The Ego-C 400mAh battery is the smallest variable voltage eGo style battery made today. It uses smart buck-boost converter technology, which allows you to adjust voltage output from 3.3v ~ 4.8v by spinning the knurled dial at the bottom. Regulated voltage output remains stable as battery is depleted. This battery is about the same size as a standard 650 eGo battery. The Ego-C 400 mAh has a sealed 510/eGo connection and will work great with most 510/eGo threaded clearomizers on the market, like the Innokin iClear 16, Smok T-Dux, Smok ARO and Aspire K1.