I had the exact same problem; chirping every 30 seconds in a hard wired smoke alarm despite changing the battery & ensuring the battery was fresh and installed properly. After frustration and considering replacing the whole dang unit, it occurred to me that the battery, although installed properly, was no longer making a good connection with the two connector plates at the end where the +/- leads connect. There was the tiniest hair-breadth play or movement of the battery. So I simply took a letter opener (or pen, screw driver, whatever), and slightly bent/pulled out each of the two metal connector plates. These had been pressed down flat over time, and the battery leads were barely touching them. As soon as I pulled them out, when I reinstalled the battery it was in really tight and the leads made a solid connection with the connector plates - VOILA, the chirping stopped and my problem was solved! Hope this helps! ... View related article.
