Comparison of battery charging algorithms for stand alone photovoltaic systems

The battery is the most common method of energy storage in stand alone solar systems; the most popular being the valve regulated lead acid battery (VRLA) due to its low cost and ease of availability. Photovoltaics are not an ideal source for charging batteries as their output is heavily dependent on weather conditions. Therefore, when batteries are used in photovoltaic systems, the performance characteristics differ significantly from batteries used in more traditional applications and the battery life is usually shortened. In conditions of varying solar radiation and load profile the battery may experience a low state of charge (SOC). A low SOC for extended periods of time will cause increased sulphation, which severely reduces the life of the battery. Typically, steps are carried out to protect the battery and to charge the battery more effectively. Such methods include intermittent charging (IC), three stage charging (TSC) and interrupted charge control (ICC), among others. This paper quantifies the effectiveness of these three battery charging algorithms and evaluates their ability to maintain the battery at a high state of charge. The measurement setup is comprised of a solar simulator, which replicates the output of a large 50 W photovoltaic panel using a low power cell. Repeatable load and solar radiation profiles and temperature control are implemented using LabView so that identical operating conditions can be set up to compare the three battery charging systems.