Ok, after alot of research and learning about wh/ah.... I think I am on track.
I came up with this :
Sealed Acid Battery 12V 7.5 AH / 20 Hrs for UPS, Seascooter and E-bike(S)
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2147
the above battery clocks in at just over 5lbs...