A battery passthrough.  There are eGo (510/eGo thread) passthroughs, or the Volt X2 (808/X2 thread).
I use the X2 myself.
Technically that's not a passthrough, but that's what they call it.  You'd be looking for something that reads "1,000 mAh passthrough" or whatever mAh rating the battery has.  Some passthroughs are just that--no battery at all.