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iBooster after power loss question

Saghost

Well-Known Member
Oct 9, 2013
8,224
7,094
Delaware
I was talking with someone about how the Volt's brake system works and how the high pressure accumulator would provide stopping power even if the car lost all power, and that got me wondering:

The new Autopilot cars all use the Bosch iBooster, right?

That's a 12V drive motor geared directly to the master cylinder piston, yes?

So what happens if the car loses 12V power?

Do you lose all brake assist immediately? (I assume Tesla designed so this wouldn't happen...)

It there a small battery or capacitor in the iBooster module that gives brake assist for the emergency stop?

Inquiring minds want to know... :)
Walter
 

EarlyAdopter

Active Member
Jun 24, 2012
2,836
2,095
Redmond, WA
I'd imagine it's driven off the 12V accessory battery. Much as the dash, turns signals, and A/C don't cut out when power from the main pack is lost, same would hold true for the brake booster.

Even if all power is lost you can still stop the car without the booster active. It just takes more muscle power.
 

Saghost

Well-Known Member
Oct 9, 2013
8,224
7,094
Delaware
I'd imagine it's driven off the 12V accessory battery. Much as the dash, turns signals, and A/C don't cut out when power from the main pack is lost, same would hold true for the brake booster.

Even if all power is lost you can still stop the car without the booster active. It just takes more muscle power.

Agree with both of those points. However, most cars I know of retain brake boost after losing the engine and 12V power, for at least a single application (including the older style Model S, I believe - vacuum boosted, right?)

I was wondering if Tesla put anything in place for that or not...
 

jerry33

(S85-3/2/13 traded in) X LR: F2611##-3/27/20
Supporting Member
Mar 8, 2012
20,510
27,524
Texas
Agree with both of those points. However, most cars I know of retain brake boost after losing the engine and 12V power, for at least a single application (including the older style Model S, I believe - vacuum boosted, right?)

I was wondering if Tesla put anything in place for that or not...

Vacuum boost (via electric pump) or electrical boost, they both work off the 12V battery.
 

Saghost

Well-Known Member
Oct 9, 2013
8,224
7,094
Delaware
Vacuum boost (via electric pump) or electrical boost, they both work off the 12V battery.

True. However, the vacuum boost is asynchronous - the pump runs at the beginning of the trip and after brake usage, and if the car lost all power you'd still have brakes for an application.

The electric boost draws power when you need it - more efficient as well as responding faster, but it means you need 12V power when you go to use it, unless I'm missing something. I think both the Leaf and the Prius use an ultra-capacitor stack for emergency braking...
Walter
 

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