Over the holidays I spent 2 weeks out of town and returned home to a totally unresponsive XC60 T8, though thankfully this was in my garage and not in long term parking.
After the fun of figuring out the emergency key and how to use it, I tested the battery terminals under the hood and found barely 4V.
One nice thing was that 'jump-starting' the vehicle was easy, since you don't need to supply enough power to actually crank the engine (the SPA T8 ICE has both no starter and no alternator (and no dip stick! WTF is the world coming to...)), so simply enough amps to boot the computers and turn the vehicle 'on' was enough for the DC to DC converter to fire up and start charging the 12V system from the HV battery.
And yes, my HV battery was full, she was plugged in the whole time. Note that only charges the HV battery pack, and the computer is supposed to be smart enough to then charge the 12V system from the HV battery periodically. Supposed to...
Searched the forums and found that "Service Campaign A39981 : BECM Software Upgrade" is supposed to fix this, emailed to my favorite service advisor and she confirmed the likely diagnosis and set up a re-programming appointment under warranty. Just got the vehicle back last night and looks like they gave me a full reprogramming of all modules ()
I find the notes interesting...sounds like there was a bug where if the HV SOC (state of charge I presume) is continuously full, the computer never cycles the DC to DC converter to charge the 12V system? Dumb logic there, I would presume it's now smart enough to cycle now and then even when the vehicle is never turned 'on'. Will look for a 2-3 week window ahead where I can strictly not drive my vehicle and see if it's really 'fixed' or not. Might be fun to figure out how to data-log the 12v system voltage during that duration too, hmmm...
Anyway, just wanted to share the news for folks who run into similar problems.
ps, no I'm not worried about the health of the 12v battery, it's AGM and designed to take some deep cycles.
pss, yes I got a warning during my trip from the Volvo app that the vehicle was going into sleep mode to save power, though I didn't keep track of exactly when during that 2 weeks I got the warning.