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