Help why is my battery terminal corroded and sparking?

If you have a multimeter set it to 10 amps and check the amp draw. If you see several amps something is left on. If less than 1 amp it is it is normal parasitic draw. My is about .4amps and will spark when I connect my neg cable to post. Don't overfill cells with water, clean connections every so often, and clean the top of your batteries also to minimize self discharge.