Sometimes, lead acid batteries give off a foul odour which resembles the smell of rotten eggs. The Rotten egg smell is the characteristic indication of the presence of Hydrogen Sulphide (H2S).
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.