[Yaworski]

Another discussion about batteries prompted this thread. I had stated that I thought that the 12V battery started the engine and was told

Not with a hybrid it does NOT.

So what does the 12V battery do? Why bother with one at all? Why not just pull 12V off the big battery pack?

If the lithium battery gets run down, how do you get the engine started?

 

[Easy Rider]

So what does the 12V battery do? Why bother with one at all? Why not just pull 12V off the big battery pack?

It boots up all the computer boards and runs some accessories......like blower fan motor and electric windows.

If the lithium battery gets run down, how do you get the engine started?

If the HV hybrid battery is dead........you can't start the car.
Jumping a ~230 V battery is NOT an option.
Most of them aren't lithium.

 

[Yaworski]

If not lithium, what are they?

Also, why don't they use the 12V battery for starting the car?

 

[Easy Rider]

If not lithium, what are they?

Also, why don't they use the 12V battery for starting the car?

Nickel–metal hydride
If they have changed to Lithium, it has been in just the last few years.

There already is an electric motor attached to the engine.
That motor runs off of the HV battery.
No need for a separate starter.

 

[Yaworski]

Nickel–metal hydride
If they have changed to Lithium, it has been in just the last few years.

I always assumed that they were Lithium but I never had any reason to investigate the issue.

 

[Easy Rider]

I always assumed that they were Lithium but I never had any reason to investigate the issue.

And you probably never will have a good reason.
In the size needed, Li is MUCH more expensive.

 

[Yaworski]

So how long can a Prius sit before the big battery discharges on its own and what do you do when that happens?

 

[Easy Rider]

So how long can a Prius sit before the big battery discharges on its own and what do you do when that happens?

Probably years.
They typically hold a charge for a LONG time.
Should it ever happen, something has caused it to fail and it's probably time for a new battery.