Page 3 of 3

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

Posted: Wed May 18, 2011 7:14 am
by aqn
BnBinSD wrote: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.
aqn wrote: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.
Found it: owner's manual, page 5-16:
"Pull up on the electric parking brake switch twice so that the maximum electric parking brake force can be applied to the vehicle."

More info on parking brake usage here.

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

Posted: Wed May 18, 2011 7:36 am
by garygid
Wow, there are more moving parts in the Parking Brake system than in the entire drive train!

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

Posted: Wed May 18, 2011 11:13 am
by Ingineer
garygid wrote:Wow, there are more moving parts in the Parking Brake system than in the entire drive train!
Yes, it's nuts. I think they screwed up and didn't include a brake force balancer in the actuator, and had to add the cable conglomeration afterwards to insure even braking force. Maybe this is why cars are in port so long!

FYI: Yes, the LEAF has a transmission, though when transmissions also include the differential, they usually are referred to as "Transaxles", so this is the proper term. Also, The FMVSS requires a mechanical "emergency" brake on all cars equipped with Hydraulic service brakes. The "Parking" brake meets this requirement, though I'd rather have a system with a pedal or a handle. (Like the Prius)

-Phil

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

Posted: Wed May 18, 2011 12:26 pm
by gbarry42
GroundLoop wrote:Emergency Brake [....] Unless it's a big handle you can yank and slide the rear, I just don't see it being that useful in crisis.
Interesting that you see that as a "feature" :) But I was wondering along the same lines, "How can you use the emergency brake without having it lock and send you going down the road backwards?" There was always this lesson in Driver's Ed., about how to apply that brake while holding it released, so that you can modulate it.

So, I noticed this, in owner's manual.
If the electric parking brake must be applied while driving in an emergency, pull up and hold the electric parking brake switch. When you release the parking brake switch, the parking brake will be released.
Not brave enough to try it, yet.

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

Posted: Mon Jun 20, 2011 11:24 am
by Nekota
Leakage current on Panasonic Gold caps (if these are used in brake backup unit) is hard to specify according to Panasonic.

Referring to the tech docs it's less than 10 uA after an hour and keeps decreasing over many hours.

http://www.panasonic.com/industrial/com ... 052505.pdf

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

Posted: Mon Jun 27, 2011 10:31 am
by tps
Ingineer wrote:1540 Farads. (Not Microfarads, FARADS!)
Wow! Way too many years ago, when I was in school, I remember being in awe when I saw a spec for a 1 Farad capacitor... I did actually use some multi-Henry chokes picked up at a surplus store to eliminate 60 Hz current on a simplex circuit of a dedicated telco pair.

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

Posted: Tue Apr 10, 2012 10:17 pm
by Ingineer
Supercaps are used instead of a small battery because they last a long time. If they installed a standard type of storage battery there, such as lead-acid or lithium-ion, there is a good chance it could decay after some years and not have power available in an emergency, which is the whole point.

4-wheel disc equipped vehicles need a lot more hydraulic pressure to perform a decent stop than cars with drum brakes, so the brake backup capacitor stores energy enough for a few stops with the boost system still enabled even if the 12v electrical system fails. It uses the 12v system to charge itself, and once charged does not consume any significant power. It also allows the complex ABS and hydraulic valve body to continue to function in some capacity.

The parking pawl system used in the Leaf is very similar to what is used in all cars with automatic transmissions, almost identical to the well-proven system used in the Prius, and thus is extremely reliable. The only difference is in the actuation mechanism, which in the Leaf (and in the Prius) is driven by a switched reluctance motor, which is a very reliable type of motor. In most normal automatic transmission cars, the parking pawl is engaged by the mechanical shifter. The parking pawl is strong enough to reliably hold the car and will not "let go". The only possibly significant problem is that if one front wheel were to slip, the car could roll. This is because the parking pawl locks the input shaft to the differential, so only one wheel would need to slip in order to allow the other one to roll. I would consider this to only be a worry if you were parking on a steep hill with a possible traction problem, such as dirt, sand, gravel, ice, etc. In those cases I would also set the parking/emergency brake, but I generally don't bother at any other time. The "roll-click" you sometimes feel when you take your foot of the brake after putting the Leaf into park (or powering it off) is simply the pawl finding the next slot in the toothed cog that allows it to positively lock. Once the pawl locks into the slot, it will not slip out, as it uses a positive locking cam mechanism. The car can roll an inch or so until this happens though, and will make some people uncomfortable but is not dangerous.

-Phil

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

Posted: Wed Apr 11, 2012 5:38 am
by TomT
I used to agree completely but I've changed my mind recently. On our other vehicle with a pedal, my wife will frequently press it down only hard enough to create a little warmth in the rear brakes when you drive off with them still on. It'll roll right out of our slightly inclined driveway... So, I can see one of the reasons why Nissan used an electric brake. Either electric or handle for me these days...

[quote="Ingineer"The "Parking" brake meets this requirement, though I'd rather have a system with a pedal or a handle. (Like the Prius)[/quote]