SMALLER Battery Packs FASTER 0-30???

WhiteKnight · Dec 22, 2011

Sorry for making a question into a new thread but I could have sworn it was said in this forum in the last few days that the SMALLER battery packs would be FASTER 0-30?

The idea was the torque curve was the same for all the packs up to 30 so the reduced weight would allow smaller packs to be faster but then the bigger packs would pull away from 30-60 mph?

Have I been hallucinating?

zdre · Dec 22, 2011

It was just a guess that assumed that the battery packs would be able to put out the same power. Now that we know they can't, 0-30 times may match or be even slower in smaller packs.

Robert.Boston · Dec 22, 2011

WhiteKnight said:
Sorry for making a question into a new thread but I could have sworn it was said in this forum in the last few days that the SMALLER battery packs would be FASTER 0-30?

The idea was the torque curve was the same for all the packs up to 30 so the reduced weight would allow smaller packs to be faster but then the bigger packs would pull away from 30-60 mph?

That logic still makes sense. I doubt that Tesla has intentionally crippled the 40kWh drivetrain in any way; the slower 0-60 would be the logical result of hitting the current limit somewhere between 0 and 60. That same logic explains the middling time for the 60kWh version.

TEG · Dec 22, 2011

Notice this LEAF chart:

The section called "Quick response" shows how the LEAF pulls well (from stop lights) at low speeds, but then flattens out once you hit the power peak. At lower RPMs, the big eMotor torque gets you going "smartly" as long as there isn't too much battery pack weight to lug, but then (as others have said), at some point the power has to be limited and acceleration mellows out.

VolkerP · Dec 22, 2011

IMO the differences in acceleration arise from different maximum power that the packs can deliver.
The PEM converts 375V DC from the battery pack to 900A AC 3~ with variable voltage & frequency for the motor. It does this from 0mph until max DC power output is reached, then power stays constant and the amps start to drop.

If the cells in the Tesla pack survive 4C for a few seconds, we get 160kW from the 40kWh pack, 240kW from 60kWh pack, and 340kW for the 85kWh pack. The latter is roughly in line with the 435hp mentioned in the Performance Features.

The performance drive train will deliver 1200A and outperform the 900A version. I guess that the Performance can reach maximum acceleration of 10m/s² whereas the standard drive train reaches 7.5m/s².

The power P required to sustain acceleration a=7.5m/s² of a car with mass m=2000kg is P=m*v*a. Solving for P=165kW I get v=11m/s = 25mph. Then, the base model might be 140kg lighter for the reduced number of cells, improving the result by 7%.
I conclude that the Model S with 40kWh pack will keep pace with 60kWh and 85Wh pack up until 25mph, then start to lag behind. The 60kWh model will keep full acceleration up to 16m/s=36mph and the 85kWh will reach 340kW at 23m/s=50mph. These are all speeds where power to overcome rolling resistance and aerodynamic drag is <15kW so we can neglect this.

WhiteKnight · Dec 22, 2011

VolkerP said:
I conclude that the Model S with 40kWh pack will keep pace with 60kWh and 85Wh pack up until 25mph, then start to lag behind. The 60kWh model will keep full acceleration up to 16m/s=36mph and the 85kWh will reach 340kW at 23m/s=50mph. These are all speeds where power to overcome rolling resistance and aerodynamic drag is <15kW so we can neglect this.

That is great info! So the three cars are equal in 0 to 25mph! Takes a little bit of the sting out of the slower 0 to 60 time. Especially since there is no stoplight in Atlanta where the speed limit is higher than 45 mph for that street.

Do you know how fast an ICE car like BMW 535i would go 0 to 25 mph?

Cobos · Dec 22, 2011

VolkerP said:
IMO the differences in acceleration arise from different maximum power that the packs can deliver.
The PEM converts 375V DC from the battery pack to 900A AC 3~ with variable voltage & frequency for the motor. It does this from 0mph until max DC power output is reached, then power stays constant and the amps start to drop.

