According to their spare parts chart (See Garfield's link above), not all use the same battery. However, I have loaned the spare battery for my Pace 400 (which originally came with a Pace 200) to a friend who has an Expilion 700 and it has fit and worked perfectly. I have also tested his battery in mine and it has worked fine.

Sent an inquiry to Cygolite asking what the difference was (thinking maybe the 700 used a higher capacity battery that would give me more runtime in the 400) but they did not respond. My suspicion is that they use the same battery and just charge more when you order it for the higher end model. If it was incompatible, they would key the housing so that it would not fit the other models.