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Power consumption of brake's back-up power capacitor?

Posted: Mon May 16, 2011 1:00 pm
by aqn
Ingineer posted this pic a while back, saying "It's the Panasonic brake backup supercapacitor assembly. Very similar to what's in the Prius, only larger."
Image
Presumably, the capacitor powers the electrically actuated parking/emergency brake in cases when there is no power from the main battery (or 12V battery?).

Does anybody know:
1. How large is this capacitor?
2. What kind power is consumed keeping it "charged up"?

Thanks.

[Edit: Changed title, from "Power consumption of parking/emergency brake's capacitor?".]

Re: Power consumption of parking/emergency brake's capacitor

Posted: Mon May 16, 2011 1:40 pm
by Ingineer
It doesn't drain really any noticeable power at all. It simply stores it in case of an electrical failure.

It powers the braking system electronics and the hydraulic booster pump if regular power fails for some reason, so you can safely get to a stop.

It's comprised of a bank of Panasonic UltraCaps, a charge circuit, and a switching boost regulator.

The LEAF's unit is larger than the one in a Prius, and it is 1540 Farads. (Not Microfarads, FARADS!)

-Phil

Re: Power consumption of parking/emergency brake's capacitor

Posted: Mon May 16, 2011 3:04 pm
by GeekEV
Ingineer wrote:The LEAF's unit is larger than the one in a Prius, and it is 1540 Farads. (Not Microfarads, FARADS!)
For those who are not electrically inclined, the proper translation of this statement is "do not short the leads or you will die!" :shock:

Re: Power consumption of parking/emergency brake's capacitor

Posted: Mon May 16, 2011 3:21 pm
by gbarry42
GeekEV wrote:For those who are not electrically inclined, the proper translation of this statement is "do not short the leads or you will die!" :shock:
Much like the battery, in that respect.

For the 'lectrically inclined, approximately how many kWh (more like watt-sec) is in there? I guess we need to know the voltage. Is this a 12V thing or a 300V thing?

Re: Power consumption of parking/emergency brake's capacitor

Posted: Mon May 16, 2011 4:12 pm
by MikeD
I haven't read any explanation of how the P position "parking lock" operates, only how the "parking brake" operates (rear drum brakes applied via two cables pulled tight by a 12 volt motor and released normally by a (the same?) 12 volt device, but optionally manually released by the brake release tool used in the cargo floor.

Can anyone link me to such an explanation? I would like to fully understand the implication of the following "TRANSIT DRIVER HANDLING TIPS" statement: "When the auxiliary (12V) battery is completely discharged [and I assume the emergency brake's capacitor is also completely discharged], use chocks because both the parking lock and the parking brake cannot be enabled." I take this to mean that under certain conditions when the car is on a hill and the car is "off", only the manual foot brakes will keep the car from rolling down the hill...

I expect that one should not rely upon the P position "parking lock" to hold the car, but only the "parking brake". And if parking on steep hills, the brake applied more than once for extra safety.

Re: Power consumption of parking/emergency brake's capacitor

Posted: Mon May 16, 2011 7:52 pm
by aqn
MikeD wrote:I haven't read any explanation of how the P position "parking lock" operates, only how the "parking brake" operates (rear drum brakes applied via two cables pulled tight by a 12 volt motor and released normally by a (the same?) 12 volt device, but optionally manually released by the brake release tool used in the cargo floor.

Can anyone link me to such an explanation?
http://nissanleafwiki.com/index.php?title=Drivetrain
explains the parking pawl mechanism.
MikeD wrote:I would like to fully understand the implication of the following "TRANSIT DRIVER HANDLING TIPS" statement: "When the auxiliary (12V) battery is completely discharged [and I assume the emergency brake's capacitor is also completely discharged], use chocks because both the parking lock and the parking brake cannot be enabled." I take this to mean that under certain conditions when the car is on a hill and the car is "off", only the manual foot brakes will keep the car from rolling down the hill...
During transit, if all power is absent (traction and 12V batteries both kaput), one won't be able to apply the parking brake nor put the vehicle into "park" after moving the car, so yes, chocks would be needed to immobilize the car. But if you have power and can apply the parking brake, then the parking brake alone is required to hold the vehicle stationery.

Federal Motor Vehicle Safety Standards's standard #105, Hydraulic and electric brake systems, require that the parking brake be frictional (S5.2) and be able to hold the car stationery without requiring continuous electrical energy, hydraulic pressure, or air pressure (can't find the reference for this just now...), on a 20 percent grade for 5 minutes (S5.2.2.2).
MikeD wrote:I expect that one should not rely upon the P position "parking lock" to hold the car, but only the "parking brake".
The Fed standards require that, with the parking brake disengaged, the parking mechanism be able to "not disengage or fracture in a manner permitting vehicle movement, when the vehicle is impacted at each end, on a level surface, by a barrier moving at 21/2mph." (S5.2.2.3)

Re: Power consumption of parking/emergency brake's capacitor

Posted: Mon May 16, 2011 10:19 pm
by MikeD
aqn: Exceptionally clear and helpful. However, on my last question about whether or not the "parking lock" is "safe enough" for most parking situations, I notice the following from the Owner's Manual: "CAUTION: To park the vehicle in cold climates, push the P position switch on the selector lever and place suitable chocks at both the front and back of a wheel with the electric parking brake released. If the electric parking brake is applied in cold climates, the brake may freeze and cannot be released.". So I gather both brake systems have their limitations and strengths.

It appears that one difference between those two brake systems is that the "parking brake" is designed so that it can be applied in an emergency while moving (say if the hydraulic brakes fail), whereas the "parking lock" is designed to be applied only when the car is not moving or moving very slowly. Has anyone tried out applying the "parking brake" while moving at speed? I don't suppose the parking brake switch provides any control on the braking force/speed ?

Question: Would it have been less confusing if Nissan had used the terms "emergency brake" and "parking brake" rather than "parking brake" and "parking lock"? Which are the commonly used terms in modern cars today?

Re: Power consumption of parking/emergency brake's capacitor

Posted: Tue May 17, 2011 12:07 am
by planet4ever
I haven't actually tried it, but I am quite confident that any request to engage the parking lock while you are moving would simply be ignored by the car. I don't think I agree with your assertion that "parking brake" is a commonly used term for the Park position on a gearshift. It seems to me that "parking brake" and "emergency brake" are used interchangeably to refer to a lever or pedal which applies a brake to the wheels.

A brake and a lock are totally different operations. A brake can allow the wheel to turn, but applies resistance to that. A lock allows no motion whatsoever. If you will forgive the pun, any successful motion with the lock engaged breaks the lock. It is my understanding that is literally true, and it would leave chunks of broken metal that must be cleaned out from in among the gears.

Ray

Re: Power consumption of parking/emergency brake's capacitor

Posted: Tue May 17, 2011 12:23 am
by GeekEV
Hitting the P button while the car is moving does nothing. I *have* tried it. :D

Re: Power consumption of parking/emergency brake's capacitor

Posted: Tue May 17, 2011 6:51 am
by Smidge204
The "P" button activates a solenoid of other actuator which mechanically jams the gearbox via a parking pawl. This is exactly the same mechanism that the "Park" position on automatic transmissions use.

Image



The parking brake lever activates another mechanism which mechanically clamps the rear disc rotors. This is exactly the same as the cable-actuated foot pedal / lever on other vehicles and is often referred to as an emergency brake. The only difference is the LEAF uses an electric actuator as an intermediate stage, for better or worse.

TonyWilliams posted this excellent photo of the system in this thread

Image

=Smidge=