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

My Nissan Leaf Forum

Help Support My Nissan Leaf Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

aqn

Well-known member
Leaf Supporting Member
Joined
Oct 1, 2010
Messages
832
Location
Austin, TX
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."
pic

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?".]
 
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
 
Ingineer said:
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:
 
GeekEV said:
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?
 
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.
 
MikeD said:
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 said:
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 said:
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)
 
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?
 
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
 
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.

800px-LEAF_parking_pawl.gif




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

IMG_0655.jpg


=Smidge=
 
planet4ever: Thanks for your response. I guess in the interest of avoiding confusion I will use the terms "parking lock" and "emergency brake". The former term seems entirely accurate in describing its function, and the latter term although not entirely accurate (since it plays a major role in parking) is an older term that is entirely accurate in that it can be used if the manual hydraulic brakes should somehow fail -- and I don't want to forget that! And most people are used to using the "emergency brake" for parking safely...
 
FYI: The Brake capacitor has nothing to do with the parking brake system or the park pawl actuator.

If the 12v system is offline, you will not be able to disengage the park pawl (or engage it), and you cannot engage the parking brake, but you can disengage it from the manual release. If you had to, I suppose you could dismantle the actuator from the transaxle and manually engage/disengage it with a pair of pliers.

The Hydraulic brakes will still function normally for a handful of stops until the capacitor depletes, then you will still have some function, but it will require substantial pedal effort. (no boost will be available)

I suspect there is at least 10 watt-hours of usable energy (12v output) in the brake capacitor. At some point I'll do an accurate test.

-Phil
 
Oh and one more thing: I believe that for normal parking situations you do not need to bother with the parking brake. If you are parking on a moderate or steep hill, I would use it in that case.

The park pawl mechanism is very reliable, it's almost the same as the system used in the Prius. I've never seen one fail unless the car was in an accident.

However, that conglomeration in the back, especially if you live in areas with high salt exposure, is suspect. Though there is an argument for occasional use just to keep things from seizing up.

If it were me, I wouldn't use it in the winter unless I had to (steep hills, ice, etc), and instead use it once a week or two in the summer for exercise.

-Phil
 
In the Owner's Manual, the parking brake is mentioned several times. In the first mention (I think), it states to "apply the parking brake twice for maximum effect" or some such, but in subsequent discussions - especially parking on hills, it's never mentioned again.

Can anyone confirm whether applying the parking brake twice truly applies more force than a single application?
 
Transmission Parking Lock may be more descriptive, or just P as used in automatic transmissions.. and put it in a pushbutton on the console along with D, N, ECO and REVERSE, in the usual order... Emergency Brake is a good one for the other one.. such complexity has to add to the cost.
 
Emergency Brake is not quite right, since it's not used in emergencies. Unless it's a big handle you can yank and slide the rear, I just don't see it being that useful in crisis. And it is used during routine parking. Parking Brake seems like the right name for it.

Is there something legitimately called a "transmission" with one gear? If so, then tranny lock sounds correct to me.


I'm on the fence.. My usual shutdown has been foot brake, parking brake, then OFF.
If I can ignore the parking brake, then it's just... Foot brake, OFF.

I just don't like the feeling of the car 'rolling into' the transmission lock. Feels wrong. Old habit, I guess. I prefer to let off the footbrake and feel the parking brake hold it right there.
Given that there have been reports of either the parking pawl failing, or damage to it from long term use as a parking stop, I guess I can let go of my parking brake habit.
 
MikeD said:
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.
Indeed. It's not uncommon for water to leak into the cable sheath of a cable-pulled parking/emergency brake. If you apply the parking brake in subzero temperatures, the water may freeze, leaving the brake stuck in the engaged position.

The parking lock ('P' in the "transmission") suffices for most situations, though I am in the habit of always engaging the parking brake, a habit from driving stick shift cars as well as a belt-and-suspenders thing. The owner's manual says to, with the foot brake depressed, "P-then-parking-brake" (page EV-18). It also recommends "parking brake-then-P" when parking on hills (page 5-20), so the parking mechanism's gear won't have to bear all of the car's weight.

MikeD said:
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.
...
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?
Federal Motor Vehicle Safety Standards spell out three different brake systems: "service" brake i.e. the "normal" brake, emergency brake, and parking brake/parking mechanism/parking lock. Each must conform to separate requirements. They can be separate systems, or two of the three can be combined, but all three can not be combined. The parking brake and emergency brake systems are normally combined, hence the confusion about whether it's a "parking brake" or an "emergency brake".

The emergency brake must be frictional (see my post above) and must be functional when all else fails: hydraulic failure, failure of main electrical system, ignition off, etc.

MikeD said:
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 ?
You can apply the parking brake while moving: pull up once to engage, pull twice to really engage. You have to be able to apply the parking/emergency brake while moving, otherwise it's not an "emergency brake"! :D

MikeD said:
I don't suppose the parking brake switch provides any control on the braking force/speed ?
The LEAF's parking/emergency brake provides a little bit of control over application force: pull up once to engage, pull up twice (doubleclick) to really engage.

Ingineer said:
FYI: The Brake capacitor has nothing to do with the parking brake system [...]
If the 12v system is offline, you will not be able to [...] engage the parking brake, but you can disengage it from the manual release. [...]

The Hydraulic brakes will still function normally for a handful of stops until the capacitor depletes, [...]
I would tend to agree with that. I checked the service manual. All references to a "brake power supply backup" are in the "BR Brake System" section, not in the "PB Parking Section". (I edited the title to reflect this info.) However, now the question becomes: which is the emergency brake on the LEAF: the service brake, or the parking brake?

Ingineer said:
I suspect there is at least 10 watt-hours of usable energy (12v output) in the brake capacitor. At some point I'll do an accurate test.

-Phil
That would be interesting information.

BnBinSD said:
In the Owner's Manual, the parking brake is mentioned several times. In the first mention (I think), it states to "apply the parking brake twice for maximum effect" or some such, but in subsequent discussions - especially parking on hills, it's never mentioned again.
This is in the service manual, page PB-17: "Pulling the parking brake switch again increases the parking brake braking force (increases the rear cable tensile force)."

I would think it's in the owner's manual as well, but I don't know for sure.

GroundLoop said:
Emergency Brake is not quite right, since it's not used in emergencies. Unless it's a big handle you can yank and slide the rear, I just don't see it being that useful in crisis.
Having a longish handle to apply and modulate the parking brake might be nice, though at the end of the day, I only care that the "emergency" brake be capable of stopping the car if the service brake fails. I don't care if I have to yank a big handle or whistle Dixie to activate it!

That said, the only gripe I have about the LEAF's parking brake is that it only has three settings: off, on, and REALLY on, and there is no room for modulation in between.

GroundLoop said:
Is there something legitimately called a "transmission" with one gear? If so, then tranny lock sounds correct to me.
It's got gears and it "transmits" power from the motor to the driveshafts. I'd call it a "transmission".
 
MikeD said:
...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....
I don't see how the parking lock can really fail. The wheels will STAY locked. The reason for not depending on it alone is that it's only locking up the two front wheels. You need the parking brake to lock up the rear wheels. On a steep, slippery slope, two wheels might not be enough.
 
There is never just one "gear", but a gear pair. The gears in the LEAF's transmission have a fixed ratio (of about 10:1) so that the electric motor is going about 10,000 rpm when the wheels are going at about 1000 rpm.

So, there is a single-speed "transmission", which probably includes the "Parking-Lock" feature.
 
Back
Top