Range for speed

There are many discussions about what accessories to select to keep power consumption to a minimum (and boost range). LED, heat pump, chargers… Sorry to disappoint all speed lovers, but keep your right foot light on the pedal if you want range.

With some simple testing I found that the power consumption is about 25% higher in 100 km/h (62 mph) compared to 80 km/h (50 mph) and more than 50% higher in 120 km/h (74 mph) compared to 80 km/h.

To put it in range terms. If you have energy in the car to go 100 km in 120 km/h you will be able to increase the range to 150 km if speed is reduced to 80 km/h.

Read more about the test and results on bmwi3owner.com - Range for speed

Great post!

To complete the picture (and if the cruise control and test area allows it), would it be possible to also add the following data points to your graphs?

Usage at 40km/h
Usage at 60km/h
Usage at 140km/h

Enjoy your new car!

I will try to find a road around Stockholm that will allow me to go 40 km/h and 140 km/h. Maybe it will be a separate low speed test. I would guess that the difference is smaller.

bmwi3ownercom said:

Read more about the test and results on bmwi3owner.com - Range for speed

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Excellent!!

bmwi3ownercom said:

If you have energy in the car to go 100 km in 120 km/h you will be able to increase the range to 150 km if speed is reduced to 80 km/h.

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As a very rough rule of thumb, the i3 has "a number" from which to estimate range, and that number appears to be about 225 (km) or 140 (mi). Subtract speed from "the number" to get range, or subtract range from "the number" to get speed.

It is a rough, but still useful estimation, only works within a limited range of practical speeds, and "the number" will decrease with outside air temperature, air conditioner use, and age/cycle related battery capacity degradation.

Also, it is encouraging to note that the i3 is more aerodynamically slick than I would have thought. 120 km/h aero drag increases 125% vs 80 km/h. yet consumption increases only 50%.

Thanks For that very handy to know

Thanks BMWi3owner, the power used at 100km/hr is what I've been looking for. Using the the given battery capacity of 18.8 kWhr gives a range of 100 km as you say. Is 18.8 kWhrs the absolute capacity of the battery? I believe the car shuts down before the battery is completely drained, and Tesla recommends their batteries should in general only be charged to 80-90% of capacity. 100 km at 120 kph is the minimum acceptable range for me. If the practical capacity of the battery is any less than 18.8 kWhr, the i3 is definitely not for me.

Nu2ecar said:

Thanks BMWi3owner, the power used at 100km/hr is what I've been looking for. Using the the given battery capacity of 18.8 kWhr gives a range of 100 km as you say. Is 18.8 kWhrs the absolute capacity of the battery? I believe the car shuts down before the battery is completely drained, and Tesla recommends their batteries should in general only be charged to 80-90% of capacity. 100 km at 120 kph is the minimum acceptable range for me. If the practical capacity of the battery is any less than 18.8 kWhr, the i3 is definitely not for me.

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18.8 is the "usable" capacity. The official absolute battery capacity is 22.2 kWh. So when proceeding to a "full" charge of 18.8 you are actually charging at 81%. So it is impossible to totally charge or totally drain the battery, in order to preserve its longevity.

However, if you take a look at http://www.mybmwi3.com/forum/viewtopic.php?f=14&t=632 , you'll notice that a Swiss user has been able to drive 143 km on one charge with a 7% juice left in the battery and an average consumption of 12.9 kWh/100 km. If I'm not mistake and if the i3's reading are reliable, this may suggest that the real-real usable capacity would be 19.7 kWh. I guess that though it is possible, it would not be a good idea to go that far into the real battery capacity.

Just a clarification. I am NOT saying that 100 km is what you can expect if you travel at 120 km/h. It can be more or it can be less. I do not know and it will depend on temperature, wind, topography, road conditions, etc. To give an answer to that would require a lot of data collection and result in an average answer.

What I did test was the relative increase in power consumption with increased speed. With same temperature, wind, etc. To make the example more concrete the figures were then applied on a range example. IF/ASSUMING you have energy in the car to go 100 km in 120 km/h...

Just so there is no misunderstanding.
That said, the range example is probably not very far from reality...

BMWi3owner, you gave the power consumption at 120 kmph at 22.7 kWhr. 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. Yes, temperature, wind, rain, gradient, load, famine and pestilence might change power consumption and thus the range.

Agj, or anyone, the Tesla recommends charging to around 80% capacity, but allows for charging up to 100% of capacity. They warn that charging to full capacity on more than rare occasions might damage the battery, and regenerative braking is unavailable when the battery is fully charged. I like that flexibility. Is it available in the BMW? Can one charge to more than 81%? Or is it forbotten because it is not permitted?

Finally, if 81% is effectively a full charge, what happens as the battery ages and its capacity decreases. Does the battery get charged to 81% of the reduced capacity? Or does the effectively full charge stay at a constant number of kWhrs?

Hi,

about the battery: BMW has done us the favour of stating the usable capacity (18.8 kWh) of the battery, where other makers seem to state total capacity. They use the remaining 15% of the 22 kWh battery not only as headroom to protect the battery from being fully charged (max. 90-95%), but also as safety cushion for an empty battery, so the battery is never below 5-10% real state-of-charge.

You cannot use this 'safety' capacity in any way, wether charging or discharging. Our friend had, after contacting BMW assistance, to have his BEV transported on a flatbed when his battery ran out, just 4 km before reaching his charge point. (Note: 0 km of range on the display really means it's empty and it will cease to operate).

