I have had about 6 months between batteries. You may have gotten a
bad batch of batteries but otherwise I would call Nike. I guess its
replaceable for a reason. I have the same gripe when comparing to my
Polar which I gave to my Mom after 3 years of use and it is still
running (well, it's "walking" now). And - that same gripe about
market share - all the aerobic equipment in my local Gym uses Polar
technology to monitor heart rate.

Jim