The main reason why you will alway have a low voltage battery is safety: It is used to actuate the contactors for the HV Battery. You would not want un-switchable HV components. For example in the event of a crash the HV battery gets completely disconnected but airbags and indicators will still work. The 12v battery will get charged off the main hv battery if it gets low btw.
to piggyback off this, 12V components are also everywhere else, so you can use off the shelf components like lights, wiper motors, etc without having a limited production run for high voltage components, which keeps costs down. Doing it this way made it possible to add hybrid systems to traditional vehicles like a camery or an escape. This also enables you to jumpstart a hybrid with another 12V system.
TLDR: money, compatibility, safety
I think James May explained it best when the whole thing failed on his car and he couldn't even get into it.
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Huh.
The 12v battery in a hybrid doesn’t start the car like you’d expect – it provides power to the computer, relays, ECU, etc. The heavy lifting of starting the engine is actually done by the hybrid battery.
I knew about the rest of the things it does because I can run my stereo and my headlights on my Prius without the battery indicator dropping. But I totally thought it also started the car.
Well in a technical sense you are also right, since the contactor for the main battery gets power from the 12v (albeit it's not a big load) Meaning if the 12v is completely dead the hv battery will not switch on and the motor will not start.
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