These new light-weight batteries from Sparkfun are perfect for any LED jewelry project. Less than a square inch in size, each cell outputs 3.7V at 20mAh. The great part — they are rechargeable (yay!). The downside — they require a specialized Lithium Polymer charger.

You can, of course, wire up a bunch together to get more amperage for other wearable projects that require more current.

Available via Sparkfun for $5.95