While not strictly a hybrid issue this describes some very neat technology that could eventually replace batteries. The Ultra Battery
I expect they may be light too compared to the same battery capacity. Which means only size would be a constraint... you could get as much storage as you have room for.
supercaps are really interesting, but have 1/10+ the storage density. Pretty favorable weight, virtually limitless rechargability and very, very fast charging. And, of course, the converse - you can get them to dump power very, very quickly, for those of us that would like clean, efficient, AND fast. There's interesting research going on with electric trains using battery & supercap storage. Interesting economics. Trains have very predictable terrain following, allowing optimal programming for such applications.
