Efficiency or Range? You Can’t Have Both – But Advanced Technology Can Help

3 years ago by Rob Kasper 28

"BMW i3 REx that might best help illustrate why a smart means of increasing the range of an EV may not necessarily be to add more battery capacity."

“The BMW i3 REx that might best help illustrate why a smart means of increasing the range of an EV may not necessarily be to add more battery capacity.”

In the world of electric vehicles, whether Battery Electric Vehicles (BEVs) or Plug-in Hybrid Electric Vehicles (PHEVs), there is a clear trade off between range and efficiency.

For a given technology, efficiency suffers as range increases due to the weight of not only additional battery capacity, but the increased structure and volume to haul that capacity around. Now that there are a significant number of plug-in vehicles being manufactured, and a recognized standard to test them, we can identify trends.

Consider Table 1 and Figure 1, a plot of efficiency (as measured in EPA MPGe) vs. range in miles for 2014 plug-in electric vehicles measured by the EPA. They are grouped into Battery Electric Vehicles (BEVs) and Plug-in Hybrid Electric Vehicles, and further identified as either conventional or advanced technology design and construction.

Conventional technology is generally characterized by a manufacturer’s use of an existing gasoline powered platform modified for battery electric drive, steel frame construction and cladding, and standard battery technology. Advanced technology is generally characterized by a clean sheet, purpose built EV design, extensive use of aluminum or aluminum plus Carbon Fiber Reinforced Plastic (CFRP) for weight savings, higher energy density lithium ion battery packs, with the bonus of performance equivalent to or exceeding the best of conventional technology plug-in vehicles.

Efficiency vs Range

Efficiency vs Range

Table 1: EPA Electric Range and MPGe

Table 1: EPA Electric Range and MPGe

Beyond the obvious observation that the price of greater range is lower efficiency within a given technology, it is important to note the significance of advancing technology.

The ground-up EV design, significantly lighter weight construction, and advanced battery technology of the BMW i3 and Tesla Model S push the blue trend line significantly up and to the right of conventional BEVs’ green trend line.

As significant is the single data point (in purple) representing the only advanced technology PHEV currently available – The BMW i3 REx.  Not only is it capable of greater efficiency and far more range than any conventional PHEV (the red trend line), it is more efficient than all but two conventional BEVs, with only slightly less range than all but the most inefficient conventional BEVs.

It is this outlier of a data point, the BMW i3 REx that might best help illustrate why a smart means of increasing the range of an EV may not necessarily be to add more battery capacity.

Battery energy is clean and well suited for powering vehicles for relatively short-range transportation but due to its weight and lengthy charge times, inefficient and inconvenient for long distances. On the other hand, the benefits of energy density and convenience make gasoline/diesel energy better suited for longer range transportation with the trade-off being greater well to wheel emissions in many parts of the world.

In the case of the BMW i3 REx, each mile of range requires either 0.15 pounds of gasoline, or 5.7 pounds of battery capacity. At 37 times the mass specific energy density of battery power, very little gasoline is required to extend range for a given tank size, and that tank can be replenished in minutes nearly anywhere in the well developed fossil fuel infrastructure that currently exists worldwide.

This capability requires a 265 pound increase in the weight(3) which imposes a 6% decrease in efficiency, but once set, that efficiency does not appreciably decrease as more energy in the form of gasoline is added to increase range. Increasing battery capacity cannot increase range as efficiently, as not only must the weight of the battery increase by 37 times the weight of gasoline per mile in the first increment, but by the weight of increased structure and volume, as well as even greater battery capacity to offset the reduction in efficiency resulting from the weight increase. There comes a point where the sacrifice in efficiency may no longer be worth the additional range to be gained. See figure 2:

Figure 2: EV Energy Storage (and Generation) Weight vs Range for Advanced Technology EVs

Figure 2: EV Energy Storage (and Generation) Weight vs Range for Advanced Technology EVs

