MS overheating on a race track

MS overheating on a race track

I took my MS85 on a longer race track (2.5 miles), and after about 15 minutes of continuous running around 70-110mph range, I got two yellow dashed lines on the power gage on the dash, limiting the output power to 160kw and regenerative - to 30kw. Found it in the owner's manual, that the car is limiting acceleration and regenerative breaking when the battery is being cooled. Which makes sense. But the question I have is this.

What are the guidelines for when it gets too hot ? Can it be proactively cooled ahead of time ? Is there any way to see what the temperature is vs. ideal ?
Just want to make sure I don't need to go off the track the next time if I can help it.


Roamer@AZ USA | December 22, 2013

You may have to modify the cooling system if you are going to run the car that hard. I can't think of any "stock" car I have ever owned that didn't need performance mods to work better in a race environment. Even my dodge viper SRT 10 needed improvements to handle hard track use.

Roamer@AZ USA | December 22, 2013

Could be something as simple as looping the coolant lines thru an ice chest filled with ice. Would require modifications and likely void the warranty.

ElectricKiwi | December 22, 2013

Is there space in the front of the car to mount another heat exchanger, or even one larger than factory standard?

TslaRcr | December 22, 2013

Thanks for the feedback ! Without doing custom mods (don't want to void the warranty), do you know if there is a way to cool it before a run ? Does turning ventilation on through climate control apply to the battery as well ? The car can turn the battery fans on on its own obviously, but can I tell it to do it when I want to ? Like if I know it may already be hot prior to a run ?

dborn | December 23, 2013

Vladt - questions like these are surely best addressed to the engineering team at the factory. Any other answer you get here can, at best, be a guess, educated or not. However, post any official answers you get, as that will obviously be widely appreciated by the general community here.

Tâm | December 23, 2013

Let me think out loud what you are saying.

When you pre-heat your cabin in winter, your battery pack is also pre-heated too.

So you want to confirm that when you pre-cool the cabin in summer, it would pre-cool your battery pack too.

However, now is winter. Even if you turn down the cabin temperature to low, your battery pack still wants to be heated up to an optimal temperature before you run. And that is what you don't want because the car would be racing and getting overheated in seconds.

Like others said, you'd better get Tesla to produce you a race car that can handle heat created by constant high speed.

ir | December 23, 2013

Tesla has mention they are working on "special autobahn tuning" for Eyropean models. I suspect it is to better address this issue.

Jolinar | December 23, 2013

I think you don't need to go off the track, just performance of Model S is limited so it won't damage itself... Roadster had this issue after a few minutes. 15 minutes for Model S looks much better, but race track tuning would definitely help too.

jat | December 23, 2013

I've taken my Model S to the track 4 times, and have had a blast. When it first starts limiting power, you don't lose much except leaping out of the corners. After a while, you have to change your lines to drive it like a big heavy Miata and preserve momentum.

Eventually, when it cuts it down to 140kW, I pull off and let it cool as it isn't fun any more. For HPDE events, this works out great, as that happens at the end of each 25min session, and it cools back down by the time I run again.

It depends on the track - at Atlanta Motorsports Park I would get some limiting after about 5 laps (it was 8 on the stock Goodyears, but with the g-Force Rivals I can stay full throttle for 37s before braking into turn 1), and about 10-11 laps before I pull off. At Roebling Road, even though it is a higher speed track it doesn't have all the elevation change and hard acceleration out of corners, so while it started limiting power after 2 laps, it only limited me to 300kW which just affected speed at the end of the straight (and I was hitting the speed limiter anyway), and I could drive the whole session before it got down to 140kW.

I asked the local service center guy (who is also a kart racer) about it, and reading between the lines of his response I don't think there is some easy upgrade to the cooling system that would resolve this problem. On the Roadsters (which had separate temperature readings), the problem was the motor rotor, and in an AC induction motor that is going to be hard to cool, as all you have is forced air cooling.

I don't think there is any useful way to pre-cool it either.

