Which battery is which?

Well, both batteries are hidden in the rear hatch area. The battery that's being charged on the MFD is the traction battery, which is under the rear seat and in the front of the cargo area (i.e. the part closest to the front of the car).

The auxiliary (12V) battery, which is the one that starts the car, runs the lights, powers the radio, etc. is hidden in the back of cargo area on the right hand (passenger's) side.

The 12V battery looks a lot like a smaller version of the batteries you're used to seeing in cars; the traction (~200V) battery is much larger.

 

The 12V battery/bus runs everything except the two Motor/Generators, and the air conditioner.

 

I thought it was the traction battery that actually starts the engine and the 12V was responsible for the computers, lights, etc.?

 

The 12 volt battery runs very little really, only filling in where the inverter falls short, it fires up the inverter and computers then the inverter takes over. The battery smoothes the power supply during peaks in demand and powers accessories when not in the ready mode such as the interior lights and radio. It also powers the headlights if they are on after the system is shut down until the door is opened. It powers the security system also.

I would say the traction battery is behind rather than under the back seat. The petrol tank is under the floor under the back seat.

 

I have always been under the impression that the 12V battery runs everything but the drive motors and the A/C. Perhaps you're refering to the fact that the traction battery charges the 12V battery when the car is in "Ready"? Here's a question that I don't know the answer to: if you were to disconnect the 12V batter while the car was in "Ready," would the headlights and the radio continue to work?

 

This experiment was done. Go to:

http://techno-fandom.org/~hobbit/cars/prius-12V/

 

Cool. Thanks. So, I guess this means that pat is right; nothing runs off of the 12V battery while the car is in "Ready." The DC-DC converter directly powers everything while the car is on.

 

The same is true of most cars - you can usually disconnect the 12V battery while the car is running. While running, the 12V power is generated by the drive system - the alternator on a normal car, or the 200V hybrid system on a Prius.

 

I wouldn't recommend doing this, however (disconnecting the 12V battery while the car is in "Ready"). You loose some regulation of the system voltage. I don't know how much regulation you loose or how much more susceptible the electronics would be to voltage problems, but I wouldn't want to find out. The results could be expensive! I suppose the design had to be done so the system would be ok (in case the battery failed).

The engine is started by the inverter/traction battery. When starting the car you press the brake pedal and the system (brake booster) draws a 1/2 sec 40 Amp pulse from the 12V battery, then you press "Start" and it draws another 1/2 sec 40 amp pulse of power (putting the computers and electronics in "Ready"). After that you will see a current -into- the 12V battery ("float charging" it). The inverter 12 Volt system charger takes over (actually running at 13.8V).

 

Never disconnect the battery of a vehicle with an alternator while it is running, you can damage the electrical system Same goes for Prius. The battery may not be providing power but it is a reservoir of electric energy.

Edit, thanks David, I posted as soon as I saw about disconnecting batteries but you had it covered.