Steven

Stevei3 said:

BMW has done us the favour of stating the usable capacity (18.8 kWh) of the battery, where other makers seem to state total capacity. They use the remaining 15% of the 22 kWh battery not only as headroom to protect the battery from being fully charged (max. 90-95%), but also as safety cushion for an empty battery, so the battery is never below 5-10% real state-of-charge.

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Note that this is not quite correct. Although I'm drawing this comparison from the LEAF, other EVs are designed similarly. There is a fairly substantial cushion at the top, that's true. It amounts to about 5%. The bottom does not have a large cushion. I made the same mistake like you did when I got my LEAF, which was one of the first in the US, and assumed about 10% buffer at the bottom. In reality, it's more like 2 to 3%, which are locked away after the car stops dead in turtle mode. So where did the additional 5-7% go? One word: losses. The battery is not perfect, and the round-trip of charging and discharging will yield noticeable losses due to internal heat development and some undesirable parasitic chemical reactions.


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Also note that the rated battery capacity is not 22 kWh. Instead, it's 21.6 kWh per manufacturer specification. I know that you mean well, and the enthusiasm on your blog is contagious. That said, it's quite important to post correct data. I don't hesitate to admit if I don't know something or had to make an assumption. It helps the reader understand better where the statement might be coming from.

More often than not, together we can figure out things in greater detail, especially when more people pitch in and collaborate.

Speaking of which, I took the liberty of creating a shared Google Doc. Please feel free to fill in the energy economy figures you have measured into the m/kWh row. That's row 7, which is also highlighted in green. Hopefully, we can make this a collaborative endeavor, and you will find the progress as useful and exciting as I do.


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surfingslovak said:

That said, it's quite important to post correct data.

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Hi,

I totaly agree and stand corrected, sincere thanks for this.

I was still using the data from the press kit of september 2013: "The high-voltage battery in the BMW i3 consists of eight modules (each with 12 individual cells), which together produce a rated voltage of 360 volts and generate approximately 22 kilowatt hours of energy.".

Interesting battery facts ! Learning something new every day.

Thanks for sharing and kind regards, Steven

Stevei3 said:

I totaly agree and stand corrected, sincere thanks for this.

I was still using the data from the press kit of september 2013: "The high-voltage battery in the BMW i3 consists of eight modules (each with 12 individual cells), which together produce a rated voltage of 360 volts and generate approximately 22 kilowatt hours of energy.".

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Happy to help, glad you didn't take it the wrong way. Press releases often take some liberties with engineering data. We found that the datasheets from the manufacturer were most reliable. The Samsung earnings report I referenced above is probably better than a PR from BMW AG. A data sheet from Samsung or Bosch would be even better than the earnings report, as it would have detailed and more accurate data.

Nu2ecar said:

...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.

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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.

rclams said:

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

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Nope. Not at all. The real beauty of electric vehicles is that their motors are nearly equally efficient across their entire range of power, as opposed to internal combustion engines, which output more atmospheric garbage than power when floored, particularly at low speed. The difference in energy consumption between flooring an EV to accelerate to a given speed, and gradually accelerating to that same speed is almost entirely explained by the aerodynamic drag of being at that higher speed for a greater amount of time. If you want to restate the observation to "forget about that addictive "high speed cruising"..." it would be more accurate. Accelerate as quickly as you like, guilt free, and with little worry about impacting range. Just don't accelerate to a very high speed and maintain it for long.

ultraturtle said:

rclams said:

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

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Nope. Not at all. The real beauty of electric vehicles is that their motors are nearly equally efficient across their entire range of power, as opposed to internal combustion engines, which output more atmospheric garbage than power when floored, particularly at low speed. The difference in energy consumption between flooring an EV to accelerate to a given speed, and gradually accelerating to that same speed is almost entirely explained by the aerodynamic drag of being at that higher speed for a greater amount of time. If you want to restate the observation to "forget about that addictive "high speed cruising"..." it would be more accurate. Accelerate as quickly as you like, guilt free, and with little worry about impacting range. Just don't accelerate to a very high speed and maintain it for long.

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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.

ultraturtle said:

rclams said:

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

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Nope. Not at all. The real beauty of electric vehicles is that their motors are nearly equally efficient across their entire range of power, as opposed to internal combustion engines, which output more atmospheric garbage than power when floored, particularly at low speed. The difference in energy consumption between flooring an EV to accelerate to a given speed, and gradually accelerating to that same speed is almost entirely explained by the aerodynamic drag of being at that higher speed for a greater amount of time. If you want to restate the observation to "forget about that addictive "high speed cruising"..." it would be more accurate. Accelerate as quickly as you like, guilt free, and with little worry about impacting range. Just don't accelerate to a very high speed and maintain it for long.

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OK, sure, enjoy your 0 to 30mph drag races off of lights & stop signs.

But brisk/fun bursts to enter freeways or to pass any cars or trucks going even slower than your 62 mph average highway pace will definitely eat into your 60 miles of highway range.

As pointed out above, even the battery itself has some resistance to current flow, as does the power cabling, the motor controller and the motor.

==> You will likely double your amps when accelerating, which means that resistance losses will be 400% higher than your steady state losses. :!:

=======

A "BMW Drive" of interest to me would be a winding country road with some tight turns and elevation changes (e.g., Highway 9 just west of Silicon Valley). After slowing for the curves, you will be accelerating back up to speed and using lots of Amps to do that in a sporting mode.

My guess is that you would not be able to get from Santa Cruz, CA to Saratoga, CA in an i3 in a "brisk, mostly legal, BMW Style".

That's 36 miles, by the way.

====================

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.

rclams said:

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.

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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.

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?