My Nissan leaf rolled off around 400ft

[klapauzius]

no no no... a bridge has cables that supply the force to prevent the bridge from swimming. a parking brake also provides force in a levering action to prevent movement. in the suggested scenario above, the creep force of Drive is negating gravity. eventually the battery runs down and shifts to neutral.

Seriously? Get a beginners physics book and look up the definition of energy!!!!

If you do believe however that a net zero force ( i.e. an electric motor compensating gravity) in static situation can lead to the consumption of energy, you should start changing the batteries in your refrigerator magnets...

The switching action on the currents in the motor that Reg pointed out is one practical explanation of energy consumption in an electromagnet that makes any sense. In addition of course there always friction (i.e. resistive in this case) losses in such systems, but they would be small.

That the switching in the Leaf's motor burns off a 1kW seems high to me, but seems probable to me.

 

[RegGuheert]

That the switching in the Leaf's motor burns off a 1kW seems high to me, but seems probable to me.

The problem is that the switching losses are a function of the voltage of the battery, the capacitances in the transistors and the switching frequency. Unfortunately, the current flowing does not affect switching losses and the frequency cannot be lowered without increasing the AC currents (and therefore losses) in the motor. There are also eddy currents generated in the magnetics of the motor which result in losses.

The good news is that future transistor technology will lower the capacitances of a transistor capable of switching the same amount of current (and blocking the same amount of voltage). In other words, this situation will improve as technology does.

 

[DaveinOlyWA]

no no no... a bridge has cables that supply the force to prevent the bridge from swimming. a parking brake also provides force in a levering action to prevent movement. in the suggested scenario above, the creep force of Drive is negating gravity. eventually the battery runs down and shifts to neutral.

Seriously? Get a beginners physics book and look up the definition of energy!!!!

If you do believe however that a net zero force ( i.e. an electric motor compensating gravity) in static situation can lead to the consumption of energy, you should start changing the batteries in your refrigerator magnets...

The switching action on the currents in the motor that Reg pointed out is one practical explanation of energy consumption in an electromagnet that makes any sense. In addition of course there always friction (i.e. resistive in this case) losses in such systems, but they would be small.

That the switching in the Leaf's motor burns off a 1kW seems high to me, but seems probable to me.

not a physics expert but zero "motion" does not mean zero force. opposing forces that cancel each other still use energy. pick up an object. hold it steady. is that zero force? no, it is not. its zero motion but you are expending calories to hold that object simply because if you let it go then gravity sends that object to the floor. gravity is not suspended because you are holding it.

 

[johnrhansen]

If you had a motor with 2 permanent magnets in it there would be a holding torque without motion with no energy usage, but it would not be a motor. For motion to occur, one of the magnets must have a variable flux. The only thing that can do this is an electromagnet. Unless you use super conductors you cannot create a magnetic field withiut resistance and the I squared r losses that are associated with it, no matter how efficient your inverter is.

 

[klapauzius]

not a physics expert but zero "motion" does not mean zero force. opposing forces that cancel each other still use energy. pick up an object. hold it steady. is that zero force? no, it is not. its zero motion but you are expending calories to hold that object simply because if you let it go then gravity sends that object to the floor. gravity is not suspended because you are holding it.

You did not look this up in a physics book or on Wikipedia? Its 5 grade level physics (that's when I learned it). Do so!...reading makes smart! Here is the link: http://en.wikipedia.org/wiki/Energy#Classical_mechanics" onclick="window.open(this.href);return false;

If there is zero net force, no energy is used.

Do you seriously think a cantilever is expending calories, while holding an object????

The reason that humans get exhausted holding stuff up is in the rather complex bio mechanics of our muscles, which cannot produce a steady, static force. The calorie loss you are experiencing in this moment are friction losses in the force generation process that your body utilizes to counteract the gravitational force.

 

[klapauzius]

If you had a motor with 2 permanent magnets in it there would be a holding torque without motion with no energy usage, but it would not be a motor. For motion to occur, one of the magnets must have a variable flux. The only thing that can do this is an electromagnet. Unless you use super conductors you cannot create a magnetic field withiut resistance and the I squared r losses that are associated with it, no matter how efficient your inverter is.

Yes, there will be resistive losses in the motor. And losses due to the particular electronics that drive the Leafs motor. Apparently those combined can be large enough to drain the battery, even though the car itself appeared to stand still.

