What will make a car battery smell bad?

you shouldn't be able to smell it at all- the water may be low- only add distilled water, and do it in a well ventilated area with no chance of sparks.

sulfuric acid, time for new battery

I would look toward the alternator. Take your car to an auto parts store they will check the battery and the alternator for free.

sulfuric acid smells a lot like stinky a rotten egg. visual inspection will probably notice deposit at the battery terminals and possible cracks in the casing. do not touch with bare skin and wear eye/face protection. repleace battery with new one, and clean off old deposit on the connections.

What happens when negligent owners don't learn the basics of cars like filling up unsealed batteries twice a year or just buying optima to begin with no acid just spiro coils and gel

Dont light a match btw.. You might die