I use an 18 amp hour AGM deep cycle battery to power my SW 12" GoTo as well as 3 dew heaters and fan.
After 6 hours use, it still has plenty to go. Quickly recharges.
And it is small enough to sit on the front edge of the mount (so no cable tangle).
It is actually a Waeco 36 aH battery, that I opened up to find was comprised of two 18aH units. So I always have a spare if needed (eg on long Astro-Camps). [But it did cost just over $300].
http://www.fridgeandpower.com.au/fri...ck-raps36.html