Emergency Lighting System batteries are like spark plugs. At 30 or 40 thousand miles, would you just change one defective spark plug? No. You will probably change them all because most likely they have the same mileage on them. Banks of batteries in large AC or DC emergency lighting systems are quite similar. When one battery fails, the balance is not far behind. As with the spark plugs in your vehicle, the replacement of all the batteries at the same time, when one fails is, in the long run, much more cost effective. A major portion of the cost to perform either of the above-mentioned tasks includes travel, labor, shop time, disposal of old material, etc. Drive to the auto repair shop 4, 6, or 8 times to replace one park plug or replace and dispose of 6, 12, 20 or more batteries one at a time versus replacing all your plugs or batteries at one time is much less costly.