IEEE Electrification Magazine - March 2017 - 76

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2020 Telsa Battery Cost Target (BEV)

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Energy Density (Wh/L)

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TEchNOLOGY LEaDErS

Figure 7. Battery cost and energy density improvements. DOE: Department of Energy; BEV: battery EV; PHEV: plug-in HEV. (Courtesy of Global
EV Outlook 2016, International Energy Agency.)

will continue the virtuous trends the
industry has enjoyed in recent years.

System Voltage and
EV Orange-Glue components
Some HEV makers retain unboosted
systems and use a 400-V or lower dc
link to use more conventional
electronics. But Toyota, Ford, and others worked more than ten years ago to
use high voltage-i.e., they use boosted systems having 600-700-V dc link
voltages. Introducing a relatively large
dc-dc converter has had a beneficial
effect on system size and costs. It
allowed the battery voltage to be
decoupled from system voltage and
the battery size to be reduced as the
specific power of HEV batteries
improved. Battery cell count could be
reduced to the bare minimum necessary for optimum cost to supply the
power. When the choice to add a boost
converter was made, the output voltage was adjusted not to just 400 V, but
to the higher voltage to reduce costs.

Figure 8. A typical high-voltage distribution
center. (Photo courtesy of Faraday Future.)

I E E E E l e c t r i f i cati o n M agaz ine / march 2017

However, most EVs have used socalled conventional dc link voltages
of 300-400 V because the battery
pack size is not dictated so much by
power but rather by the energy. Energy on an EV has been on the rise to
meet the ever-growing appetite for
higher driving ranges, resulting in an
increased battery cell count. A higher
number of cells can be configured to
support practically any dc bus voltage level, without an additional dc-
dc converter. It is no secret that today
some OEMs are using higher dc EV
bus voltages on their new EVs.
Familiar power electronics and
interconnect size benefits are realized
when higher voltages are used. This is
perhaps most significant when considering higher power charging of an
EV. Fast-charge power levels are limited by physical size factors of electrical
contacts and cables and their thermal
limits. The German-led CharIn consortium and the combined charge
standard connection system have
been public about higher power charging using an 800-V standard to support
charge rates up to 350 kW, up from
today's Tesla Supercharge max rate of
150 kW. While 800 V is not groundbreaking new technology, it marks a
shift to new materials and components for use in the automotive world.
Transition to the higher voltage will
need to include not just the inverters and motors, but the high-voltage

distribution and control elements
throughout the modern EV (Figure 8).
This includes the ubiquitous orange
cables and connectors and more
tactical components that create the
propulsion system. Notably, this also
includes the relay and fuse systems
that turn on, make safe, and bootstrap the car. In other words, 800 V
drives more interesting high-voltage
architectural choices that must be
made to consider the possible preferred voltage levels and interconnects of the other major loads, like
air conditioner compressor motor
drives, resistance heaters, accessory (12-V) converters, and onboard
ac chargers.

Inverters and Silicon carbide
More than 100% of an EV's driving
energy passes through its power
inverter. Of course, this includes one
pass to discharge the energy from
the batteries to drive the car down
the road and an additional two passes
through the inverter for the braking
energy recouped through regeneration and, subsequently, reused for
additional driving. To extend driving
range and make use of the precious
current, inverter efficiency is paramount. Materials and device engineers have innovated to bring us a
new generation of power semiconductor devices with extraordinarily
low losses that can boost inverter

Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2017