I tried this on a moderate hill today, but the leafs power gauge is not good enough to show any of these "small" losses. Simulating the "creep-vs-gravity" situation by holding the pedal in a position that held the car still, no significant output was reported.
But a kilowatt would not show up on the gauge (I used the one in the energy screen not the one on the dash), so maybe someone with a more sophisticated "Leaf"-meter (there are plenty out there) can try this and report what the current draw on the battery is in such a situation?

 

[kubel]

so maybe someone with a more sophisticated "Leaf"-meter (there are plenty out there) can try this and report what the current draw on the battery is in such a situation?

With the parking brake on, selector to D, pedal to the medal, I'm getting 10kW reported use by the motor in LEAF Spy Pro. The car never broke traction. Not sure if that's an accurate value. The rear suspension did compress quite a bit though, so I know the motor was cranking out some torque.

 

[johnrhansen]

In physics as I remember energy is work or force applied over a distance. That is in theory. Using that same theory, you could drive down to the grocery store and return with exactly the same charge as when you left. The same real world losses that are responsible for that sad state of affairs is also responsible for the leaf's traction motor drawing power when it isn't going anywhere.

 

[RegGuheert]

so maybe someone with a more sophisticated "Leaf"-meter (there are plenty out there) can try this and report what the current draw on the battery is in such a situation?

With the parking brake on, selector to D, pedal to the medal, I'm getting 10kW reported use by the motor in LEAF Spy Pro. The car never broke traction. Not sure if that's an accurate value. The rear suspension did compress quite a bit though, so I know the motor was cranking out some torque.

Wow! That's a bit more than I suspected! In any case, all of this is loss, since no work is done. Most of this power would simply be the average current flowing though the windings (to create the electromotive force in the electromagnet) times the battery voltage. That power would be dissipated in the main switching transistors, the wiring and in the motor windings. If we assume that is 9kW of the total and battery voltage is 380V that means you had about 23.7A flowing through an equivalent in-phase winding. (It's a three-phase motor, so even if one winding is at its peak current position, the other two will each be 120 degrees away, so they will be also drawing some current and therefore contributing to the EMF. So likely no single winding had more than about 15A in this test.)

Would you mind running the same test but with no pedal force? This should give us a more accurate indication of the power consumed in the condition which may have occurred for the OP. Readings for both 'D' and 'ECO' might be interesting! TIA.

 

[klapauzius]

Wow! That's a bit more than I suspected!

Yes...I wonder what the Leaf does with all the heat coming from this 10 kW boiler?
One would hope that the electronics shuts down eventually, or limits the current? Or is the cooling sufficient to handle that kind of waste?

I guess this would be the equivalent of the Leaf climbing a very steep road?

 

[DaveEV]

Or is the cooling sufficient to handle that kind of waste?

I guess this would be the equivalent of the Leaf climbing a very steep road?

Cooling is most likely sufficient for this. Similar losses are probably experienced at heavy throttle. 10 kW of waste heat is nothing compared to the heat that an ICE sheds under even light load.

 

[kubel]

Would you mind running the same test but with no pedal force? This should give us a more accurate indication of the power consumed in the condition which may have occurred for the OP. Readings for both 'D' and 'ECO' might be interesting! TIA.

I'm not getting consistent readings from LEAF Spy Pro for the idle test. In both D and ECO, sometimes it shows 0kW, other times it's showing 0.7kW. I wonder if creep is switched off at a certain point if the LEAF detects the car is stopped and brakes are enabled. Or maybe LEAF Spy Pro isn't showing an accurate reading.

 

[klapauzius]

Would you mind running the same test but with no pedal force? This should give us a more accurate indication of the power consumed in the condition which may have occurred for the OP. Readings for both 'D' and 'ECO' might be interesting! TIA.

I'm not getting consistent readings from LEAF Spy Pro for the idle test. In both D and ECO, sometimes it shows 0kW, other times it's showing 0.7kW. I wonder if creep is switched off at a certain point if the LEAF detects the car is stopped and brakes are enabled. Or maybe LEAF Spy Pro isn't showing an accurate reading.

Can you try without brakes, e.g. on a slight incline?
Basically push the pedal until the car stops rolling backwards and stands still. I assume that simulates the OPs situation?

