range penalty from fast starts?

I have found that fast starts impact range, but not as much as ICE environments. In normal day-to-day driving you will find that a "slow" start from a tesla is still much faster than everyone else at a light. In temperate weather spirited driving will give me an aggregate of 350-400 wh/mile conservative drivin closer to the 310-320.

There is still an energy penalty for fast starts:

1) Tire slippage increases wear on the tires and wastes energy.
2) Increased power demand results in more power losses through the inverter/power electronics, as well as more thermal losses in the battery itself converting chemical energy to electrical.
3) Greater forces on the drivetrain increase friction.
4) Other reasons I'm not thinking of at the moment.

Having said all that, the cool thing is that there is much less energy loss doing a fast vs. slow start in a Tesla vs. a gas-powered engine. In other words, gunning it in a Tesla wastes significantly less energy than gunning it in an ICE. This gap widens even more when you consider that at the next red light, the Tesla recovers about 80% of the kinetic energy that the ICE cars are wasting to heat in the brakes.

DaveVa said:

In normal day-to-day driving you will find that a "slow" start from a tesla is still much faster than everyone else at a light.

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Yep. I drive a non-Performance 85 and just accelerating at a rate that seems "normal" to me now, I still notice the other cars from the light way back behind me in the rear view mirror.

mknox said:

Yep. I drive a non-Performance 85 and just accelerating at a rate that seems "normal" to me now, I still notice the other cars from the light way back behind me in the rear view mirror.

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In any Tesla thats definitely true. When leaving a light, I have to 'try' and drive like a grandma with a broken foot to go slow enough people will hang with me. The car off the line is just so easy to get moving with little throttle input and effort. Every time I take off normally people disappear in my rear view mirror. Then again, 50% of the time if I am at the front of the light in the P85D I am running to 60MPH in 3.2 seconds or so just because it puts a smile on my face. In all honesty, I didn't buy the car to save much on gas or better the environment.. the performance of the P85D's electric motors were essentially 98% of the reason I had to have the car

electric losses quadruple when power doubles. 4 times the power means 8 times the losses and so on. It means higher power demand doesn't lead to just higher losses, it decreases the efficiency dramatically. Both in the motor but more so in the battery. How bit the real world difference is, I'm not sure. Lots of things add up. If I had time to kill I would set up a test, but I don't

There is one case where the losses can actually help in a sense. If the battery is cold, the efficiency is worse and you get less range. The battery will give you a better range if you first use part if it's capacity to warm itself up. So even though you are losing some energy to heat the battery, you still come out on top in the end. Since most of the power losses when accelerating fast are in the battery, maybe that warms up the battery and helps. Maybe.

The obvious and straightforward fix to this is to just switch over to high temp superconductor cables, inverters and motors. Problem solved, (assuming the liquid nitrogen chillers don't use too much power).

Fast acceleration reduces range in the following ways:

1. Your average speed over the entire trip will be faster if you accelerate fast. The higher your average speed, the more friction and aerodynamic drag, and hence the more energy used.
2. During acceleration, the electric motor operates more efficiently than when driving at constant speed (provided you don't accelerate too fast). With slow acceleration, you are accelerating longer and operating the motor more efficiently longer.
3. If you accelerate quickly, more of the power from the motor is wasted as heat and in other forms of internal energy (e.g. noise) rather than be used to propel the car.
4. You will get less energy out of the battery with greater power demands (Peukert's Law).

Efficiency (and hence range) is greatly affected by what electrical engineers call "I-squared-R" losses. That is, the watts dissipated equals the resistance times the square of the current. During jackrabbit starts, the current is very high, leading to heating in the wiring, electronic control pack, and motor (proportional to the square of the current). This heat is lost energy and can't contribute to range (second law of thermodynamics). So yes, there will be a range penalty for hot starts (although it probably won't be huge because the hot start does not last all that long).

I'd be more concerned about the extra stress on the drive train. The hot start can only put extra wear on gears, bearings, and contribute to fatigue of the metal of the gearbox. (Maybe that's one factor as to why so many people have mentioned that Tesla replaced their motor because it started making noise).

mknox said:

Yep. I drive a non-Performance 85 and just accelerating at a rate that seems "normal" to me now, I still notice the other cars from the light way back behind me in the rear view mirror.

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+1
I think I have a pretty normal Tesla acceleration off the light, but am surprised how quickly those cars behind me get really tiny

After going back to my 2009 Tahoe (I use it to tow)... even normal Tesla starts feel like warp speed.

Actually, let me rephrase that, any other car feels like AN ETERNITY to get MOVING. and don't even get me started on normal car STOPPING.

Once you get used to the regen, normal cars are almost scary to drive... they just keep rollin!

Of course on some roads I've been able to average almost 1200 wh/mile for up to 5 miles... whoops so... only 4x normal use. I should have got a pic.

The car is too damn fun to drive 'slow'... tires be dammed, I'm having fun!

It may depend on how fast you're getting up to. I notice a pretty good hit on the kW/m average on the energy graph if I boot it from stop to 50mph or above.
But around here those opportunities are rare, so more often I'm just getting up to 30, 35 or 40mph. I can pretty much stomp on it and hardly notice a hit at all.
I wonder if it has more to do with the power required to accelerate from higher speeds. Can I state 0-30!=30-60? (Though it feels almost the same!)

