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Recommended Practices for Charge Controllers

Eric P. Usher and Michael M.D. Ross
CANMET Energy Diversification Research Laboratory

Full Text of Report
Link to CETC-Varennes
Link to IEA PVPS Programme

Note on Authorship:

This article was jointly authored by Eric P. Usher, formerly of the CANMET Energy Diversification Research Laboratory (currently known as CETC-Varennes) and now with the United Nations Energy Programme (UNEP), and Michael M.D. Ross, formerly of CETC-Varennes, and now principal of RER Renewable Energy Research.


This report was prepared by the CETC-Varennes as part of its contribution to Task III of the International Energy Agency (IEA) Photovoltaic Power Systems Programme (PVPS).


Eric P. Usher and Michael M. D. Ross. Recommended Practices for Charge Controllers, Report IEA PVPS T3-05: 1998, CANMET Energy Diversification Research Laboratory, Natural Resources Canada, Varennes, Québec, August 1998.


In stand-alone photovoltaic (PV) systems, charge controllers regulate the power from the PV array in order to protect the battery against overcharge. In addition, most controllers have a low-voltage-disconnect which regulates the power to the load thereby protecting the battery against deep-discharges. The charge controller is therefore the energy manager in a stand-alone PV system, ensuring that the battery is cycled under conditions which do not reduce its ability to deliver its rated capacity over its expected lifetime.

Most controllers will initially allow all the current output from the PV array to pass to the battery, then, as the charging nears completion, will taper or apply intermittent current to maintain the battery on a float charge. In addition, some controllers will intentionally overcharge the battery periodically to equalize the cells and mix the electrolyte.

Although the controller is one of the least costly components in a stand-alone PV system, its appropriate operation is fundamental to the long-term reliability and recurring costs of the PV system. Choosing the best controller for a particular system and application, and configuring it correctly, are paramount for successful PV system operation. These Recommended Practices are intended to provide PV system users, operators, and integrators with the most current information on how to choose, configure and maintain controllers in PV stand-alone systems. In addition, it includes sections helpful to manufacturers of charge controllers.

It should be noted that these Recommended Practices apply only to controllers of lead-acid batteries, and should not be used with systems employing nickel-cadmium or other battery technologies. This is not a serious restriction, since the vast majority of PV systems use lead-acid batteries.

This document first gives a general introduction to charge controller terminology and configurations, then discusses issues of controller selection and setpoint determination, and finally provides suggested practices for procurement, installation, and maintenance. Section 1 introduces the few basic concepts underlying charge control for lead-acid batteries, and defines the terms used in subsequent sections. Section 2 deals in more depth with the how charge controllers should function in order to achieve full charging of the batteries under various conditions. Section 3 examines how best to terminate discharge, such that the battery is not discharged to dangerously low levels. Section 4 overviews features that have been found useful on many charge controllers. Section 5 is aimed at purchasers of charge controllers, and should help them decide which controller best meets their needs. Section 6 is a brief introduction to methods for charging batteries in systems containing power systems other than PV arrays. It focusses on the question of when a generator should be started, and how long it should run, in order to minimize the cost of a hybrid system with a generator. Installation and maintenance procedures are described in Section 7; a trouble-shooting chart constitutes Section 8, and Section 9 lists the testing a charge controller should undergo in order to verify that it will function satisfactorily. The document finishes with a glossary, a bibliography, and a list of the mailing addresses of many manufacturers of charge controllers.

The field of charge control, while seemingly quite straightforward, has turned out to be one of the most problematic issues in photovoltaic system operation. These Recommended Practices are gleaned from a variety of sources representing current understanding of how best to treat lead-acid batteries in photovoltaic systems. A careful reading of these sources, however, reveals that a consensus does not exist on many matters related to charge control. It is clear that these Recommended Practices will require modification and improvement as understanding of the field changes. Therefore, the reader should be cautioned that this report is a good primer on charge control, but can not be considered definitive.