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Power Management
Texas Instruments Incorporated
Getting the most battery life from
portable systems
By Keith Keller,
Power Analog Field Applications,
and M.A. Banak,
Electronics Design Consultant
Introduction
Run time of portable systems is increasingly important as
devices become more feature-rich with power-hungry
processors, transmitters, receivers, and media playback, to
name just a few. Using a battery “fuel gauge” is the natural
step to increasing run time of the system. Unfortunately,
previous-generation gauges are up to 15% inaccurate,
depending on usage profiles and cell aging. In this article,
we describe a third-generation technology from Texas
Instruments (TI) called
Impedance Track
™, which can
provide up to 99% accuracy for the entire lifetime of the
battery pack and extend its life.
Common problems in Li-ion battery applications
The most important considerations in the design of a bat-
tery subsystem are its safe use and the reliability of the
available power; but even the safest design is of little value
if the battery life is unpredictable. No one expects a
charged battery to last forever, and fault mechanisms are
an established fact of life. Thus, the battery-system
designer must devise a means of telling the user how
much charge is left in the battery and of protecting the
cells from various fault conditions. Much of this attention
centers around the management of the battery near the
point where it is considered fully discharged.
Customers do not like to have their systems abruptly
halt or, even worse, halt and lose data. With previous-
generation battery-monitoring technology and an “aggres-
sive” system implementation that did not take into account
real-life battery behavior, unpredictable system shutdown
was a real possibility. Inaccuracies in true capacity would
creep in over time. We could only make an educated guess
as to how the individual cells would age over time (develop
an increased internal impedance of the electrolyte anode/
cathode material) from normal charge/discharge cycling.
Figure 1 shows that with normal cell aging, 500 charge/
discharge cycles can increase cell impedance such that the
run time is half that of a new cell. (A cycle is defined as a
transfer of greater than 70% of the total energy out of and
into the cell.)
Figure 1. Impedance change with charge/discharge aging
4.2
4.0
Cycle 1
3.8
Cycle 100
Cycle 200
3.6
Cycle 300
3.4
Cycle 400
Cycle 500
3.2
System Termination Voltage
3.0
0.4
0.8
1.2
1.6
0
2.0
Run Time (hours)
8
High-Performance Analog Products
www.ti.com/aaj
4Q 2008
Analog Applications Journal
Texas Instruments Incorporated
Power Management
Li-ion cells have certain known characteristics.
Impedance is extremely dependent on tempera-
ture during discharge. High temperatures and a
minute overvoltage on a cell cause a large degra-
dation to cell lifetime. Figure 2 shows that charg-
ing a cell even 50 mV higher than its specified
maximum will decrease cell life by up to 50%.
1
Figure 3 shows that cells discharged more than
80% will see a fivefold increase in DC impedance
(from approximately 300 mW to greater than
1.5 W) from room temperature to 0ºC.
1
Overanextendedperiodoftime,itispossible
for cells in series to become imbalanced. The
usable life of the pack can be reduced if one cell in
the stack reaches the cell undervoltage (CUV)
sooner than the others. At that point the cell pack
needs to report zero capacity and shut down. An
analogy is a chain, which is only as strong as its
weakest link. In extreme circumstances, one of the
low-cell-voltage protectors can trip and immediately halt
all further discharge. The system shuts down without
warning, yet previous-generation fuel gauges would report
more than adequate time remaining.
Indeed, the mistracking of protection devices with fuel-
gaugeregistershasbeenaperennialproblem.Onestrategy
for coping with this has been to pad the fuel-gauge report
with considerable margin. This allows the system designer
Figure 2. Charge voltage affects battery service life
1100
1000
900
800
700
4.2 V
4.25 V
4.3 V
4.35 V
600
500
400
0
100
200
300
400
500
600
Number of Cycles
to guarantee that zero capacity is reported at a cell level
that is well above the low-voltage shutdown threshold.
This will prevent an unexpected shutdown while the
system is in service; but typically the 15% margin needed
to guarantee reliability is a high price to pay, and this
margin may need to be further increased to allow for the
uncertainties of cell aging, temperature effects, and user
discharge profiles.
