal resistance. A prototype has been developed to power the nodes of a bee hive monitoring network. Since a good quality super-capacitor has a life-time of approximately 20 years, this architecture leads to a highly reliable and maintenance-free network. 2. DESIGN REQUIREMENTS The main goal of this prototype is to determine if a cost-efficient solution can be developed around a single EDLC cell. The end device must be simple, easy to manufacture, and reliable. The author decided to use only one super-capacitor to eliminate the need for cell balancing in series architectures. Most balancing circuits are dissipative and a sensor node cannot afford to waste any energy. Additionally, such a circuit would increase the price of the final product. Affordable super-capacitors operate at a maximum voltage of 2.5 to 2.7V (depending on the manufacturer), while most common sensors and micro-controllers run at 3.3V. This introduces the need for an efficient step-up converter, with low current consumption. Since the device will be operated outdoor, solar energy is the obvious choice to charge the capacitor. However, because the capacitor has a far lower energy density than a rechargeable battery, every minute of sunshine matters. So, an efficient charger is required, preferably one that implements a MPPT (Maximum Power Point Tracking) architecture. The cheapest solar panel consists of a single cell and outputs an open circuit voltage of up to 0.7V (depending of the manufacturer and cell quality). After a thorough research, I found a solution which fits the requirements: low start-up