Evaluation of the batteries and charge controllers in small stand-alone photovoltaic systems

Abstract

In this paper, we report the results of long-term tests on 14 separate small stand-alone PV systems. These tests were carried out at two separate laboratories in Albuquerque, New Mexico and in Cape Canaveral, Florida. The systems were heavily instrumented to help determine what effects voltage regulation set points have on system water loss, battery capacity lifetime and system reliability. We find that under the proper conditions, the flooded lead-acid batteries in these PV systems can achieve cycle-lifetimes comparable to manufacturer's expectations for conventional battery applications. We also find that both the overcharge and overdischarge protection provided by the system charge controllers were necessary for proper operation of these systems. Details of the data and a number of conclusions for system designers are presented. I NTRO D UCTlO NEarly loss of battery capacity is one of the most common problems restricting the use of small stand-alone PV systems, both in developed countries and in third world applications. 12,3,4 A better understanding of how charge controller settingsand algorithms affect specific types of batteries would allow users to improve PV system reliability and achieve more predictable battery lifetimes.In this paper we report the results of two separate long-term tests of flooded lead-acid batteries and charge controllers in fourteen separate PV systems. Previous papers 56,7 have reported shorter term experiments involving batteries and charge controllers and given an interim report on the tests reported in this paper. The tests reported here, which were designed to evaluate what parameters affect a battery's stateof-charge, lifetime, and water loss, were performed at Sandia National Laboratories (Sandia) in Albuquerque, New Mexico and at the Florida Solar Energy Center (FSEC) in Cape Canaveral, Florida. Seven complete stand-alone PV systems were tested at each location. The tests lasted slightly less than two years at each location, with the systems operating at Sandia from

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“…Lead-acid batteries are ideal to store harvested energy. These render greater lifet imes than expected in stand-alone PV systems than conventional batteries [19,20].…”

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“…Despite hyperenthusiastic statements by some charge regulator manufactures, independent laboratory evaluations have never been conclusive on this issue. 38,48 Instead, rather simple mistakes in charge regulator design, leading to excessive voltage drops or lack of protection in common accident situations, have often been reported. 3,18,28 As already mentioned for batteries, the uncertainty associated with such a lack of empirical evidence has led to widespread inconsistencies between different existing technical standards.…”

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“…Previous testing and evaluation of charge controllers and batteries at Sandia and the Florida Solar Energy Center provided insight into recommended set-points for charging flooded lead-antimony batteries. [2,3]. …”