In an off-grid photovoltaic solar system, the battery is often in a floating charge and discharge state. It can be roughly considered that the amount of sunshine in the summer is large. In addition to supplying power for the load, there is excess electric energy to charge the battery. In winter, the amount of sunshine is small, and the power generation of the square array is insufficient, and it is necessary to use the stored energy of the battery. On the basis of this seasonal cycle, a much smaller daily cycle is added: the battery is charged when there is excess during daytime power generation, and the load power is supplied from the battery at night. Therefore, it is required that the battery has a long cycle life, a small self-discharge, an overcharge and discharge resistance, a small temperature influence, and a high charging efficiency, and of course, consideration of low cost, convenient use, and simple maintenance. Currently used batteries are lead-acid batteries and nickel-cadmium batteries.
In the selection and design of the battery system, the factors considered mainly include:
●Electrical characteristics: voltage, capacity, charge and discharge rate, etc.;
●Performance: The relationship between cycle life and discharge depth, system self-discharge;
● Physical properties: size and quality;
● Maintenance needs: immersion or valve-controlled seals;
● Installation: location, structural requirements, environmental conditions;
● Safety and auxiliary systems: rack tray fire;
● Cost: warranty terms and availability.