Range for speed

[Nu2ecar]

BMWi3owner gave a power consumption at steady 120 kph. That's 75mph. An 18.8 kWhr battery would give a range of 62 miles at 75 mph under those conditions.

Stevei3 gave a power consumption at a steady 65 mph. An 18.8 kWhr battery would give 73 miles at 65 mph in those conditions.

Rclams measured battery depletion over 10 miles of driving at a steady 60 mph, and calculated the range of an 18.8 kWhr battery to be 63 miles.

It's silly to worry about too much about the third or fourth significant figures. YOUR MILEAGE WILL VARY. But measuring power in an electrical circuit is easy and accurate; it's simply volts times amps. Measuring the energy content of a battery is more difficult and subject to more error.

Until I get more authoritative estimates, I'm going to use 73 miles at 65 mph and 62 miles at 75 mph as my guide for the range of an i3. The battery might use extra power in an initial drive as it warms to optimum temperature. It will be use more in cold conditions and in headwinds. For me, this is the range is at the edge of acceptable. Some will find it completely unacceptable, others won't find it constraining at all.
Nu2Ecar

 

[ultraturtle]

Leaving aside the more technical comments that have been posted in the meantime, I was intrigued when I mulled over ultraturtle's posting on Sun to the effect that energy usage depended largely on speed rather than acceleration.

As I understand it, the two Eco modes not only limit top speed but also soften the throttle response - if acceleration was largely irrelevant why would the Eco modes limit the throttle response at all?

The overview of the ECO PRO modes in the manual (p 145) states "With ECO PRO and ECO PRO+ various settings are made in the relevant mode.... Limitation of speed to an adjustable maximum between 90km/h, 55 mph and 130km/h, 80 mph..." It lists only a limit on top speed, not on acceleration. In the "Principle" section of the introduction, however, it does state "ECO PRO and ECO PRO+ support an efficient driving style. To do this, the control of the drive is optimized for moderate acceleration..." There is good reason to control extremes of acceleration, as there is a slight falloff of efficiency beyond approximately 70% maximum torque for a typical EV motor. It's actually at a slightly lower threshold at lower speeds, and a bit higher (up to ~ 100%) at higher speeds, but 70% would be a prudent limit for these modes, if such an acceleration limit actually is programmed in. My point should have clarified that even 70% power gives pretty neck snapping acceleration, and the falloff in efficiency beyond it is actually pretty slight.

I have some numbers to back up the assertion.

After getting help from a bunch of PiP owners smarter than I, obtaining the actual efficiency curves for the motor, and making multiple test runs to measure, I found that there was effectively no loss in efficiency up to 70% max torque (see results about halfway down the page at http://priuschat.com/threads/step-on-it-have-some-fun.113916/page-3). Using this info, I made multiple 7.2 mile closed loops (to zero out effects of wind and terrain) at both quick acceleration and roughly half that acceleration. The end result was a 4% savings in power consumption at the milder acceleration rate, but at a cost of 3 mph average, and 12.7% of time. The lower average speed of the mild acceleration runs resulted in 22% less aerodynamic drag, which would explain pretty much all of the savings in power consumption.

Can't wait to hook up an OBD-II tool to an i3 and take some measurements! Gots to get my nerd on.

One of the first things I'd like to do with the car is to set up a protocol that accelerates madly to a lower top speed, and compare it to mild acceleration to a higher speed such that both loops take the same amount of time. If both runs consume nearly the same amount of energy, the point is better made.

 

[Nu2ecar]

Correction!


BMWi3owner gave a power consumption at steady 120 kph. That's 75mph. An 18.8 kWhr battery would give a range of 62 miles at 75 mph under those conditions.

Stevei3 gave a power consumption at a steady 65 mph. An 18.8 kWhr battery would give 73 miles at 65 mph in those conditions.

Rclams measured battery depletion over 10 miles of driving at a steady 60 mph, and calculated the range of an 18.8 kWhr battery to be 73 miles.

 

[surfingslovak]

Leaving aside the more technical comments that have been posted in the meantime, I was intrigued when I mulled over ultraturtle's posting on Sun to the effect that energy usage depended largely on speed rather than acceleration.

As I understand it, the two Eco modes not only limit top speed but also soften the throttle response - if acceleration was largely irrelevant why would the Eco modes limit the throttle response at all?

