Analysis of battery current microcycles in autonomous renewable energy systems A.J. Ruddell a,* , A.G. Dutton a , H. Wenzl b , C. Ropeter b , D.U. Sauer c , J. Merten d , C. Orfanogiannis e , J.W. Twidell f , P. Vezin g a Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX Oxfordshire, UK b TU-Clausthal, Leibnitzstraße 28, 38678 Clausthal-Zellerfeld, Germany c Fraunhofer Institut fu ¨r Solare Energiesysteme, Heidenhofstraße 2, 79110 Freiburg, Germany d Trama TecnoAmbiental, Ripolle ´s 46, E-08026 Barcelona, Spain e ESCO, 122 Tatoiou, GR-14671 Athens, Greece f AMSET Centre, Horninghold, LE16 8DH, England, UK g Vergnet, 160 rue des sables de Sary, 45770 SARAN, France Received 12 December 2001; received in revised form 23 July 2002; accepted 16 August 2002 Abstract Battery currents in autonomous renewable energy systems (RES) are generally predicted or measured in terms of mean values over intervals of 1 min or longer. As a result, battery charge–discharge cycles with periods less than the averaging period are ignored, and the actual battery ampere hour (A h) throughput and resulting battery wear may be seriously underestimated, leading to optimistic prediction of battery lifetime. This paper considers short charge–discharge cycles or microcycles, arising from the characteristics of autonomous renewable energy systems, including generators, regulators, loads, and load inverter. Simulation results are used to show that inverters operating directly from the battery can cause microcycles, resulting in significantly increased battery throughput. Initial experimental results of the effects of microcycles on battery capacity and charging characteristics, and the contributing processes, are discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Lead–acid battery; Cycling; Microcycles; Charge–discharge; Wind turbine; Photovoltaic systems 1. Introduction The accurate prediction of battery lifetime in autonomous renewable energy systems is desirable in order to achieve reliable and economic designs, however there are many factors with complex interdependence making an accurate prediction difficult. Key operational issues affecting the battery lifetime are: (a) the system sizing for the particular operational conditions; (b) the operational temperature; (c) the operational control of charging, including the charge controller characteristics; and (d) the generation and load profiles. Methods of estimation of the battery lifetime in PV systems have been published by Spiers and Rasinkoski [1,2], Drouilhet and Johnson [3], and Symons [4]. The factors affecting lifetime may be broadly considered under two categories, the ‘cycling lifetime’, resulting from ampere hour (A h) throughput causing wear, and the ‘float lifetime’ resulting from processes during standby causing mainly positive plate corrosion. These lifetimes are defined for the two modes of operation (continuous cycling, or standby), and do not include the effects of additional acid stratification or irreversible sulphation that are likely to be caused by cycling at partial state-of-charge. Therefore, it is not surprising that the prediction of battery lifetime is frequently inaccurate, and the actual lifetime under the duty encountered in a real application is much shorter than anticipated. A systematic definition by the Fraunhofer Institut Solare Energiesysteme (Fraunhofer ISE) of performance para- meters for lead–acid batteries in renewable energy systems, identified four major classes of operating conditions based on battery currents and state-of-charge (SOC) cycles, and four classes of operating temperature conditions [5]. The battery current classification by Fraunhofer ISE is shown summarised in Table 1. The lifetime determining mechan- isms resulting from a particular combination of operating Journal of Power Sources 112 (2002) 531–546 * Corresponding author. Tel.: þ44-1235-445-551; fax: þ44-1235-446863. E-mail address: a.j.ruddell@rl.ac.uk (A.J. Ruddell). 0378-7753/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0378-7753(02)00457-3