Direct current is used in nearly all electronic systems as their power supply and also to charge batteries, which are the primary sources of DC. Some railway propulsion use direct current, too, specially in urban areas. High voltage direct current is used to interconnect alternating current power grids.
Direct current is commonly found in many low-voltage applications, specially where these are powered by batteries, which can produce only DC, or solar power systems, since solar cells can produce only DC. Most automotive applications use DC, although the alternator is an AC device which uses a rectifier to produce DC. Most electronic circuits require a DC power supply. Applications using fuel cells, mixing hydrogen and oxygen together with a catalyst to produce electricity and water as byproducts, also produce only DC.