The overview of the ECO PRO modes in the manual (p 145) states "With ECO PRO and ECO PRO+ various settings are made in the relevant mode.... Limitation of speed to an adjustable maximum between 90km/h, 55 mph and 130km/h, 80 mph..." It lists only a limit on top speed, not on acceleration. In the "Principle" section of the introduction, however, it does state "ECO PRO and ECO PRO+ support an efficient driving style. To do this, the control of the drive is optimized for moderate acceleration..." There is good reason to control extremes of acceleration, as there is a slight falloff of efficiency beyond approximately 70% maximum torque for a typical EV motor. It's actually at a slightly lower threshold at lower speeds, and a bit higher (up to ~ 100%) at higher speeds, but 70% would be a prudent limit for these modes, if such an acceleration limit actually is programmed in. My point should have clarified that even 70% power gives pretty neck snapping acceleration, and the falloff in efficiency beyond it is actually pretty slight.

I have some numbers to back up the assertion.

After getting help from a bunch of PiP owners smarter than I, obtaining the actual efficiency curves for the motor, and making multiple test runs to measure, I found that there was effectively no loss in efficiency up to 70% max torque (see results about halfway down the page at http://priuschat.com/threads/step-on-it-have-some-fun.113916/page-3). Using this info, I made multiple 7.2 mile closed loops (to zero out effects of wind and terrain) at both quick acceleration and roughly half that acceleration. The end result was a 4% savings in power consumption at the milder acceleration rate, but at a cost of 3 mph average, and 12.7% of time. The lower average speed of the mild acceleration runs resulted in 22% less aerodynamic drag, which would explain pretty much all of the savings in power consumption.

Can't wait to hook up an OBD-II tool to an i3 and take some measurements! Gots to get my nerd on.

One of the first things I'd like to do with the car is to set up a protocol that accelerates madly to a lower top speed, and compare it to mild acceleration to a higher speed such that both loops take the same amount of time. If both runs consume nearly the same amount of energy, the point is better made.

Once again, acceleration is not just about rpms, motor efficiency or aerodynamic drag. Some serious heat develops in the battery when you discharge it at high currents, which happens when you "step on it". This energy is irrevocably lost. In terms of priority, the worst thing you can do is drive fast, the second worst thing you can do is use your the cabin heater, and the third worst thing you can do is do jack-rabbit starts. Trust me on this. I think this is getting overly analytical for no good reason. There is plenty of empirical data from comparable vehicles. That said, you might as well enjoy the car if you don't need the range.

 

[surfingslovak]

...The usable capacity of the battery (thanks very much agj) is 18.8 kWhrs. Thus the car could be operated for 18.8/22.7 hrs in the conditions of your test. 0.83 hrs at 120 kmph gives 99.38 km, which I rounded to 100 km.

So, this means you have about 60 miles of highway range at a steady 62 mph.

So, forget about enjoying that addictive "brisk acceleration" unless you're just doing a grocery run.

Meh.

This sounds incorrect to me. The i3 should have 73 miles of range at steady 65 mph. Something does not add up. How did you get the economy data you quoted above?

 

[surfingslovak]

I like the looks and interior of the i3, and I enjoyed my two test drives. The car is truly quick and handles well, but when I tested the steady state 60 mph range of the i3 on a flat freeway, it came out a little less than the results I got for the LEAF we had at the time.

The Test: We drove 10 miles in both directions on Hwy 101 (very flat) at exactly 60 mph (using the Navigon GPS app to set the speed; i3 was indicating 62 mph at 60 mph real, BTW). It was around 10am, with some very typical cross wind and the temperature was in the low 60's, IIRC.

The steady state 60 mph i3 range I calculated was 73 miles, assuming 18.8 kWhr of usable battery. Note that this "range" does not account for lights or heating, accelerating to highway speed, dicing with other cars & trucks or traversing hills (e.g., San Francisco).

So I reluctantly concluded that the "test case" of a nighttime winter round trip to San Francisco (which required some hypermiling to accomplish for us in the LEAF) would be barely possible in the i3.

So I withdrew my deposit.

We are now leasing a Chevy VOLT as we wait 3 years to evaluate the "Gen 3"/Bluestar Tesla.

This sounds similar in observed range to my 2013 Leaf @ a steady 60mph (77 miles) @ 40-45F, with the difference that the stretch of highway I tested on was not flat - there are a number of gradual, rolling hills. I tested over a 65 mile stretch. I'm interested to see how the i3 range compares over the same stretch.

Sorry, this looks suspect too. The LEAF gets about 82 miles at steady 60 mph when the battery is new. How did you measure this and how old was your LEAF at the time, if you don't mind? I cannot over-stress the importance of good process and accurate data.

 

[agj]

That said, you might as well enjoy the car if you don't need the range.

+1

 

[i3atl]

I like the looks and interior of the i3, and I enjoyed my two test drives. The car is truly quick and handles well, but when I tested the steady state 60 mph range of the i3 on a flat freeway, it came out a little less than the results I got for the LEAF we had at the time.

