Application
Note:
Safe
Use
and
Charging
of
Lithium
Ion
Batteries
Matthew
Beutler
ECE
480
–
Spring
2013
Abstract
This
paper
discusses
the
usefulness
of
the
high
density
lithium
ion
batteries
of
present
day,
as
well
as
some
of
the
dangers
that
they
pose.
Presented
are
some
safety
tips
and
a
step
by
step
instruction
on
designing
a
fast
lithium
ion
battery
charger
with
built
in
temperature
protection.
Background
Batteries
allow
the
use
of
all
modern
day
electronics
and
are
an
essential
part
of
making
electronics
mobile.
As
processing
power
has
been
shrinking
exponentially
since
computational
electronics
were
first
invented,
power
electronics
has
struggled
to
keep
pace.
Lithium
Ion
batteries
are
currently
one
of
the
highest
capacity
for
weight
batteries
on
the
market
today.
Most
batteries
consist
of
two
electrolytes
and
rely
on
a
chemical
reaction
to
produce
the
power
that
batteries
offer.
One
of
the
first
batteries
that
could
deliver
a
decent
amount
of
current
was
the
Daniel
cell,
rated
at
1.1
volts;
it
used
a
copper
sulfate
solution,
sulfuric
acid
solution,
and
a
zinc
electrode.
[1]
Nickel
Cadmium
batteries,
invented
in
1893,
have
an
energy
density
of
50-‐150W*h/L,
which
has
still
been
one
of
the
most
energy
dense
batteries
until
recent
years.
Alkaline
magnesium
batteries,
invented
in
1949,
also
have
a
high
energy
density,
but
there
capacity
is
very
dependent
on
the
load,
so
the
application
defines
their
usefulness.
[1]
Lithium
ion
batteries
were
first
invented
in
the
1970’s,
and
far
surpass
its
predecessors
with
maximum
energy
densities
up
to
620
W*h/L
at
the
present
day.
[2]
Lithium
ion
batteries
have
a
much
lower
self-‐
discharge
rate,
compared
to
other
batteries,
and
they
don’t
have
“memory”
which
means
charging
and
discharging
without
giving
them
a
full
cycle
causes
them
to
retain
less
charge
over
time.
Lithium
ion
batteries
are
actually
best
utilized
when
the
charge
remains
closer
to
the
middle
of
its
capacity
as
opposed
to
either
end
of
the
spectrum.
[2]
Safety
Concerns
Lithium
Ion
batteries
have
been
particularly
criticized
because
of
many
safety
concerns
they
poses
if
not
carefully
used.
Overcharging
or
allowing
the
batteries
to
overheat
can
cause
thermal
runaway,
which
may
cause
the
cells
to
rupture
or
combust.
[3]
Over
discharging
the
cells
may
cause
physical
short-‐
circuits
within
the
cell
which
can
render
it
useless
and
dangerous
to
attempt
to
charge.
Fortunately
laws
within
the
U.S.
prevent
manufacturers
from
selling
lithium
ion
cells
without
protection
circuitry,
though
it
is
possible
to
purchase
them
from
outside
the
US.
It
is
also
very
cheap
and
easy
to
buy
protection
circuit
modules
for
a
wide
range
of
lithium
ion
batteries.
Most
protection
circuitry
provided
thought,
only
protects
against
over
voltage
charging,
and
under
voltage
discharging.
Designing
a
Lithium
Ion
Battery
Charger
-‐
Basics
The
first
consideration
to
take
into
account
is
the
size
and
ratings
of
your
battery
pack.
The
voltage
is
the
main
concern,
but
the
C
rating
and
capacity
is
very
important
to
determine
the
max
charge
rate
which
can
be
safely
applied
to
the
battery.
For
example,
in
the
charger
designed
in
this
example,
the
battery
pack
has
a
nominal
voltage
of
7.4
volts
and
a
6600
mAh
capacity,
the
C
rating
defines
the
maximum
charge
and
discharge
of
the
pack.
