I use an old (clean) 2 liter coke bottle and replace the cap with a top that came off of an unused bottle of gear oil. (the kind that you snip off) I added a 2 foot piece of clear plastic tubing to the top.
Fill the "coke" bottle with distilled water. Replace modified cap. Put the hose in the desired hole in the battery, and turn the bottle upside down. When it gets full turn the bottle back upright. (if you're in a hurry, you can squeeze the bottle to make it flow a little faster)
If your batteries are under part of the top cap like mine are, you can see in the hole by getting a lighted "dentist's" mirror at Wally World.
Works Great...