By constructing these devices in thin layers divided with graphene you can get phenomenal improvements in simple battery performance ~ try: http://www.torquenews.com/1080/battery-thomas-edison-invented-finding-new-life-cars for starters! (that's a pun)
Rossi's industrial size battery case ordinarily contained some quite ordinary iron hydride anodes ~ but he'd modified pieces of the circuitry ~ by themselves a full load of batteries could hold and discharge an appreciable amount of energy for about 5.5 hours (the time period he held his experiment to).
If he was getting more charge out of the units than he put into them I'd look at something besides his jumba juice and nickel powder ~ maybe some part chock a bloc full of nanoparticles of some kind ~ maybe even produced by accident, but otherwise reliably produced in some quite understandable but ignored process.
Folks have been working with these super high capacity units for in-line emergency power sources for a couple of decades ~ and the newer graphene sheet augmented units do the same job and more ~ graphene appears to have almost magical power (according to some of the guys working with it) and has been proposed for inclusion in hot fusion operations as well ~ for what purpose?
Now's the time to pick up the battery cases (about 1/4 the size of an international transportation container) and anodes ~ they've quit subsidizing the windmills which were one of the major users of the newst sorts of storage batteries. They were smoothing the power output with them.