Smoke Alarm

Why is it that your smoke alarm battery never goes dead at a convenient time?  Ours seems to always go dead late at night when we are relaxing and getting ready for bed or in the middle of the night when we are sound asleep.  This morning, the smoke alarm starts beeping at 6am.  It jolts me out of bed and sends the dogs in to a barking frenzy.  By the time I get awake enough to figure out what is happening, everything is quiet.  I look over at hubby and he is still sound asleep, so maybe I was just dreaming.  I go back to sleep and about an hour later it happened again.  This time I am not dreaming because it woke hubby too.  I get the ladder so hubby can help me change the batteries (I am too short even with a ladder) and when I look in the box of batteries, we are short one battery.  Since the smoke alarms are all connected, they still beep even is one is low, so I made a mad dash to Walmart to get a battery.  I know they said you should change the batteries once every 6 months or when the time changes, but I have it on my calendar to change them every 3 months now.  No more being jolted out of bed.