Hypothetical 60kWh Performance Option

[LithiumBob]

Tesla should undoubtedly offer a Performance Motor Option with the S2. I am an enthusiast who values efficiency and performance and a Model S reservation holder. Also, $90K is a bit over my budget .

Based on the motor spec sheet here: Model S Options Pricing | Tesla Motors, I have plotted the power curves of S3 Performance, S2 Standard and my proposed S2 Performance here:

From the graph above, from 0 to 3000 RPM, the S2 Performance has the same power as a S3 Performance, but is a few hundred pounds lighter. What this means is that the S2 Performance package will be the fastest Tesla off the line - even quicker than the Roadster Sport from 0 to 30 MPH! In fact, if Tesla optimizes the battery pack, I imagine they could off more than 225 KW with this package, making the 0 to 60MPH as fast if not faster than the S3 Performance.

Besides off the line acceleration, from a customer standpoint there are other advantages of the S2 Performance over a similarly priced S3:

1. Much lower cost.
2. Much better vehicle dynamics since it is a few hundred pounds lighter, the S2 Performance Option will out brake and out corner the heavier S3.
3. More efficient since it is lighter and will have lower rolling resistance. Efficiency is one of the reasons why we buy an electric car. I don't need 300 miles range and do NOT want to carry the extra battery weight that comes with a S3 every day.

Tesla could price the Performance Option upgrade easily from $5K to $10K. If interested in a S2 Performance Option, please respond to this post so Tesla sees that customers want this.

 

[Kipernicus]

The forum can correct me if I'm wrong, but I think not many people outside of Tesla employees refer to the 60kWh battery as "S2" and 85 as "S3". You must already have an inside line to Tesla.

While "S2 Perf" may be possible, its not so good from a product management perspective. The way they have it now is very clean and straightforward - Performance only with the biggest battery as the top of the line model. A Gen3 performance that is less expensive and faster than MS 85kWh non-perf may make sense, but a MS 60kWh performance makes for an inconsistent lineup and will only anger all current P85 customers.

 

[EarlyAdopter]

A 60 kWh Perf would cannibalize sales of the 85 kWh Perf at what I suspect would be lower gross margins for Tesla ( both percent and absolute dollars). Not likely to happen. It's important to note that everything, and I do mean everything, at Tesla right now is focused on getting profitable in order to build out manufacturing capacity and get to Gen 3.

For a lighter, faster car you'll probably need to wait for the Gen 3 based supercar that Elon has hinted at.

 

[CanuckS#69]

I thought that the 85kwh battery was needed in order to supply enough power for the bigger motor and inverter on the perf model. With electric conversions, this is often an issue as you need more battery amps in short burst than you can provide without having a large number of cells, which increases weight.

 

[Todd Burch]

The reason there isn't a 60 kWh performance version that performs better than the 85 kWh perf has nothing to do with marketing or business (however if technically feasible those might be reasons not to do it).

If you want a battery to live a long, healthy life, there is a limit to the amount of discharge current (read power) you want to draw from the pack. (Refer to "C" and "E" rates here: (http://mit.edu/evt/summary_battery_specifications.pdf). As the capacity of the pack increases, you can draw more power from it while still not exceeding the safe limit. Likewise, a smaller capacity pack allows less maximum power to be delivered while still preserving pack life.

So the real (primary) reason is that the amount of power pulled from the pack to make this happen would dramatically shorten the life of the smaller capacity battery.

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I should add that I'm sure Tesla's squeezing as much power as possible from the pack while still maintaining pack life.

 

[GSP]

Yes, but the OP's suggestion doesn't draw any more power from the 60 kWh battery than the standard 60 kWh car does.

GSP

PS. I like this idea. The marketing concerns could work out OK in the end. It still will not have the power of the 85 kWh performance car. The P85 will have better high speed passing acceleration, and more range.

 

[Todd Burch]

Yes, but the OP's suggestion doesn't draw any more power from the 60 kWh battery than the standard 60 kWh does.

GSP

Not so sure about that. We're looking at a graph of motor horsepower in kW--not power draw from the battery in kW. When you're flooring it, I think there are a lot of electrical reasons (such as increased internal resistance with increased current draw/torque) that result in actual power draw from the battery markedly exceeding power output by the motor.

To get better acceleration in the fixed gearing, more torque is needed--so more current draw is required from the battery. This increases the C (or to be pedantic, E) rate. Increased current draw increases the amount of Joule heating (http://en.wikipedia.org/wiki/Joule_heating) and the internal resistance of the battery (http://en.wikipedia.org/wiki/Internal_resistance).

