The D-40 is a completely different camera than the I-40 or M-40. In addition to the fact that they have a sleep mode, they don't seem to get nearly the same battery life as the I-40.

If it has been cold, the alkaline batteries might go down a little faster. The literature that moultrie puts out gives the I-40 a 150 day battery life and the D-40 a60 day battery life in good conditions.

If you still can, you might try to return the D-40 for an I-40 or M-40. You'll end up saving the price difference in batteries over a couple years and the I-40 and M-40 have significantly better trigger times and picture quality as well.