I agree on a couple notes: (1) MBZs tend to "trickle" a battery to the point of not being able to crank the engine over. It's happened to me on a number of occasions; and with a fresh battery (I only drive my car once per week). Hence, my reason for purchasing a "battery tender". (2) I have been keeping my car under wraps in the garage with the battery tender in place. I think the principle is somewhat different than a trickle charger; the battery tender has some "intelligence" built into it. That said, the amount of gases produced (if any) could not possibly be harmful (IMHO). However, for safety's sake, I always wear safety glasses whenever connecting/disconnecting leads to the battery terminals.