That's not good. Are your water levels high enough? Are you overcharging? Are the batteries properly vented?
Over-charging a lead acid battery can produce hydrogen sulfide. The gas is colorless, very poisonous, flammable and has the odor of rotten eggs. Hydrogen sulfide also occurs naturally during the breakdown of organic matter in swamps and sewers; it is present in volcanic gases, natural gas and some well waters. Being heavier than air, the gas accumulates at the bottom of poorly ventilated spaces. Although noticeable at first, the sense of smell deadens the sensation with time and potential victims may be unaware of its presence.
As a simple guideline, hydrogen sulfide becomes harmful to human life if the odor is noticeable. Turn off the charger, vent the facility and stay outside until the odor disappears.