There are several possible explanations as to why the low battery warning signal goes off if the battery is new.
Does your smoke detector have a pause button?
If so, you may have pressed this button accidentally. This means that the alarm will beep once every minute while in pause mode for the next 15 minutes (max). Wait a moment and see if the detector keeps beeping.
Are you sure it's your smoke detector?
There are other devices that give off similar beeps or warning signals.
Even "new" batteries are not always really new
If batteries are stored away somewhere, especially in cold rooms, they can run out more quickly. Always check the expiration date on the package when purchasing new batteries. The batteries included with your detector can also be affected by this. An included battery may not last as long as expected, which is normal for those kind of batteries. If this happens, replace the battery and check to see if the problem continues.