Figure 3. Li-ion impedance dependence on temperature and depth of discharge
3
0°C
10°C
20°C
30°C
40°C
50°C
2
1
0
0
20
40
60
80
100
State of Discharge (%)
9
Analog Applications Journal
4Q 2008
www.ti.com/aaj
High-Performance Analog Products
Power Management
Texas Instruments Incorporated
A complete model of battery capacity under
all conditions
The state of charge for a Li-ion cell can be fully predicted
if the following parameters are included in a comprehen-
sive model:
•Thecell’stotalchemicalcapacity(Q
max
). This is initially
specifiedasthedatasheetcapacity(e.g.,2400mAhfor
an18650cylindricalcell),butthefuelgaugeautomati-
cally updates it after the first charge/discharge cycle of
the battery.
•Theamountofelectricchargethathaspassedintoor
outofthecell,whichismeasured/acquiredbythe
coulomb-counting process.
•Thepresentloadcurrentinthesystem(average
and peak).
•Thecell’sinternalresistancewhiledeliveringcurrent.
Thisvarieswithtemperaturechanges,effectsofcell
aging,andthecell’sstateofcharge.
•Thecell’srelaxedopen-circuitvoltage(OCV).Thisis
measuredatlightload(<C/20)withachangeinbattery
voltage of less than a few microvolts over a sampling
period.Whenfullycharged,thecellrequiresashorter
rest period than when it is deeply depleted.
Aprecisecapacityestimatecanbecalculatedbymea-
suringthecell’sopen-circuit(relaxed)voltage,monitoring
thevoltageprofileofthecellunderload(findingthecell’s
impedance),andintegratingcurrentinandoutofthe
battery.AllLi-ionbatterieswiththesamechemistryand
anode/cathode material have extremely similar relaxed
OCVprofiles.Thisvoltagemeasurementdirectlycorrelates
tothecell’sstateofchargeandisamazinglyindependent
ofthecellmanufacturer.Forexample,withLiMnO
2
cells
fromeitherSonyorPanasonic,anOCVmeasurementof
3.9Vwillequateto90%fullcharge.Keepinmind,though,
thatLiMnO
2
cellsdonothavethesameOCVprofileas
LiCoO
2
orLiFePO
4
cells.
TI’s
Impedance Track
™ solution
Overthelastseveralyears,advancedbattery-management
solutionsfromTIhaveevolvedfromatwo-ICchipsettothe
currentlyavailablesingle-ICsolutionslikethebq20z75and
bq20z95.(Forsingle-cell“1s”applications,pleaseseethe
bq27500-v100andbq27540.)TheseSmartBattery
Specification(SBS)1.1-compliantICsareprotection-
enabled and implement
Impedance Track
fuel-gauging
technologytocontinuouslyanalyzethebatteryimpedance
andmaximumcapacity(Q
max
),resultinginsuperiorfuel-
gauging accuracy.
Improved accuracy means greater available capacity
Fuel gauging is used to provide a graceful system shutdown
as the battery approaches its end of discharge. The remain-
ing charge is estimated and used to trigger shutdown when
thebatteryisgettingclosetoempty.Asmentionedearlier,
most designs using legacy devices must allow for an
inaccuracymarginofupto15%becauseerrorscreepin
whenafulldischargedoesn’toccur.Withthe
Impedance
Figure 4. Extended capacity available with
Impedance Track
technology
4.5
Legacy System
15% Reserve Cutoff
4
7%
3%
0%
3.5
3
0
1
2
3
4
5
6
Capacity (Ah)
Q
max
Track
system,whichcanprovideupto99%accuracy,we
canprogramtheterminatevoltage(V
Term
)atwhichzero
capacityisdefined.Ifwehavetrue99%accuracy,thenthe
14%capacitywegainbackcanbeusedtosignificantly
increasetheruntimeofthesystem(seeFigure4).
Forexample,at7%capacity,theoperatingsystemcould
warntheuserthatashutdownisimminent;and,at3%,a
shutdowncouldbeforced,includingsavingthedata.That
last1to2%ofcapacitycouldbesavedforservicingafew
extrareboots,wheretheuserisremindedthatthebattery
is empty and needs to be recharged. The
Impedance
Track
system also automatically balances the cells during
chargetaper,thussqueezingadditionalcapacityfromthe
cell stack and extending the life of the pack overall.
Elimination of charger banks on the production floor
LegacyfuelgaugesrequireQ
max
calibration with a four-
step charge/discharge cycle:
1.Assemblepack;calibratevoltage,current,andtempera-
ture; run final electrical check.
