TEG
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Re: Event - REFUEL at Mazda Raceway Laguna Seca 07/01/12

Thu Jul 05, 2012 1:08 pm

gascant wrote:...
In Tony's video, you can see him roaring past me, driving the track's only silver LEAF. Small wonder that I finished last among the LEAFs--I guess I didn't trust the car (and brakes) enough. In retrospect, I let off the accelerator far too early and lost a lot of speed around the turns. As a result, I was only able to get speeds of around 70 MPH up the straightaways...
In my experience, having ridden with the faster track racers (including Nader), their style tends to be closer to "full acceleration until the moment you need to start hard braking for the turn." As much as I have heard instructors talk about being "smooth and gentile", I find the faster drivers tend to have an "speeding up vs slowing down" approach and not much into coasting or maintaining a stable speed. (With that said, LEAF max acceleration and speed forces one to be stuck at constant speed on long straights and up hills.) That technique of coming towards a corner "hot" and scrubbing just enough speed at the last possible minute helps with lap times, but it also works the brakes really hard. Thankfully the stock LEAF brakes are quite competent even on the track.

TEG
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Re: Event - REFUEL at Mazda Raceway Laguna Seca 07/01/12

Thu Jul 05, 2012 1:09 pm

KeiJidosha wrote:
TEG wrote:
tbleakne wrote:...The picture of the Tesla roadsters cooling off after short runs at high power was dramatic. The Tesla S is supposed to have more robust cooling of both its battery and motor.
Don't forget the inverter ("PEM"). The Roadsters' air cooled inverter is one of the more likely components to overheat at the track. The Model S liquid cools the battery/pack, motor, and inverter.
Tesla roadster and ActiveE would probably have benefited from ability to turn off regenerative braking. As you brake into a corner the motor, inverter and batteries are at full regen, recycling a lot of the power (and heat) you just dumped thru them to get the car up to 90mph. Ability to turn off regen could have reduced the heat load by ~30%. Assuming the brakes were up to the task by themselves.
Yes, but I think those running stock Roadster, ActiveE, and LEAF brakes probably benefit from the regen helping somewhat. If you could disable regen, then I would think (at least) a race brake pad upgrade would be in order.

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Re: Event - REFUEL at Mazda Raceway Laguna Seca 07/01/12

Thu Jul 05, 2012 1:13 pm

Here's our CarWings summary for Sunday, 45.7KWh in five partial fast charges, plus an L2 topup on the way home.

Total was 51.2KWh consumed, 6.7KWh regen, 1.2KWh accessory

Image
Last edited by adrianco on Thu Jul 05, 2012 1:23 pm, edited 1 time in total.

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Re: Event - REFUEL at Mazda Raceway Laguna Seca 07/01/12

Thu Jul 05, 2012 1:17 pm

TEG wrote: Thankfully the stock LEAF brakes are quite competent even on the track.
I was braking very hard from ~80mph several times per lap and I notice some slight vibration since the event, so I think the rotors overheated and warped slightly. It's not enough to be a big issue, and I didn't get any brake fade during the event.

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Re: Event - REFUEL at Mazda Raceway Laguna Seca 07/01/12

Thu Jul 05, 2012 1:57 pm

adrianco wrote:I was braking very hard from ~80mph several times per lap and I notice some slight vibration since the event, so I think the rotors overheated and warped slightly. It's not enough to be a big issue, and I didn't get any brake fade during the event.
That's a result of uneven pad deposits on the rotor, typically caused by insufficient cool-down after heating up your brakes and coming to a stop... Re-bedding in your pads may help - otherwise you might get away with turning your rotors, but otherwise you'll need new rotors.

http://www.stoptech.com/technical-suppo ... ther-myths" onclick="window.open(this.href);return false;

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Re: Event - REFUEL at Mazda Raceway Laguna Seca 07/01/12

Thu Jul 05, 2012 1:59 pm

drees wrote:
adrianco wrote:I was braking very hard from ~80mph several times per lap and I notice some slight vibration since the event, so I think the rotors overheated and warped slightly. It's not enough to be a big issue, and I didn't get any brake fade during the event.
That's a result of uneven pad deposits on the rotor, typically caused by insufficient cool-down after heating up your brakes and coming to a stop... Re-bedding in your pads may help - otherwise you might get away with turning your rotors, but otherwise you'll need new rotors.

http://www.stoptech.com/technical-suppo ... ther-myths" onclick="window.open(this.href);return false;
Thanks, I'll try that.

