Yes, a "jump starter" with a cigarette lighter output such as this
http://www.jaycar.com.au/productView.asp?ID=MB3692 would be fine to power a telescope.
Or for a cheaper, lighter, more compact option, you could look at a 12V SLA (Sealed Lead Acid battery) and charger, and make up your own cable to connect the battery to the scope. (For my Celestron SLT / Meade LX90, I just cut the cigarette lighter adapter off, and soldered on some red and black terminal lugs to clip to the battery terminals - make sure you get correct polarity before you plug it in!!!)
See this link for some SLAs and chargers - less than half the cost of a jump-starter (but of course, a jump starter can be used to power more devices, and for longer):
http://search.jaycar.com.au/search?w=sla&view=list
Something the size of a CG-5 probably pulls one or two amps peak when slewing at high speed, but more like half an amp (or less) while tracking, so a 7 Amp-Hour 12 volt SLA would pack enough juice for at least 14 hours operation - more than enough for a couple of night's viewing between charges.