Low voltage battery...

It does have to pump up the brake pressure, but everything else is controlled by the HV battery and inverter. All you need in the 12v battery is enough juice to throw the relays that connect the HV battery, then the HV takes over.
I found that it took a spike of 30A for less than a second on SWMBO's Gen 2 Prius, so you can start the car with any old 12v battery as long as it can take a 30A surge for a second.
The 12v battery on the UK Prius+ (7 seater v) is rated at 45Ah, larger than the Gen 3 and Gen 2 Pruis.
I left the rear interior light on for 2 days, running the car once in between, on mine to no ill effect.

 

how do you turn off smart key?

 

It's the button just below the steering column.

SM-G900H ?

 

^ Think that's only applicable on second gen.

 

You may be correct. That is what I have.

SM-G900H ?

 

Voltage is just as important for electronics. So even if it can deliver a 50A surge, if the voltage drops it might not get the job done. Which is just what tends to happen when a batt gets old.

 

But ALL the 12VDC battery has to do is provide a short term voltage/current to "pull-in" the Traction battery connection relay. Once the Traction battery is connected all 12VDC comes from the inverter. The only way for that source to fail is if the 12VDC battery presents a load, i. e. has a shorted cell, that exceeds the capabilities of the inverter.

SM-G900H ?

 

SM-G900H ?

 

1: Boot ECU
2: Prime the brakes
3: "pull in" traction battery connection relay

Right?

 

No, there is more than just flipping that relay. The 12V battery also powers the power brake accumulator, which begins pressurizing the moment the driver door is opened and may take 10-ish seconds, even before the driver touches any controls inside. It must also power the dome and door lights, and boot multiple ECUs which then must run through various diagnostics and decide whether or not the car is in condition to go to 'Ready'. Navigation, entertainment, and climate control systems may also start running immediately, before 'Ready', if they were still 'on' when the car was previously shut down.

So there is a fair amount 12V stuff happening before that traction battery relay is ever flipped.

 

Apart from the alarm system and the brake accumulator, nothing else powers up until the start button is pressed and the HV battery is activated, all the other 12v systems you mention are powered off the inverter via the HV battery.

 

How then do you explain the dome light coming on when you approach the vehicle with a smart key, or the fact that you can turn on the parking and headlights, or honk the horn before the car is Ready?

 

Then why can I turn on most of those other systems in my car without powering up that inverter at all? My ScanGauge is clearly showing normal battery voltage, not the higher inverter output voltage.

 

Besides all that, the brake lights must be on, plus the dash instrumentation, plus whatever else we've forgotten. To avoid reciting such a long list is why I referred in my previous post simply to " ... everything else that has to happen before the system is "READY."