REVIEW PAPER A review of the present situation and future developments of micro-batteries for wireless autonomous sensor systems J. F. M. Oudenhoven * ,† , R. J. M. Vullers and R. van Schaijk Holst Centre/IMEC, High Tech Campus 31, 5656 AE Eindhoven, The Netherlands SUMMARY Wireless sensor nodes (WSNs) are expected to play an increasing role in multiple application areas. These application areas vary from networks around the human body, sensors in smart tires, sensor networks that can control the safety and comfort levels throughout smart buildings, sensors that monitor the necessity for maintenance and sensors that track the conditions of food throughout the distribution chain. These wireless sensors need energy, which can be supplied by a battery or an energy harvester. However, even when an energy harvester is applied, energy storage is required to serve as energy buffer. In this review, the requirements that different types of wireless sensor networks impose on these batteries are explored, and several suitable types of batteries are reviewed. Moreover, the trends in battery development are described, and the future improvements are predicted. Finally, the possibilities are discussed to select a battery with properties that are matched to the requirements of the sensor nodes. Copyright © 2012 John Wiley & Sons, Ltd. KEY WORDS wireless autonomous sensor system; battery; micro-battery; future trends Correspondence *J. F. M. Oudenhoven, Holst Centre/IMEC, High Tech Campus 31, 5656 AE Eindhoven, The Netherlands. † E-mail: jos.oudenhoven@imec-nl.nl Received 19 March 2012; Revised 27 June 2012; Accepted 6 July 2012 1. INTRODUCTION Wireless sensor nodes (WSNs) are employed today in many different application areas, ranging from health and lifestyle, automotive, smart building, predictive maintenance (e.g. of machines and infrastructure), smart packaging to active radio frequency identification tags. However, currently these devices have limited operational lifetime as they require significant operating power. In Figure 1, the basic building blocks of a wireless sensor node are depicted. The analog data from the sensor is converted to digital values, processed by a digital signal processor, and the result is sent to the outside world by the radio. The power is delivered by a micropower system, which either consists of a battery or an energy harvester. At this moment, most examples of WSN are single point-measurements, where the sensor node communicates with a base station. In the future, wireless mesh networks will be applied, which increases the need for autonomous solutions. Many efforts are ongoing to decrease the power consumption of the sensor nodes. It is expected that many applications will be able to reduce their power consumption to below 100 mW [1]. In 2008, we estimated the battery lifetime in case of a 100 mW average power and 1 cm 3 volume. A battery can only deliver the required energy for around 2 months [2]. Clearly, for these small devices, energy harvesting is the only alternative. This could limit the use of WSNs because of the need for large batteries. It has been reported that the energy density increase of batteries has been limited to a factor of two to three per decade [3]. However, many recent publications have claimed improvements in materials or new device concepts that may lead to an accelerated energy density increase. This will affect the application window for wireless sensor nodes. It is the subject of this paper to review the new improvements and to predict how this will influence the autonomy of a 1 cm 3 size WSN. As has been pointed out by others [4], care has to be taken to interpret some of the claims stated in research papers, as many of the properties reported are only valid for subparts and not for complete packaged battery systems. We will, on the other hand, only focus on complete batteries and first discuss the requirements for batteries for the WSN applications, and then we will present the current status of battery technology. Finally, the progress in the field will be discussed, and from estimates, the future limits of battery-operated WSN. INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.2949 Copyright © 2012 John Wiley & Sons, Ltd.