1 — EPA testing protocol does not account for approximately 4 miles of range remaining after REx fuel exhaustion when publishing a 72 mile battery powered electric range before REx activation, but does account for it in the total range calculation of 150 miles: 72 electric miles + 1.9 gal x 39 mpg + 4 electric miles = 150 EPA range (76 electric + 74 gasoline). 76 miles of range is also the result of dividing the EPA measured total i3 wall to wheel consumption of 22.0 kWh by the i3 REx EPA measured consumption rate of 0.288 kWh/mile. This value is further corroborated by the CARB BEVx designation awarded to the i3 REx which requires the electric range not only be at least 75 miles, but that it must exceed the gasoline range, neither of which would be possible without accounting for the ~4 miles of range remaining after REx fuel exhaustion.
2 — The EPA’s 95 MPGe rating of the Toyota Prius Plug-In Hybrid includes 0.2 gallons of gasoline operation plus 29 kWh of electric operation per 100 miles. Subtracting the 10 mile of gasoline operation contribution to the total (0.2 gal X 50 mpg) yields 29 kWh per 90 miles, or 32.2 kWh per 100 miles, which results in 105 MPGe for electric only operation. (MPGe = 33,705 divided by watt hours per mile.)
3 — While EPA rated at 87 miles of range in its base form, purchasers of the Mercedes-Benz B-Class can choose to pay an additional $600 for the Range Package, which makes an additional 17 miles of range available. There is no difference in total battery capacity between the two configurations, only the percentage of SOC made available to the driver.
4 — The 8 BMW battery pack modules weigh 55 lbs. each, for a total of 440 lbs. Reference page 17 of the BMW i3 Service Managers Workshop Participant Guide at http://darrenortiz.com/website_pdfs/ BMWi3PG.pdf.
5 — 265 lbs for the REx engine and all associated equipment is the difference in weight between the i3 BEV and i3 REx as published on BMW’s spec pages: http://www.bmwusa.com/Standard/Content/Vehicles/2014/i3/BMWi3/Features_and_Specs/BMWi3Specifications.aspx http://www.bmwusa.com/Standard/Content/Vehicles/2014/i3/BMWi3RangeExtender/
Features_and_Specs/BMWi3RangeExtenderSpecifications.aspx. Adding the 440 lb. battery weight makes the total energy production and storage weight at 76 mile of range 705 lbs. This increases by 11.4 lbs. of gasoline for every 74 miles driven beyond 76.
6 — Widely quoted in other sources, Car and Driver claims the Telsa Model S 85 kWh battery pack weighs 1,323 lbs: http://www.caranddriver.com/reviews/2013-tesla-model-s-test-review. This is exactly 600 kg, making it appear to be an estimate, but it is the only number we have to work with, as Tesla does not publish the spec.
7 — Weight of the 60 kWh Tesla Model S battery pack is estimated from the 85 kWh figure to be 60/85 X 1323 lbs. = 934 lbs.

Tags: , ,

28 responses to "Efficiency or Range? You Can’t Have Both – But Advanced Technology Can Help"

  1. jsmay311 says:

    With all due respect, this analysis exaggerates the relationship between range and efficiency.

    A perfect example of this is comparing the Volt and ELR. They use the same drivetrain and battery (except that the ELR uses more of the battery’s charge). So here we see a large difference in efficiency between 2 vehicles which is driven entirely by vehicle design — not battery size/weight. And yet, without the ELR’s point on the above chart, the negative slope of the EREV trendline becomes much flatter.

    Also, in the “conventional BEV” trendline, the slope relies very heavily on the RAV4 EV and the BYD e6, one of which is CUV and the other a poorly engineered Chinese vehicle. If you look only at the compacts and sedans from more respected/mature auto companies, the negative trendline almost disappears.

    Obviously, larger batteries = heavier vehicles = lower efficiency. But general vehicle design is just as important as using lightweight materials for boosting efficiency. Again, look at the Volt vs ELR — or, even more strikingly, the Leaf vs the Smart fortwo ED. The Smart weighs ~35% less than the Leaf and 19% less range, yet it also has 6% LOWER efficiency.

    Vehicle design is key.

    1. It exaggerates the relationship between weight and efficiency.

      Aerodynamic drag is far more important that weight in electric cars, in particular. On a typical car, the aero drag at just 28MPH is more than half of the load on the drivetrain.

      Aero drag is a total loss. Always.

      Weight / moving mass on the other hand can be “regained” in two ways: coasting is the best way to move the car forward with no additional energy. And regenerative braking is great for when you want to slow the car down.

      Case in point is the Illuminati Motor Works ‘7’ – it weighs 2,900 pounds and yet it goes 220+ miles on just a 33kWh pack.

      With a high efficiency drivetrain, and low aero drag, you can get longer range from a smaller battery. And a smaller battery costs less and weighs less, too.

      1. Peder says:

        very well said.

