The image shows India's first (19.04.1975) experimental indigenous satellite, covered in solar panels. A set of solar panels like this is called an array. Each solar panel has several solar modules and each solar module has many solar cells connected in series. If you connect two voltage sources in series, they behave like a single voltage [...]
Let's take an energy source like a battery or a PV module. When you draw no current from it, the voltage at it's terminals is called it's open-circuit voltage. When you connect any electrical load to it, current flows through it and it's terminal voltage decreases because some power is  wasted by it's internal resistance. [...]
Our batteries are expensive components of our off-grid solar installations. So it is a good idea to protect them from over-charging and deep discharge. To do this we use charge controllers. Charge controllers sense the voltage on the battery. If it is lower than a given threshold then they disconnect the battery from the load. [...]
 http://www.motherearthnews.com/renewable-energy/solar-power-zm0z12aszphe.aspx#axzz32lrUwzhu
