I run the Waeco battery system for both the Lx200 and laptop when I am in the field. These batteries are great but they cost over $400. Surveyors and serious campers use them for their reliabililty. It will run a laptop and LX200 for over nine hours without recharge.
Here is the web site it you are interested.
http://www.waeco.com.au/products.asp?id=179&cat=2
Good luck with your selection.