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02.12
Electric Vehicle
Charging at Work
By Bob Bruninga, PE
American driving habits are
based on a century of fossil fuel powered
vehicles and gas tanks, which has created a
significant misunderstanding of the electric
vehicle (EV). The gas-tank, run-until-empty
scenario and then fill-to-full at a public gas
station is not how EVs are used. As shown in
figure 1, a commuter EV will be plugged in at
home and at work to maintain a full charge at
the start of each trip in the morning and
afternoon. Rarely will it be run to empty such
that a search for a higher cost public charge
will be desired.
Figure 1. A battery is not a gas tank. Source: Overlooking L1 Charging-at-work
in the Rush for Public Charging Speed” in the proceedings of the 2012 IEEE
International Electric Vehicle, Conference 4-8 March 2012 Greenville, SC
All EVs are designed to be
charged overnight from a standard 115v outlet
(level-1) which can give at least 32 miles of
range in eight hours. The overnight charge plus
another eight-hour, 115v charge at work, can
give the EV driver a daily range up to 64 miles
(independent of the size of the battery). This
is far above the national average 40 mile
commute and satisfies 90 percent of USA commute
distances.
The EV is not intended to
replace all gasoline vehicle usage, but is ideal
for the commuter that can plugin at home and
plugin at work. The commuter car spends at
least 16 hours a day just sitting at home and at
work as shown in figure 2. During these times,
low-cost, low-speed L1 charging from 115v
outlets is ideal.
Figure
2. The average vehicle spends 90 percent of its
time either at home or at work[1].
EV Charging: The
General Motors model for charging an EV is shown
in Figure 3. GM assumes the majority of all
charging will be at home[2]. Next will be
routine charging at work and only the tip of the
charging pyramid will be at public charging
facilities. To this pyramid we have added the
probable electricity costs at each location.
Public charging at three times higher electric
rates than at home, is like looking for $10 gas;
a tactic usually avoided by most drivers.
Figure 3. The three layer GM charging pyramid and added notes on electricity
costs[2].
Commuter Distances: For
the full implications of the charge-at-work
scenario, the US Department of Transportation
(DOT) statistics [3] for one-way distance to
work are shown in figure 4. These data show
about 90 percent of all U.S. commutes are 32
miles or less. The narrower green bars show the
low cost per day, and the resulting total cost
per month for each distance at the bottom in
red.
Figure 4. DOT one-way commute distances overlayed with daily
and monthly
electricity cost data.
Level-2 Charging: Figure
5 uses the same data to show how placement of
Level-2 chargers at-work or other eight-hour
daily parking lots wastes more than 80 percent
of the available charging capacity. The red
bars show the hours to charge (L2) for each of
the commute distances. In almost 70 percent of
the cases, the EV is fully charged in under an
hour, leaving the L2 charger unavailable to
anyone else during the remaining seven or more
hours of the day. Even 90 percent of all
commuters are fully charged in under two hours.
Figure 5. DOT commutes with L2 charge hours showing wasted capacity.
L1 Charging (115v outlets):
In contrast, 115v outlets at work can fully
charge 90 percent of all commuters in eight
hours or less (L1). This data is quite reliable
since vehicles leave home in the morning after
an overnight full charge and usually go straight
to work, leaving errands and activities for the
afternoon/evening. Thus, commuter EVs arrive at
work with a charge-need well correlated with
their individual distance to work. After being
plugged in all day (L1), then, they leave work
fully charged as well.
Figure 6. DoT commuting distance with charge hours
using 115v outlets (Level-1
charging).
Charging Distance versus
Charging Time: Range anxiety and public
charging fears are further perpetuated or
misdirected by the common presentation metric of
hours-to-full-charge-from-empty as shown
in the left hand table of figure 7. Instead,
this same data can be presented as
miles-per-8-hour-charge for at-home and
at-work charging as shown on the right.
Compared to the typical 10-15 mile commute for
50 percent to 70 percent of all EV owners, the
eight-hour overnight and at-work charge is
completely adequate up to 64 miles a day for
all EV car models independent of their overall
battery capacity. What counts is only the
length of the commute:
Figure 7. Presenting miles-per-8hr-charge instead of
hrs-to-full-charge is less alarming to commuters.
 Payin-to-Plugin:
For 50 cents to $1 a day for charging-at-work, there is little need for complex
metering and payment systems. Employers can simply collect a monthly charging
fee of $10 to $20 a month and issue a “charging pass” for EV employees to plug
in to available outlets, as shown in figure 8. This informal system can be
managed by the employer as easily as they currently manage parking passes and
monitor handicapped spaces in their employee lots or garages [4].
Figure 8. (depicted at
right) A monthly employer charging pass costing
the same as the electricity used is an easy way
to implement charging at work using existing or
future 115v (L1) outlets.
Recommendations:
An EV is generally not a
one-for-one replacement for a wide
ranging-general purpose gas car but is optimum
for the commuter with a place to plug it in at
work. Every EV comes with a standard 115v
charging cord. The public focus on fast
public charging (L2) is off-target based on
the century old gas-tank legacy. The EV is more
of a commuting appliance, to be plugged in at
home and at work, than the legacy go-fill-up-fast-at-a-public-charging-station
experience of the American public.
We should:
-
Not oversell EVs as gas car
replacements across the board and where
inappropriate
-
Recognize that
charge-at-home and charge-at-work covers 90
percent of all USA commutes
-
Encourage Employer
charging-at-work to double the EV commuter’s
range
-
Encourage informal monthly
pay-in-to-plug-in programs for the ~ $1/day
cost of electricity
-
Educate the public of the
advantages of L1 charging at home and at
work
-
Encourage L1 charging to
avoid peak loads and neighborhood utility
clustering problems
-
Encourage L1 charging from
standard outlets to avoid expensive
electrical work
-
Place L2 fast chargers at
shorter-duration spaces for easy access to
more cars per day
-
Discourage L2 chargers in
long-term/daily lots where 1-hr charged cars
block usage
-
Recognize that utility load
leveling can be easily accomplished in bulk
employer lots
-
Avoid magnifying L2
issues/concerns/problems where L1 solutions
also exist
The 200 million EV goal in 30
years will not be met with the single-minded
focus on public L2 charging. The EV is a very
significant part of the solution to our future
energy, environmental and national security
problems. We must not let misinformation, and
public confusion based on a century of
quick-fill-up-gas-tank legacy undermine or slow
this radical new technology. As the commuter
learns the benefits of EVs and charging at-work,
the expensive quick public EV charger will
eventually be as little used as the
spare-gas-can-in-the-trunk is used by gasoline
drivers.
[1] 2001 National Household
Travel Survey; GM Data Analysis (Tate/Savagian)-SAE
paper 2009-01-1311
[2] Presentation by
Dr. Mary Beth
Stanek, Director, Environment and Energy Policy
and Commercialization, General Motors Company,
Washington DC EV Forum, 12 Dec 2011
[3]
US DoT, Bureau of Transportation Statistics,
Omnibus Household Survey. Research and
Innovative Technology Administration.
[4]
http://aprs.org/payin-to-plugin.html
Bob Bruninga, PE, is senior
research engineer at the U.S. Naval Academy,
Aerospace Engineering Dept. , bruninga@usna.edu.
Website: http://aprs.org/payin-to-plugin.html
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