Just to let you know, that this has been done.
Back in the 80's Orlando Helicopters (the group that created the WhisperJet and Hummingbird kit) successfully flew a Sikorsky S-52 (HO5S-1) on battery power. I believe they were using lead-acid batteries and a torpedo motor. I'm sure it was a pretty short flight, but proved (?) the concept.
BTW, Do you really think they're keeping it up for seven minutes, or does it just
seem like seven minutes?
HOSS-1