This lightweight foot pod transmits distance and pace data. It's small enough to attach to shoelaces or fit in the mid-sole pocket of compatible shoes.
Compatible with:

Forerunner 305
Forerunner 405
Forerunner 405CX
Forerunner 310XT
FR60
Forerunner 50

Does it come with Batteries and what type of Battery does it take?
Yes it does come with a battery. Replacement Batteries are model: CR2032

1.8 Inches by 1 Inch in Size.
Light Weight.

98% accuracy (calibrated) for most users; 95% median accuracy out of the box
Measures cadence, or number of steps per minute, to optimize training
Small enough to attach to shoelaces or fit in the mid-sole cavity of compatible running shoes; automatically turns on with movement
A small, replaceable coin cell battery powers the pod for at least a year of training (>400 hrs)
Unlike simple pedometers, the foot pod uses high-precision sensors (accelerometers) to measure a runnerfs unique foot motion in 3-D, providing greater accuracy
Highly responsive to gait changes, allowing you to run speed intervals away from the track

Compatible with:
Forerunner 305
Forerunner 405
Forerunner 405CX
Forerunner 310XT
FR60
Forerunner 50

Garmin Foot Pod