I too am interested, especially if the SLA pack can be made an option.
I have Nilar NiMH 24V/10Ah, 1/2 the weight of SLAs, fits in old Currie eFolder rear pack (though doesn't slide into custom rack, rats!).

Yes, battery costs as much as rest of kit, but this is interchangeable among other Curries and a Giant LAFree Lite (got from local pawn shop!)