I would try using a q-tip with 91% rubbing alcohol (get at any pharmacy). Don't overdo it as you could easily ruin the battery. 91% alcohol will evaporate somewhat faster and with less residue than regular 70% alcohol will.
You might want to look at your filling habits too. If you are getting "gunk" on the battery, it could be that you are flooding the atty by overfilling the cart/overdripping. It probably wouldn't hurt to GENTLY blow out the battery with compressed air, as there is probably some liquid in it as well. From 6-12" away, GENTLY blow into the battery with the air. If it's a manual, you might be able to gently blow around the button too. Did I mention to do so gently?