The battery backups can be switched in very quickly so that there is no glitch or drop in the power supply, that is the main point of them, not to hold up a supply for a longer time, not like other backup systems which take time to be brought online but can hold up the line voltage for much longer.
It's not the least charm of a theory that it is refutable. The hundred-times-refuted theory of "free will" owes its persistence to this charm alone; some one is always appearing who feels himself strong enough to refute it - Friedrich Nietzsche