Are you able to disconnect the battery when you park it for the 8 hours? If so, you could do that at work or home. If battery still dead hours later when reconnected to load, then it's battery. If not, you may have parasitic drain. Odds are your battery is shit, especially if older nom-agm and cold climate. Maybe could check electrolyte (distilled water) if lead acid battery. If you replace, Costco has great warranty policy on the interstate brand they peddle. Much better about honoring it than actual interstate retailer lol. Had people with stories of warranty replacement last month of eligibility.
If battery, get a new one, install with anti corrosive pads. Use steel wool and clean the car contacts with contact cleaner. Use dielectric grease when you reconnect.
Just my 2-cents. If your electrical is parasitic, you might install a relay to toggle the main off bypassing car electrics. Other things could be alternator not pushing enough juice to recharge it, but you'd probably lose power while driving if alternator bad. Those usually easy to swap out of located well on engine.
This post was edited by RedFromWinter on May 22 2023 09:13pm