The Test: We drove 10 miles in both directions on Hwy 101 (very flat) at exactly 60 mph (using the Navigon GPS app to set the speed; i3 was indicating 62 mph at 60 mph real, BTW). It was around 10am, with some very typical cross wind and the temperature was in the low 60's, IIRC.

The steady state 60 mph i3 range I calculated was 73 miles, assuming 18.8 kWhr of usable battery. Note that this "range" does not account for lights or heating, accelerating to highway speed, dicing with other cars & trucks or traversing hills (e.g., San Francisco).

So I reluctantly concluded that the "test case" of a nighttime winter round trip to San Francisco (which required some hypermiling to accomplish for us in the LEAF) would be barely possible in the i3.

So I withdrew my deposit.

We are now leasing a Chevy VOLT as we wait 3 years to evaluate the "Gen 3"/Bluestar Tesla.

This sounds similar in observed range to my 2013 Leaf @ a steady 60mph (77 miles) @ 40-45F, with the difference that the stretch of highway I tested on was not flat - there are a number of gradual, rolling hills. I tested over a 65 mile stretch. I'm interested to see how the i3 range compares over the same stretch.

Sorry, this looks suspect too. The LEAF gets about 82 miles at steady 60 mph when the battery is new. How did you measure this and how old was your LEAF at the time, if you don't mind? I cannot over-stress the importance of good process and accurate data.

Suspect how (do you want it to have more or less range)? I measured it with the SoC display over a 65 mile stretch. I had the Leaf for 6 days before this test, but I don't know how long it was on the lot before I purchased.

 

[ultraturtle]

..., and the third worst thing you can do is do jack-rabbit starts. Trust me on this.

'Druther trust the data.

I think this is getting overly analytical for no good reason.

I mean no offense, but the whole point of this sort of analysis is to overcome stale, difficult to change ways of thinking.

Please don't forget the internal resistance of the battery. It's relatively low for lithium-ion cells, like the i3 uses, but not entirely negligible. High discharge currents when accelerating hard or climbing hills at a brisk pace will cause more heat development in the battery. The discharge process is quite efficient, but there are some losses, and they do increase when not driving moderately.

A point on which we agree, but let's look a little closer. Many folks interested in EVs come from a solar energy background where internal resistance of a given battery is a very big deal, and a significant factor in sizing a lead acid battery bank. The higher the discharge rate, the higher the internal resistance, the greater the heat generated, and the lower the total energy storage capability of the battery. The capacity of a lead acid battery discharged quickly, say in 2 hours, is roughly 58% of the capacity of that same battery discharged slowly, over the course of 100 hours. For this reason, lead acid battery spec sheets do not state a single ah capacity, but rather a broad range of them based on discharge rate, with most manufacturers standardizing somewhat on the 20 hour rate for comparison purposes. So, how does a wise EV engineer deal with internal resistance?

• 1. She chooses the right battery technology for the job. Ever wonder why Lithium Ion battery capacity is never stated at a "20 hour" rate but rather as a single number? It is because all of the factors (to include internal resistance) that might otherwise contribute to reduced capacity at high discharge rates are inherently so small that they do not significantly affect a Lithium Ion battery's capacity. Consider this comparison of AGM (a lead acid technology) to lithium ion:
  
  
  
  
  
  2. She chooses the best battery chemistry for the job. The NMC (Nickel-Manganese-Cobalt Oxide) chemistry BMW chose for the i3 has the lowest self heating rate of any Lithium Ion battery chemistry currently available for use. It is lower than the LOM chemistry used in the Nissan Leaf, Chevy Volt, and Tesla Model S. (http://batteryuniversity.com/learn/article/types_of_lithium_ion)
  
  3. She sizes a massive battery bank so that even at high total discharge rates, each individual cell is discharged relatively slowly. Ever wonder why a 60 kWh Tesla Model S accelerates more slowly than an 85 kWh model? Them Tesla engineers are some smart folks, and intentionally limit the maximum discharge rate of individual cells, making the power draw on each cell similar for the two cars even though they are capable of accelerating at different rates.
  
  4. She designs an effective thermal management system. You cannot do better than the active, liquid coolant system of the i3.
  
  5. She limits the maximum power draw. Ever wonder why power is limited to 170 hp and top speed is limited to 93 mph / 150 km/h? It ain't because it is significantly more expensive to make a more powerful motor. It is more likely to limit power draw on the battery bank.

That said, you might as well enjoy the car if you don't need the range.

