I generally agree with the other posters re: back up battery, but I was going to suggest the "Target" needs cleaning of accumulated dust, but contrary to bright-spark, the best way to clean it (it's a small, maybe 2 inch in diameter silver colored cup-shaped device where smoke particles settle to be detected) is to disable the device by pulling the correct circuit breaker or other protective electrical device, removing the cover and simply wiping the cup clean with a soft tissue. Replace the snap-on cover and turn electrics back on...