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Re: Event - REFUEL at Mazda Raceway Laguna Seca 07/01/12

Thu Jul 05, 2012 2:13 pm

adrianco wrote:Here's our CarWings summary for Sunday, 45.7KWh in five partial fast charges, plus an L2 topup on the way home.
Total was 51.2KWh consumed, 6.7KWh regen, 1.2KWh accessory
...
Here is my day driving from Palo Alto to the track, (not racing), doing mobile QC, then driving back home again:
Image

(note, to get over 5 miles/kWh I was drafting behind big trucks for much of the way. Even so, I still stopped for over an hour of L2 along the way to just make it to the track.)

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Re: Event - REFUEL at Mazda Raceway Laguna Seca 07/01/12

Fri Jul 06, 2012 11:11 pm

TEG wrote:In my experience, having ridden with the faster track racers (including Nader), their style tends to be closer to "full acceleration until the moment you need to start hard braking for the turn." As much as I have heard instructors talk about being "smooth and gentile", I find the faster drivers tend to have an "speeding up vs slowing down" approach and not much into coasting or maintaining a stable speed. (With that said, LEAF max acceleration and speed forces one to be stuck at constant speed on long straights and up hills.) That technique of coming towards a corner "hot" and scrubbing just enough speed at the last possible minute helps with lap times, but it also works the brakes really hard. Thankfully the stock LEAF brakes are quite competent even on the track.
The key to fast driving is getting as close to the limits of tire adhesion as possible. In short, if the LEAF can sustain 0.60 G around a corner, the driver's task is to consistently attain 0.53-0.58G or so. Driving this close to the "limit" means that the driver can't make any 'sudden moves', which might induce >0.60G and cause the car to spin. So, when instructors say "smooth and gentle", they are talking about avoiding sudden moves (aka "inputs") when you are near the car's limits of adhesion. That doesn't apply on straights, where you aren't close to the car's adhesion limits.
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Re: Event - REFUEL at Mazda Raceway Laguna Seca 07/01/12

Sun Jul 08, 2012 9:24 am

EricH wrote:Driving this close to the "limit" means that the driver can't make any 'sudden moves', which might induce >0.60G and cause the car to spin. So, when instructors say "smooth and gentle", they are talking about avoiding sudden moves (aka "inputs") when you are near the car's limits of adhesion. That doesn't apply on straights, where you aren't close to the car's adhesion limits.
This is a good explanation of the dynamics involved, but as a Porsche club driving instructor for the last 13 years, I would offer a couple of refinements to the concepts in play here. "Smooth and gentle" definitely applies to not upsetting the car by demanding more from it than the 4 little contact patches of the tires can provide, and keeping inputs smooth and progressive is essential in that respect. But in trying to keep the car as close to the edge of the ideal boundary of the "friction circle" for as much of the lap as possible, you are bound to exceed the available grip at some point. This is when "sudden moves" or inputs are absolutely essential--in correcting a slide when one end or the other of the car loses traction. When a driver is praised for having "quick hands," that is why--they are catching the car to prevent a spin or off-track incident before it's too late, and this often involves instantaneous and abrupt inputs to both steering and throttle, in different measures and directions depending on whether it is oversteer or understeer that is the problem, and how severe it is. These reactions can be somewhat counter-intuitive, but must be mastered and made nearly unconcious and automatic before advancing very far down the performance driving path.

The friction circle limits still apply on the straights, but only to acceleration and braking. In a Leaf, with limited torque and ABS brakes, it is indeed mostly a non-issue, but with a higher-powered race car without ABS, available grip can easily be exceeded even in a straight line.

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Re: Event - REFUEL at Mazda Raceway Laguna Seca 07/01/12

Sun Jul 08, 2012 9:50 pm

tbleakne wrote: The battery impedance can vary, but I am confident that my calculation is reasonably close (+- 20%) because it is based upon the DC voltage drop I have observed under load with the Gid meter.
Tom, I believe that I found your original post about this. While I agree with your findings, I had to ask, since my own test indicated somewhat lower internal resistance in January. Phil reportedly measured 92 milliohm last month. Someone just reminded me that internal resistance varied as a function of temperature, SOC itself, and the phase of the moon ;-) I like the 1% of nominal motor power approximation, which should be close enough for our purposes.
tbleakne wrote: Since my post I realized we can calculate the worst-case temperature rise from battery discharge by neglecting all heat transfer out of the battery pack.

