If your vehicle is showing rotten egg or sulfur smell coming from the battery area, here are the most likely causes and the parts you'll need to fix each one.
As batteries age, internal lead plates can degrade, warp, and touch each other, creating an internal short circuit. This causes the battery fluid to boil and release hydrogen sulfide gas (rotten egg smell) even under normal charging conditions.
If the voltage regulator fails, the alternator may supply excessive voltage (over 15 volts) to the battery. This severe overcharging literally boils the battery's electrolyte mixture, venting dangerous sulfur gas into the engine bay.
Physical damage to the battery case allows acid to leak and sulfur gases to escape. This is frequently caused by over-tightening the battery hold-down bracket during a recent installation or from severe physical impact.
Loose or heavily corroded battery cables create massive electrical resistance. This resistance generates extreme localized heat that can melt the battery casing around the post and cause the acid inside to boil and vent.