2002 Hyundai Sonata battery lights

Engine problem
2002 Hyundai Sonata Engine Size unknown Wheel Drive Type unknown Automatic 60000 miles

several times my batter light has come on, and each time I check the battery, there would be corrison and a slight chemical smell, soI would clean it off. Then the light will go off. Recently the same thing happend, but when I cleaned the battery this time the light stayed on. So a got a new battery. But the light is still on. A friend says that it's my alternator, and my brother says it's the wiring to the battery or engine. The cost of an alternator job would be about $2000. Can you give me a clue as to what to check next. I do not want to spend money needlessly