All EVs have a DC to DC converter, to convert High voltage (~ 400V or so) to Low voltage side (12V)
Since car keep some load active (cell comm, remote, etc), the 12V battery still monitor (* Note: when EV is recharging, the main DC contactor has to be close to allow current flow thru to the high voltage battery, otherwise if not running, it should be open to isolate from the HV battery) if required from time to time but I'm not aware of all fine details and may vary widely from one manufacturer to another...
Same YTuber, having problems with his e-Niro dead 12 Volt battery
https://youtu.be/CRMNKi84_yw?t=260 has a battery monitor plugged into OBD port (which potentially kill the battery), to check activity level maintain while at rest.
His battery replaced under warranty, check about the update
https://youtu.be/oZi5q20ND1Y?t=123Edit to annotate about DC contactor (isolation purpose) status at rest, charging or not.
12 volts battery management...?
https://youtu.be/A4NUdNWyocM
IONIQ 5’s 12-volt battery is dead: here's how I jump-started it
Likely some accessory light left on. I'd imagine engineers who design electric cars would know how to keep a 12v battery charged ![Wink ;)](https://www.autos.ca/forum/Smileys/CarTalk/wink.gif)
But they don’t. Seem to be the #1 issue with EV’s for some odd reason. My son just replaced the original battery in his 9 year old Kia Rio so go figure.. ![Huh ???](https://www.autos.ca/forum/Smileys/CarTalk/huh.gif)
That's just strange (the EVs' 12V battery issue). I suppose in an ICE car (with exception of hybrids) every time the key is turned on, the alternator is running, so basically the 12V battery can charge whenever it wants to. What is the case with EVs? Do they charge the 12V battery only when connected to 120/240V or can they use their traction battery for this purpose?