Smoke detector question

[yellowstang]

Our smoke detector has 3 short beeps. All have had batteries replaced, yet 1 still beeps 3 times every couple minutes. Turned the power to them all off (thank goodness the breaker is marked!) Any idea's? Is it just bad and needs to be replaced? They are the originals when the house was built, so 7 years old.
They are USI-1204 model, all wired together with a 9 volt battery backup.

If I get replacements, any recommendations on a new type? Do most come with the same standard plugin for the hard wired part, or am I stuck with USI brands only?

[talisman]

Modern smoke detectors are designed to go out every 7 years for some stupid ass reason, so it is time to replace all of them.

[yellowstang]

Modern smoke detectors are designed to go out every 7 years for some stupid ass reason, so it is time to replace all of them.

Yeah, they actually have an expiration date on the bottom, 2016! I sent an email to a vendor for this model asking if they have a similar model, or these in stock so I can replace them all. Amazon had just one in stock for a seller I found there. The battery cage on this model sucks ass, it is a PITA to put a new battery in. Cheap ass plastic holder, broke one trying to get it lined up and snapped back in place.