This is an extremely tiny and light weight battery based on the new Polymer Lithium Ion chemistry. This is the highest energy density currently in production.
Each cells outputs a nominal 3.7V at 40mAh. This may sound like not so much power, but the cell is really tiny... like, about half an inch square!

Comes terminated with a standard 2-pin JST-PH connector - 2mm spacing between pins.

These batteries require special charging.
Do not attempt to charge these with anything but a specialized Lithium Polymer charger. Battery includes built-in protection against over voltage, over current, and minimum voltage.

Note: Be careful with the JST connectors. They can stick in pretty good and tugging on them can damage the connector. Check this tutorial for an easy way to remove them safely.

Dimensions: 12 x 16 x 5mm 

Weight: 2g

Features:

• Excellent long-term self-discharge rates (<8% per month)
• Robust power source under extreme conditions (-25 to 60C)

Documents:

• Datasheet (Protection Circuit)
• JST connector
• MSDS