Solar modules, some wind generators, hydrogen fuel cells and most micro-hydro generators create DC or Direct Current electricity. This is the type of electricity found in batteries. The most typical DC current is 12 volts DC which is what we find in automobile batteries. To keep things simple lets use the 12 volt battery as the building block for our educational system.

Typically, but not always, solar modules make DC electricity directly from sunlight. It is used to charge a battery and this stored energy is held in the battery for use later. This is very convenient because we don’t always need electricity when the sun is shining. Several solar modules can be wired in parallel to speed the rate at which a battery is charged. See the section titled Basics of Electricity for more on this subject.

If we have a 12 volt battery that is completely discharged it will have a terminal voltage of less than 11 volts when read with a volt meter. A fully charged 12 volt battery will have a terminal voltage of approximately 12.7 volts when at a full state of charge. However, to fully charge a 12 volt battery we must slowly raise the voltage of the battery to approximately 14.5 for a wet cell (lead-acid) battery or 14 volts for a sealed, maintenance free or gelled cell battery. Once the battery terminal voltage has been brought up to 14 or 14.5 volts and held there for a period of time we can say the battery has been fully charged. Once we stop charging the battery and allow the voltage of the battery to “settle” it will drop down to the 12.7 volts mentioned earlier. Because we have to raise the battery level to 14.5 and sometimes even higher, most solar modules, wind generators, fuel cells and micro-hydro systems have a maximum output voltage of approximately 17 volts. This higher voltage is required because heat from the sun, friction or other outside sources can significantly reduce the peak output voltage of our system. By allowing a few extra volts output, we can be assured of having the required 14.5 volts required to completely charge the 12 volt battery.