Susan,
When charging a lead acid battery the charging process produces sulpher dioxide gas (amongst other gasses as well) & yes it does smell like rotten eggs, however this is usually indicative of over charging the battery.
Unless you have a very sophisticated battery charger it won't show you the state of charge in the battery, if the dial reads amps then it's showing you the charging rate.
If the dial shows volts then it is showing you the state of charge in the battery.
As most battery chargers only show the charging rate, it's important that you do not exceed 1/10 of the battery capacity as the rate of charge, for instance an 85 amp hour battery should not be charged at more than 8.5 amps, otherwise you are in danger of "cooking" the battery.
So my question would be, does your battery feel quite warm or hot when it's being charged and producing the eggy smell? if the answer is yes then you're over charging the battery & may have cause irreversable damage to the internal plates of the battery.
One final thing, the gasses produced by charging the battery are highly flammable and in the right concentration explosive, as you can smell the gasses then you do have a potentially explosive atmosphere, DO NOT disconnect the charger from the battery whilst it is connected to the mains as this will produce a spark & KABOOM! so switch your charger off from the mains and ventilate the room pronto!