    2. Just_Chris says:

      You are right about the conventional bev line being skewed by 3 vehicles. I would rather plot the data comparing like for like so lets put the Tesla drive trains together and the i3 with the conventional BEV’s (since it has a similar battery pack to the others) and ignore the e6 which is based on a totally different battery chemistry. Re-draw the lines and what happens? The Merc and the Rav 4 suffer because of their shape with the model-s doing much better with the same drive train. The smaller battery pack gives better MPGe is this a real effect or an effect of the test procedure? All of the Tesla drive trains are less efficient which I suspect is due to the way they do the battery management and probably power management, motor type, fatter tires, etc.

      What is happening in the conventional BEV graph now? oh, as range increases efficiency goes up! God only knows why this is the case but it is unlikely to be anything to do with pack weight.

      As for comparing the i3 battery with the model S battery and drawing a curve WTF? the relationship between what BMW include in their battery pack weight and what Tesla include in their battery pack weight is irrelevant, batteries are modular you want to go twice as far you put twice as many in, linear, you may save some weight through reduce packaging by making the cells bigger but really not that much.

      Overall if you plot all this data on a graph with a range of conventional hybrids and petrol cars you find that it is pretty much that all ev’s sit in a little cluster at around twice the best conventional cars.

  2. Dr. Kenneth Noisewater says:

    I’d add a couple of columns to that: Mass and CdA.

  3. Ocean Railroader says:

    What this shows is that a 5% to 25% raise in energy density can really shed the pounds for a EV.

    I remember reading a story that a guy took a old EV that he had made in his garage with lead acid batteries. He replaced the lead acid batteries with lithium and ended up dropping 60% of the car’s weight while keeping the same range.

  4. Holger says:

    Statements like “each mile of range requires either 0.15 pounds of gasoline, or 5.7 pounds of battery capacity” are grossly misleading as you need a 330 lbs additional machine to convert gasoline into a usable form, whereas the battery supplies it ready for use.
    More telling is the figure 2… only 20% improvement in specific energy is needed to be on par.

    1. scott franco says:

      +1

    2. ultraturtle says:

      The full statement “In the case of the BMW i3 REx, each mile of range requires either 0.15 pounds of gasoline, or 5.7 pounds of battery capacity.” should not be misleading at all. It is factual statement that the i3 REx (which already has the 265 lb REx system installed) can travel one EPA mile on 0.15 pounds of gasoline (1 mile divided by 39 mpg times 6 pounds per gallon) or 5.7 pounds of battery (87% [useable capacity] time 440 lbs divided by 76 miles). These numbers come straight from the manufacturer and the EPA.

  5. DonC says:

    Because of regen, aero and drive train efficiency are far more important than mass. Mass will effect performance. Range not so much. Even electrical loads are more important than mass.

    This is why the cost/mass trade-off doesn’t favor the use of aluminum much less carbon fiber bodies in EVs. Tesla isn’t making a $45K car out of aluminum for a reason.

  6. Gadge says:

    What is missing in this article is any discussion ‘driving style’ has on efficiency. My 2014 LEAF is currently getting 5.5 miles/kWh as a result of conservative in city driving using B/ECO mode. Calculating MPGe with the EPA formula factors out to be 185!

    1. Brian says:

      I don’t follow your math. First of all, when you say 5.5 miles/kWh, are you talking about wall-to-wheels (i.e. including charger efficiency) or battery-to-wheels (i.e. what is reported by the car itself). If it is the latter, with 21kWh usable in the Leaf, that only works out to 115 miles. If it is the former, to get 185 miles, you need 8.8 miles/kWh battery-to-wheels. This seems impossibly high to me. It also means that your charging efficiency is only 62%! You should be getting more like 75% on L1 or 85% on L2.

      1. Brian says:

        Disregard my comment – I misread yours as 185 miles range, but reading it again, clearly you said 185MPGe.

  7. ultraturtle says:

    “Aerodynamic drag is far more important that weight in electric cars, in particular”

    For some reason many folks have come to believe that electric cars are somehow subject to different laws of physics than gasoline cars. Not sure where that came from, but perhaps it is simply a perception, as EV drivers are consumed with range – and efficiency as a means of extending it, whereas ICE vehicle drivers are generally far less focused on either.

