'Rotten eggs' smell is H2S (hydrogen sulfide) smell. Batteries smelling like that pose a security risk.
From http://wiki.answers.com/Q/Why_does_charging_lead_a... :
"When a lead-acid battery is discharged, the electrolyte (sulfuric acid) and the active material on the plates of the battery (lead) are consumed to produce water and lead sulfate and current flow. The chemical equation for a lead-acid battery during discharge is: PbO2 Pb 2H2SO4 -->PbSO4 2H2O +Electrical energy. The chemical equation for a lead-acid battery during charge is the reverse with lead, sulfuric acid, and heat being yielded as well as some Hydrogen gas. Ideally, all of the lead sulfate is recombined with the hydrogen from the water to replenish the sulfuric acid. When a lead-acid battery is charged, electrical energy is added to the battery, causing the water and lead sulfate to be recombined to produce electrolyte and the active plate material. During normal charging, hydrogen gas is given off, however if internal damage to the plates or low electrolyte levels exist, internal gassing may create hydrogen sulfide gas. In sealed batteries this would normally not vent to the outside."