Why my Wh/mi so bad?

[ZenRockGarden]

the power (energy per unit time) is greater.


I completely agree. Energy per unit time (known as power) is higher. Alas, the battery capacity is in units of power * time aka Kilo Watt HOURS.

Accelerate any way you want, the only thing the battery cares about is energy, not power.

 

[ZenRockGarden]

My physics knowledge is lacking, but this doesn't ring true to me.

Given a fixed speed, getting to it in a shorter amount of time should take more energy. Reducing the time to full speed isn't free.

Right?

WRONG. Ask any educated person. It takes higher power levels for a shorter time, and exactly identically the same energy.

Literally go to any person you know with a 4 year degree in physics or who graduated with an A in the subject from high school. Tell them "I believe it requires MORE energy to reach accelerate an object to a given speed quickly, as opposed to doing the same thing more slowly." Write back with your results. Seriously. Go do it.

 

[NYMY]

Thank you. I didn’t read all of the pissing match above, but @ZenRockGarden claims to know physics but really only has a rudimentary understanding. He/she assumes an ideal, lossless system where all energy is conserved and the efficiency of the batteries and motors is perfect and unrelated to power/current draw. All of these assumptions are false. Furthermore, many of the inefficiencies are nonlinear. There is likely an optimal rate of acceleration that optimizes motor load & efficiency with battery efficiency. It would be interesting to see what that is but I haven’t seen or ready anythign about it.

@ZenRockGarden - for starters, go read up on internal resistance of batteries then come back and correct your posts.

Edit - human exercise physiology is completely different from automotive power and efficiency so there’s zero point in comparing the two.

@ZenRockGarden is making a reasonable and solid point: if you accelerate for 20 seconds vs 40 to get up to speed and then drive 50 miles, whatever efficiency difference there is in that brief acceleration period (sizeable or no) is not the primary loss of energy, which is the sustained loss during the 50 miles. Since that goes like v^2, your overall efficiency per mile drops like 1/v (roughly). I don't think this is super controversial?

Now if you're driving around town and are constantly on the brakes, it is a different story...

 

[ZenRockGarden]

@ZenRockGarden - for starters, go read up on internal resistance of batteries then come back and correct your posts.

Edit - human exercise physiology is completely different from automotive power and efficiency so there’s zero point in comparing the two.

The internal resistance of the batteries is LOW compared to the overall 95%+ efficiency of the system. Can we please just use common sense here and understand basic facts that accelerating fast to a given speed, and doing it more slowly uses the SAME total energy, give or take incredibly small differences in losses from heating in the system?

And yes, can we also understand that human muscles DONT work the same way?

Sheesh. It's like middle-school-physics nightmare here.

 

[sleepydoc]

The internal resistance of the batteries is LOW compared to the overall 95%+ efficiency of the system. Can we please just use common sense here and understand basic facts that accelerating fast to a given speed, and doing it more slowly uses the SAME total energy, give or take incredibly small differences in losses from heating in the system?

And yes, can we also understand that human muscles DONT work the same way?

Sheesh. It's like middle-school-physics nightmare here.

*sigh* That’s the problem. You’re treating it like high school physics and ignoring all the nonidealities in the system. I’m done trying to have an educated discussion here but please, if all you know is high school physics, don’t pretend you know more.

 

[ZenRockGarden]

*sigh* That’s the problem. You’re treating it like high school physics and ignoring all the nonidealities in the system. I’m done trying to have an educated discussion here but please, if all you know is high school physics, don’t pretend you know more.

I agree - if knowing high school physics, and the difference between power and energy are too hard for you.... we're done. Buh Bye!

 

[father_of_6]

Literally go to any person you know with a 4 year degree in physics or who graduated with an A in the subject from high school. Tell them "I believe it requires MORE energy to reach accelerate an object to a given speed quickly, as opposed to doing the same thing more slowly." Write back with your results. Seriously. Go do it.

No need... I wasn't arguing, just discussing. We're not enemies.

 

[thesmokingman]

No need... I wasn't arguing, just discussing. We're not enemies.

The real data on this is all over the forum. It's been discussed ad nausea for who knows how long. Hell it is even printed clearly in Tesla's manual, getting maximum range. It explicitly states to avoid frequent and rapid acceleration, use chill mode. There's no need to go in a circle with this poster.

 

[ZenRockGarden]

It's just hard when folks can't even argue from the basic facts of physics. Accelerating at any rate to a given speed involves the same energy. Yes, we could then get into secondary heat losses in the system... but they are actually... secondary, and the basic physics is the base from which to discuss further, not "well I think walking is easier than running so physics is stupid!"

 

[ZenRockGarden]

The real data on this is all over the forum. It's been discussed ad nausea for who knows how long. Hell it is even printed clearly in Tesla's manual, getting maximum range. It explicitly states to avoid frequent and rapid acceleration, use chill mode. There's no need to go in a circle with this poster.

People who need "chill mode" for self control are the same people who have to have "the brake pedal makes Jesus cry" explained to them.

