IEEE Talks Transportation Electrification: Dr. Sheldon S. Williamson
Innovation in both on-board battery management and off-board battery charging is advancing electric transportation.
Dr. Sheldon S. Williamson is at the forefront of research and development (R&D) in power electronics to both help lengthen the lifetime of a vehicle’s battery pack and drive standardized approaches to charging for different applications. He is Canada Research Chair in Electric Energy Storage Systems for Transportation Electrification and a Professor within the Department of Electrical, Computer and Software Engineering, in the Faculty of Engineering and Applied Science, at the University of Ontario-Institute of Technology. Noted author and co-author of over 150 papers and several books as well as book chapters on electric transportation and energy storage systems, Dr. Williamson is a Senior Member of IEEE and a Distinguished Lecturer of the IEEE Vehicular Technology Society (IEEE VTS).
Please tell us a little about trends related to battery research for electric transportation.
Currently, there’s a big drive, both for commercial vehicles as well as mass transit, to go electric. Battery technology is where the main hurdle lies. Our group’s research at the University of Ontario-Institute of Technology (UOIT) over the past six to eight years has been twofold really. Under the broad umbrella of battery management is onboard battery management on the vehicle itself, which has to do with how power electronics plays a role in lengthening the lifetime of a battery pack, so a battery can last longer on an electric vehicle (EV). And then there’s the offboard issue, which is charging the EV battery pack. Because of the varying philosophies, the market has been really slow in deploying charge stations of any one particular type or standard.
What is the problem to be solved with regard to onboard battery management?
The main problem with today’s battery technology for EVs is the driving range that a battery can give you. The driving range is at best 400 or 450 kilometers per charge. Moreover, what nobody notices is that the range dwindles by roughly about 10-15 km per year, due to natural capacity fade of Lithium-ion batteries and/or aggressive driving patterns. These numbers are just not good enough, when you compare them with the range of a conventional vehicle internal combustion engine.
One of the common mistakes that we as consumers make is that we think of an EV in the same way that we do as a conventional vehicle with an internal combustion engine. That should not be the case because, with an EV, we do not let the battery discharge completely. The philosophy is we top off a battery pack from 50- or 60-percent charged to 100 percent. We don’t let it go all the way down to zero.
So, really, the problem we have to solve is to use the pack’s capacity with a limited number of cycles. Typically, Lithium-ion batteries come with about 1,500 to 2,000 charge and discharge cycles; this is very limited. This is what defines the lifetime of a battery pack (or range of the EV). So when you have only 1,500 cycles to play with, that essentially amounts to about 6-8 years of lifetime for a battery pack on an EV. With today’s conventional vehicles, on the other hand, you’re looking almost like an average of about 10 years of usage. So you’re deleting about 4 years’ time for using a vehicle, when you compare an EV to a conventional vehicle. Add to that fact that the EV cannot be resold for much value with an old battery pack, and you see the socioeconomic impacts that lifetime of a battery pack has on EV sales.
Where does R&D in power electronics help?
What’s been happening is we found that we can actively manage voltages on each cell on board, as your vehicle is running, via battery management systems, which are comprised of power electronic circuit boards—basically, DC-to-DC converters.
The cells are unbalanced to start with, in terms of their capacity to hold charge. Because Lithium is so susceptible to even constructional damages or defects that, if it is not 100 percent accurately done, they have different capacities right off the bat. And so you’re looking at some cells which are 3.50 Volts, some that are 3.70 Volts, 3.40 Volts, so on. Add to that the fact that there are about 100 or so cells connected in a series in a Lithium-ion or Lithium-Polymer EV traction battery pack. The overall effect of unbalanced voltages (due to unbalanced capacities and capacity fade) can lead to catastrophic results.
Capacity fade, as the name suggests, means the battery pack fades in capacity to hold charge over time. So, in a series of 100 cells, if one or two fail to hold any charge over time, you’re looking at either causing a short circuit or a fire or an explosion on board.
The whole purpose of doing active battery management is to share energy from an over-charged, low-capacity cell with an under-charged, high-capacity cell—the cell with the higher charge shares its current and charges the lower voltage cell—thus, the string of 100 or so cells can be voltage balanced.
Right now, you buy an EV, and it says you get 300 miles per charge. However, this range will dwindle by as much as 8-10 miles a year because of capacity fade. By the time you’re in your 6th or 7th year of the vehicle, you’re looking at about 50-60 mile differential from what you bought it at. R&D around active battery management is helping us better hold that driving range at near 300 miles for longer periods of time.
How is the challenge different or similar in research around off-board battery charging?
We are part of a number of different research areas in the off-board space.
Some of the research that’s ongoing is off-board DC fast charging—taking the charger out of the vehicle and putting it off board and having DC for charging a battery pack rather than plugging into an AC power outlet. Some of the power electronic converters that we’ve been working on are anywhere between 10 kW to about 125 kW, to be installed as off-board DC charge ports, for various electric mobility applications. We are also developing high-efficiency (97% and up) 250 kW DC fast charge AC/DC conversion systems for electric mass transit buses. Mass transit electrification is one of the new initiatives at UOIT, funded by related industry as well as the Federal and Provincial Governments.
UOIT also boasts its own Campus Microgrid, the first-of-its-kind on a Canadian University Campus. DC charge ports with the capability to integrate renewable energy systems have been developed by our research group. More recently, our group has successfully built and tested a high-efficiency, single-conversion-stage 5.0 kW prototype of an integrated EV/Photovoltaics-PV/Grid charger interface.
Reducing energy conversion stages is another research topic that we’re working on. When you’re using photovoltaics, you’re going to generate voltage at varied levels depending on the sun’s intensity; whereas, with the grid, it’s a stiff AC voltage at some level—it’s well-defined, well-regulated AC power. And then you have your EV battery pack, which is really very dependent on a nice smooth current level.
When you have two different sources like PV and an AC Grid at different voltage levels, you need a power electronic conversion unit, which can regulate these, and bring them to a common DC voltage bus. Conventionally, they have used multiple stages, and you lose a lot of power doing conversion from step to step every time. What we’ve done is combine all of these units into a single conversion stage, which is highly novel.
So that’s some of the cutting-edge work on integrated power electronics. There’s also ongoing work on wireless power transfer for fast charging as well as in-motion charging of e-buses and various forthcoming autonomous e-mobility systems. We are looking at power levels upwards of 125 kW for wireless fast charging systems. Our group has already constructed and demonstrated 3.7 kW and 11 kW wireless charging systems.
It sounds as though each application is being treated as its own space for innovation.
Definitely. It’s interesting how the transformation is going to take place, and the technology is growing as the applications come along. The applications drive the innovation in the technology itself.