    As for the claim that aerodynamics are more important than weight, the facts simply do not support it. At a CdA of 6.2 sq ft, such a claim would predict the Tesla Model S to be 24% more efficient than the BMW i3 REx, at 7.4 sq ft. At 117 MPGe, however, the EPA measures the BMW i3 REx to be 31% more energy efficient than the Tesla Model S at 89 MPGe. This measurement is right in line with the rule of thumb that percent energy consumption increases by roughly half of the percentage increase in weight – At 4,674 lbs, the Tesla Model S is 61% heavier than the BMW i3 REx’s 2,899 lbs, so one would expect it to increase consumption by about half that percentage, and at a 31% increase it does.

    1. RussB says:

      The idea that EVs benefit less from mass savings comes from a comparison with ICEVs. A 100kg mass savings in an ICEV (petrol) will net between .15 and .30 l/100km fuel savings. That same 100kg mass savings in a BEV will get you only about .05 l(equivalent)/100km savings. These numbers look similar on a percentage basis because the BEV is so much more efficient to begin with, but it is the absolute savings that counts.

      This is why, from a total energy perspective, it doesn’t make sense (as DonC says above) to use lots of aluminium and CFRP in BEVs. It gains you some range, but it costs you a lot of energy.

      As for aero being more important than mass, the relationship is also somewhat exaggerated in a BEV. The aero difference between the 6.2 and 7.4 CdAs you mention is equivalent to a mass difference of about 200kg. That is admittedly a pretty big aero difference, but 200kg is a pretty significant mass difference, too.

      1. ultraturtle says:

        Just reporting the facts. As much as I would like Tesla’s Model S numbers to be reported as more efficient (because I really, really want to justify such a purchase), I cannot. Fudge as you may, there is simply no way that one can make the efficiency case of a Tesla over any but the most impractical options available. All three current Tesla models rank in the bottom 25% of EPA efficiency.

        I’m not saying that the Tesla Model S is not the best possible option for those poor bastards that have to deal with their poor life choice of driving 3 or more hours every day between charging opportunities. I’m just saying that potential purchasers of that vehicle would be smart to draw a 110 mile radius around their home charging station, and ask themselves if they either can find and are willing to wait 3 or 4 hours for the charge opportunity to venture beyond that radius, or would be willing to settle for a gasoline range extender, thereby guaranteeing that they will always have the potential to drive more EV miles than any silly BEV “purist”.

      2. ultraturtle says:

        “The idea that EVs benefit less from mass savings comes from a comparison with ICEVs. A 100kg mass savings in an ICEV (petrol) will net between .15 and .30 l/100km fuel savings. That same 100kg mass savings in a BEV will get you only about .05 l(equivalent)/100km savings.”

        So you are saying that the laws of physics do not apply equally to ICE vehicles and EVs? An equal increase in mass results in 3 times the energy used to move one vehicle vs the other simply because of the difference in powertrain?

        I cannot follow the logic. Please post a source.

        1. RussB says:

          I must not have explained very well, because the logic is pretty simple. There are two primary reasons that you get less benefit from reducing mass in a BEV than in an ICEV:

          The BEV can take advantage of regenerative braking, so while a lighter car requires less energy to accelerate, there is also less energy available to be regained when braking.

          The BEV is much more efficient to start with. It takes the same amount of energy at the wheels to move a BEV and an ICEV of equal mass (ceteris paribus), but you have to put far more energy into the ICEV to get that energy out of the powertrain and to the wheels.

          The idea that there is less benefit to lightweighting a BEV vs an ICEV is not at all controversial.

          Here is one of many studies describing this. Note that when viewed as a percentage, the mass effect looks quite similar, but the difference is obvious on an absolute basis.

          http://avt.inel.gov/pdf/prog_info/SAEWorldCongress2013MassImpactPaper.pdf

          1. ultraturtle says:

            “The BEV can take advantage of regenerative braking, so while a lighter car requires less energy to accelerate, there is also less energy available to be regained when braking.”

            EVs with a Range Extending engine (REx) are the comparison here, not antiquated ICE only vehicles. Both have equal capacity to regenerate energy upon braking, so this point is not valid.

            “The BEV is much more efficient to start with. It takes the same amount of energy at the wheels to move a BEV and an ICEV of equal mass (ceteris paribus), but you have to put far more energy into the ICEV to get that energy out of the powertrain and to the wheels.”

            Comparison is tank/battery pack to wheels. While the electric drivetrain typically operates in the range of 90% efficiency while the gasoline drivetrain operates in the 20% range, it matters not. From the energy source to the road, an i3 REx requires either 0.15 pounds of gasoline, or 5.7 pounds (37 times the weight of gasoline) of battery capacity to move it one mile. Efficiency of conversion has nothing to do with the argument presented.

