2002 Hyundai Sonata my battery light has come on

2002 Hyundai Sonata Automatic 60000 miles. Several times my battery light has come on, and each time I check the battery there would be corrosion and a slight chemical smell and I would clean it off. Then the light will go off. Recently the same thing happened, but when I cleaned the battery this time the light stayed on. I relpaced the battery with 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 $2,000.00. Can you give me a clue as to what to check next? I do not want to spend money needlessly.