Battery box filling up with water.

I had a battery box with small vents on the top, where water could get in. But why they had vents at all was the great question - it wasn't a sealed box by any means, and plenty of room around where the cables when in, even around the edge of the lid. I just sealed the top vents up with caulk.

Battery box is supposed to keep the corrosive stuff in - I wouldn't drill drainholes.