This issue has happened to us twice in the one month since we got the Ioniq 5.
The first time around, I thought it could've been me not having put the car to Ready (ie, start/stop pressed without the brake pedal) while playing around with the menus for the first time setting up profiles etc., but fortunately charging the high voltage main battery apparently solved the issue.
Don't actually know for sure though whether a) I had indeed mistakenly started it without brake pedal b) whether it was able to start later due to the time elapsed since my last attempt or due to the trickle charge of the main battery.
If the high voltage main battery isn't full, I would personally just charge that instead of charging the 12v through smart charger using jump leads because charging the main battery should charge the 12v too (stated in the manual) and I don't really want to use the jump leads if I can avoid that because technically, in line with what the manual says, shouldn't jump charge the 12v without disconnecting it from the car first (haven't seen anyone disconnect the 12v from the car first though prior to jump charge, certainly the AA man didn't do that - see further below).
Second time it happened, I'm sure I had the vehicle in Ready and then changed it to Util mode so I could use heating and music whilst the car was parked. The 12v got low (maybe, I don't know). Failed to go to Drive gear. Then failed to go to Ready mode once switched off and back on. AA called. 12v jump charged. No dice. Lock-unlock-lock-unlock (ie twice) per AA's direction, no dice. AA man disconnected and reconnected 12v battery, car goes okay to Ready mode.
This implies potential culprits as:
- Even in Util mode via Ready mode, the car uses 12v for either/both of audio and heating, or the car uses 12v for the two screens always no matter which mode and 40 minutes showing those two screens is enough to drain the 12v battery enough for the car to refuse to shift to Drive mode.
- If you put the car to Util mode, you are in for trouble when you attempt to come out of it and attempt to start driving, for some people sometimes (doesn't happen always)
This is hugely frustrating when it happens. Prior to reading this thread, I wasn't aware of zombie mode and thought I genuinely had low battery somehow (even now, I think that might be the case but even if so, once jump charged, it still has to be disconnected and reconnected, so definitely a software glitch as well).
The Hyundai manual cryptically uses 'etc' giving a couple of examples of what would run using the high voltage main battery when in Util mode.
I want to know if any of a) heating, b) screens and/or c) audio playback uses 12v regardless of whether the car is in the Util mode. I'll then not use those systems whilst the car is parked (it would be a bummer if that's the heating as it's the onset of winter in the UK now).