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EVDRIVER
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Re: "D/B" on shifter?

Tue May 18, 2010 7:28 pm

Bottom line, they have engineered some solution. My guess is the cigarette lighter glows bright under these conditions.

daniel
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Re: "D/B" on shifter?

Tue May 18, 2010 7:51 pm

Regardless of what maximum percent SoC they allow the battery to reach, once it reaches that, they either overcharge the battery, revert to 100% friction breaking, or use some other method to dump energy. And regardless of how rare the situation of a full battery at the top of a long hill is, if it can happen, then they have to have some way to prevent overheating of the friction brakes, because the alternative could be disastrous.

All I'm asking is for Nissan to tell us if they've thought of this (I presume they have, but I'd like to hear them say it) and what particular method(s) they've implemented to deal with it. There are no technical difficulties involved, as there are various ways to do it. I'd just like to know, because it's not a trivial issue. Conventional cars and hybrids have a backup in the form of compression braking. I'd like to know what the backup is for the Leaf. Not speculation about what might be, but the straight word from Nissan about what is.
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LTLFTcomposite
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Re: "D/B" on shifter?

Tue May 18, 2010 7:56 pm

Don't diesel-electric locomotives use giant resistors with fans on downgrades?

For the Leaf you should make it down safely from a big mountain pass on the friction brakes if they are sized appropriately, although I never like to ride the brakes down a hill even in a car let alone a truck. As pointed out this seems like it would only be an issue if your house was on the top of the hill, but there are certainly cases where that is true.

Maybe we'll start to see signs that say "EV's Welcome!" on runaway truck ramps.

If they are relying just on friction brakes for this scenario, that raises an interesting question... sized for this worst case scenario that would never happen for most drivers, combined with the regen those brakes will probably last the life of the car.
LTL
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LTLFTcomposite
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Re: "D/B" on shifter?

Tue May 18, 2010 8:00 pm

Or deploy a parachute.
LTL
White 2012 SV delivered 10 Dec 2011 returned 25 Nov 2014 replaced with stopgap ICE Sentra
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AndyH
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Re: "D/B" on shifter?

Tue May 18, 2010 9:06 pm

daniel wrote:All I'm asking is for Nissan to tell us if they've thought of this (I presume they have, but I'd like to hear them say it) and what particular method(s) they've implemented to deal with it.
Daniel - It's good that you care about having brakes that work. But let me start with a question: What do you drive now? Do you have a statement from the manufacturer that they sized the brakes correctly for the car? Actually, you do. If the car is certified to be on US roads, it must meet US safety standards - and that includes brake performance.

"These Federal safety standards are regulations written in terms of minimum safety performance requirements for motor vehicles or items of motor vehicle equipment. These requirements are specified in such a manner "that the public is protected against unreasonable risk of crashes occurring as a result of the design, construction, or performance of motor vehicles and is also protected against unreasonable risk of death or injury in the event crashes do occur."

Standard No. 135 - Light Vehicle Brake Systems - Passenger Cars (Effective 3-6-95), Multipurpose Passenger Vehicles, Trucks and Buses (Effective 12-1-97)

This standard specifies requirements for vehicles equipped with hydraulic and electric service brakes and parking brake systems to ensure safe braking performance under normal conditions and emergency conditions. Manufacturers of passenger cars and multipurpose passenger vehicles, trucks and buses with a gross vehicle weight rating less than or equal to 3,500 kg (7,716 lbs.) may certify compliance with either FMVSS No. 105, described earlier in this booklet, or FMVSS No. 135. The options expire on September 1, 2000 for passenger cars and on September 1, 2002 for other vehicles, on which dates compliance with FMVSS No. 135 is mandatory.


How will this affect you? The FMVSS standards are designed to assure new vehicles are capable of stopping within a certain distance deemed necessary for safe driving. FMVSS 135 is the current standard and applies to 2000 and newer cars, and 2002 and newer light trucks. Compared to the earlier FMVSS 105 standard, FMVSS 135 requires roughly a 25% reduction in pedal effort for the same stopping distance.

FMVSS 135 says all vehicles under 10,000 lbs. gross vehicle weight (GVW), except motorcycles, must be capable of stopping within a distance of no more than 230 feet (70 meters) from 62 mph (100 km/h) with cold brakes (under 212 F or 100 C) and with no more pedal effort than 368 ft. lbs. (500 N).

In July 2005, these same requirements were extended to trucks and buses weighing more than 10,000 lbs. Formerly, only school buses had to meet the same stopping requirements as passenger cars and light trucks.

The FMVSS 135 standard also specifies a required stopping distance for vehicles should the power brakes fail (no power assist), or if one of the two hydraulic circuits fail. Under these conditions, the maximum stopping distance from 62 mph (100 km/h) is not to exceed 551 feet (168 meters) with a maximum pedal effort of no more than 368 ft. lbs. (500 N). FMVSS 135 also has a stopping requirement in the event of an anti-lock brake (ABS) system failure. The rules require the stopping distance not to exceed 279 feet (85 meters) with a maximum pedal effort of no more than 368 ft. lbs. (500 N).

