One thing I never understood is why hybrids have the 12 volt battery at all. Couldn't the entertainment system etc be powered from the 200V EV battery (after stepping it down obviously)?
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.
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.)
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.
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.
There's a lot of Tier 3 supplier…stuff…in a car that was architected for 1978 and has never had a thorough re-think, because the profit margin is like nothing.
12v is such a common top-rail voltage for electronics of all kinds, I think its a bit more that than "car makers are too greedy to manufacture 400v headlights."
No. Or, at least, not easily. The high voltage battery isn't generally connected until "the computers are happy" with the state of things, for various hybrids/EVs/etc.
There's also a legal requirement that "marker lights and such" still work (presumably, also power locks and such) after a prime mover failure. Basically, if the engine quits, you should still be able to signal, get over, turn your 4-ways on, etc.
The easiest solution is to just put a lead acid 12V battery in the car for that. Lithium, in particular, is a problem below freezing because you can't safely charge it, whereas lead doesn't have that problem.
Because they want to completely disconnected the big battery when not in use to prevent phantom drain, and you need a little power to run passive systems that need to be on still (like remote keys etc).
Some EVs like Tesla now use a separate lithium ion 12v battery that should last forever, instead of a lead acid 12v battery with a limited life span.
Even if they don’t have a 12v battery they still have to have a 12v system for reasons others mentioned and because it’s a requirement to sell cars in the USA. My Porsche Taycan has jump start lugs for the benefit of other cars and so I can have my low voltage systems operate with some help from another car.
My mild hybrid Audi is the same way. Gas engine, 48v accessory belt run battery, and that 48v battery works in tandem with a small 12v battery for the low voltage system.
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.