Ahhh. Def don't wanna lose that, so I think DC to AC converter in the battery is the way to go.
Come to think of it, I have a UPS with a battery a bit smaller than the footprint of the HD20, and that runs up to 150 minutes, so I could def get 90 minutes on older, less efficient equipment. Maybe even 120!
Lugging that heavy battery around is crossing into performance art territory. I'm so doing this - when I find one that isn't stupidly overpriced.