NiMH and NiCd batteries self discharge at a MUCH faster rate than alkaline batteries. In fact, at "room temperature" (about 70 degrees F) NiMH and NiCD batteries will self discharge a few percent PER DAY. Storing them at lower temperatures will slow their self discharge rate dramatically. NiMH batteries stored at freezing will retain over 90% of their charge for full month. So it might make sense to store them in a freezer. If you do, it's best to bring them back to room temperature before using them. Even if you don't freeze your NiMH batteries after charging them, you should store them in a cool place to minimize their self discharge.