Speed vs Efficiency with respect to total travel time

[Anonymous]

To the OP's point, and what's been discussed here, it looks like a 600 mile trip @ 75 mph would involve around 5 hours on the charger. While it can be done, it's a crazy amount of time to waste.

I guess in Oilerlord's world, every long distance trip involves 4 super-caffienated college students taking turns driving with only bathroom breaks and take-out food consumed in the car! :lol:

 

[PV1]

I believe the physics of the current state of lithium-ion batteries is such that the efficiency drops off above 52 MPH, so driving at 85 MPH in a Bolt EV will eat up your storage capacity like a hot knife through butter.

Has nothing to do with the physics or current state of the art Lithium-Ion batteries.

It has evertything to do with the aerodynamics of the car. Which in the Bolt's case is not great.

So you are claiming that range is linear?

Lithium ion batteries will support 50% of range at 100 MPM than they will at 50 MPH so long as the aerodynamics of the car is properly designed?

I don't think so!

Where did he say that range is linear?

The drop in range with increased speed is due almost purely to aerodynamic drag, same as how a gas car gets worse mileage at 80 than it does at 65. Aero drag is definitely not linear. A more aerodynamic car (the EV1 for instance) would experience the same thing, but not as bad as the Bolt because overall aero drag is lower. Take air out of the equation, and the Bolt would use just about the same power to maintain 90 MPH as it would 30 MPH (slightly more drag in the bearings and perhaps an efficiency drop in the motor because it's spinning faster).

 

[Anonymous]

Has nothing to do with the physics or current state of the art Lithium-Ion batteries.

It has evertything to do with the aerodynamics of the car. Which in the Bolt's case is not great.

So you are claiming that range is linear?

Lithium ion batteries will support 50% of range at 100 MPM than they will at 50 MPH so long as the aerodynamics of the car is properly designed?

I don't think so!

Where did he say that range is linear?

The drop in range with increased speed is due almost purely to aerodynamic drag, same as how a gas car gets worse mileage at 80 than it does at 65. Aero drag is definitely not linear. A more aerodynamic car (the EV1 for instance) would experience the same thing, but not as bad as the Bolt because overall aero drag is lower. Take air out of the equation, and the Bolt would use just about the same power to maintain 90 MPH as it would 30 MPH (slightly more drag in the bearings and perhaps an efficiency drop in the motor because it's spinning faster).

So isn't that linear?

If you take out the effects of aerodynamic drag completely, based on what you are saying, shouldn't the Bolt EV go one third the range at 90 MPH than it can at 30 MPH?

That is not my understanding of the physics of pulling higher amounts of kilowatt-hours out of lithium ion batteries.

 

[PV1]

Think of it this way. Imagine wading through water vs. trying to run through water in a swimming pool. You can walk without much effort, but running is almost impossible. If drag was linear, then it would only be twice as hard to double your speed. However, you can feel that resistance increases much faster than your speed through the water. The same thing applies to the car moving through air (a fluid just like water), and this is where your power drain is at speed. The battery drains faster because the car needs exponentially more power to move through air faster.

The lack of air resistance allow satellites to orbit Earth for years with no need for continuous thrust (any thrust they do have is for tweaking their orbit and only used in short bursts). Momentum keeps them moving, and gravity keeps them in orbit.

Regarding efficiency of the batteries, there is some truth to this, but it applies more to lead-acid batteries than lithium ion. A smaller draw can siphon more energy out of a battery, but with the size of the Bolt's battery pack and the power draw used to maintain speed, the difference is so little that you'd never notice it. Power tools, though, will show this effect way more than a car will.

It takes a lot more energy to move air at 65 MPH than it does at 60 MPH. I can't remember the exact ratio, but it is far from linear (the difference between 75 and 70 MPH is much bigger than the difference between 65 and 60 MPH).

 

[redpoint5]

Take air out of the equation, and the Bolt would use just about the same power to maintain 90 MPH as it would 30 MPH (slightly more drag in the bearings and perhaps an efficiency drop in the motor because it's spinning faster).

Rolling resistance has a lot to do with tire deformation. Continuously bending the tire generates heat, which is linear with speed. Go twice as fast, and you get about twice the rolling resistance. Range would be roughly similar though since at twice the rolling resistance, you get to the destination in half the time.

People living on mars will get exceptional efficiency and range. WIth just 1% of the atmospheric pressure of Earth, aerodynamics isn't even a concern except for very fast moving vehicles.

 

[gpsman]

Why are peeps quoting & mis-quoting and reposting my post out of context over and over.

STOP IT!

 

[gpsman]

I believe the physics of the current state of lithium-ion batteries is such that the efficiency drops off above 52 MPH, so driving at 85 MPH in a Bolt EV will eat up your storage capacity like a hot knife through butter.

Has nothing to do with the physics or current state of the art Lithium-Ion batteries.

It has evertything to do with the aerodynamics of the car. Which in the Bolt's case is not great.

