Lithium battery replacement for Norman 400b

I would love to be able to power my 400b with batteries with more power/longer run times. I'm willing to charge them out of the case. Does anyone sell such a thing? Does anyone know the specs on the "regular" (super?") batteries as far as output requirements? I'm thinking it should be possible to build a battery with cells that would solve the need. I'd even consider an "external" battery that did not fit in the case - giving up a little portability of course.

Thoughts?

Thanks!