Our house is 7 months old. At about 3 AM one day last week it started chirping. Pulled the ladder out replaced battery. By the time I put the ladder away and got back in bed, it started chirping again.
Pulled ladder out again and removed unit from wall. Watched infomercial until time to get up. Called warranty, they came out 2 days later and replaced. Said they have been having problems with the combination units. Both batteries tested good. Took an afternoon nap!