ROTTEN EGGS is what my batteries smelled like when they were being boiled dry due to over charging when on a single mode 13.6 Converter/Charger unit...
My 85AH Interstate 12V battery did not last long after I found it boiled down below the internal cores... Game over
It was a new battery with around six months on it... The remaining three Batteries I purchased at the same time in 2008 lasted until last season after I started using a good working three mode converter/charger setup... Eight or so years sounds good to me...
Roy Ken