2.Dischargepacktoempty.
3.Chargepacktofullcapacity.Thefuelgaugehasnow
learnedQ
max
.
4.Drainoffcellstoapproximately40%capacityforstorage.
The entire process can take nearly an entire production
shift,andtheproduction-gradechargersneedmainte-
nance.Atypicalfactorywillhavedozensofsuchchargers
onthefloor,whileproductiontechniciansspenddizzying
hours keeping them in repair.
With
Impedance Track
technology,weneedonlycharac-
terizeatypicalbatterypackfromthetargetedcellmanufac-
turerandthensavethosecharacterizationparametersfor
download to all cell packs. Production packs are assembled
withthecellsatthechargelevelsetbythemanufacturer,
andasimple,inexpensiveprogrammingfixtureisusedto
program the onboard flash memory with the design
parameters.
Withthismethod,thereisnolostproductiontime,no
lostfactoryspaceforallthosechargers,andnoneedfor
special development of factory-grade chargers.
10
High-Performance Analog Products
www.ti.com/aaj
4Q 2008
Analog Applications Journal
Texas Instruments Incorporated
Power Management
Smart charger/smart battery interface
Conformance to the SBS allows smart chargers to com-
municate directly with the battery pack. When paired with
an SBS-compliant battery charger, an
Impedance Track
fuel gauge like the bq20z75 can request optimal charge
currents to properly top off cell capacity via the SMBus
interface. Also, status flags in the charger are visible to the
host processor, which will broadcast timely warnings to
the charger to stop charging. In short, the entire business
of optimizing charger design is resolved by using this SBS-
compliant battery monitor/fuel gauge.
During field operation, all standard SBS commands can
be requested of the battery pack and fuel gauge, such as
AtRateTimeToEmpty (0x06), Temperature (0x08), Voltage
(0x09),Current(0x0a),RelativeStateOfCharge(0x0d),
RemainingCapacity (0x0f), and CycleCount (0x17), to
name just a few (see Figure 5). Support for many of these
commands already exists within operating systems such as
WinCE. Please note that SMBus communication is only
point-to-point. Multiple batteries are not allowed to com-
municate on the same lines. There are several I
2
C/SMBus
expanders to help us with multiple-pack systems.
Up-front design effort saves time and lowers costs
The advantages of using
Impedance Track
technology are
realized from the engineering effort that is invested up
front. The ICs come preloaded with defaults that require
refinement for a specific target application. With proper
configuration, an optimal design can be made to run
trouble-free in the field.
It is important to note that the comprehensive nature of
the
Impedance Track
system actually makes for a rigor-
ous checklist of design considerations for the engineer.
Taking the time to work through each configuration regis-
ter makes battery-pack design issues become evident. It is
Figure 5. SBS data screen
11
Analog Applications Journal
4Q 2008
www.ti.com/aaj
High-Performance Analog Products
Power Management
Texas Instruments Incorporated
common for designers to discover overlooked areas of
concern well before product release.
Configuration of the “golden-unit” battery pack is accom-
plished with a device evaluation module (EVM), an EV2300
USB interface board, and device-specific profiles in the
support software, all available from TI. The software has a
graphical user interface (GUI) that allows the entry of
configuration data for the battery-pack design. The soft-
ware also includes a system-setup wizard called bqEASY™,
which allows the designer to answer questions about
the system and then configures the dataflash constants
automatically.
For example, the
Impedance Track
analog front end
(AFE) will service a complete menu of alarm options,
covering overcurrent during charge and discharge. These
alarm threshold values are entered into the software’s GUI
menus, from which they are downloaded into the EVM
board. In each case, these limits must be based on the
expected system parameters. The specifications in the
datasheet for the particular cell size employed must also
be considered.
Likewise, the cell manufacturer’s requirements for
charge/discharge current levels and battery capacity must
be entered into the bqEASY wizard or the GUI menus (see
Figure6).Otherconfigurationfeaturesincludeunitsof
capacity and SMBus communication options.
It is rare that the defaults in the IC configuration are
adequate for a particular design. Along with the bqEASY
wizard software, TI offers a set of quick-start instructions
to get past the defaults for the most common applications.
Figure 6. bqEASY configuration wizard
12
High-Performance Analog Products
www.ti.com/aaj
4Q 2008
Analog Applications Journal
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