            “The idea that there is less benefit to lightweighting a BEV vs an ICEV is not at all controversial.”

            I agree. The proposition is too absurd to be taken seriously.

            1. RussB says:

              Sorry, I was responding to what you wrote, not what you were thinking 😉

          2. ultraturtle says:

            You might want to actually read the study, which in the abstract clearly states my case and refutes yours:

            “The results of the testing and analysis showed that for a given vehicle, the road load shows a slightly non-linear trend of decreasing road load with decreasing mass. This trend appears to be consistent across vehicle powertrain architectures (i.e., conventional powertrain, hybrid electric, or all-electric).”

            or put more succinctly from page 10 of the slide show of the same study at http://energy.gov/sites/prod/files/2014/03/f10/vss074_francfort_2012_o.pdf:

            “The mass impact on vehicle road load appears to be independent of vehicle powertrain technology…”

            1. RussB says:

              We appear to be talking about two different things – road load (you) and the energy consumption needed to meet that load (me).

              Just in case anyone else is still following this thread 🙂 and does not understand the difference, road load is the amount of energy needed at the wheels to move the vehicle, and the energy consumption of the vehicle is how much energy you have to put in to the powertrain to get out enough energy to meet the road load.

              My initial comment was a response to that part of your comment about not knowing why people think a mass change affects an electric vehicle differently than a gasoline vehicle. My point is that this thinking arises from the fact that, in terms of energy consumption, EVs do respond differently to mass change than ICEs. I don’t think that anyone is arguing that the road load that must be met is significantly different, but it is not the road load that matters. It is the amount of energy that you have to put into the car to meet that load that matters. This is why it is a question, ceteris paribus, of powertrain efficiency.
              If I need 2 joules at the wheels to meet the road load, with an ICE (20% efficiency) I have to put 10 joules in the tank. With a BEV (90% efficiency) I only need to put in 2.2 joules. If I reduce the mass of both vehicles such that the energy I need to meet the road load goes down to 1 joule, I now need to put 5 joules into the ICE and 1.1 joules into the BEV. The same mass reduction has saved me 5 joules in the ICE, and 1.1 joules in the BEV.
              So if the technology that allowed me to get that mass change (i.e. CFRP) costs 4 joules, it does not make sense to use it on the BEV, but may make sense on the ICE.

              Again, maybe we are talking about two different things, but this is all I was trying to say.

  8. protomech says:

    “Efficiency or Range? You Can’t Have Both”

    Tell that to these guys:
    http://insideevs.com/tag/sunswift/

    It’s true that adding additional weight by way of a larger battery can slightly hurt efficiency at the same time it dramatically improves range – when comparing within the same vehicle design. But if you compare across vehicle designs, improved efficiencies through powertrain engineering or improved aerodynamics will also improve range, to the point that you can have a vehicle with significantly higher range and efficiency like the Sunswift car.

    Imagine dropping a LEAF or i3 powertrain into the 1st generation Honda Insight – an aero-first design that was willing to compromise significantly for improved efficiency. Compared to the host vehicle, both range and efficiency would go up dramatically.

    Now build one of those with newer materials technology like the i3.. instead of a CUV..

    1. ultraturtle says:

      “adding additional weight by way of a larger battery can slightly hurt efficiency at the same time it dramatically improves range”

      I’ll agree with that statement only to the point supported by facts: “adding additional weight by way of a larger battery can massively hurt efficiency at the same time it minimally improves range””

      1. RussB says:

        Actually, a mass change has a much larger affect on range than on efficiency, on a percentage basis (have to use percentages here, of course). As you rightly mention above, a 1% mass change will get you about a .5% efficiency change. The same mass change will get you about a 1% change in range.

        1. ultraturtle says:

          “a 1% mass change will get you about a .5% efficiency change. The same mass change will get you about a 1% change in range.”

          Pretty much the point of the article. Your assertion that a 60% increase in range results in a 30% increase in consumption (or equal decrease in efficiency) helps demonstrate a point of diminishing returns for adding battery weight.

          1. RussB says:

            Again, just responding to your statement that adding mass changes efficiency “massively” while changing range only “minimally”.

            1. ultraturtle says:

              “Again, just responding to your statement that adding mass changes efficiency “massively” while changing range only “minimally”.”

              And I do appreciate that you finally realize this to be the case.