Cell covers typically have gas diffusers built into them that allow safe dispersal of any excess hydrogen that may be formed during overcharge. They are not permanently sealed, but are designated to be "maintenance free".
HTH[/QUOTE]
Just a word of caution regarding the hydrogen. My wife had a battery explode in her car when she turned the ignition to start it. We assumed the hydrogen was ignited by a spark, possibly from a loose terminal arc, but we don't know for sure. It blew the top off the battery, sprayed acid all over the engine and hood and buckled the hood out to the extent they had to replace the hood. If anyone has been standing around the battery with the hood open they would have been seriously injured.
Always be extremely careful taking cables and chargers off battery post.
It is a miniature bomb in the making.
Mark 1:17 ...I will make you fishers of men