Back when I had an EQ6 I did some testing:
It draws ~0.5A when idle, just after powering on. (Alignment on screen)
~1.5A with both axis slewing(no-load), and ~0.75A when tracking(no-load).
While the mount was slewing I loaded it up with my hands(as hard as I could) and it peaked at around 2.2A.
I would guess a well balanced mount would be drawing similar to the no-load figures.
So.... multiply those figures by the number of hours you reckon you'll be using it in a session, then go from there.
Don't forget to get long life(years, not session length
) from a Lead-acid battery, you want to discharge it as shallow as possible.
If we say, for example, 1A average draw, over 5 hour session, then 5Ah capacity would be used from the battery. If you wanted the battery to be discharged no more than 30% DOD, then: 1/0.3*5 = 16.67 so a battery around 20Ah should be fine.