PaceyWhitter | December 23, 2013

I sincerely doubt that the autobahn tuning Tesla is doing will include anything for battery cooling. The reason the battery overheats on the track is due to the rapid acceleration and deceleration, not the top speed.

I'm assuming the autobahn tuning will merely be a raising of the electronically limited top speed and perhaps some suspension adjustment.

Haeze | December 23, 2013

Hmm... I never thought people who race cars were worried about the warranty.

jat | December 23, 2013

@PaceyWhitter - top speed matters quite a lot -- if you really remove the speed limiter, your top speed will be where the maximum power available (318kW on a S85) exactly matches drag and rolling resistance. At that speed, you will be generating a ton of heat in the drivetrain - a substantial portion of that is lost just through resistive heating. People have also said that driving at the speed limiter on the Autobahn quickly brings on the power limiter, so I think the big limitation now is in fact heat.

After you fix that though, the battery charge won't last very long at those high speeds.

viranjit | December 23, 2013

@vladt - I have taken the Model S on HPDE events twice now and have faced the same overheating issues that @jat and you described. The problem was a lot worse with higher ambient temperatures (96 degrees in May at Thunderhill at Willows, CA, vs. 70 degrees at Sears Point in Octorber at Sonoma, CA).

I have spoken with one of the test drivers at Tesla Motors and he offered a good suggestion: turn down the regenerative braking setting since charging the battery also adds heat. Other than that, he candidly said that Tesla didn't make a car for every kind of customer. He thought that the airflow behind the plastic nosecone was sufficient and wasn't the factor limiting cooling of the battery pack or the motor/inverter subsystems.

I am going for another HPDE at Thunderhill Raceway in early January and am hoping that the cool weather will allow me more laps before limiting power. I also changed my driving style a bit and don't try to go too hard on the straight sections of the track; one of my instructors said that there isn't much of challenge or learning to be had there. Otherwise, apart from driving slowly in the paddock after each session to cool down, there isn't much we can do.

The precise throttle control and exit acceleration out of corners is very gratifying, though. The weight definitely shows itself and I had to use trail braking aggressively to get the weight on the front wheels before turning in.

Did any of you (@vladt or @jat) notice any other gremlins?


jat | December 23, 2013

@viranjit - the tracks I have been on have few point-by places except for the straight, so I can't really let off there if I am trying to pass anyone.

The only place I noticed the weight much is back-to-back corners such as through esses - otherwise, the torque hides the weight penalty pretty well, and I was passing 911s and Vettes coming out of tight corners because I could accelerate so much more quickly (at least before the power is limited). I left regen on standard, as I found it served the function of trail braking (keeping weight on the front wheels to turn). I didn't notice too much difference based on the ambient temperature as it was never over 80 when I was at the track so far, but it isn't surprising that would make a difference. Too cold though and your tires don't work well.

The only other thing I would mention is that the HVAC is running hard for a while after you exit to cool down the drivetrain, so when you are planning on pulling off make sure you account for that and don't have to charge an extra hour in order to get home.

viranjit | December 23, 2013

@jat - regen generates heat, so it is better to use the brakes for weight transfer to postpone the overheating issue in the battery/drive/electronics modules.

HVAC runs hard after the track sessions but it needs airflow as well so driving around for a few minutes in the paddock helps. I often skip the last session of the last day to keep enough charge to drive home. Thankfully, a new Supercharger has just come on line at Vacaville, CA, about 95 miles from Thunderhill and it is on the way home to the Bay Area. Otherwise, I would stick around till the track closed while plugged into the 240V/40A RV outlet to get to the Folsom Supercharger.

