The PMC8 needs about .5 amps. A battery with a capacity of 5000 ma hours (milliamp hours) or 5.0 amp hours will run the mount for about 5/.5 or 10 hours. Now derate this by about 50% for safety and you have 2.5 hours. A battery with 25000 ma hours would provide 25/.5 or 50 hours...derate by half and get 25 hours. Pretty impressive.
The thing is when you are observing you will need dew heaters in summer and sometimes in winter too. These things will pull about 2 amps at least for the main objective and the eyepiece. Then you need one for the guide scope and maybe two for your finder scope so that's another 2 or three amps. So say you need 4 amps of dew control. All of a sudden the 5000 ma battery looks dead in 5/4.5 or about an hour. The 25000 ma hr one is done in 5 without derating. So expect less.
I have a huge lead acid battery solution so I never worry. (100 amp hours...haha at 40 pounds). But I would surely go with as big a battery as you can afford. But be careful these big lithium batteries might not be able to be carried on aircraft...as in taken in vacation with you. So check that out. In fairness I haven't and won't haul my lead acid along on a flight...probably couldn't anyhow.
Don't plan on using an ac-dc adapter powered by a battery (which has a built in 120 volt inverter) as this will be a huge power waster. I would imagine you would suffer huge reductions in use time. Be sure to get a battery which has a built in 12v output capability.
Wes, Southport NC
PMC-8, ES ED 127, 10" LX200GPS, Astro-Tech 8" Newt, ETX-90
Polemaster, Orion ST-80 and SAG
Electrical Engineer, Retired