What exactly is the 12V battery used for?

My understanding is that it is used primarily for starting the car, and also for the fans, lights, etc. (Air-conditioning?)
While the car is on, the high voltage (aka HV, or traction) batteries will supply voltage to keep it charged, as well as the generator powered by the gas engine. (I'm not sure if that's in parallel or in series, but all power ultimately comes from the gas engine). In any case, you should leave the car on for awhile after getting a jump, or the battery won't be recharged. People have reported a variety of weird problems, including "triangle of death" on the dashboard display when the 12V battery goes bad.

Note also that getting and giving a jump is a little different than a standard car, because of the smaller size of the 12V battery. Be sure to read the owners manual before doing this.

But in a 2010, you shouldn't have any problems for many years.

 

I had understood that it did just about everything that non-hybrid 12V batteries do, EXCEPT start the engine. The 12V needs to boot up all the computers and controllers and flip the relay to energize the HV system, but the HV system actually spins up the electric motors and gas engine.

From my understanding, these engine stops should not matter. The 12V battery is charged from a DC-DC converter from the HV battery whenever the car is in 'Ready' mode. And when the HV battery gets too low, it automatically starts the gas engine for recharging.

 

No. Once in Ready mode, the 12V bus is powered by a DC to DC converter from the HV system. After you boot to Ready, you can physically remove the 12V battery and the Prius will run just fine. The ICE (engine) starting and stopping makes no difference, as the ICE is started by power from the HV battery.

Simplifying a bit, when you push the Power button, enough power is drawn from the 12V battery to boot the computers and close an HV relay. The HV relay connects the HV battery, and from then on everything is powered by the HV system.

The Prius doesn't really have a starter. It has two HV motor-generators (MGs) called MG1 and MG2. MG2 is the big electric motor used to push the Prius and for regenerative braking. MG1 connects to the ICE and is used to start the ICE, and also as a generator to charge the HV battery and supply power for virtual gearing (the electrical power path from the ICE to MG1 to MG2 to the wheels).

As for how long it takes to recharge a dead 12V battery, it all depends. The 12V charger isn't very powerful, so it takes awhile. A completely dead 12V battery may take hours to recharge. If it is dieing, it may never fully recharge.

Tom

 

Three reasons:

1) Most automotive accessories are designed for 12 volts, so without a 12V battery you would need to run the DC to DC converter all of the time.

2) To make absolutely sure that the HV battery is never run down. If you allowed a power draw without being able to recharge, a person could run down the HV battery. This is bad for a very expensive battery, and nearly impossible to jump start since there are very few HV chargers.

3) In an emergency, the HV battery can be disconnected while power steering and power brakes still function.

Tom

 

It would be nice if the HV vattery could be used to start the car when the 12v fails. From personal experience: If the car won't start at first it may start after the 3rd try. Depending on the state of the battery you may be able to recharge it overnight. I was able to drive about 20 miles to the dealer but it didn't hold a charge so they had to replace the battery to move it.

 

WARNING!
Reverse polarity jumper can blow the control computers (or at least it did in the earlier models!)

We do not know if the current generation control computers are diode protected from reversed polarity jump start. It is not the type of experiment that anyone wants to replicate. But there have been scattered reports from the earlier models.

Bob Wilson

 

Actually this is the main reason why they have to have a separate battery. When the Prius is Off (or in ACC or IG-On mode) the HV battery is isolated by the System Main Relay(s). When you push the Power button with your foot on the brake pedal, you should hear a series of clicking noises coming from the rear of the car. This is the sound of the relays closing (actually one relay closes and opens again). The relays require the 12v from the 12v battery to energize the coils which in turn close the contacts to connect the HV battery to the rest of the system.

This is not an arbitrary design decision - it provides the means to quickly isolate the HV battery in the case of a ground fault or if the SRS (air bag) is deployed.

 

This would seriously complicate the job of emergency responders, who want to know that the hazardous electrical systems are dead. With the current design, disconnecting the 12V system kills everything, disconnecting the HV system inside the battery case.

 

I stand corrected. It makes sense and I should have known this too, but apparently forgot it at some point. Thanks.

 

I'm curious whether the 12v powers the cabin a/c ventilation fan, which I found on other cars is a powerful fan with high current draw. Running on the highest speed in one car a long time blew the alternator. You can also see lights dimming when that fan ran highest and the engine was idling. This must be the highest power load on the 12v system if that's what powers it.
I notice a delay when starting before the a/c comes on, which may be for the time to engage the traction battery.
Another fairly high load is headlights, at least the incandescent ones, fairly powerful lights, not at all like a flashlight -- never seen the wattage rating. But compare to a 120v floodlight PAR lamp, must be a 100watts at least to get such beams.

 

When in Ready mode, the 12V battery is, in effect, powering nothing. The 12V system is being powered by a DC-DC converter off of the HV battery. When not in ready mode, everything is powered from the 12V battery.

 

On a conventional vehicle with an alternator, it generates less than a few hundred watts at the idling speed.

On the Prius in READY mode, it supplies approx 1.2 kW max. via DC/DC converter.
The power is enough for A/C fan and lights.

Ken@Japan

 

The other use of the 12V battery is to store a small amount of water after it is dead.

Tom

 

Absolutely correct! In fact it may be the most important use ... to some...

 

Wow! That means stop and go traffic on a hot night would be killer. (charge and mpg?)

 

Stop-and-go traffic is an MPG killer in every car; less so in a Prius because of energy storage and regenerative braking, but more obvious because of the MPG display. The 12V battery doesn't care because it's not involved, unless you actually shut the car OFF and turn it ON again repeatedly.

 

The one limitation of the hybrid system I can see is it cannot be used to jump start another regular car through booster cables, as the 12v supply does not support the cranking amperage, either from the converter or the battery. Maybe if the engine were hot already and the load relatively small.