If the cells in the Tesla pack survive 4C for a few seconds, we get 160kW from the 40kWh pack, 240kW from 60kWh pack, and 340kW for the 85kWh pack. The latter is roughly in line with the 435hp mentioned in the Performance Features.

The performance drive train will deliver 1200A and outperform the 900A version. I guess that the Performance can reach maximum acceleration of 10m/s² whereas the standard drive train reaches 7.5m/s².

The power P required to sustain acceleration a=7.5m/s² of a car with mass m=2000kg is P=m*v*a. Solving for P=165kW I get v=11m/s = 25mph. Then, the base model might be 140kg lighter for the reduced number of cells, improving the result by 7%.
I conclude that the Model S with 40kWh pack will keep pace with 60kWh and 85Wh pack up until 25mph, then start to lag behind. The 60kWh model will keep full acceleration up to 16m/s=36mph and the 85kWh will reach 340kW at 23m/s=50mph. These are all speeds where power to overcome rolling resistance and aerodynamic drag is <15kW so we can neglect this.

So in essence the 40kWh will have the same motor but it will not be able to get as many horsepower out of the engine as the bigger batteries? That might be relevant for countries with fees based on power.

Cobos

VolkerP · Dec 22, 2011

Cobos said:
So in essence the 40kWh will have the same motor but it will not be able to get as many horsepower out of the engine as the bigger batteries? That might be relevant for countries with fees based on power.

Yup. out of the pack, that is.

My understanding of AC motor technology is you design for maximum current. Excursion:

The full current produces the full torque. At 0 RPM you need nearly 0 Volts to make the motor turn - with full torque! As the motor spins up, it acts as a generator - it produces its own voltage. That voltage is in opposition to the current applied from outside. You have to increase the voltage to keep the current up. Same current, more volts = more power. Finally, your power supply maxes out. You cannot keep the current up, and thus the torque. Power output reaches a constant value. Finally, adverse electromagnetic effects (that I don't understand at all) make it impossible to raise voltage any further.

That leads me to assume, all standard power trains for 40kWh, 60kWh, and 85kWh battery pack size are the same 900A capable wires, PEM, and motor.
The Performance power train runs 1200A with upgraded wiring, PEM, and motor. But it has approximately the same peak power like the standard one.
Think of the smaller packs like a throttling an ICE engine to a lower power level. This is common to circumvent legal/tax regulations!

Dave EV · Dec 22, 2011

VolkerP said:
IMO the differences in acceleration arise from different maximum power that the packs can deliver.

Nice calcs - do you think the 40 kWh pack will have the same voltage as the 60 / 85 kWh packs despite having fewer cells? I guess they could easily arrange them so that fewer of them are in parallel assuming they are the same cells as used in the 60 kWh pack.

stopcrazypp · Dec 22, 2011

VolkerP said:
Think of the smaller packs like a throttling an ICE engine to a lower power level. This is common to circumvent legal/tax regulations!

The best analogy of a battery pack I can come up with is to assume the fuel pump of a car scales directly with the fuel tank size. The battery power (analogous to fuel pump speed) is the limiting the "engine" in this case.

VolkerP · Dec 23, 2011

drees said:
Nice calcs - do you think the 40 kWh pack will have the same voltage as the 60 / 85 kWh packs despite having fewer cells? I guess they could easily arrange them so that fewer of them are in parallel assuming they are the same cells as used in the 60 kWh pack.

I'd rather they stick to the same nominal pack voltage around 375V and implement the power limit in the PEM software. The PEM identifies the pack capacity and selects according power limit. Would ease up the battery swap process.

Changing the voltage might require changes to PEM, the chargers, and the battery management system.

VolkerP · Dec 23, 2011

stopcrazypp said:
The best analogy of a battery pack I can come up with is to assume the fuel pump of a car scales directly with the fuel tank size. The battery power (analogous to fuel pump speed) is the limiting the "engine" in this case.

Yes, and in ICE that limitation is usually implemented by limiting supply of the other component required for combustion - air.

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SMALLER Battery Packs FASTER 0-30???

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