Rotten egg smell can come from both a fried battery and a bad cat. If you experience an overcharging condition, then you should replace your battery anyhow. The excess heat from overcharging can and usually does warp the plates inside of the battery. This may not kill the battery right away, but it will fail. This happened to me last summer. Alternator failed so I replaced it and the belt, but kept getting a charge light flickering on the dash. Took me all summer to figure out because the battery seemed to hold a charge. I would do a complete charging system check just for peace of mind.