Research and development (R&D) in power electronics over the last half-dozen years is yielding real-world advances in battery technologies and promises to overcome some of the hindrances that have held back electric transportation thus far. Innovation is taking place in two broad areas:
- On-board battery management on the traction battery pack itself (power electronics R&D is helping lengthen the lifetime of a battery pack).
- Off-board battery charging (power electronics R&D is helping the industry coalesce around more standardized approaches to charging for different applications).
On-board Innovation
Battery technology is where the primary hurdles lie to increased proliferation of electric transportation. The main problem is the driving range that current-generation batteries deliver—at best, up to 400 or 450 kilometers per charge (or about 250 to 280 miles) when new. In the minds of many users, that is just not good enough when compared directly with today’s fuel tanks.
There is an expectation problem here that might have to be overcome in time. A common mistake that we as users make is that we think of an electric car in the same way as a conventional vehicle with an internal-combustion (IC) engine. But the truth is that, in the coming world of widely enabled and deployed electric transportation, we will be using electric vehicles (EVs) and their battery packs in a fundamentally different manner than we use fuel tanks for IC engines today.
Instead of burning most of the fuel for an IC engine and then refilling the tank when it is almost empty, batteries for EVs will tend to be recharged more frequently and not be discharged completely. More often, the battery pack will be “topped off” from 50- or 60-percent charge back to 100 percent, making the driving range of the battery pack feel longer to the driver.
However, the problem here isn’t strictly psychological. Even still, there is a thorny issue to be dealt with in that typical Lithium-ion batteries come with only 1,500 to 2,000 charge and discharge cycles. That works out to an average lifetime for the battery pack of an EV of six to eight years—or, two to four years less than today’s conventional IC vehicle. Add to this fact that the EV with an end-of-life battery pack has very low resale value, and it’s easy to grasp the socio-economic obstacles that an EV has to uptake.
Where R&D in power electronics is helping the cause is by limiting the number of spent cycles per battery through better energy management of the pack. As an EV is running, power electronics can be used to actively balance the voltages across the 100 or so cells connected in a series within a battery pack. Active battery management systems (BMS) are comprised of power electronics circuit boards (basically bidirectional DC/DC converters) to share currents in a bidirectional manner, which eventually balance the voltages across the cells.
The cells are inherently unbalanced to start with. Lithium is so susceptible to construction damages or defects that cells within the pack range considerably from 3.10 to 3.90 Volts at the outset of usage. Furthermore, there’s capacity fade to contend with—the capacity of the cells shrinks over time, resulting in driving range for the battery pack dwindling by as much as 15 to 20 miles per year.
Active BMS equalizes voltage across the adjacent or non-adjacent cells, by sharing overflowing currents from lower capacity cells to those with higher capacities, thus balancing the voltage across the entire string of cells. Cells with higher current overflows are used to charge those with lower capacities, until all are returned to 100 percent. By doing this, the impact on improving cycle life as well as calendar life is substantial. Through the Canada Research Chairs (CRC) program, R&D is taking place at the University of Ontario-Institute of Technology in the design and development of such power electronics controller boards and control systems for dynamically estimating cell voltages and capacities as the vehicle is running. This leads to significantly more efficient on-board battery management. The result is fewer spent cycles, offsetting the effect of capacity fade, and ultimately helping conserve the original driving range of the battery pack over the lifetime of the EV.
Off-board Innovation
Concurrent innovation is taking place in tailored approaches to off-board battery charging for different applications of electric transportation.
Power electronics R&D in DC fast charging is helping the EV industry take the charger out of some vehicles and put it off board for some driving applications. Small-scale prototypes of 10 kW DC charge ports are currently being developed at UOIT. The idea is to scale up to about 200 kW (1000V, 200A), to comply with SAE J1772 standards. Campus microgrid-level charging ports are being researched, where renewables such as solar power are integrated. EVs come in to charge from DC plug points rather than AC. A 5.0 kW prototype has already been built.
Reducing power conversion stages in fast chargers is another critical research effort that is being undertaken.
As previously discussed, EV battery packs are very finicky because of the inherent fragility of Lithium. They are very dependent on a nice smooth current level, such as produced by the traditional electricity grid, with its fixed, dependable AC voltage level that is well defined and regulated. With photovoltaics (PVs), or solar panels, however, voltage is generated at varied levels depending on the sun’s intensity. So, introducing PVs as an energy source demands a power electronic conversion unit to account for their irregularity.
Conventionally, the conversion has taken place over multiple stages. The PV must be converted to a regulated DC and from that DC level to the DC level required for the traction battery pack. In addition, the AC grid has another conversion unit dedicated to converting AC to DC, and then regulating that DC level and bringing it back to the AC grid. Thus, power is lost at each conversion step to the tune of 1-2 percent. UOIT is developing a conversion process, which combines all of these units into a single conversion stage, thus bringing overall conversion efficiencies up to about 97-98 percent.
In general, integrated converters for both DC fast charging, PV/Grid/EV interface, and wireless power transfer for fast charging of future electric transport and autonomous e-mobility are some of the major R&D focal points. These research questions are especially important to be answered for particular applications, such as future autonomous vehicles and electric mass transit traversing dedicated routes. The EV would not need to plug in; you just park, and the EV charges on the fly or on the go via in-motion (dynamic) or static charging. UOIT is currently developing 120 kW wireless fast chargers for futuristic electric mass transit applications (e-buses).
Conclusion
The R&D conducted at UOIT is mainly funded through the Canada Research Chairs (CRC) program from the Canadian government’s Natural Sciences and Engineering Research Council (NSERC). The group’s research is also strongly funded by several national and international industry partners. The chief mandate of the CRC program is to find solutions for electric energy storage systems for transportation electrification. Canada has established a national goal by 2030 to reduce greenhouse gas emissions by 30 percent below 2005 levels. To achieve success, the automotive industry must provide affordable and reliable EVs for personal and mass transit. Power-electronics R&D in both on-board and off-board battery management for electric transportation is making crucial strides forward in the pursuit of the ambitious national goal.
