What kind of battery do you have right now? Some of them are interchangeable, and some are not. An ego has two types of threads: a 510 connection that will fit most clearomizers and RBA's as well as some cartomizers, and an "eGo cone threading" that will fit other types of mini-clearomizers. If you have a little cig-a-like battery, it might use 510-threaded cartos, or it might use something else like 808. Find out what kind you have, and if it's not 510-threaded, you can probably find an adapter. Or, it might use a proprietary connection that's not compatible with any standard thread on the market. In that case, you probably won't be able to use your existing carts with the new battery, and would have to upgrade both halves at the same time.