There is also a hot performance stopping requirement for fade resistance. With the brakes hot, the maximum stopping distance for the second of two back-to-back panic stops is not to exceed 292 ft. (89 meters) with the same pedal effort as before (368 ft. lbs. or 500 N). The parking brakes are also covered by FMVSS 135. The rules specify conditions under which the parking brake must be able to hold the vehicle on both an uphill and downhill incline.

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garygid
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Re: "D/B" on shifter?

Wed May 19, 2010 12:02 am

Coming down out of the mountains, 10 to 30 miles of downhill grade is not an uncommon situation.

If you charge near the top of a long grade, it might help to NOT charge to "full".

But, overheating mechanical brakes can be a real, significant issue.
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daniel
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Re: "D/B" on shifter?

Wed May 19, 2010 6:35 am

garygid wrote:Coming down out of the mountains, 10 to 30 miles of downhill grade is not an uncommon situation.

If you charge near the top of a long grade, it might help to NOT charge to "full".

But, overheating mechanical brakes can be a real, significant issue.
I'm not sure the Leaf will have an option for charging to less than full, and there's still the scenario where you need a full charge if you're driving east, but you're going to hit a long downhill if you drive west, and when you plug in the car at night you don't yet know where you'll be going the next day. And Nissan cannot put the burden on the driver, as most drivers will not be able to figure such planning into their charging routine. The car needs to be able to deal with it.
AndyH wrote:
daniel wrote:All I'm asking is for Nissan to tell us if they've thought of this (I presume they have, but I'd like to hear them say it) and what particular method(s) they've implemented to deal with it.
Daniel - It's good that you care about having brakes that work. But let me start with a question: What do you drive now? Do you have a statement from the manufacturer that they sized the brakes correctly for the car? Actually, you do. If the car is certified to be on US roads, it must meet US safety standards - and that includes brake performance.
Actually, my daily driver now is a Zap Xebra, which has virtually no safety features at all, since it is classified and registered as a three-wheel motorcycle, and therefore does not fall under any safety regulations. It does have excellent brakes, and a hand brake, but no backup system comparable to compression braking. I made the decision that in order to be able to drive electric, three years ago, when there was virtually no other alternative, I was willing to accept the risks. Its principal safety feature is the fact that it only goes 35 mph. The Xebra at 35 mph is actually safer than a conventional car at 70 mph (the freeway speed limit here). And with a 40-mile maximum range (aftermarket batteries), there are no mountains I can reach.

I also own a Prius, but I only drive it when I have to go farther than about 35 miles, which means summer road trips to Canada for hiking in the mountains, and a few times a year when I go out of town.
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"You can order" email late September, but was out of the country so...
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EVDRIVER
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Re: "D/B" on shifter?

Wed May 19, 2010 7:19 am

Yes, you can charge less than full and many people do this who reside at the top of large hills.

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evnow
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Re: "D/B" on shifter?

Wed May 19, 2010 8:04 am

I'm about 2 miles from the top of a hill (we are actually on a plateau) - and then I go down probably for a mile without any breaks. There are a lot of homes near the edge of the plateau who go down the hill as they come out of the home. So, this is definitely not a theoretical question in Seattle/San Francisco metro areas.
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AndyH
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Re: "D/B" on shifter?

Wed May 19, 2010 8:35 am

daniel wrote:Actually, my daily driver now is a Zap Xebra, which has virtually no safety features at all, since it is classified and registered as a three-wheel motorcycle, and therefore does not fall under any safety regulations. It does have excellent brakes, and a hand brake, but no backup system comparable to compression braking. I made the decision that in order to be able to drive electric, three years ago, when there was virtually no other alternative, I was willing to accept the risks. Its principal safety feature is the fact that it only goes 35 mph. The Xebra at 35 mph is actually safer than a conventional car at 70 mph (the freeway speed limit here). And with a 40-mile maximum range (aftermarket batteries), there are no mountains I can reach.

I also own a Prius, but I only drive it when I have to go farther than about 35 miles, which means summer road trips to Canada for hiking in the mountains, and a few times a year when I go out of town.
That makes sense, Daniel. A car has three independent brake systems. The hydraulic system has two circuits - one controls the left front and right rear wheel; the second controls the right front and left rear wheel. Either circuit is required to safely stop the car even when the other is completely inoperative. In addition, the brake system must safely stop the car even if the power brake system dies, or the anti-lock system dies. In addition, the parking brake system is independent of the main brakes.

Engine compression and/or regen - though they might be available - are not part of the brake calculation. The friction brake system must be able to stop the car in both normal and emergency use. Period.

I hope that helps,
Andy

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