For a 650 lb battery pack, this yields (51/650) = .079 deg F per min, or 2.4 F rise at 30 kW for 30 minutes.
At 50 kW power level, battery loss scales quadratically, so we have (50/30)^2 * 2.4 = 6.6 F at 50 kW for 30 min.
Yes, that's exactly what I was considering as well. Thank you for outlining it so eloquently. I did some back-of-the-envelope calculations, and my values were higher. It looks like BTUs assumes water, which has a specific heat of 4.18 J/g/K. I wanted to suggest that we used steel with specific heat of 0.49 J/g/K instead, but then I found a battery conference report, which pegged the specific heat of lithium-ion batteries at 0.8 J/g/K. It looks like there is quite a bit of aluminum in the battery as well (0.9 J/gK), and I suggest that we used 0.8 J/g/K for the entire pack. This means that the temperature delta from ambient you calculated would have to be be multiplied by 5.22:

Adjusting your figures for a 650 lb battery pack, this yields (51/650*5.22) = 0.412 deg F per min, or 12.5 F rise at 30 kW for 30 minutes. And using the handy quadratic scale for 15 and 50 kW power levels:

(15/30)^2 * 12.5 =3.1 F at 15 kW for 30 min.
(50/30)^2 * 12.5 =34.7 F at 50 kW for 30 min.

The 3.1 F is what I roughly measured during a 31-minute test run with about 15 kW average power output this week. It would be interesting to get some other power levels for comparison. This would also mean that the majority of the waste heat is contained within the battery pack itself, and that it's not dispersed by the chassis of the vehicle.

Although this might seem odd, I believe that I measured elevated temps at the lower door body panels, and was able to see the consequences of solar loading as well. Both of these effects are measurable, but fairly benign. They yielded about 1 or 2 degrees difference.
tbleakne wrote: The same process that causes the t^.5 loss of capacity also raises the battery impedance, so this heating will rise with age, but it should still not be a concern under most circumstances.
Yes, I thought so too and I was hoping that we would see the square-root-of-time relationship we saw in the NREL report. However, given the anecdotal evidence of disproportional range loss and the fact that I'm unable to get the same energy economy like last year when following the same driving protocol, I have a sneaking suspicion that internal resistance might be rising faster than anticipated. I'm really curious if Phil will be able to get 92 milliohm six months from now.
tbleakne wrote: The graph I posted in another thread shows the impedance rising rather rapidly at very low SOC, so high power down here is not advised (and ultimately limited by the BMS). It also shows the charging process as being slightly endothermic at low SOC levels, and then becoming exothermic as the SOC level rises. This is consistent with the QC tapered charging profile.
I don't disagree with the report, and I find it interesting. I did not find a reference to the lithium-ion chemistry they used however and I was unable to find any evidence that the charging process is endothermic at any SOC even though I tried. I used a Fluke 62 IR gun to collect quite a bit of data on my own vehicle and on others as well. Additionally, I found the quote I've been looking for, and according to members of the Leaf design team, the vehicle develops more waste heat during charging than during vehicle operation. I believe that this contradicts the report you found and it would be another reason to take it critically:

LeafHopper wrote:My mistake. I was typing faster than my brain. My intent was to write conductive cooling. Certainly there could be convective cooling also even in a sealed container, but it is unimportant at these temperatures compared to conductive cooling. There is no fan inside with the batteries and no air from outside is pushed through the container from the front grill.

Convection would not stop if the container is closed, it would just reach a steady state temperature when the conductive heat transfer out was balanced by the heat produced. But the product designer said that the temperature does not vary more than 1 or 2 degrees F. Photos of the prototype do not show any fans within the battery compartment and I was told that there were no fans.

I also asked them about excessive heating during charging. The answer was that no excessive heating occurred although it was greater than during operation of the vehicle.
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tbleakne wrote: The picture of the Tesla roadsters cooling off after short runs at high power was dramatic. The Tesla S is supposed to have more robust cooling of both its battery and motor.
Indeed! Both of the Tesla vehicles use cylindrical 18650 cells, which supposedly have much worse thermal properties than the pouch cells used in the Leaf and in the ActiveE. We knew from last year that the Leaf will perform well, but I was baffled by ActiveE'a poor showing. I collected some additional data on my own vehicle during the course of this week, and couldn't help noticing that battery temperature can rise 20 F above ambient when driving the ActiveE hard on the freeway for 10-15 minutes.

This simply isn't the case with the Leaf, as your calculations have shown above, and it must be the result of higher motor power in the ActiveE, battery insulation and smaller battery mass. I believe that they used NMC, which is a bit more dense than what the Leaf is using. When you realize that BMW starts limiting motor power around 102 F, which roughly corresponds to seven temp bars on the Leaf, you know why the ActiveE didn't place better. Image I just find it surprising that active battery cooling didn't buy BMW anything on the track last Sunday.

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