The smoke alarm should not work at all if the batteries are not installed correctly.
New batteries could have low power level just from long shelf life, so it's possible it's still a battery problem. Have you tried a pair of batteries from another package, or tried the batteries in another device? A flash light that uses AA is a good low tech way to see if batteries are good or not [bright light vs dim] when a battery tester is not available.
It is possible the smoke detector failed.
Another possibility is the smoke alarm is at it's EOF (end of life). The beep may be an indicator that it is not longer at peek effectiveness, check your manual or online to see if your smoke alarm is smart enough to indicate it reached it's EOF.
Answered on 10/18/2013 by Terry from Quakertown PA