Faulty batteries tend to get overcharged. The high current during overcharging causes the production of Hydrogen Sulphide gas. This leads to the characteristic odour.
If you find that there is a smell coming from the battery bank, you need to identify the defective battery. This needs to be done as early as possible as there is a risk of battery explosion. Hydrogen Sulphide is extremely flammable. The battery will have to be replaced.
Battery Rooms should also be provided sufficient ventilation to vent out the gas.