About the gremlins, when traction control engaged steering became very hard. According to the test driver I contacted at Tesla, there will be a software patch to address it which will be tested and, over time, released to the general public. I was hoping to test it out this January but the patch may not be ready in time. I am not experienced enough to turn off traction control on this car. I only have a few track days under my belt, and then only HPDEs.

viranjit | December 23, 2013

Finally, a BIG thank you to @dortor for pointing out the only 240V outlet at Sears Point (Sonoma Raceway/Infineon Raceway) and for pointing me to the adapter needed to use it.


jat | December 23, 2013

I also have left TC on (I was going to try it off in the afternoon of my last session but it rained so I left it on), and I haven't noticed any problem steering - the car is well balanced and pushes under acceleration and gets a bit loose under braking while turning, as expected.

Regen is maxed out at 60kW, which is pretty low current (and dissipated heat is proportional to the square of the current) compared to 318kW for max acceleration. Charge level is also an issue since I can only get 40A charging at the track (and no Superchargers anywhere near me, though one will soon be near Roebling Road), so regen helps a bit there.

TslaRcr | December 23, 2013

Thanks everyone for the helpful comments ! I drove at Willow Springs raceway in CA. The temp was about 55 degrees, but it was my fourth 15 minute session of the day before the power got limited. Wondering what it's going to be like in the summer when it's 100 degrees ...

Yeah, I get it that you can keep driving, but you lose track position with other cars starting to pass you, and your lap times go out the window.

The only handling 'issues' I experience with the car is on auto-cross courses. The car is way too heavy for small courses with sharp turns. Making the best of it though :) Hoping to get racing tires that should improve the situation.

I tried turning off TC drag racing. Can't say it made any difference. In cold temp on a slick track the tires were spinning about the same with and without TC. For me TC adds a piece of mind, if nothing else.

dchan415 | December 24, 2013

How was battery life on track? Where did you guys charge at Thunderhill?

jat | December 24, 2013

@dchan415 - At AMP I averaged about 1200Wh/mi, and at Roebling Road about 1100Wh/mi. I got about 4.5 hours of charging at 40A between/before sessions, which was just enough to let me run most of the day. I got enough charge while changing tires and getting cleaned up to make it to a friend's house with an HPWC, and I charged there for an hour to make it back home.

@vladt - I'm really happy with the BFG g-Force Rivals; at AMP it let me stay full throttle for 37s, and I was 10mph faster at turn 1 and could brake at least 100ft later.

The HPDEs I have gone to have been 5x 25min sessions, so that is probably why I saw more of an issue than you since your sessions were shorter.

Out4aDuck | December 24, 2013

To make a quick estimate, the motor and the power electronics each operate at about 95% efficiency. So when you are demanding 300 kW from the powertrain, you are also dissipating 30 kW of heat in these two devices. Additionally, heat will be dissipated from the battery pack, but I don't have a feel for battery efficiency. Can anyone help?

Also, I'm guessing that the power limitation is primarily due to motor temperature. The Tesla motor is very, very small for its maximum power output. Compare it in size to an industrial 3-phase induction motor which is designed to run at rated horsepower forever. Again, does anyone know for sure?

viranjit | December 25, 2013

@dchan415 - Thunderhill has several 240V/50A NEMA 14-50 outlets for RVs in the paddock area. While there are many outlets, some keep tripping on a 40A charge. On those, limiting charge to 30A solved the problem.

Due to low cell phone coverage at Thunderhill, it is difficult to monitor charging progress using the phone app, so it is wise to keep checking that charging is uninterrupted initially (while deciding on charging current limits).

@vladt - please read the battery and vehicle warranties carefully. There is a section on autocross use that could affect your coverage. I think autocross is considered competition, while HPDEs are educational driving schools. I disagree with that interpretation since I think autocross is driving practice, but I have reluctantly given that up for now.

viranjit | December 25, 2013

@vladt - would you consider moving this thread from "public" to "private" (registered owners only)? The information is mostly relevant to people who already own the car and it would limit some of the troll and fanboy posts.

lolachampcar | December 25, 2013

You guys are awesome. I though driving a Stock Car on a street course was a challenge. I can not imagine trying to get 4600lbs to change direction :)

TslaRcr | December 25, 2013

@viranjit - thanks for the note re warranty. I checked, and it's listed under the driving over rough surfaces area. Need to clarify. Autocross refers to couple different styles. The one you are thinking of is country road autocross. That would without a doubt damage this car, it makes perfect sense that they exclude it from the warranty, and it's most definitely *not* what I'm doing.

