If you’re looking for a method to beat battery weight or add some big volts in a tiny space, try experimenting with polymer lithium ion batteries. Better known as LiPoly, lipo, and LipIon, these batteries are very small and lightweight, but they can deliver 3.7V @ 100mAh.
Be forewarned, however, LiPoly batteries require some special handling. For example, only a special polymer lithium battery charger circuit can be used for charging these batteries. Also, soldering the battery’s positive and negative terminal tabs to your project board can be very challenging. Do not exceed a 350F soldering iron temp and work swiftly; using a soldering time of less than 5 seconds.
You can purchase LiPoly batteries from Spark Fun Electronics. A single 100mAh batteries costs around $7.
(Image: Spark Fun Electronics)