GM researchers posit simple model to assist in balancing high energy density vs fast charging in EV design
With an eye toward balancing higher energy density batteries and fast charging capability, General Motors researchers Mark Verbrugge and Charles Wampler have derived and implemented a simple model to assist in evaluating factors such as cell performance, cost, life, and fast-charge capability in the design of electric vehicles.
In a paper published in the Journal of Power Sources, they suggest that their approach is useful in terms of comparing and contrasting battery systems and shedding light on technological tradeoffs.
We are at a crossroads in terms of balancing two promising technologies: (1) higher energy density (Wh/L) and specific energy (Wh/kg) batteries, relative to today’s conventional graphite/metal-oxide lithium ion systems, and (2) fast-charge capability, defined here as greater than Level 2 charging, or greater than about 20 kW. Currently in the United States, conventional Level 2 charging of 6.6 kW is available in homes and various community locations. In the ideal case, high energy batteries would be able to accommodate fast charge, but two of the most promising high-energy cell technologies, i.e., cells employing Si-enhanced or Li-metal negative electrodes, are problematic insofar as they cannot at present accept fast charging without significant degradation in cell life.
… The tradeoff between high-energy, as provided by cells with Li-Si and Li metal negative electrodes, and fast-charge capability, which can be obtained from conventional lithium ion cells employing lithiated graphite or titanate negative electrodes, for examples, poses a dilemma in the design of electric vehicles (EVs).—Verbrugge and Wampler
For their analysis, Verbrugge and Wampler considered costs associated with the cells, added mass due to the use of larger batteries, and charging, as well as a new cost input—the cost of adaption, corresponding to the days a customer would need an alternative form of transportation, as the EV would not have sufficient range on those days.
The end result is a qualitative model that can be used to calculate the optimal EV range (which maps back to the battery size and performance), including the influence of fast charge. Verbrugge and Wampler cautioned that the results are qualitative, given the complexity of the problem; their approach does, however, identify key factors to be considered in battery sizing.
Of particular note is the cost of adaptation. When we exercise the model with inputs one can associate with a battery like that of the Chevrolet Bolt EV, we find that the net cell volume and the vehicle range are consistent with an adaptation cost of $165/day, three times the average cost per day to rent a car in the United States. This adaptation cost would be appropriate for customers having a strong desire to avoid relying on some alternative method of transportation for the days in which the EV could not supply the needed miles. … the Bolt EV’s 238 miles range would mean that the 75th percentile drivers would need an alternative form of transportation—that is, they would need to adapt—about 5 days per year.)
The results … allow one to assess whether fast-charge of a conventional lithium ion battery is superior to the implementation of a high-energy density cell that cannot be fast charged. For the parameters chosen, fast-charge of a conventional lithium ion battery offers superior value to the customer relative to the high-energy density cell.—Verbrugge and Wampler
Mark W. Verbrugge, Charles W. Wampler (2018) “On the optimal sizing of batteries for electric vehicles and the influence of fast charge,” Journal of Power Sources, Volume 384, Pages 312-317 doi: 10.1016/j.jpowsour.2018.02.064