[MetroMPG]

I replied:

I've never heard of the requirement to open the caps while charging golf cart type batteries we use, and don't know of anyone who does.

Perhaps it is a difference in cap design?

But the gas is able to escape these batteries while they are closed (which is the reason some people build enclosed battery boxes & vent them to the outside). The seal is not air tight. In fact electrolyte can seep out if the battery is not sitting upright.