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Power Management
Texas Instruments Incorporated
Li-ion battery-charger solutions for
JEITA compliance
By Jinrong Qian
Sector Manager, Battery Charge Management – Advanced Portable
Introduction
Lithium-ion (Li-ion) batteries tend to become dangerous
when they are overcharged at high temperatures. Safely
charging these batteries has become one of the most
important design specifications in battery-powered porta-
ble equipment. Progress has been made in establishing
industry standards such as the Japan Electronics and
Information Technology Industries Association (JEITA)
guidelines for improving battery-charging safety. This
article addresses safety requirements and battery-charger
solutions that meet these requirements in both notebook
and single-cell handheld applications.
Battery-charger safety and the JEITA guidelines
Widely used in consumer electronics from cell phones to
laptops, Li-ion batteries have the highest volumetric and
gravimetric energy densities among the rechargeable
batteries, with no memory effect. They also have a self-
discharge rate that is 10 times lower than that of NiMH
batteries, and they can provide the instant power required
by the system; but are they safe?
Everyone in the industry has seen pictures of exploding
laptops and heard about the massive and unprecedented
recalls of Li-ion batteries due to cell safety concerns. Such
battery explosions or fires originated within the manufac-
turing process. Batteries contain several metal parts that
can sometimes result in undesirable metal impurities within
the cell. These impurities are typically sharp metal shards
from the battery casing or from electrode materials. If
these shards get between the battery’s electrode and sepa-
rator, battery cycling in the negative electrode can eventu-
ally cause the shards to puncture the separator. This
results in a microshort between the positive and negative
electrodes, producing high heat that may ultimately result
in fire and/or an explosion.
High temperatures, fire, and explosions are all results of
thermal runaway—a condition whereby a battery enters
into an uncontrollable reaction. Thermal runaway is a
process in which the internal temperature of a battery
with LiCoO
2
as the cathode material and graphite as the
anode material reaches approximately 175°C. This is an
irreversible and highly exothermic reaction that can cause
a fire, usually when the battery is charging.
Figure 1 shows the charge current and charge voltage
over temperature commonly used in the older Li-ion-
battery-charging systems that are prone to thermal run-
away. Both the battery charge current and charge voltage
are constant over the cell temperature from 0 to 45°C.
High cell temperatures not only speed up battery aging
but also increase the risk of battery failure.
To improve the safety of charging Li-ion batteries,
JEITA and the Battery Association of Japan released new
safety guidelines on April 20, 2007. Their guidelines
emphasized the importance of avoiding a high charge
current and high charge voltage at certain low and high
temperature ranges. According to JEITA, problems in the
Li-ion batteries occur at high charge voltages and high cell
Figure 1. Upper-limit charge current and charge voltage
in older Li-ion-battery-charging systems
Upper-Limit Charge Current: 1C
Charge
Current
Upper-Limit Voltage: 4.25 V
(4.2 V Typical)
Charge
Voltage
No Charge
No Charge
TI
(0°C)
T4
(40 to 45°C)
Temperature
8
High-Performance Analog Products
www.ti.com/aaj
1Q 2010
Analog Applications Journal
Texas Instruments Incorporated
Power Management
temperatures. Figure 2 shows the JEITA guide-
lines for the charge current and charge voltage
over cell temperature for batteries used in
notebook applications. These batteries have
LiCoO
2
as the cathode active material and
graphite as the anode active material.
In the standard charging temperature range
from T2 to T3, a Li-ion cell can be charged in
the optimal conditions of the upper-limited
charge voltage and the upper-limited charge
current recommended by the cell’s manufac-
turer for safety.
Charging at low temperatures
If the cell’s surface temperature becomes lower
than T2 during charging, the lithium ions could
each gain one electron and become metallic lith-
ium. This metallic lithium is likely to deposit on
the anode, because at low temperatures the
transfer rate decreases and the penetration of
lithium ions into the negative electrode carbon
slows down. Such metallic lithium could easily react with
electrolyte, causing permanent loss of the lithium ions,
which degrades the battery faster. In addition, the chemi-
cal reaction between metallic lithium and the electrolyte
generates a lot of heat, which could lead to thermal run-
away. Therefore, the charge current and charge voltage
are reduced at low cell temperatures. If the temperature is
further reduced to T1 (0°C as an example), the system
should not allow charging.
Charging at high temperatures
If the cell’s surface temperature rises
above T3 (45°C as an example) dur-
ing charging, the cathode material,
LiCoO
2
, starts to become more active
and can chemically react with the
electrolyte when the cell voltage is
high. If the cell temperature is
further increased to T4, the system
should prohibit charging. If the cell
temperature reaches 175°C with a cell
voltage of 4.3 V, thermal runaway may
occur and the battery may explode.
Similarly, Figure 3 shows the
JEITA guidelines for charging Li-ion
batteries in single-cell handheld
applications, where the charge
current and charge voltage are also
functions of the cell temperature.
The maximum charge voltage of
4.25 V includes the battery charger’s
full tolerance. The battery can be
charged at up to 60°C with a reduced
charge voltage for safety.
