You DO have a vent line connected to your battery, don't you? There's a good reason why hydrogen should not build up inside the car.
There are very good reasons why you want to vent any battery in the passenger compartment, but they all have to do with the safety of humans, not with the performance of the battery.
Oh Jimbo Do your homework on Optima Batteries. Knowledge is the key for success. Wake up the future is here iPhone ?
While the chances of a battery being so severely overcharged, that it vents toxic & flammable fumes, is remote, it is still possible. For that reason, we recommend any battery installed in an enclosed area (including trunks and interiors) be properly-vented to the outside atmosphere. The Optima DS46B24R YellowTop does have a vent port for hose attachment. Jim McIlvaine eCare Manager, OPTIMA Batteries