Electric Vehicle (EV) Characterization & Battery Development for Transportation

The transport industry is undergoing a revolution in vehicular power sources. As petroleum and road tax prices rise, manufacturers are increasingly turning to low carbon vehicles in order to meet the demands of their target markets and safeguard their businesses against oil depletion. The fully electric vehicle (FEV) has become the focus of development for many large manufacturers. However, the current battery technologies upon which the vast majority of such machines are based are approaching their maximum theoretical specific energy.

Euan McTurk, triple degree medallist in the Renewable Energy undergraduate program at University of Dundee, is leading the development of a next generation battery technology for the automotive industry and studying for his PhD within the Nano-Materials Research Lab (NMRL).

Battery Development for Transportation

Battery	Year	Energy Density Wh/kg	Problems
Lead Acid	1859	~ 25	Low energy density
Nickel Cadmium	1906	~ 45	Banned in the EU due to harmful chemicals
Sodium Nickel Chloride	1985	~ 90	High operating temperatures required; electrolyte can solidify
Lithium Ion	1991	~ 110	Prone to thermal runaway and explosion

As can be seen, these battery technologies have their own distinct limitations, and each has insufficient energy density for use in a cross-country electric vehicle. Thus, focus is turning to novel, next generation battery designs in order to produce a high capacity, robust and affordable cell that can fully satisfy the criteria for an automotive traction battery.

Vehicle Characterization

Our research is utilising a Peugeot 106 Electrique for advanced battery caharcterisation. This FEV is clutchless and gearless and is powered by nickel cadmium flooded batteries, which under idea conditions provided a range of 50 miles per charge. At 13.2 kg each, these batteries are a major contributor to the 1087 kg kerb weight of this small vehicle.

The 106 had to be restored to fully operational condition prior to undergoing any tests or alterations, both for the sake of vehicle health and consistency in the juxtaposition of old and new cell technologies. This involved careful servicing the traction batteries to replenish the deionised water levels had depleted through evaporation and the formation of hydrogen during the absorption stage of the charge cycle.

For characterisation, the vehicle was connected to a laptop via a custom made evLite diagnostic and maintenance tool, which facilitated the direct and comprehensive recording of numerous vehicle parameters. A GPS data logging system was also used to obtain terrain and distance information, and a video camera was mounted in the vehicle to marry the numeric data with a visual representation of the environment. The car was then driven on a test route around the West End of Dundee which subjected the vehicle to a range of driving conditions, and a graphical profile of the vehicle was created from the obtained data.

Vehicle Profile with NiCd and LiFePO4 Batteries

ScreenHunter_04 Mar. 22 12.58

During the 14 km test route, the capacity of the NiCd batteries dropped from 67% to 47%, indicating a range of between 30 and 40 miles per charge, depending on driving conditions. The car produced an average output of 4.02 kW during this journey, and recaptured 20% of expended power under regenerative braking. Peak power of 25.1 kW (33.7 bhp) was delivered at the same instance as maximum current of 234.6A , but interestingly not under hardest throttle or minimum voltage. Although 0 – 30 mph acceleration time was impressive at 7.5 seconds, further acceleration proved to be slower, with a 40 – 50 mph time of 10 seconds. This could be improved with a higher system voltage, such as that provided by lithium air cells, provided that the current delivering capability of these cells is adequate.
Another Peugeot 106 Electrique has been converted to market leading lithium iron phosphate batteries, shedding 50 kg of kerb weight and doubling the vehicle range to 100 miles per charge. This vehicle also benefits from a higher system voltage and therefore sharper acceleration than the NiCad version. However, the LiFePO₄ batteries require strict monitoring at all times, ensuring that voltages do not stray outwith slender extremities in order to avoid damaging the cells.

Next Generation Batteries: The Concept and Advantages of Lithium Air

Lithium air technology is based on a solid lithium anode and a porous carbon cathode. A catalyst, e.g. manganese-based oxide, is sometimes infused into the carbon structure. During the discharge cycle, lithium metal oxidises to form lithium ions, which traverse the electrolyte membrane and intercalate in the cathode structure. Atmospheric oxygen is drawn into the cell through the porous cathode membrane, where it combines with lithium ions within the carbon structure and electrons from the external circuit to exothermically produce lithium oxides (mostly Li₂O₂ with some Li₂O).
As lithium air cells for terrestrial applications do not require the oxygen reagent to be permanently contained within the battery, the specific energy of this technology is significantly greater than that of other lithium ion variants, with a theoretical maximum of 11,140 Wh/kg. Additionally, these cells have less dependency on expensive materials and so a fully developed commercial design is likely to cost less per watt hour capacity than current cell technologies. These factors have been influential in the selection of this battery design as the basis of this study.

Lithium air schematic v3 15032010791

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