MichaelLAX specifically said that the efficiency of the lithium-ion batteries drops off above 52 MPH.

I wanted to take the BATTERY CHEMISTRY out of the equation, and out of this discussion. It is irrelevent where the Watts come from. It could be lead acid, nickel cadmium, nickel metal hydride, zinc carbon, or alkaline. It could be from a really long extension cord, bank of super capacitors, solar cells, or "Mr. Fusion" cold fusion reactor. A watt is a watt.

The efficiency of the batteries changes with temperature and age, not speed!

 

[redpoint5]

The efficiency of the batteries changes with temperature and age, not speed!

For the purposes of easy calculation, we assume that battery efficiency is constant, but that's not true. Batteries are less efficient the more rapidly they are discharged. This is why Ah ratings are given along with the discharge time or C rate. The same battery fully discharged at 2C (30 min), will have lower capacity than the same battery discharged at 0.5C (120 min).

So, while battery efficiency has nothing directly to do with speed, it does have to do with discharge rate, which goes up with speed. It's a very minor consideration compared to aerodynamic drag though.

 

[gbobman]

I wanted to take the BATTERY CHEMISTRY out of the equation, and out of this discussion. It is irrelevent where the Watts come from. It could be lead acid, nickel cadmium, nickel metal hydride, zinc carbon, or alkaline. It could be from a really long extension cord, bank of super capacitors, solar cells, or "Mr. Fusion" cold fusion reactor. A watt is a watt.

The efficiency of the batteries changes with temperature and age, not speed!

Nicely stated

 

[Anonymous]

I wanted to take the BATTERY CHEMISTRY out of the equation, and out of this discussion. It is irrelevent where the Watts come from. It could be lead acid, nickel cadmium, nickel metal hydride, zinc carbon, or alkaline. It could be from a really long Extension Cord, bank of super capacitors, Solar cells, or "Mr. Fusion" cold fusion reactor. A watt is a watt.

The efficiency of the batteries changes with temperature and age, not speed!

For the purposes of easy calculation, we assume that battery efficiency is constant, but that's not true. Batteries are less efficient the more rapidly they are discharged. This is why Ah ratings are given along with the discharge time or C rate. The same battery fully discharged at 2C (30 min), will have lower capacity than the same battery discharged at 0.5C (120 min).

So which of these two mutually exclusive statements is correct?

 

[WetEV]

I wanted to take the BATTERY CHEMISTRY out of the equation, and out of this discussion. It is irrelevent where the Watts come from. It could be lead acid, nickel cadmium, nickel metal hydride, zinc carbon, or alkaline. It could be from a really long Extension Cord, bank of super capacitors, Solar cells, or "Mr. Fusion" cold fusion reactor. A watt is a watt.

The efficiency of the batteries changes with temperature and age, not speed!

For the purposes of easy calculation, we assume that battery efficiency is constant, but that's not true. Batteries are less efficient the more rapidly they are discharged. This is why Ah ratings are given along with the discharge time or C rate. The same battery fully discharged at 2C (30 min), will have lower capacity than the same battery discharged at 0.5C (120 min).

So which of these two mutually exclusive statements is correct?

The second, by a tiny amount. The difference isn't large for Li ion cells at normal C rates. Capacity of a Li ion will be very slightly different at 2C or 0.5C. Effect is rather larger for lead acid, or for Li ion at very high discharge rates (like 10+C).

Air resistance is roughly proportional to velocity squared. The big effect.

 

[SparkE]

I wanted to take the BATTERY CHEMISTRY out of the equation, and out of this discussion. It is irrelevent where the Watts come from. It could be lead acid, nickel cadmium, nickel metal hydride, zinc carbon, or alkaline. It could be from a really long Extension Cord, bank of super capacitors, Solar cells, or "Mr. Fusion" cold fusion reactor. A watt is a watt.

The efficiency of the batteries changes with temperature and age, not speed!

For the purposes of easy calculation, we assume that battery efficiency is constant, but that's not true. Batteries are less efficient the more rapidly they are discharged. This is why Ah ratings are given along with the discharge time or C rate. The same battery fully discharged at 2C (30 min), will have lower capacity than the same battery discharged at 0.5C (120 min).

So which of these two mutually exclusive statements is correct?

The second, by a tiny amount. The difference isn't large for Li ion cells at normal C rates. Capacity of a Li ion will be very slightly different at 2C or 0.5C. Effect is rather larger for lead acid, or for Li ion at very high discharge rates (like 10+C).

Air resistance is roughly proportional to velocity squared. The big effect.

For some reason, the most important piece got cut out of a previous post (i.e., wasn't quoted) :

So, while battery efficiency has nothing directly to do with speed, it does have to do with discharge rate, which goes up with speed. It's a very minor consideration compared to aerodynamic drag though.

The big thing that cuts down on efficiency (for ANY vehicle) at higher speeds is aerodynamic drag.

 

[Anonymous]

So which of these two mutually exclusive statements is correct?