The
data
sheet
gives
a
maximum
discharge
current
as
.75C,
and
a
maximum
charge
current
as
.265C.
This
means
the
battery
pack
can
supply
up
to
5
amps
of
current
to
the
application
it
is
being
used
for,
while
the
maximum
charge
current
is
1.75
amps.
Some
battery
packs
will
allow
much
greater
discharge/charge
rates
so
it
is
important
to
read
the
specs
associated
with
the
battery
pack
and
design
to
meet
the
needs
of
your
application.
When
charging
lithium
ion
batteries,
there
are
two
stages
of
charging:
constant
current
and
constant
voltage.
During
the
constant
current
stage,
the
maximum
current
allowed
(or
designed
for)
will
flow
into
the
battery
as
the
voltage
level
increases
to
the
rated
voltage
of
the
battery
pack.
Once
the
voltage
is
reached,
current
will
continue
to
decrease
as
the
capacity
of
the
battery
is
filled;
during
this
stage
the
voltage
stays
constant.
http://powerelectronics.com/site-‐files/powerelectronics.com/files/archive/powerelectronics.com
/portable_power_management/battery_charger_ics/804li-‐ion-‐battery-‐life-‐Figure01.jpg
Some
needed
and
important
components
for
a
good
battery
pack
charger
include
a
transformer,
bridge
rectifier,
voltage
and
current
regulator.
For
extra
safety,
a
temperature
sensor
is
included
in
the
design
which
requires
an
extra
voltage
regulator
for
a
reference
voltage.
The
Design
A
battery
operates
on
a
DC
voltage,
and
since
this
design
includes
the
use
of
a
wall
outlet,
it
will
require
the
transformation
from
120
volts
AC,
to
a
usable
DC
voltage.
The
regulators
used
in
this
design
allow
for
15
volts
to
be
dropped
across
them
without
affecting
the
current
limit,
so
a
standard
transformer
was
used
to
bring
the
wall
outlet
voltage
down
to
9
volts
AC.
Next,
a
bridge
rectifier
was
required
to
acquire
DC
voltage
and
a
large
capacitor
was
placed
across
the
output
to
reduce
the
voltage
ripple.
The
LM317
is
a
regulator
that
can
be
used
to
set
a
voltage
or
limit
current.
In
the
data
sheet
several
examples
are
shown,
one
in
particular
shows
its
use
as
a
battery
charger
[4].
Using
this
example,
with
a
few
modifications,
we
can
design
a
custom
battery
charger
with
temperature
protection.
Setting
the
voltage
desired
to
charge
the
battery
pack
to
is
the
first
consideration
to
make,
and
this
depends
on
the
values
of
the
resistors
R1
and
R2
as
shown
in
the
schematic.
The
voltage
of
the
battery,
Vbat,
can
be
used
to
solve
for
R1
and
R2.
Since
the
application
of
this
battery
pack
was
to
power
an
RC
car,
a
higher
voltage
was
desirable.
The
battery
charger
was
in
turn
designed
for
7.8
volts.
(Each
lithium
ion
cell
can
be
charged
between
3-‐4.2
volts).
R2
𝑉𝑏𝑎𝑡 = 1.25 1 + = 7.8
R1
Solving
this
equation
for
R1
and
R2,
the
ratio
of
R2/R1
is
about
5.25.
I
chose
R2=1600
and
R1=300,
which
gave
me
a
ratio
of
5.33.
This
will
set
the
voltage
across
the
battery
pack
as
7.9125
volts,
which
is
still
under
the
maximum
voltage
rating
for
the
battery
pack.
The
LM317
can
provide
currents
above
2.2
amps,
so
if
the
battery
pack
isn’t
rated
for
that
much
current,
a
current
limiting
resistor
needs
to
be
added
from
the
negative
side
of
the
battery
pack
to
ground.
