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The urge for better batteries

Part 1: Size and price requirements

Today, battery electric vehicles (BEVs) constitute a negligible share (<1%) of the total global passenger car market. This is in spite of public awareness about the environmental benefits an electrification of road transport could bring. The current generation of BEVs are propelled by energy stored in lithium-ion battery packs (LIBs) and therefore have zero tailpipe emissions. This unique selling point, however, seems to be clouded by the fact that the price tag is high, and the driving range is limited compared to conventional internal combustion vehicles (ICVs). Can the battery technologies of the (near) future help increase the competitiveness of electric vehicles?

 

To answer this question, we need to gain a deeper understanding about:

 

  • The size and price requirements for a battery in order to propel an electric vehicle with the same technical maturity as a conventional car
  • The potential of new battery technologies to take us beyond current limits

 

Here, we’ll focus on size and price requirements. The potential of new battery technologies will be explored in Part 2 of this article series.

Measuring the mobility power of a vehicle

We recently introduced a technical index called the Mobility Diffusion Coefficient (MDC), which is the algebraic product of the driving range and max-speed of a vehicle. This coefficient enables us to more accurately juxtapose BEVs with ICVs from a technical and price point of view.

 

MDC is simply a rough measure of the mobility power of a vehicle. Vehicles characterised by a higher MDC can drive a longer distance in a shorter period time with a full tank or a fully charged battery. While this index correlates with the engine and tank size of an ICV, in a BEV, it is mainly determined by the energy capacity of the battery pack, which is reported in kilowatt-hours (kWh).

The MDC of battery electric vs. internal combustion vehicles

To better understand the importance of MDC, let’s analyse a group of BEVs and ICVs from the passenger car market in the year 2016-2017.

 

In a recent study, we built a sample composed of 13 different BEV/ICV pairs, together with three BEV models from Tesla (S-60D, S-75D, and S-90D). In each of the 13 pairs, technical specifications (speed, torque, number of seats) were closely shared between the BEV and ICV, and both vehicles were selected from the same car manufacturer: Smart, Peugeot, Citroen, Chevrolet, Nissan, Volkswagen, Fiat, Renault, Kia or Ford.

 

Unsurprisingly, we found a decent correlation between the MDC of the BEVs and the size of battery packs. This correlation is visualised with the help of a contour map in Figure 1. BEVs (white circle) and ICVs (white squares) from our sample set together with Tesla cars (white diamonds), are scattered on this contour map. The MDC is colour coded according to the colour bar on the right side of the figure. Every black solid line corresponds to a different battery pack size and helps us to size the battery for a desired combination of maximum speed and driving range.

 

Explaining figure 1.

An approximate contour plot for the capacity of a LIB battery-pack in a mid-size BEV as a function of driving range (km) and maximum speed (km/h). Black solid lines are the iso-cap lines for the capacity of the current generation of LIB packs, and the MDC coefficients are colour coded according to the colour bar. Two rectangular zones (dashed–dotted) define the technical boundaries for the two groups of potential BEV customers, i.e., early adopters (zone 1) and late majority adopters (zone 2). The BEVs (white circles) and ICVs (white squares) from our sample, together with Tesla cars (white diamonds), are superimposed on the map for comparison. ( Source: M. Safari, Energy Policy, (2018) 115: 54-65.)

 

The rectangular zones 1 and 2 represent the technical-satisfaction ranges for the mainstream BEV drivers and ICV drivers, respectively. These two zones are distinguished by a significant difference (> 80%) in their characteristic MDC coefficients which, in turn, reveals a clear contrast in mobility culture between the two groups of drivers. Zone 1 represents the vehicles of choice for true environmentalists, conspicuous consumers, and tech-savvy drivers—namely ‘early adopters’— who are willing to overlook the speed/driving-range limitations of BEVs for other reasons. Zone 2, on the other hand, typifies a group of drivers who do not want to give up on any of the technical attributes of ICVs, plus those who cannot afford the price premium of a BEV—namely ‘late majority’ adopters. 

 

Figure 1 suggests that the battery size for the electric vehicles of choice among the two groups of early adopters and late majority adopters are 22 and 79 kWh, respectively. If assembled with the current generation of lithium-ion batteries, battery packs of such sizes would cost $6,000 and 20,000 given the current estimations for the price of lithium-ion batteries, i.e., ~250$/kWh.

Are there any prospects for developing cheaper and more energetic batteries in the future?

The next generation of batteries

Extensive research is currently underway to develop the next generation of batteries. Batteries that are less expensive, safer to use, and more environmentally friendly than current state-of-the-art lithium-ion batteries. In the next part of this series we will discuss how the use of new battery chemistries will enable the much-needed batteries of the future. 

 

References:

  1. https://sciencetrends.com/battery-electric-vehicles-new-mobility-mindset-or-better-batteries/.
  2. Safari, Energy Policy, (2018) 115: 54-65

 

Author

Mohammadhosein (Momo) Safari – Associate Professor, Department of Engineering Technology, Hasselt University & EnergyVille, Thor Park 8310, BE-3600 Genk, Belgium

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