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Power Management
Texas Instruments Incorporated
Host-side gas-gauge-system design
considerations for single-cell
handheld applications
By Jinrong Qian,
Applications Manager, Battery Management Applications,
and Michael Vega,
Applications Engineer, Battery Management Applications
Introduction
It is desirable to determine the remaining capacity of a
battery for handheld devices such as smartphones, portable
media players (PMPs), and personal digital assistants
(PDAs). Many handheld portable devices have used voltage
measurement alone to approximate the remaining battery
capacity, but the need for a more accurate method has
become critical in some applications. A host-side gas
gauge has become more attractive than the traditional
pack-side gas gauge since it can reduce the cost of a new
battery pack when the life of the original battery is over.
This article focuses on improving gas-gauge accuracy and
on host-side battery-management design considerations
such as high-accuracy Impedance Track™ gas gauges,
battery insertion, and coordinating operation with the
battery-charging system.
Problems of existing gas gauges
The traditional gas gauge is located in the battery pack as
shown in Figure 1 and is always connected to the Li-ion
cell. The gas gauge monitors the charging and discharging
activity and uses an embedded algorithm to report the
remaining battery capacity. When the battery life is over,
the battery cell along with the pack electronics circuit will
be thrown away, wasting the gas gauge that is still in good
operation. The end user has to buy not only another battery
pack but also another gas gauge. In the host-side gas-gauge
system, the gas gauge is located in the motherboard, while
the battery cell and pack-protection circuit are in the pack
side. With this configuration, the user will not have to pay
for a gas gauge when purchasing a new battery pack; but
there are several design challenges, including battery-
chemistry detection, battery-insertion detection, and
coordinating operation with the battery charger.
A common erroneous belief is that the shrinking run
time of a Li-ion battery is primarily due to depletion of the
battery capacity. However, it is generally not the capacity
loss but the increasing battery impedance that results in
early system shutdown. The battery capacity actually drops
by less than 5%, while the internal DC resistance of the
battery increases by a factor of 2 after approximately 100
cycles. A direct effect of the higher resistance of an aging
Figure 1. Traditional battery pack
PACK+
1k
Ω
10 k
Ω
bq27210
VCC
BAT
0.1 µF
0.1 µF
1k
Ω
SCL
SCL
SRP
0.1 µF
0.1 µF
I
2
R
SNS
20 m
Ω
SDA
SDA
SRN
0.1 µF
1k
Ω
RBI
PGM
VSS
GPIO
PACK
–
RT: 103AT
TS
Protector Controller
12
Analog Applications Journal
High-Performance Analog Products
www.ti.com/aaj
4Q 2007
Texas Instruments Incorporated
Power Management
battery is a higher internal voltage drop in response to a
load current. This voltage drop causes the aging battery to
reach the minimum system operating voltage or battery
cutoff voltage earlier than would a fresh battery.
Conventional gas-gauging technologies—mainly the
voltage-based and coulomb-counting algorithms—have
obvious performance limitations. The voltage-based
scheme, widely adopted in handheld devices such as cellu-
lar phones due to its low cost and simplicity, suffers from
changes in the battery resistance over time. The battery
voltage is given by
Figure 2. Battery-discharge characteristics
over cycle life
4.2
Cycle 1
3.9
Open-Circuit Voltage
3.6
IxR
BAT
3.3
Cycle 200
3.0
Shutdown Voltage
V
=
CV
−
I
×
R
,
2.7
BAT
BAT
where OCV is the battery open-circuit voltage and R
BAT
is
the battery internal DC resistance. Figure 2 shows that the
lower voltage of an aging battery causes the system to
shut down earlier than it would with a fresh battery.
Load conditions and temperature variations can change
the available battery capacity by up to 50%. Most end users
have experienced early system shutdown in portable
devices that lack a true gas gauge. The coulomb-counting
scheme takes the alternative approach of continuously
integrating coulombs going to and from the battery to
compute the consumed charge and state-of-charge (SOC).
With an established value for full capacity, coulomb count-
ing allows the remaining capacity to be determined. The
drawback of this approach is that self-discharge is difficult
to model with accuracy, and without periodic full-cycle
calibration the gauging error accrues over time. None of
these algorithms addresses resistance variations of the
battery. The designer must reserve more capacity by
terminating system operation prematurely to avoid the
unexpected shutdown, leaving a significant amount of
energy unused.
Single-cell Impedance Track gas gauge
What makes Impedance Track technology unique and much
more accurate than other solutions is a self-learning mech-
anism that accounts for the changes in chemical capacity
(Q
MAX
) and the increasing battery resistance that is due
to aging. An Impedance Track gas gauge implements a
dynamic modeling algorithm to learn and track the battery
characteristics by first measuring and then tracking the
impedance and capacity changes during battery use. With
this algorithm, no periodic, full-cycle capacity calibration
is required.