SabrToothSqrl said:

Once you get used to the regen, normal cars are almost scary to drive... they just keep rollin!

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Regen is the big difference. An ICE without hybrid regen capability loses all that acceleration energy when braking. Unless you overdo it and use your friction brakes, an EV recaptures much of the energy that went into vehicle kinetic energy.

The main reason, IMO, that you will experience quite a big "range penalty" (as the OP put it) is that in real life a lot of fast starts will also mean a lot of braking and fast deceleration.

If in fact we were only talking about fast starts and all other things being equal I'd bet the range penalty is actually quite negligible.

It takes the same amount of energy to accelerate the 2 ton Model S to say 65 mph regardless of if you do it in 5 seconds or 15. This is basic physics.

Johan said:

It takes the same amount of energy to accelerate the 2 ton Model S to say 65 mph regardless of if you do it in 5 seconds or 15. This is basic physics.

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That does not take into account real world effects. The energy from the battery is not transmitted as efficiently to power the wheels with faster acceleration. With fast acceleration, you are traveling more of the trip at higher speeds. Higher speeds have more aerodynamic drag and friction consuming more energy. I have measured the difference in energy consumption on my car (not Tesla) between fast and slow acceleration. There is a difference. Faster acceleration consumed about 25% more energy over a distance of 0.25 miles.

lphe said:

That does not take into account real world effects. The energy from the battery is not transmitted as efficiently to power the wheels with faster acceleration. With fast acceleration, you are traveling more of the trip at higher speeds. Higher speeds have more aerodynamic drag and friction consuming more energy. I have measured the difference in energy consumption on my car (not Tesla) between fast and slow acceleration. There is a difference. Faster acceleration consumed about 25% more energy over a distance of 0.25 miles.

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Well if you travel more of the trip at higher speed you'll either travel the same distance in a shorter period of time OR travel further if you keep going for a certain period of time. So yeah, not apples to apples quite.

My point was this: imagine I have a daily commute where there's one on-ramp where I always beed to accelerate and and two red lights where I always have to start from standing still. For one month I accelerate slowly on the on-ramp and from the lights. The next week I accelerate aggressively at these points. Apart from that my driving is exactly the same (same decelerations, same top speeds, same cruising speeds). My claim is that the difference in energy use is negligible. My real world experience confirms this.

Johan said:

It takes the same amount of energy to accelerate the 2 ton Model S to say 65 mph regardless of if you do it in 5 seconds or 15. This is basic physics.

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That... is not accurate at all.

It will take WAY more energy to accelerate it quickly... while in the S the difference isn't massively apparent, the difference is there.

With a daily commute of under 40 miles... I just punch it off the line... almost every time... don't really care lol.

Johan said:

It takes the same amount of energy to accelerate the 2 ton Model S to say 65 mph regardless of if you do it in 5 seconds or 15. This is basic physics.

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I thought basic physics taught that the energy is equal to the initial energy plus the work done? (where work is property of force, and force is a property of acceleration. So accelerating "hard" uses more force, hence more work, hence more energy).

<- not a physicist

SabrToothSqrl said:

That... is not accurate at all.

It will take WAY more energy to accelerate it quickly... while in the S the difference isn't massively apparent, the difference is there.

With a daily commute of under 40 miles... I just punch it off the line... almost every time... don't really care lol.

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I'm sorry but it is accurate.

Work = integral(F(t)v(t) dt, 0 to T) where F(t) is Force, v(t) is velocity at time t, T is total time of acceleration. But under constant acceleration, v(t)=Ft/m where m is mass and force F is constant.
= integral(F^2 * t/m dt, 0 to T)
= F^2 * T^2 / 2m
But, F=ma, so T is inversely proportional to F, T = V/a = Vm/F where V is target velocity.
= F^2 * (Vm/F)^2/ 2m = F^2 * V^2 * m^2 / (F^2 * 2m )
= mV^2/2
This is independent of rate of acceleration.

(Disclaimer: The equations written out above are stolen from ItsNotAboutTheMoney on this board, from this post: http://www.teslamotorsclub.com/show...erating-slowly?p=649186&viewfull=1#post649186)


In simpler terms: rate of acceleration is irrelevant since the work required to add a certain amount of kinetic energy to a given mass is the same (i.e. to accelerate a 2 ton Model S to 65 mph requires the same amount of energy regardless of if you do it fast or slow).

Now in real life there are some factors to consider:

Higher heat losses in the drive train and possibly also a bit in the tires with faster acceleration. Higher omhic losses in the battery and inverter with higher current draw. The electric motors is very efficient from 0 RPM so that doesn't matter much, and in fact electric motors are more efficient at higher loads, which in fact favors faster acceleration.

I stand by my claim that the difference is negligible and that any experienced real world differences are due to these other factors:

More braking, less overall efficient driving style, going faster in general.

Please don't make the mistake of confusing accelerating quickly with going faster. Going faster obviously uses more energy, but also gets you there faster.