This
charger
was
designed
to
limit
current
to
about
1.5
amps
because
of
the
.4
ohm
resistor.
This
resistor
will
have
a
considerable
amount
of
current,
and
therefore
must
be
able
to
handle
a
considerable
power
load.
The
resistor
I
used
was
rated
for
5W,
though
a
3W
power
rating
should
suffice.
In
order
to
add
some
extra
safety
to
this
charger,
a
temperature
sensor
can
be
placed
near
where
the
battery
pack
will
be
while
charging.
By
taking
the
output
of
the
temperature
sensor
and
comparing
it
to
a
set
threshold
voltage
with
a
comparator,
we
can
use
the
output
to
control
a
BJT
as
a
switch,
so
if
the
battery
pack
gets
too
hot,
the
charger
will
stop
charging.
The
LM34
outputs
10
mV
*
the
temperature
reading
in
degrees
Fahrenheit,
so
if
it
is
70
degrees
in
the
room,
the
output
would
read
.7
V.
[5]
The
data
sheet
for
the
lithium
ion
battery
pack
I
was
designing
for
gave
temperature
recommendations,
so
I
designed
the
charger
to
shut
off
it
a
temperature
above
137
degrees
was
reached.
In
order
to
have
the
temperature
sensor
compare
the
current
temperature
to
the
threshold
temperature,
a
DC
voltage
of
1.37
V
Temperature
Sensor
needs
to
be
the
positive
input
for
the
comparator.
It
is
possible
to
transform
the
supply
DC
voltage
using
a
buck
converter,
but
a
voltage
regulator
was
chosen
here
for
simplicity.
Again
the
LM317
was
used
and
an
example
in
the
data
sheet
shows
an
adjustable
regulator
with
improved
ripple
rejection.
Using
this
example,
I
designed
an
output
voltage
of
1.37
volts
by
setting
R1=466
ohms,
and
R2=55
ohms.
Now
by
inserting
a
BJT
between
the
first
LM317
and
the
battery
pack
we
can
turn
of
the
current
if
the
temperature
exceeds
the
set
137
degrees
Fahrenheit.
This
BJT
needs
to
be
able
to
hand
a
lot
of
current,
so
a
TIP31
was
used.
[6]
Conclusion
Designing
a
battery
pack
charger
requires
careful
considerations
for
safety
precautions
and
timely
charging
rates.
Given
the
procedure
described,
many
battery
configurations
could
be
designed
for.
There
are
also
several
other
improvements
that
could
be
made
on
this
design,
including
a
hysteresis
or
permanent
charger
shut
off
when
the
temperature
exceeds
a
threshold
(opposed
to
continuing
charge
when
the
temperature
drops
back
below
the
threshold)
as
well
as
current
sensing
so
when
the
battery
pack
is
almost
full,
it
will
stop
charging.
Inside
finished
charger
Outside
finished
charger
References
[1]
www.allaboutbatteries.com/history-‐of-‐batteries.html.
Retrieved
4
April,
2013.
[2]
“Panasonic
Develops
New
Higher-‐Capacity
18650
Li-‐Ion
Cells;
Application
of
Silicon-‐based
Alloy
in
Anode".
greencarcongress.com.
Retrieved
4
April,
2013.
[3]
Spotnitz,
R.;
Franklin,
J.
(2003).
"Abuse
behavior
of
high-‐power,
lithium-‐ion
cells".
Journal
of
Power
Sources
(Elsevier).
Retrieved
4
April,
2013.
[4]
Fairchid
Semiconductors.
http://www.fairchildsemi.com/ds/LM/LM317.pdf.
Retrieved
2
April,
2013.
[5]
Texas
Instruments.
http://www.ti.com/lit/ds/symlink/lm34.pdf.
Retrieved
2
April,
2013.
[6]
Fairchid
Semiconductors.
http://www.fairchildsemi.com/ds/TI/TIP31C.pdf.
Retrieved
2
April,
2013.
