Besides replacing batteries yearly, think about actually replacing smoke detectors every 5-10 years. We've lived here almost 19 yr and last Christmas (of course middle of the night and with infant g-boys here!) the hardwired/battery back-up detectors all over the house (7 total)were going off. We replaced batteries on all and of course the next night the same thing happened (never in the daylight--go figure). I finally called our electrician and he asked if we had the same ones he installed 19 yrs ago and then laughed when I said yes. He says he replaces the ones in his house every 5 yrs. That they are not made to last forever and dust can make them nonfunctioning. So $85 later and lots of ladder time for DH we had all new detectors. Feel much safer--we live in country & rely on volunteer FD.