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From what I've read, I think it uses a "12V" Lithium-Ion battery that's electrically separate from the main pack but physically located within that enclosure.

You need a low voltage system for accessories, and you need a battery (or capacitor stack in the future?) to stabilize the voltage on that system and to run the computers to certify the high voltage safe before it connects the high voltage.
 

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Oh, that would make sense. Technically once the car is on, the DC-DC inverter should output a perfectly steady 14V source (stable and clean enough to run sensitive computer components), but when it's off, like you said, it has to be able to run its checks to make sure the HV system is a-go.

I suppose technically depending on the design of the battery, they could just isolate all but 4 cells of the battery, but it might be better to have the low-voltage portion more easily replaceable.

I wonder if the Ioniq can receive or provide a jump start...
DC-DC converters by nature produce ripple current/voltage at some level. You need capacitance in the circuit to clean that up, either from a battery or from capacitors.

Using some cells from the main pack has several problems. The largest of these is that you lose some of the isolation.

As currently designed, all the modern production EVs I've seen isolate both sides of the high voltage pack from the frame - and will open the HV contactors if either side begins to have even a slight connection to ground (or +12V I believe.)

As long as the computers and contactors react faster than the incident, this can even protect you from an extreme accident where both sides are shorted directly to the frame after an impact, because in the milliseconds it takes for the accident to progress from the first connection to the second, the car takes action to isolate.

With four cells hooked in to the 12V, I can't completely isolate both sides.

You also have the problem that those cells will be wearing differently than the rest - with implications for total pack capacity, since the most worn cells will limit the ability of the rest to take charge unless you use extensive (and expensive) active pack balancing.

There's no way that the risks and penalties of using part of the main pack are justified to eliminate the cost and weight of the 12V battery, especially since the battery can be much smaller/lower energy than an ICE 12V because it doesn't have to start an ICE.
 
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