The autocross I referred to is a short cone course on a paved surfaced. Similar to a regular race track, but much shorter (3/4 mile) with 20+ turns. The speeds are mostly 20-40mph, and the highest you can ever reach is 60mph. The only real stress in this type of racing is on tires. I took a video of it - please take a look:

I'm sure you'll agree this is a fairly mild stress on the car.

Re making the thread private, I can't find an option to do that.

jat | December 25, 2013

@lola - not everyone gets to drive real race cars :)

@viranjit - what trolls/fanboys have posted on this thread?

viranjit | December 26, 2013

@vladt - I have participated in parking lot autocross in my previous car and I agree that it shouldn't a warranty red flag. Given how much I stretched to get this car, I don't want to take a risk without some official TM clarification. I also didn't know about cross country autocross till you mentioned it :-).

@jat - there haven't been any troll/fanboy posts on this thread but there had been some on similar threads in the past. Most were from fanboys who would weigh in on the (un)suitability of anyone using this car on the track. They may be right but seem to miss the fact that it isn't about racing, but about learning how to drive this car better in a controlled environment. The weight certainly needs getting used to.
About trolls, I have tended to limit any negative feedback on the car to verbal or private communication with the folks at TM. The car is fantastic as a daily driver and I didn't want to feed criticism of the company from outsiders based on unconventional use.

@lola - I have only one car and I didn't want to abandon my driving education. Your posts are the ones I look forward to reading.


GeekEV | December 26, 2013

There are tracks under the car that are use by the machines at the factory to hold the battery during transport. I wonder if there would be any advantage to designing a heat sink that can slot into those tracks to increase the surface area for heat dissipation while racing? Or is most of the heat in the motor/inverter?

jat | December 26, 2013

@GeekEV - we don't know what the weak link in the heat dissipation is, but the speculation is that it is the motor. Regardless, there is already a liquid cooling system running through the battery which I am sure is capable of exchanging more heat than anything mounted externally on the shielding. If the issue is just more cooling, then I think it would be better to increase the heat exchanger capacity.

chrisdl | December 27, 2013

Here's a link to my calculation of the losses in the battery:

A more accurate calculation is available at TMC, but the result is the virtually same.

In short minimal loss with a new battery pack at full output is 52 kW (Ri of 60 mOhm)
A loss of 86kW may be more realistic, especially as the battery ages (Ri of 100 mOhm)

This does not (yet) take the losses in the motor and the inverter into account.

jkn | December 27, 2013

I get slightly different result than chrisdl, but conclusion is same: At full power battery generates lot of heat.

Maximum discharge current is 12.5 A per cell with 3.6 V.
Power out from battery is 7104 * 12.5 * 3.6 = 320 kW

Internal (chemical) voltage of full cell is 4.2 V. Loss is 0.6 V.
Power loss in battery is 7104 * 12.5 * 0.6 = 53 kW
Internal resistance of cell is 0.6/12.5 = 48 mOhm

With 100 mOhm and 12.5 A I cannot get 320 kW from battery. Both cannot be correct.

I don't know exact efficiency of Teslas motor+inverter, but this is related:

I believe transferring heat from components to liquid is easy. (Rotor might be a problem.) Transferring heat from liquid to air is not easy.

Regen produces more heat than using same power. Motor and inverter are not optimized to be generator. Battery chemistry generates heat when charged.

Out4aDuck | December 28, 2013

@chrisdl and jkn,
Thanks so much for the excellent information! I always like to understand the engineering challenges.

S4WRXTTCS | December 28, 2013

Makes me think a race between multiple Tesla's on a race track would be an interesting event in strategy.

Since you could't go all out all the time.

NKinne | December 29, 2013

BMW specify that the i8 is not for track use. Is probably the same issues.