Figure 2. JEITA guidelines for charging Li-ion batteries in
notebook applications
Upper-Limit Charge Current
Charge
Current
Upper-Limit Voltage: 4.25 V
(4.2 V Typical)
Charge
Voltage
4.20 V
No Charge
No Charge
T3
(45°C)
T4
T1
T2
(10°C)
Temperature
Battery-charger solutions for meeting
JEITA guidelines
The smart battery pack, which includes a fuel gauge,
analog front end, and second-level protector, is commonly
used in notebook applications. The fuel gauge provides the
battery’s cell voltage, charge and discharge current, cell
temperature, remaining capacity, and run time to the
system through SMBus for optimizing the system perform-
ance. The bq20z45 and bq20z40 fuel gauges with
Impedance Track™ technology, recently developed by
Figure 3. JEITA guidelines for charging Li-ion batteries in
single-cell handheld applications
Maximum Charge Current: 1C
0.5C
Charge
Current
Maximum Charge Voltage: 4.25 V
(4.2 V Typical)
4.15 V Maximum
Charge
Voltage
4.10 V Maximum
T1
(0°C)
T3
(45°C)
T4
(50°C)
T5
(60°C)
Temperature
T2
(10°C)
9
Analog Applications Journal
1Q 2010
www.ti.com/aaj
High-Performance Analog Products
Power Management
Texas Instruments Incorporated
Texas Instruments (TI), include a series of flash-memory
constants for flexibly programming the battery’s charge
current and charge voltage based on the JEITA guidelines.
The temperature thresholds are user-programmable and
provide flexibility for meeting different specifications with
different applications. The fuel gauge usually broadcasts
the charge current and voltage information to the smart
battery charger or keyboard controller for periodically
setting the proper charge current and voltage. An SMBus-
controlled battery charger, such as the TI bq24745, can be
used as a slave device to get the charge voltage and cur-
rent information from a smart battery pack with either the
bq20z40 or the bq20z45 fuel gauge.
Figure 4 shows a schematic of a smart battery charger
with a smart battery pack that complies with the JEITA
guidelines for notebook applications. This SMBus-
controlled battery charger with a synchronous switching
buck converter can support Li-ion batteries with one to
four cells and a charge current of up to 8 A. The dynamic
power-management function allows charging the battery
and powering the system simultaneously without increas-
ing the adapter’s power rating.
The battery pack in single-cell portable devices usually
has the cell and a safety protector but uses the charger
instead of a fuel gauge to monitor the cell temperature
and adjust the charge voltage and current. TI’s bq24050
single-cell linear battery charger was designed to meet the
JEITA specifications for handheld devices. It reduces the
charge current by half when the cell temperature is
between 0°C and 10°C, and reduces the charge voltage to
Figure 4. Smart battery charger bq24745 with fuel gauge bq20z40 or bq20z45
RAC
10 m
D1
Adapter
System Load
R1
430 k
10
1 µF
0.1 µF
CSSN
CSSP
DCIN
R2
66.5 k
bq24745
C8
1 µF
ACIN
ACOK
VDDP
10 k
VREF
C6
10 µF
BOOT
C4
1 µF
R7
200 k
Q3
ICREF
UGATE
RSN
10 m
L
5.6 µH
To Smart
Battery Pack
R8
50 k
0.1 µF
R9
1.4 M
PHASE
GND
ICOUT
C3
10 µF
Q4
LGATE
R4
10 k
PGND
+3.3 V
VDDSMB
CSOP
R11
10 k
R6
10 k
C7
0.1 µF
Keyboard Controller or
Smart Battery Pack with
bq20z40 or bq20z45
CE
CSON
VFB
SDA
SMBus
SCL
0.1 µF
R9 7.5 k
VICM
EAO
C5
100 pF
C23
51 pF
C21
2 nF
R10
20 k
C22
130 pF
R11
200 k
EAI
FBO
10
High-Performance Analog Products
www.ti.com/aaj
1Q 2010
Analog Applications Journal
Texas Instruments Incorporated
Power Management
4.06 V when the cell temperature is between 45°C and
60°C. Figure 5 shows a typical application circuit with the
bq24050 linear charger. The charger monitors the battery’s
cell temperature via the thermistor (TS) pin and adjusts
the charge current and voltage when the monitored tem-
perature reaches the threshold.
Conclusion
Charging Li-ion batteries safely is critical and has become
one of the key specifications for charger design. Reducing
the charge current and voltage at lower and higher
temperature ranges as JEITA recommends can significantly
improve the safety of charging these batteries. Both
switch-mode and linear battery-charger solutions that
comply with JEITA guidelines have been presented.
Related Web sites
power.ti.com
www.ti.com/sc/device/
partnumber
Replace
partnumber
with
bq24050,
bq24745,
or
bq24747
Figure 5. Typical single-cell application circuit with JEITA-
compliant linear battery charger
bq24050
Adapter
Q1
IN
OUT
CHG
VSS
C1
103AT
1 µF
R2
R2
ISET
TS
R1
R1
PRETERM
ISET2
Host
USB Port
LDO/CE
D
–
D+
bq24050
ISET/100/500
VBUS
D+
D+
D–
D–
GND
GND
11
Analog Applications Journal
1Q 2010
www.ti.com/aaj
High-Performance Analog Products
TI Worldwide Technical Support
Internet
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E121709
Impedance Track is a trademark of Texas Instruments. All other trademarks are the property of
their respective owners.
© 2010 Texas Instruments Incorporated
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