I ran into this same scenario a few years ago. Our ups backed up lighting needed the batteries replaced. We are a five prison complex, and it was estimated that it was going to cost around $300k to do it. The Life Safety Code stated that the exit lights were to illuminate at all times, of course, and we have internal batteries on ours, and we were able to do away with the battery backup on the other lighting due to the Life Safety Code stating that the lights had to be back online within 20 seconds, and our gen sets spin up within 15. This was the 1999 edition.