For those of you who have this model (i have two from different serial ranges), the battery is a 395/399 SR927SW. There was another thread here that listed a different battery size, which I believe is incorrect. Or, it may be correct for that serial range in the event you have one that differs from mine.
You will need one of the finer watch screwdriver sets to remove and replace the battery cover once you get the watch back off. I believe I used a 0.8mm common screwdriver to remove.