Out of curiosity, why does it have a 12v battery (& system) ? I always assumed it didn't have one, and that accessories would be powered off the "big" battery.
It has a large, high voltage battery that could easily be stepped down (and in fact is, since it charges the 12V battery), and yet a separate 12V battery is required to power the electronics to start the car.
Since the 12V battery is only used to power the electronics (the high volatage battery powers the starter motor), it is very small and consequently, drains quickly (a few weeks) if the car is not in use. I don't drive much and regularly find it dead when I try to drive somewhere.
The upside of it only being required to power the electronics is that I can (and regularly do) jump start it with a NiMH remote control car battery.
I understand that if the high voltage battery goes flat in a Prius, you're somewhat screwed, which is perhaps why they don't allow it to power the electronics for starting, but it's still a pain.
Disclaimer: this is an older Prius, newer models may be different
Nobody got the answer even remotely correct WRT the EE issues.
Battery bulk current handling capacity is provided by bulk lead in the plates and copper wires. Its extremely cheap to build a lead acid battery that can momentarily squirt out a thousand (or so) amps to start an engine on a cold winter night. Maybe $100 or so.
Unfortunately power converters / supplies of all sorts scale at a higher than polynomial rate. So although a 20 amp smart battery charger might only be $50, a similar power converter capable of 1000 amps peak starting currents might be way over $2000. Thats about 10KW and thats about the right price for non-industrial non-automotive inverter hardware so its probably higher cost.
The total system cost to provide momentary 1000 amp starting currents is minimized by a $50 charger and a $100 battery rather than a $2000+ inverter.
My wife has one and I'm slightly jealous, I know ham radio guys who've spliced in inverters to run their entire house off the prius, using it as a stationary generator basically. There have been articles in QST magazine. Anyway one issue they and everyone else runs into is the primary battery is physically disconnected from the world until the boot up self tests pass. Leakage current to chassis, stuff like that. Until the tests pass, power is not connected by the giant relay. So its chicken and the egg time, where how do you get enough power to run the self tests of the electrical system if the primary battery isn't connected until after all the tests pass? So you need a secondary battery thats never physically disconnected, the 12V lead acid traditional batt.
Now an excellent firmware question is why not have the firmware self test and partial boot the car every 6 hours to top off the 12 volt battery for 5 minutes? Probably some mechanic who thinks the car is off will manage to set it on fire by not being careful while replacing the battery or something like that. "Oh of course the main power cables are powered down, the whole car is powered down... err... whoomph" and then you need a new car and mechanic.
A design change to connect the 12V to the primary battery would then mean routing even more junk thru the primary battery compartment. There really is no perfect solution.
(Have to reply to mikeash as an edit. Yes I researched this after your comment and you are correct and MG1 is powered from the HV not the 12V to start, I did not know that. I knew it charged the HV and helped out at full throttle, along with MG2 that does "most of the work".
Anyway the TLDR summary still stands, from a scaling perspective there is an economic crossover point at a surprisingly low current, at least around the 90s's to 00's when this was designed, where its much cheaper to use a battery/charger than a really beefy inverter above a certain (medium-ish) current level. Someday that'll probably change as semiconductors advance and that crossover point becomes 50 kiloamps or something.)
The required 12V startup current for a Prius turns out to be surprisingly low. The author measures it for a 2005 model directly at around 30 amp (with one test showing a brief spike to 60 amp).
Based on this, I bought a pack of 10 5Ah sub C NiMH batteries for jump starting (unfortunately there is no outlet in my car park for a trickle charger).
Your comment about peak current confuses me. Was that just meant to address normal cars? Because the starting current for a Prius comes from the high-voltage battery, not the 12V battery.
As for topping off the 12V battery every so often, I don't think there would be much point. Most people drive their cars at least once every couple of weeks. If they don't, a cheap trickle charger will keep the 12V system happy while it sits idle. No point in adding equipment to solve a 0.01% case that's easily solved by the people who need it.
Most of the car's internal electronics (lights, airbags, ignition, touchscreen, instrument cluster etc.) are based on traditional components that require a 12V connection. The 12V battery in the Model S also powers the high voltage contacts that transmit power from the main battery to the motor.
Chemical limit of the materials. If you go out to buy battery bigger than 12v, for example 24v, then you will see that it is actually made of 2 12v or may be even 16 1.5v, depends on what material they use to make a battery. I have asked your question about Tesla and low voltage batteries to my friend who was studying as Physics Teaching, he explained me these things.
