Unfortunately, the sun may not shine just when you need electrical energy. The reverse is also true: energy may not always be required when the sun supplies plenty of it! In addition to the solar cell array, a stand-alone system requires another important component: an electrical energy storage device.
The first device that comes to mind for this function is the battery, which is available in many different shapes and structures. Apart from special battery types, including chloride-zinc, iron-sulphide, lithium, nickel-iron, silver-zinc and sodium-sulphur, a number of which are still under development, familiar types such as the lead (gel) acid, NiCd and NiMH batteries are widely used for this purpose. These batteries feature a high and fairly constant capacity, nearly loss-free current acceptance and delivery, and extended durability despite many charging/discharging cycles. Lastly, they are almost maintenance free.
The lead plates of special batteries for solar systems have selenium or calcium doping instead of antimony as used in car batteries. These special batteries are marked by high cycle repeatability, excellent charge efficiency, low self-discharging, high immunity against deep discharging and overcharging, and, unfortunately, a high price! Of course, there are exceptions. Some online stores sell top products for low prices. Check before you buy!
Just as with solar cells and modules, batteries may be connected in parallel or in series. When doing so, it is essential to use batteries of the same type, with the same capacity, nominal voltage and charge condition. You can also connect the batteries in parallel and in series (few batteries in series and then few such lines in parallel). But this increases voltage and current that is needed to charge them.
Reverse Current Protection
The reverse current protection diode prevents the battery from discharging itself via the solar cell. Irrespective of its operating principle, the control circuit avoids overcharging of the battery. A deep-discharging protection should be integrated in all cases.