To answer the question: This is almost always a lead-acid battery failure causing the battery to vent hydrogen sulfide (H2S). The battery needs to be replaced as soon as possible.
As an additional note, H2S can be extremely dangerous at higher concentrations. If you experience eye irritation or difficulty breathing or your ability to smell the odor deteriorates noticeably, the concentration of the gas is dangerously high and you should see a doctor. At that point, you may need to hire a hazmat cleanup service to remove the battery and clean up the area.
Wikipedia says this on H2S toxicity:
- 0.00047 ppm or 0.47 ppb is the odor threshold, the point at which 50% of a human panel can detect the presence of an odor without being able to identify it.
- 10 ppm is the OSHA permissible exposure limit (PEL) (8 hour time-weighted average).
- 10–20 ppm is the borderline concentration for eye irritation.
- 20 ppm is the acceptable ceiling concentration established by OSHA.
- 50 ppm is the acceptable maximum peak above the ceiling concentration for an 8-hour shift, with a maximum duration of 10 minutes.
- 50–100 ppm leads to eye damage.
- At 100–150 ppm the olfactory nerve is paralyzed after a few inhalations, and the sense of smell disappears, often together with awareness of danger.
- 320–530 ppm leads to pulmonary edema with the possibility of death.
- 530–1000 ppm causes strong stimulation of the central nervous system and rapid breathing, leading to loss of breathing.
- 800 ppm is the lethal concentration for 50% of humans for 5 minutes exposure (LC50).
- Concentrations over 1000 ppm cause immediate collapse with loss of breathing, even after inhalation of a single breath.