I think that looking at the maximum motor power output (in kW) as the limiting factor is misleading.

We need an EE from Tesla on this board

 

[Jason S]

I'm trying to track this in terms of cost. 85 vs P85 is 10000. 60 vs 85 is 10000. ... So a 60 perf would equal cost of 85, maybe? Prolly not, given that the 85 seems undertuned (stay with me...).

And you're making the HUGE assumption that the 60 can actually put out the power required to keep up with an 85 perf. If anything, given the performance difference between the 60 and 85 then the 85 is undertuned while the 85 perf is the one actually using all the available power.

 

[Todd Burch]

And you're making the HUGE assumption that the 60 can actually put out the power required to keep up with an 85 perf. If anything, given the performance difference between the 60 and 85 then the 85 is undertuned while the 85 perf is the one actually using all the available power.

I think the P85 is using the maximum discharge current the battery can support while still maintaining desired useful life. The 85 non-perf is obviously using less.

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BTW, the caveat to this is that if a P85 driver and 85 non-perf driver floor it all the time while driving--all else being equal--the non-perf battery will last longer. How much longer depends on the maximum discharge rate (E value) that Tesla chose.

 

[aronth5]

A 60 kWh Perf would cannibalize sales of the 85 kWh Perf at what I suspect would be lower gross margins for Tesla ( both percent and absolute dollars). Not likely to happen. It's important to note that everything, and I do mean everything, at Tesla right now is focused on getting profitable in order to build out manufacturing capacity and get to Gen 3.

For a lighter, faster car you'll probably need to wait for the Gen 3 based supercar that Elon has hinted at.

Well everything hopefully doesn't forget finalizing the Model X and it's manufacturing build out :smile:

 

[efxjim]

Is the pack fewer cells or same # cells just lower capacity? I seem to remember they are using same # cells with lower capacity.

 

[Todd Burch]

Is the pack fewer cells or same # cells just lower capacity? I seem to remember they are using same # cells with lower capacity.

I believe that's correct. I think the 60 has more cells than the 40...and the 85 has the same # of cells as the 60, but with higher capacity.

 

[vfx]

I believe that's correct. I think the 60 has more cells than the 40...and the 85 has the same # of cells as the 60, but with higher capacity.

Makes me wonder if all the other auto manufactures are buying one of each. S1, S2, S3...

 

[LithiumBob]

....To get better acceleration in the fixed gearing, more torque is needed--so more current draw is required from the battery.
....

Not true, torque is proportional to motor current, not battery current. So the 60KWh performance option will NOT have higher peak currents if the peak power is maintained at 225KW.

 

[Todd Burch]

Not true, torque is proportional to motor current, not battery current. So the 60KWh performance option will NOT have higher peak currents if the peak power is maintained at 225KW.

I'm not sure how what I said contradicts what you said. You are absolutely right that more motor current yields more torque. My point was that demanding lots of torque (motor current, which obviously must come from the battery pack) in a short period of time really heats up the battery, which increases the battery pack's internal resistance and therefore requires even more current to maintain that torque.

 

[gregincal]

Theoretical output graphs notwithstanding, what really counts is the actual draw off the battery in an 85kWh performance when you punch it from a stop. It's been a while since my test drive in a performance, but I believe the current draw on the battery went up very quickly to max. Any performance owners care to share a picture of energy usage to do the 4.5 0-60?

 

[derekt75]

I have no reason to doubt the OP's curves. The power draw certainly does not get to 320 kW instantly. I can't say exactly when it gets there as my eyes are usually more focused on the road when I'm pulling 320 kW but somewhere around 40 mph sounds right.

I'll also note that the max torque for the 40 kWh battery is very similar to the max torque for the 85 kWh non-perf model. I think that lends credibility to the OP's conjecture that the 60 kWh could have a torque similar to the P85 if they did the performance treatment to the 60 kWh.
I also think I would have been interested in getting that configuration. Personally, I didn't want to pay for extra range, but I wanted high performance car, so I had to get the big battery. That being said, I did get it, so that's not really a good reason for Tesla to offer something else.

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oh, and for those who haven't studied physics for a while,
with a single speed transmission, torque is proportional to acceleration.
power is proportional to torque times RPM. (or more precisely, power (in W) = torque (in N-m) * RPM * 2 * pi / 60 ). That's why power draw linearly increases for thousands of RPM (until the acceleration starts to fall off).
and lastly, battery current draw is proportional to power usage.

