Do leave caps off or on when charging battery?

By all means, wear a hat. The battery should wear its caps as well.

ANS2:The battery needs to be able to breathe. If you know for a fact that the caps are clean and the vents are open you should leave the caps on.

Back in the day, when the caps screwed in and had little tiny vent holes, that was a good idea to leave the caps off or loose because the battery could build pressure enough to blow out the side.

The vent caps, today, will pop off before the battery case blows out. If you leave the caps on, you are going to retain a large volume of explosive hydrogen gas if you don't top the cells up with distilled water beforehand. That hydrogen can be ignited by any nearby sparks.