The second, by a tiny amount. The difference isn't large for Li ion cells at normal C rates. Capacity of a Li ion will be very slightly different at 2C or 0.5C. Effect is rather larger for lead acid, or for Li ion at very high discharge rates (like 10+C).

Air resistance is roughly proportional to velocity squared. The big effect.

For some reason, the most important piece got cut out of a previous post (i.e., wasn't quoted) :

So, while battery efficiency has nothing directly to do with speed, it does have to do with discharge rate, which goes up with speed. It's a very minor consideration compared to aerodynamic drag though.

The big thing that cuts down on efficiency (for ANY vehicle) at higher speeds is aerodynamic drag.

Most important to you, perhaps; because you are focus on the overall picture.

My question had to do with lithium ion chemistry: whether or not increased speed and hence increased KW draw has an effect on the usable capacity on a lithium ion battery: one answered yes, the other answered no!

WetEV answered there is a difference, but not by a large amount!

 

[oilerlord]

How to achieve 452.8 miles with a Tesla Model S on a single charge? Easy. Drive 24 mph. The trip only took 18 hours, 40 minutes. LOL!

https://www.youtube.com/watch?v=Z5W4LJ5zL9g

 

[redpoint5]

How to achieve 452.8 miles with a Tesla Model S on a single charge? Easy. Drive 24 mph. The trip only took 18 hours, 40 minutes. LOL!

Neat experiment.

I can cover that distance in 6 hrs, 30 minutes at 70 MPH in my Prius and burn only 9 gallons of fuel. I bet I could go 1000 miles on a tank of fuel if I drove 25 MPH. Perhaps even further since my engine would be off most of the time, only kicking on to periodically charge the battery.

 

[HotPotato]

Tony Williams has done the calculations. Here's all the details:

http://insideevs.com/planning-a-long-journey-with-your-chevrolet-bolt-ev-or-opel-ampera-e/

According to his graph, 75 mph is fastest---based on charging frequently but partially.

"When you leave your home or hotel, charge up all the way to 100% if you want to. But, you really want enroute chargers to be about 75-125 miles apart for the lowest overall travel time. The car will charge the fastest from 0% to about 65%. The strategy is to burn down the battery at your first charge location to 10-20% remaining (or lower, if you are adventurous) and then charge for 30-60 minutes to add 75-125 miles of range each time. In addition, you really only want to use the fastest chargers, and those that are the best are the ones that are 125 amps."

 

[redpoint5]

Tony Williams has done the calculations.

According to his graph, 75 mph is fastest---based on charging frequently but partially.

This is close to my calculated sweet spot of 71 MPH.

His comment on speed is:

don’t drive slowly between charging spots to save energy. Drive normal freeway speeds of 65-75mph (105km/h to 120 km/h) to get the fastest overall speed in your journey. Above 75mph is not recommended, due to the relatively poor aerodynamics of the Bolt EV.

For some people, 75 MPH is not normal freeway speed. I know that's slow on California interstates, and some states have 80 MPH speed limits.

 

[Anonymous]

This is close to my calculated sweet spot of 71 MPH.

The fastest (and safest) speed that gets you to the next DCFC is always best, as DCFC are outputting fastest when the energy remaining is lowest.

For some people, 75 MPH is not normal freeway speed. I know that's slow on California interstates, and some states have 80 MPH speed limits.

California always has a right lane for slower traffic!

In addition, there is the unofficial "California Courtesy" which is if you are driving the left lane at no matter what speed and someone approaches you at a faster speed from behind, you move over to the right and let them pass, and then move back to the left lane.

Not only does California have the best traffic engineers (for example, HOV lanes that interchange to HOV lanes), but, for the most part, the best drivers, too!

 

[oilerlord]

His comment on speed is:

"don’t drive slowly between charging spots to save energy. Drive normal freeway speeds of 65-75mph (105km/h to 120 km/h) to get the fastest overall speed in your journey. Above 75mph is not recommended, due to the relatively poor aerodynamics of the Bolt EV."

For some people, 75 MPH is not normal freeway speed. I know that's slow on California interstates, and some states have 80 MPH speed limits.

Further, I see this as the main takeaway from the article:

"Again, to have the lowest overall travel time, it is not advantageous to drive slower to get improved energy consumption, since the fast chargers can upload power so much faster than you can consume it."

Very interesting. This flies in the face of hypermiling in terms of driving like grandpa, and blowing the better part of a day sitting at chargers. It appears you actually need to drive a Joe Public 75 mph to arrive in the shortest time. Of course, that assumes the hypothetical that a public charger is available every 75-125 miles along the route, they all hypothetically charge at 125 amps/50kW, and are all hypothetically in working condition.

 

[Anonymous]

Very interesting. This flies in the face of hypermiling in terms of driving like grandpa, and blowing the better part of a day sitting at chargers...

No one ever suggests hypermiling as a means to get anywhere faster!

It is a means to stretch your distance between charges at the sacrifice of time!