Impedance Track technology enables compensation for
load and temperature to be modeled accurately. Most
important, gas-gauging accuracy can be maintained during
the whole lifetime of the battery. Because system design no
longer requires a premature-shutdown scheme, the battery
capacity can be fully utilized. Impedance Track gas gauges
determine the remaining battery capacity more accurately
than either coulomb counting or cell-voltage correlation.
They actually use both techniques to overcome the effects
of aging, self-discharge, and temperature variations.
2.4
Battery Capacity
Impedance Track devices constantly maintain database
tables to keep track of battery resistance (R
BAT
) as a
function of depth of discharge (DOD) and temperature.
To understand when these tables are updated or utilized,
it is helpful to know what operations occur during different
states. Several current thresholds can be programmed into
the gas gauge’s nonvolatile memory to define a charge; a
discharge; and “relaxation time,” which allows the battery
voltage to stabilize after ceasing charge or discharge.
When a handheld device is turned on, the gas gauge
determines the exact SOC by measuring the battery
open-circuit voltage (OCV) and correlating it with the
OCV(DOD,T) table. After completing OCV measurement,
the gas gauge applies the load, starts the integrating
coulomb counter, and continuously calculates the SOC.
The total capacity, Q
MAX
, is calculated through two OCV
readings taken at fully relaxed states when the battery-
voltage variation is small enough before and after the
charge or discharge activity. As an example, before the
battery is discharged, the SOC is given by
Q
Q
MAX
1
SOC
=
.
1
After the battery is discharged with a passed charge of
Δ
Q, the SOC is given by
Q
Q
MAX
2
SOC
=
.
2
Taking the difference of these two equations and solving
for Q
MAX
yields
1
Q
Q
=
,
MAX
SOC
−
SOC
2
where
Δ
Q = Q
1
– Q
2
. This equation illustrates that it is not
necessary to have a complete charge-and-discharge cycle
to determine the battery’s total capacity. The battery’s
time-consuming learning cycle during pack manufacturing
can therefore be eliminated.
13
Analog Applications Journal
4Q 2007
www.ti.com/aaj
High-Performance Analog Products
Power Management
Texas Instruments Incorporated
The battery’s R
BAT
(DOD,T) table is updated constantly
during discharges, and the resistance is calculated as
battery termination voltage (typically 3.0 V), the SOC
corresponding to this voltage is captured as SOC
FINAL
. The
remaining capacity, RM, is calculated as
OCV DOD T
(
,
− Battery Voltage Under Load
Average Load Current
)
R
BAT
(
DOD T
,
)
=
.
RM
=
(
SOC
−
SOC
)
×
Q
.
START
FINAL
MAX
The gas gauge uses R
BAT
to compute when the termination
voltage will be reached at the present load and temperature.
It also uses R
BAT
to determine the remaining capacity (RM)
by using a voltage-simulation method in the firmware. The
simulation starts from the present SOC
START
and calcu-
lates the future battery-voltage profile under the same
load currents with consecutive SOC decrements. When
the simulated battery voltage, V
BAT
(SOC
I
,T), reaches the
Design considerations for host-side gas-gauge
and battery-charging system
Figure 3 is a circuit diagram of a host-side battery-
management system including the battery charger and gas
gauge. The bq24032A is a power-path-management battery
charger that can simultaneously power the system while
charging the battery.
Figure 3. Host-side gas-gauge and battery-charging system
bq24032A
V
OUT
AC Adapter
System Load
4
15
AC
OUT
USB
20
16
C0
10 µF
USB
OUT
R4:2k
Ω
C1
10 µF
C2
10 µF
2
17
STAT1
OUT
R5:2k
Ω
3
9
High
Enable
STAT2
CE
R6:2k
Ω
I
CHG
19
5
USBPG
BAT
C4: 10 µF
R7:2k
Ω
18
6
ACPG
BAT
High: 500 mA
Low: 100 mA
7
8
High: AC, Low: USB
ISET2
PSEL
R3: 35.2 k
Ω
10
13
ISET1
DPPM
R1: 1.33 k
Ω
I
= 800 mA
R8: 10 k
Ω
CHG
Safety Timer: 5 h
V
14
12
TMR
TS
= 4.0 V
R2: 51.1 k
Ω
DPPM
11
1
VSS
LDO
Q1
100 k
Ω
V
OUT
0.1 µF
TPS71525
2.5
-
V LDO
0.1 µF
12
5
1.82 M
Ω
BAT_GD
VCC
11
2
SCL
BI/TOUT
I
2
Battery Pack
18.2 k
Ω
10
3
T
SDA
TS
103AT
1
4
BAT_LOW
BAT
PACK+
bq27500
6
9
0.1 µF
VSS
NC
Safety
Protector
SRN
SRP
8
7
PACK–
R
: 10 m
Ω
SNS
14
Analog Applications Journal
High-Performance Analog Products
www.ti.com/aaj
4Q 2007
Texas Instruments Incorporated
Power Management
There are several host-side gauging-system design con-
siderations. The first one is to get a new battery’s initial
capacity when it is inserted. Since there is a solid correla-
tion between the battery OCV and SOC, the OCV must be
measured before the battery charging or discharging starts.
