Maybe I'm being too narrow minded, but I just don't think you will ever see batteries (stored energy using inverters or whatever similar gadget) as the primary power source for rotorcraft. You might get away with that for fixed wing, because they can turn the prop on and off when needed (& they can generally carry more weight). If it's going to happen in a rotorcraft, my money is on a hydrogen system that provides energy directly to a fuel cell, which provides power for the electric motor and systems. Again, fuel cell technology is coming along nicely... how to carry enough hydrogen (safely) is the real problem.
Heck, you can purchase a retail version of a fuel cell power generator that will provide 1,000 watts of electricity as long as hydrogen gas is supplied. It's quite, nearly maintenance free, and was designed to run INDOORS (you don't have to worry about CO poisoning in the cockpit)...
http://www.fuelcellstore.com/cgi-bin...94/product=393
http://www.ballard.com/popup_airgen_virtual_tour.htm
So, how can you carry enough H2 fuel (read throughput) for the larger systems (currently used in Ford & Chrystler protypes)? For rotorcraft, it just can't be done... today. That's just one challenge. With this kind of configuration, it is likely that you will need a complicated transmission to maintain RRPM in all fight regimes... simply adding more power or less power might not be an option anymore.