My 10" Synscan GOTO draws 1ah according to the manual. On that basis I bought this for $99.00 from Jaycar.
http://www.jaycar.com.au/12v-3-in-1-...ttery/p/MB3730
After 4hrs use I still had 57% battery life. The only fault I found was that the socket connection for the plug was a bit on the loose side so I wedge it with a matchstick.