It must have been real bad luck to have an incline that just matches the creep...

 

[johnrhansen]

I think the safest way to test this would be to gently nose the car into a wall leave it in drive pushing against it, then measure the power draw. It would also be interesting to have a bathroom scale between the wall and the car to see how much force is exerted by the car in creep mode. If you really want an accurate measurement you could also get a spring scale attached to the same wall to see how much force is exerted in reverse. This way the error from the incline is canceled out when the readings are averaged. Of course have some wheel chocks positioned to limit the cars roll if it gets away from you. No point getting hurt in the name of science!

 

[Staque]

If setting the parking brake disables creep, you might have discovered a hack way to disable creep. Just wire an extra switch to fool the car into thinking the parking brake is on. Of course it will light up the brake light on the dashboard, and maybe fail some diagnostics or sanity checks (OMG we're going 65mph with the parking brake on!). Or maybe it would just work. Or maybe it wouldn't work at all.

(On the Gen-1 Honda Insight, some folks wire a switch in parallel with the clutch switch to disable the IMA system. The car thinks the clutch is pressed, and suppresses assist or regen. Same idea, and there are no side-effects on the Insight.)

 

[donald]

FWIW, the Fluence I have is devised to have a completely separate braking system from the motor. So if you engage the handbrake, the motor works fully against it. With no acel pedal, the dash gauge (that measures digitally in units of kW only) indicates a draw of 1 kW on the flat. Parked on a gentle uphill (that it would otherwise roll back on if not braked) it can indicate 2 kW!

It is a different motor to the Leaf to be sure, but I don't suppose the zero-speed losses are hugely different. In fact, they should be less for the Leaf than the Fluence because Fluence has an externally excited rotor.

I would be extremely surprised if the OP managed to leave it perfectly balanced in 'creep'. The system, AFAIK, is not designed to do that. Instead the power will gradually increment, if there are no braking operations, so it should creep up the slope whatever he left it on.

The discussion on force and energy is hilarious! Permanent magnet motors do actually have a cogging torque so in theory it could be possible to set the car so that it is just on the point of rolling away and it needs just a little shove to get over that, without any power expended to maintain that very delicate balance. But any active torque will still demand power even if the car remains stationary in balance with gravity. Current must flow around the stator, and Leaf doesn't use superconducting magnets so it's gonna use power to do that!

Just a question - could this be more like an electric brake failure, or at least some error of 'logic' in the braking? What was the charge state of the 12V battery?

 

[IrvingHarlem]

Devin wrote:


Nubo wrote:

I'll never understand why people choose to not use the parking brake. Do you imagine something bad will happen if you set the parking brake "unnecessarily"?

This is really the most important take away here. When I learned to drive, I learned to set the parking brake as a part of the procedure for turning off a car. It doesn't matter if you're on a hill or not. It also doesn't matter if you're in the country where nobody is likely to come steal your car - you should treat a car the same way you should treat a loaded firearm. If you are leaving it unattended it needs to be properly powered down and the key goes with you. As the owner of a car (which is a privilege, not a right) this is just basic safety responsibility. People need to be a little more aware that a car is not just an appliance to get you from A to B, but like any power tool is capable of causing great bodily harm or death if not treated with adequate respect. I know many people around here who do not set the parking brake. Why not? Because in wet, cold, winter weather, you can have a situation where wet brake shoes or pads will freeze to the drum overnight. The result can be an undriveable car.

I have had the brake pads on the LEAF freeze to the discs a couple of times, but I was able to get them to break free. Drum brakes can be much worse.
_________________
RegGuheert
==================
As a Chicago flatlander I almost never set the parking brake. And, when I do I forget and drive off with it engaged. No car or truck I have ever driven has prevented me from driving. I assume that has to be bad for the brakes. So, I save those parking brakes for the rare out of state trip.

 

[johnrhansen]

Might be a good idea for nissan to put in a circuit that pushes the park button if the driver's door is opened.

 

[lion]

Might be a good idea for nissan to put in a circuit that pushes the park button if the driver's door is opened.

I've seen too many people open their door a little when trying to park in reverse, so I could see this solution cause even more problems.

The car already switches to P when turning the car off. Personally, I would love to see the parking brake set when turning the car off (but still require manual disengaging), but since they switched back to mechanical parking brakes in the 2013 model year, I don't see that happening.