The specs on line say it has an (impressively large) 9.6v 45Ah internal battery, good for 10 hours operation
I suspect 45Ah may be a typo and the battery is actually 4.5Ah even though the 45Ah figure appears everywhere on different sites.
Compare for example with the effectively identical Celestron branded version, here where a space has been used instead of the decimal point (9 5v 4 5Ah) https://altairastro.com/celestron-nexstar-evolution-925×22-sct-6537-p.asp
That would be ok for the claimed 10 hrs just tracking (at ~0.5A) but a lot of slewing would take more power. What life do you typically get with the internal battery ? (If the internal battery is 4.5Ah then say 3 x that time with the 17Ahr power tank would seem reasonable given that you don’t want to run down lead acid batteries too low to prolong their life)