Accelerating in an EV is pure joy, without the guilt of significantly reduced efficiency of the magnitude you would experience doing the same in an internal combustion engine powered vehicle . There are losses, but they are insignificant in comparison. Bottom line. Have some fun. Step on it. You will not significantly affect your range accelerating quickly to speed,

 

[rclams]

Sorry, this looks suspect too. The LEAF gets about 82 miles at steady 60 mph when the battery is new. How did you measure this and how old was your LEAF at the time, if you don't mind? I cannot over-stress the importance of good process and accurate data.

Remember, this was a brief Test Drive (with a line of people waiting their turn), not a multi-day, data-recorder evaluation. :roll:

Here's what I did:

January 25, 2014 (Sunny, Cool (55-60*F), Breezy from the Southwest):

A) We drove our nearly 3 year old, 11 Bar (32,000 miles, 1 capacity bar lost) LEAF Northbound on Hwy 101 to the BMW dealer. I set the cruise control to give 60 mph per the Navigon app (62 mph indicated). Once up to speed, I reset the Miles/kWhr energy display. Just before exiting the highway, I noted the LEAF display at 4.5 miles/kWhr.

B) We waited a bit for our i3 demo car, then my wife drove it back to Hwy 101 and we proceeded Southbound. The i3 center display had a Bar Graph showing kWhr/100km on 1 minute intervals. We set the cruise control to give 60 mph per the Navigon app (also 62 mph indicated). During a 5 minute interval at a steady 60 mph, the displayed energy consumption was about 17 kWhrs/100km (I took pictures of the screen with my iPhone).

C) We exited the highway and switched seats. I then proceeded Northbound, set the cruise at 60 mph per Navigon, and during a 5 minute interval at a steady 60 mph, the energy consumption was a bit less than before, about 15 kWhrs/100km.

D) With a two-way average of 16 kWhr/100 km, I get a two-way average of 3.9 Miles/kWhr at 60 mph (actual GPS speed). With the semi-official 18.8 kWhr of usable capacity, this suggests a range of about 73 miles under the conditions indicated (climate control was set on ECO as I recall).

E) We then returned home on Hwy 101 Southbound, with an indicated LEAF energy rating of 4.1 miles/kWhr in that direction at a true 60 mph. This gives the LEAF, in similar conditions, an average indicated energy consumption rate of 4.3 Miles/kWhr, about 10% better than the i3. (I assume that the odometer reading is more accurate than the indicated speed, subject to the wear on the tires.)

We were in the i3 for 27 minutes and drove a total of 14.3 miles. FWIW, the indicated "Average Energy Use Since Charged" was 16.9 kWhr/100km, and the Range Remaining display (working backwards and including some surface street travel) suggests a Full Charge range of 77 miles.

Literally, YMMV (your mileage may vary). Cheers.

 

[ecoangel]

http://bmwi3.blogspot.co.uk/2014/03/an-aerospace-engineer-from-uk-compares.html

I like the i3 but it is sadly not as aerodynamic as it could be:

"The Cd is 0.3 (about as bad as a 1992 Toyota Camry or 1993 Subaru Impreza). The i3 is much wider than most pure 4 seaters and the MPV styling and battery floor makes it quite tall. Frontal Area of 2.38 m2 x 0.3 results in a CdA of 0.714.

Compare this to a 2001 Audi A2 1.2 TDI : CdA 0.544 or 2013 VW XL1: CdA 0.279.

At 100km/h (62mph) the i3 BEV creates 326 Newtons of drag. The REX model a little more at 336 Newtons. The A2 a mere 257 Newtons.

More meaningful to the average punter is BHP absorbed by drag. Here we are not considering the drag from the drive-train / single gear or tyre friction and this is for a flat road with nil wind.

At 40 mph the i3 consumes a minimum of 3.33 BHP (2.5kW) in drag alone. Not a lot! But accelerate to 60 mph and it goes up a factor of 4 to 12.5BHP (9.317kW). Get on the freeway in Montana or de-restricted Autobahn and at 93 mph the i3 requires 42.14 BHP (31.44 kW)!

It is no wonder that i3 test drivers have noticed a massive reduction in range when driving on faster roads. If you add in tyre resistance, wheel well turbulence, and a less than optimal gear ratio (optimised for acceleration rather than cruising then expect about 60 mile range from a BEV and a bit less all electric from the REX).

What if BMW had optimised just the Aero side?

40mph:
A2: 2.54 BHP (1.894kW)
XL1: 1.30 BHP (0.964kW),

60mph:
A2: 9.52 BHP (7.10kW)
XL1: 4.88 BHP (3.64kW)

93 mph:
A2: 32.1 BHP (24kW)
XL1: 16.5 BHP (12.28kW)"