 

[Todd Burch]

I have no reason to doubt the OP's curves.

I don't think anyone here is doubting the OPs power curves--at least not that I've seen. The issue, I think, is that they're showing power delivered by the motor, not power delivered by the battery. And that is key. The battery's acceptable healthy discharge rate is the bottleneck, not the motor's maximum torque output.

I'll also note that the max torque for the 40 kWh battery is very similar to the max torque for the 85 kWh non-perf model. I think that lends credibility to the OP's conjecture that the 60 kWh could have a torque similar to the P85 if they did the performance treatment to the 60 kWh.

That's a big assumption. This only tells me that the 40/60 kWh might achieve the 0-60 of the 85 non-perf. We already know the 85 non-perf is dialed down from the performance.

Again, the two issues here are:

-We're looking at power delivered at the wheels, not by the battery.
-To maintain battery health, smaller battery packs must deliver smaller maximum power.

Back to the OP, I believe it's possible for the 40 to achieve the 0-60 times of the performance--but allowing that would probably destroy the battery in 2-3 years due to the high discharge rate. Would you put up with battery replacement every few years to get that extra bit of performance? I know I wouldn't!

 

[derekt75]

I don't think anyone here is doubting the OPs power curves--at least not that I've seen. The issue, I think, is that they're showing power delivered by the motor, not power delivered by the battery. And that is key. The battery's acceptable healthy discharge rate is the bottleneck, not the motor's maximum torque output.

Well, one person was saying that they thought that the power draw quickly approached 310 kW. I'm not sure how one defines quick, but you don't see that on the screen until around 40 mph. I thought that the screen showed battery power being supplied and not engine power, but maybe not?

Is the suggestion that the battery is supplying some 320 kW shortly after stepping on the pedal, but only 80 kW is getting to the wheels at 10 mph and the other 240 kW is being lost to heat? That would be a $#!+ load of lost power. My company makes instruments where we struggle to remove 20 W of heat with a fan. I can't imagine trying to remove 240000 W of heat from the inverter and motor.

I think it's far far more likely that at 10 mph, the battery in my perf is only delivering about 80 kW and over 80% of that is going to the rear differential.

If a 60 kWh battery can deliver 225 kW at 100 mph, I don't see why it couldn't deliver 80 kW at 10 mph.

That's a big assumption. This only tells me that the 40/60 kWh might achieve the 0-60 of the 85 non-perf. We already know the 85 non-perf is dialed down from the performance.

Again, the two issues here are:

-We're looking at power delivered at the wheels, not by the battery.
-To maintain battery health, smaller battery packs must deliver smaller maximum power.

Actually, I don't think that the smaller batteries can match the 0-60 of the 85 non-perf. I think they should be able to match the 0-20 times of the non-perf as the torque at low end is similar for all models. Once they're getting above 30 mph, they're limited by how much power you can draw out of the battery. Before that, though, it seems that they're limited by something else (such as the inverter or motor).

Regarding issue #1, I think power delivered to the wheels should at least be a majority of the power supplied by the battery. If the power at the wheels is less than half of the power coming out of the battery, Tesla is doing something wrong, and something somewhere is getting really hot really fast.

Regarding issue #2, I'm totally in agreement. I just think that max power out of the battery should not have a significant impact on the 0-20 times. The critical factor there is how much torque your inverter/motor can create/handle, not how much power your battery can supply. Only after torque is not the limiting factor does power handling become important.

Back to the OP, I believe it's possible for the 40 to achieve the 0-60 times of the performance--but allowing that would probably destroy the battery in 2-3 years due to the high discharge rate. Would you put up with battery replacement every few years to get that extra bit of performance? I know I wouldn't!

I think the 40 kWh battery matched with the motor and electronics in the performance car could achieve similar 0-20 times as the perf 85 without causing undue damage to the battery.
The 60 kWh battery with a performance treatment should be able to get to maybe 30 mph before falling behind the perf 85, and then at some point, I would expect it would even fall behind the non-performance 85 kWh battery.

Anyway, if what you care about is getting off the line fast, or maximum acceleration, I would think that a perf 60 could be as fast as a perf 85 from 0-30 mph without doing any damage to the battery.

 

[LithiumBob]

No. More torque will NOT heat up the battery more if the power is maintained. The high motor currents are in AC and are really decoupled from the DC battery currents.

Also, one other correction, increase in battery temperature actually lowers resistance. ALthough, you could argue if this happens frequently you can accelerate degradation of the battery pack.