distilled water

My battery has been very weak if it sits overnight in the cold after a few short trips (<2 miles), and in the morning the car wouldn't start. Come to find out, the water in the battery was practically bone dry. I filled up the six cavities with new distilled water, and so far, so good, but...

I know the battery is supposed to be a no-maintenance item, but there's a window on the side that shows how much liquid is inside. I tried to look at the level but I couldn't distinguish the liquid from the plastic shell. Maybe it was too dark. Does anyone have any experience replenishing the...