Journal
of
Power
Sources
209 (2012) 105–
119
Contents
lists
available
at
SciVerse
ScienceDirect
Journal
of
Power
Sources
jo
ur
nal
homep
age:
www.elsevier.com/locate/jpowsour
Direct
solar
photovoltaic
charging
of
a
high
voltage
nickel
metal
hydride
traction
battery
Nelson
A.
Kelly
∗
General
Motors
R&D
Center,
480-106-224,
Chemical
Sciences
and
Materials
Systems
Laboratory,
30500
Mound
Road,
Warren,
MI
48090-9055,
USA
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
26
January
2012
Received
in
revised
form
21
February
2012
Accepted
23
February
2012
Available online 6 March 2012
Keywords:
Photovoltaic
(PV)
High
voltage
battery
Solar
energy
Electric
vehicles
Optimized
system
a
b
s
t
r
a
c
t
The
electrification
of
vehicle
power
trains
using
batteries
and
fuel
cells
is
an
important
technological
step
forward
in
the
effort
to
improve
the
efficiency
and
reduce
the
tailpipe
emissions
of
vehicles.
The
production
of
electricity
and
hydrogen
in
a
renewable
fashion,
such
as
using
solar
energy,
can
provide
a
clean
and
sustainable
energy
source
for
electric-powered
vehicles.
In
the
present
work
we
develop
a
charging
system
to
prove
the
concept
of
direct,
efficient,
solar-powered
charging
for
battery-electric
vehicles
using
solar
photovoltaic-powered
charging
of
the
high-voltage
nickel-metal
hydride
(NiMH)
battery
that
is
used
in
the
GM
2-mode
hybrid
system.
We
utilize
a
protocol
for
high-voltage
battery
charging
that
was
developed
in
an
earlier
study
that
included
a
DC–DC
converter
to
boost
the
low-
voltage
produced
by
the
PV
array
to
the
high
voltage
needed
to
charge
the
battery.
However,
no
power
conversion
was
used
in
the
present
study.
Rather,
we
use
a
high-voltage
solar
array
capable
of
outputting
a
wide
range
of
voltages,
from
approximately
250
to
400
V
in
50
V
increments,
at
the
photovoltaic
system
maximum
power
point.
By
varying
the
solar
system
output
voltage
we
measured
the
solar
energy
to
battery
charging
efficiency
under
a
variety
of
conditions
to
determine
the
optimal
photovoltaic
system
configuration.
The
system
and
methods
developed
in
this
work
can
be
used
to
efficiently
charge
a
range
of
battery
electric
vehicles
by
adapting
the
PV
system
output
to
the
battery
charging
voltage.
© 2012 Elsevier B.V. All rights reserved.
1.
Introduction
In
order
to
increase
vehicle
efficiency,
reduce
dependence
on
petroleum,
and
reduce
emissions
of
air
pollutants
and
greenhouse
gases,
there
is
an
increasing
trend
toward
vehicle
electrification.
This
move
toward
electric
power
trains
is
part
of
the
“new
DNA”
for
future
vehicles
[1–4].
At
General
Motors
(GM)
we
are
pursu-
ing
this
goal
by
developing
a
test
fleet
of
fuel
cell
electric
vehicles
(FCEV)
powered
by
hydrogen
[1,2]
as
well
as
marketing
a
range
of
electrified
vehicles,
from
hybrid
electric
vehicles
(HEV)
to
plug
in
and
extended
range
electric
vehicles
(PHEV
and
EREV)
in
which
internal
combustion
engines
and
electric
motors
are
both
utilized
[5,6]
to
achieve
the
aforementioned
goals.
In
order
to
have
a
sustainable
transportation
system
it
is
impor-
tant
that
vehicles
are
powered
by
renewable
energy.
In
support
of
this
long-term
goal
we
have
studied
the
production
of
hydrogen
for
FCEV
[7–11]
and
electricity
for
battery
charging
[11–13]
using
solar
energy.
Solar
energy
has
the
potential
to
supply
an
ever
increasing
fraction
of
the
total
energy
needed
in
the
future
and
in
particular
the
DC
electricity
produced
by
solar
photovoltaic
(PV)
cells,
mod-
ules,
and
arrays
is
ideally
suited
to
split
water
to
produce
hydrogen
∗
Tel.:
+1
586
986
1623;
fax:
+1
586
986
1910.
E-mail
address:
nelson.a.kelly@gm.com
and
to
charge
batteries
[14–16].
Solar
powered
hydrogen
and
bat-
tery
charging
systems
can
be
scaled
from
small
widely
distributed
systems,
including
individual
home
owners
and
parking
structures
[11],
to
large-scale
installations
that
collect
the
solar
energy
from
hundreds
of
square
miles
of
land
and
store
it
as
chemical
energy
in
charged
batteries
or
as
hydrogen
produced
via
water
electroly-
sis
[15–19].
Solar
energy
can
provide
a
clean,
renewable
source
of
energy
to
charge
PHEV,
EREV,
and
battery
electric
vehicles
(BEV)
to
maximize
the
environmental
benefits
of
eliminating
greenhouse
gases
and
other
emissions
associated
with
fossil-fueled
electric
generation.
However,
a
major
challenge
to
using
solar
energy
tech-
nology
is
the
need
to
design
an
inexpensive,
safe
solar
battery
charger
with
optimum
efficiency.
In
our
first
study
on
charging
a
high-voltage
traction
battery
using
a
PV
system
we
decided
to
use
our
low
voltage
arrays
with
a
DC–DC
converter
to
increase
the
50-V
output
to
350
V
necessary
to
charge
the
nickel
metal
hydride
(NiMH)
used
in
the
GM
2-mode
hybrid
[13].
That
study
served
as
a
proof
of
concept
for
charging
a
high-voltage
battery
with
PV
electricity.
To
our
knowledge,
this
was
the
first
time
a
high-voltage
HEV
battery
was
charged
using
solar
energy
with
charge
control
provided
by
the
internal
battery
computer.
Because
battery
packs
for
electric
powered
and
electric
assisted
vehicles
have
electronic
controls
to
monitor
and
control
the
charging,
the
PV
system
only
needs
to
be
designed
to
have
a
maximum-power
point
voltage
near
the
charge
cut-off
voltage
0378-7753/$
–
see
front
matter ©
2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jpowsour.2012.02.106