Interview with Sheldon Williamson - June 2nd

Sheldon Sheldon S. Williamson (S’01–M’06–SM’13–F’20) received his Bachelors of Engineering (B.E.) degree in Electrical Engineering with high distinction from the University of Mumbai, Mumbai, India, in 1999. He received the Masters of Science (M.S.) degree in 2002, and the Doctor of Philosophy (Ph.D.) degree (with Honors) in 2006, both in Electrical Engineering, from the Illinois Institute of Technology, Chicago, IL, specializing in automotive power electronics and motor drives, at the Grainger Power Electronics and Motor Drives Laboratory. Currently, Dr. Williamson is a Professor at the Smart Transportation Electrification and Energy Research (STEER) group, within the Department of Electrical, Computer, and Software Engineering, at Ontario Tech University, in Oshawa, Ontario, Canada. He also holds the prestigious NSERC Canada Research Chair position in Electric Energy Storage Systems for Transportation Electrification. His main research interests include advanced power electronics and motor drives for transportation electrification, electric energy storage systems, and electric propulsion. Prof. Williamson is a Fellow of the IEEE.

In this interview, Sheldon answers questions from his webinar, Smart Battery Energy Management and Health Conscious Fast Charging for Future Transport, originally presented on June 2, 2020.

Question: How does fast charging lead to reduced battery life?

DC Level-2 fast charging of 50 kW and above tends to cause capacity fade at a faster rate, which is mostly influenced by the growth of the solid electrolyte interphase (SEI) layer within the cells. This increases the cell’s internal resistance as well as the quantity of free lithium capable of cycling. The increase in SEI predominantly occurs with ambient temperature (Ta) above 25°C, which in turn, decreases calendar life of the cells (and the pack).

See: L. Patnaik, A. V. J. S. Praneeth and S. S. Williamson, "A Closed-Loop Constant-Temperature Constant-Voltage Charging Technique to Reduce Charge Time of Lithium-Ion Batteries," in IEEE Transactions on Industrial Electronics, vol. 66, no. 2, pp. 1059-1067, Feb. 2019.

Question: What is the effect of partial charging and discharging on the SOH/SOL of Li-ion cells?

Partial discharge on Li-ion is fine. There is no memory and the battery does not need periodic full discharge cycles to prolong life.

Question: When does active cell voltage balancing start becoming a cost-effective option?

Target costs for active cell balancing topologies is 5-10% of the battery pack cost. Battery OEMs usually wish this cost to be < 1%. For the detailed economics of using an active battery cell voltage balancing unit and the return on investment (ROI), please refer to the paper below.

See: P. A. Cassani and S. S. Williamson, "Significance of battery cell equalization and monitoring for practical commercialization of plug-in hybrid electric vehicles," 2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition, Washington, DC, 2009, pp. 465-471.

Question: Since the temperature along the surface of the battery cell is not even, can you please comment on the placement of the sensor on the cell surface for accurate results?

While testing the proposed health-conscious fast charging method in the lab, the cells are given time to reach thermal equilibrium, compared to ambient temperature. Thus, accurate placement of the temperature sensor is not that critical.

Question: Why does your health-conscious CT-CV fast charging technique use a feedback as well as feed-forward aid?

It must be pointed out that this is the first real attempt in literature of actually feeding temperature back via a control loop in order to monitor temperature rise of Li-ion cells whilst fast charging. Since the time-constant of temperature is large (in minutes/hours), a PID controller is good enough for the feedback controller. For the feed-forward term, a simple way to arrive at a current profile that meets the fast charging requirements is to use an exponentially decaying current profile, going from initial value of say, for example, 2C, to a final value of 1C. Thus, we need a current feed-forward term to superimpose this on the system. For further details on the controller design, please refer to the paper cited below.

Question: What are the long-term ageing effects of CT-CV charging compared to CC-CV charging?

In order to prevent degradation and eventual ageing, the cell surface temperature has to be maintained constant, which can be achieved by controlling cell current, as is proposed by the new CT-CV charging technique. Please refer to the paper below, to see the effect of this charging technique over varied temperatures. The paper addresses charging at high C-rates and different chemistries, such as the NMC cathode based cell.

See: V. A. Marcis, A. V. J. S. Praneeth, L. Patnaik and S. S. Williamson, "Analysis of CT-CV Charging Technique for Lithium-ion and NCM 18650 Cells Over Temperature Range," 2020 IEEE International Conference on Industrial Technology (ICIT), Buenos Aires, Argentina, 2020, pp. 947-952.