If you're in the market for a 12v powerpack (battery), I've just seen some for sale at GraysOnline auctions.
The powerpacks are 18AH units and also have a compressor built in (can't think of an astronomical use for a compressor but then I'm only a beginner
)
Here's the link:
http://www.graysonline.com.au/catalo...ALE_TYPE=THUMB