For accurate OCV measurement, the bq27500 does not
allow the battery to be charged or discharged after it is
inserted. It first determines if the battery is present or not
by putting the BI/TOUT pin in high-impedance mode and
detecting battery insertion when the BI/TOUT pin voltage
is pulled down. Battery charging is disabled when the
temperature-monitoring pin is pulled to ground by turning
on MOSFET Q1. After the OCV reading is fin
ished and
the
initial battery capacity is accurately learned, BAT_GD is
pulled low, which turns off MOSFET Q1. When the battery
is inserted without an adapter, the gauging system should
wait for a few milliseconds to measure the OCV before
applying power to the system load.
The second design challenge is how to monitor the
battery temperature for charging qualification and for
adjusting the battery capacity. To minimize battery degra-
dation, the gas gauge prohibits battery charging, typically
when the cell temperature is out of the 0 to 45°C range.
The gas gauge also has to monitor the cell temperature to
adjust the battery impedance and capacity. The bq27500
can be configured to monitor the cell temperature through
its TS pin, while the temperature threshold to qualify
charging can be set through the data flash constants. To
minimize power consumption, the gas gauge measures cell
temperature every 1 second by internally pulling BI/TOUT
high and measuring the voltage across the TS pin.
If the
cell temperature falls outside of the preset range, BAT_GD
is pulled high and turns on the MOSFET Q1 so that the
battery charger is disabled until the cell temperature
recovers. In the meantime, the temperature information is
used to normalize cell impedance and adjust the capacity.
Another design consideration is how to minimize the gas
gauge’s total power consumption, since the gas gauge is
always connected to the battery as long as the battery is
inserted. There are four operation modes: NORMAL,
SLEEP, HIBERNATE, and BAT INSERT CHECK. In
NORMAL mode, the gas gauge measures current, voltage,
and temperature and periodically updates the interface
data set. Decisions to change states are also made. The
most power consumed is typically 80 µA. When the
SLEEP-mode bit is set and the average current is below a
programmable sleep current, the bq27500 enters the
SLEEP mode. It periodically wakes to take data measure-
ments and update the data set, then returns to sleep to
minimize current consumption, typically down to 15 µA.
To further reduce current consumption, the gas gauge
enters HIBERNATE mode and consumes only 4 µA if the
average current is less than the HIBERNATE current value
programmed in the flash memory and if the HIBERNATE-
mode bit is set. A cell voltage measured lower than the
HIBERNATE voltage value programmed in the flash
memory can replace the HIBERNATE bit requirement. The
BAT INSERT CHECK mode manages when charging and
discharging are allowed so that OCV measurements can be
taken when a battery pack is inserted into the system. No
gauging occurs in this mode. Once battery insertion is
detected and OCV readings are complete, the gauge
proceeds to NORMAL mode.
Another important design consideration is how to safely
and accurately indicate low battery capacity so that data
can be saved and the system can be safely shut down.
Traditionally, the low-battery indication has been based
purely on the battery voltage because of simple hardware
implementations and cheap solutions. When the battery
voltage is below the preset threshold, the BAT_LOW pin
changes the state and can be used to control the system
for possibly reducing functionality and providing a warning
signal to the end user. However, this method may not be
accurate, since the battery voltage is a function of the load
current, aging, and temperature. The status indicator may
flicker for a pulsating load in handheld applications. Another
method by which BAT_LOW could be configured is based
on the relative SOC, which is more accurate than the pure
voltage measurement. With this method, BAT_LOW will
change its state when either the battery voltage or the
relative SOC reaches the preset threshold. Therefore, the
microprocessor can safely prepare for data saving and
system shutdown in advance.
Conclusion
The host-side Impedance Track gas-gauging system pro-
vides high-accuracy gauging and a low system cost. The
bq27500 is an ideal solution for handheld devices such as
smartphones, PMPs, and PDAs. Understanding the
Impedance Track technology and host-side design
challenges is critical.
Related Web sites
power.ti.com
www.ti.com/sc/device/
partnumber
Replace
partnumber
with
bq24032A,
bq27210,
bq27500,
or
TPS71525
15
Analog Applications Journal
4Q 2007
www.ti.com/aaj
High-Performance Analog Products
TIWorldwide Technical Support
Internet
TI Semiconductor Product Information Center Home Page
support.ti.com
TI Semiconductor KnowledgeBase Home Page
support.ti.com/sc/knowledgebase
Product Information Centers
Americas
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