The 2nd gen motion sensor has a little label under the battery that says "NOTICE! USE ONLY: Panasonic or Dureacell Battery (CR2450)"
Seems they mean it. Bought a bunch of Energizer 2450's and they work fine in the multi sensor but no light at all in the motino sensor. The tab in the sensor that touches the outer rim of the battery is poorly designed at an angle that attempts to make contact at the bottom of the battery toward the negative side. The Energizer battery has a thinner side profile so the contact is never made.
Obviously this is irritating, no way to know if any of the generic battery's on Amazon or eBay will work either as the thickness doesn't appear to be standard as one would expect.