Abstract: The current invention is comprised of a method and apparatus for monitoring the condition of backup batteries, especially those associated with an on-line, uninterruptible power supply driving a critical load. This is accomplished by measuring an AC ripple current induced into the backup batteries by the UPS, comparing the amplitude of the AC ripple current to a low level limit and to a high level limit determined by the UPS operating parameters and the critical load, and indicating an alarm condition when the AC ripple current falls outside the range defined by the low level and high level limits.