[JonathonW]

Everything on the 12V system essentially is powered from the traction battery (the big one that can power the motors), once the car is turned on.

In Toyota, at least, the traction battery is completely disconnected (via a relay) when the car is off. The 12V battery is needed to power anything on the 12V system up until the car starts-- that includes the car's computer, which is what (after doing all its self-checks and whatnot) activates that relay and connects the traction battery to everything else in the car.

The traction battery isn't always connected probably mostly for safety reasons (having 400-someodd volts energized across the whole car even when it doesn't need it isn't great), but that also keeps it from getting excessively drained if something in the car malfunctions. It's pretty cheap to replace a 12V lead-acid battery if it's overdischarged after you left the lights on... the big hybrid battery, not so much.

[sokoloff]

Some EVs will monitor the 12V battery and periodically connect the traction battery to the DC-DC converter to maintain the 12V battery when the car is parked for an extended period of time. (On the older Smart EVs, this doesn’t have a limit, so the traction battery will kill itself trying to maintain a weak 12V battery. There’s a firmware patch for it.)

[Stratoscope]

My Kia EV6 does that, and when the 800V battery pack is charging the 12V battery while parked, an orange light on top of the dashboard goes on.

This had me worried the first few times it happened. Then I found out it is a warning to mechanics/tow truck drivers/first responders that the high voltage system is energized.

[VectorLock]

Thats interesting; do they put the light somewhere that it isn't a nuisance to look at when you're inside the car?

[Stratoscope]

The light is on top of the dashboard, right in the middle. It's intended to be very visible, especially from outside the car.

It is only on if the car is turned off and parked and the high voltage system has turned on to charge the 12V battery. When you're driving or just have turned on the car while parked, the light goes off.

So the only occasion the light might be annoying is if you're taking a nap in the car at night. And then you could just throw a towel on top of it.

[VectorLock]

Is it visible to the driver, while driving?

[Stratoscope]

The light never goes on when you are driving.

You expect to have the high voltage system energized when driving, otherwise the car wouldn't go anywhere. There would be no purpose for this light to go on.

The light is specifically to indicate that the high voltage system is on when the car is parked. It's a safety warning for that situation.

[rwmj]

Makes sense, thanks for the explanation.