 

[MY-Y]

This isn't the healthiest discussion, so I might regret chiming in.
1. kWh is energy, not power. 1 kWh = 3.6E6 Joules.
2. Accelerating from 0-60 imparts the same energy on the car regardless of the time taken. However, this does not mean that the same energy is used because the losses aren't linear with power draw. Why?
A. Resistive losses are a function of current squared. If you draw twice the current, your resistive losses are 4x.
B. The losses in the battery are also non-linear. You get less energy from the battery at high current draws than more modest draws. When drawing higher currents, you can use scanmytesla to see the voltage drop. The battery has (roughly) a fixed number of electrons to move from one side to the other (AH), so when the voltage drops at higher draws, the losses are quite observable.

There are a few other losses at high acceleration (e.g. friction losses, magnetic coupling in the motors). I didn't include these above because they are harder to argue about and aren't as important to understanding why hard acceleration consumes more energy.

This doesn't mean you need to accelerate super gently for good efficiency. Moderate acceleration is fine. For city driving, moderate acceleration (including regen braking), is key for efficiency.

 

[sleepydoc]

This isn't the healthiest discussion, so I might regret chiming in.
1. kWh is energy, not power. 1 kWh = 3.6E6 Joules.
2. Accelerating from 0-60 imparts the same energy on the car regardless of the time taken. However, this does not mean that the same energy is used because the losses aren't linear with power draw. Why?
A. Resistive losses are a function of current squared. If you draw twice the current, your resistive losses are 4x.
B. The losses in the battery are also non-linear. You get less energy from the battery at high current draws than more modest draws. When drawing higher currents, you can use scanmytesla to see the voltage drop. The battery has (roughly) a fixed number of electrons to move from one side to the other (AH), so when the voltage drops at higher draws, the losses are quite observable.

There are a few other losses at high acceleration (e.g. friction losses, magnetic coupling in the motors). I didn't include these above because they are harder to argue about and aren't as important to understanding why hard acceleration consumes more energy.

This doesn't mean you need to accelerate super gently for good efficiency. Moderate acceleration is fine. For city driving, moderate acceleration (including regen braking), is key for efficiency.

Yes! - Someone else who understands more than high school physics! Thank you!

 

[Terpslax]

For the OP, I live in NY; when I drive in the 5 boroughs in traffic I‘m in the 260-280 range. When I’m on highways that allow me to get to 70-90 mph I’m in the 300-340 range. So many factors effect the range its crazy. Somet8mes I look down and I’m shocked that I’m in the 350 range and other t8mes I look down and I’m at 230.

If you have home charging just don’t pay attention to it

 

[Pianewman]

Terpslax: To clarify: those are Wh/mi numbers, right? NOT miles per full charge???

 

[STS-134]

The real data on this is all over the forum. It's been discussed ad nausea for who knows how long. Hell it is even printed clearly in Tesla's manual, getting maximum range. It explicitly states to avoid frequent and rapid acceleration, use chill mode. There's no need to go in a circle with this poster.

There is zero reason to ever put these cars in chill mode. All it does is software limit the total power to the motors; you can do exactly the same thing by not pressing down on the accelerator pedal so much. Except when in chill mode, if you ever actually need the extra power to get out of a certain situation, you won't be able to do it, because it won't be possible to go into the configuration and change things back quickly enough. So it's all downsides and no upsides to putting the vehicle in that mode.

 

[Terpslax]

Terpslax: To clarify: those are Wh/mi numbers, right? NOT miles per full charge???

Yup.

 

[sleepydoc]

There is zero reason to ever put these cars in chill mode. All it does is software limit the total power to the motors; you can do exactly the same thing by not pressing down on the accelerator pedal so much. Except when in chill mode, if you ever actually need the extra power to get out of a certain situation, you won't be able to do it, because it won't be possible to go into the configuration and change things back quickly enough. So it's all downsides and no upsides to putting the vehicle in that mode.

I’ll take the opposite view - there’s zero reason to ever take it out of chill mode. Even in chill mode you have more power and acceleration than 90% of the cars on the road. I had mine in chill mode for a year and never missed the extra power but it was easier to keep from speeding and easier to accelerate at a more reasonable rate. Passing is still more thane adequate, too.

 

[boulder.dude]

The biggest cost of hard acceleration is wearing out your tires faster! Lots of posts on TMC from people who burned through their tires after getting their car or buying Accel Boost. (Btw the battery provided the energy that went into wearing down those tires).

 

[sleepydoc]

The biggest cost of hard acceleration is wearing out your tires faster! Lots of posts on TMC from people who burned through their tires after getting their car or buying Accel Boost. (Btw the battery provided the energy that went into wearing down those tires).

you forget - there's no difference between accelerating hard and acceleration gently because the car's final speed and kinetic energy is the same. Extra energy spent wearing the rubber of the tires is purely fictitious!

 

[STS-134]

I’ll take the opposite view - there’s zero reason to ever take it out of chill mode. Even in chill mode you have more power and acceleration than 90% of the cars on the road. I had mine in chill mode for a year and never missed the extra power but it was easier to keep from speeding and easier to accelerate at a more reasonable rate. Passing is still more thane adequate, too.

And if you ever find yourself in a situation where there's a truck barreling at you and the only way out is with maximum acceleration, you're screwed. If you don't want the extra power 99.9% of the time you're driving, then fine -- just don't use it by never pressing the accelerator pedal past a certain point! I can accelerate as quickly or as slowly as I want in any mode just by modulating the amount of pressure I'm putting on the accelerator. Sometimes I can get < 200 Wh/mi if I am driving for maximum efficiency.