Quickstart Guide for bq27441-G1 User's Guide

User's Guide
SLUUAP7 – December 2013
Quickstart Guide for bq27441-G1
The bq27441-G1 is the easiest-to-use lithium battery gauge in the industry. For fast time-to-market and
virtually no development effort, battery characterization, nor learning cycle is required. Once assembled
into the end-system, only a few simple registers need to be configured before accurate gauging results
can be read from the IC. This document outlines the minimum procedure required.
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Contents
Overview .....................................................................................................................
Choose Between bq27441-G1A and bq27441-G1B ...................................................................
Hardware .....................................................................................................................
Programming the Configuration ...........................................................................................
Reading the Gauge Registers .............................................................................................
Gauge Learning .............................................................................................................
Recommended Flowchart ..................................................................................................
Other Configuration Options ...............................................................................................
Summary .....................................................................................................................
Related Documentation from Texas Instruments .......................................................................
Revision History .............................................................................................................
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List of Figures
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Flowchart for Updating the Gauge Configuration Parameters ........................................................
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Gauge Register Commands ...............................................................................................
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Recommended Flowchart ..................................................................................................
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List of Tables
Impedance Track is a trademark of Texas Instruments.
Mathcad is a trademark of Mathsoft, Inc..
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1
Overview
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Overview
The bq27441-G1 fuel gauge utilizes a standard lithium battery profile that matches typical batteries
available in the market. On IC power-up, default settings are loaded into the gauge RAM. These should be
over-written by the system to match the actual battery capacity. Additionally, the full and empty battery
conditions should be updated to match the system requirements. By configuring only these four
parameters, the gauge's battery capacity predictions can be utilized with confidence. During field operation
in the end-equipment, the Impedance Track™ algorithm continually optimizes accuracy by learning the
battery capacity and resistance profile to account for cell-to-cell variations and battery aging.
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Choose Between bq27441-G1A and bq27441-G1B
Because the bq27441-G1 utilizes standard battery profiles, the system designer can select between two
different flavors of the fuel gauge. The bq27441-G1A should be used if the cathode is a traditional LiCoO2
chemistry and/or the battery data sheet recommends a maximum charging voltage of 4.2 V. The bq27441G1B should be selected if the cathode is a lithium-manganese type and/or the maximum recommended
charging voltage is 4.3 V or 4.35 V. These two types of rechargeable lithium chemistries are the most
popular in the industry and other types are not currently supported by the bq27441.
To give further confidence in the suitability of bq27441-G1A or bq27441-G1B for a particular battery
application, the corresponding EVM can be ordered and tested with the target battery. The results of a
battery cycle can be logged with the GaugeStudio companion software and reviewed for accuracy.
The log of a battery cycle can also be analyzed using the Mathcad™ Chemistry Selection Tool (SLUC138)
to assist in the choice.
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Hardware
The bq27441-G1 fuel gauge comes in a tiny 9-pin, 1,62 mm × 1,58 mm, CSP package and only requires a
single 0.47-µF capacitor to be connected between the VDD and VSS pins. Optionally, a 1.0-µF capacitor can
be also connected between the BAT and VSS pins. The fuel gauge has the ability to provide interrupts to
the system through the GPOUT pin. The GPOUT pin is an open-drain output that can be configured to
use the internal pullup resistor of the gauge to 1.8 V or an external 4.7-kΩ pullup resistor can be used.
For accurate gauging the fuel gauge needs to be able to detect that the battery has been inserted into the
system. The fuel gauge has the ability to detect battery insertion through the use of the BIN pin ([BIE] bit
in Op Config register is set) or with the host sending the BAT_INSERT subcommand ([BIE] bit in Op
Config register is cleared). If using the BIN pin for battery detection, then one of the following options can
be used:
• Host can drive the BIN pin to a logic low to indicate the battery is inserted, or
• A weak 1.8-MΩ pullup resistor can be connected between the BIN and VCC pins to pull the BIN pin
high. When a battery pack with a pulldown resistor (typically the battery pack thermistor) is connected
to the system, a logic low is generated, or
• If the battery will remain in the system permanently, then the BIN pin can be tied to VSS through a weak
4.7-MΩ pulldown resistor.
NOTE: The BIN pin must not be connected directly to VSS.
NOTE: The GPOUT internal pullup voltage is 1.8 V. If an external pullup resistor is used, then the
internal pullup resistor must be disabled (default), and the GPOUT pin can be connected to a
voltage between 1.8 V and 3.6 V through a 4.7-kΩ resistor.
2
Quickstart Guide for bq27441-G1
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Programming the Configuration
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4
Programming the Configuration
A number of configuration parameters are available in the bq27441-G1 so that it can be tuned to match
the target battery as well as the system requirements. Most of the defaults can be left alone if desired, but
there are four important parameters that should be configured in order to achieve accurate gauging of the
target battery. These parameters are Design Capacity, Design Energy, Terminate Voltage, and Taper
Rate. If you are using the bq27441-G1 EVM, then GaugeStudio can be used to configure the gauge to fit
the target battery. You can download GaugeStudio at www.ti.com/product/bq27441-G1.
Design Capacity should be set to the nominal battery capacity printed on the battery label or found in the
battery datasheet. It gives a starting point for the gauge's predictions, and actual battery capacity (which
varies from battery to battery and changes over time) is learned during operation.
Design Energy should be set to be Design Capacity × 3.7 if using the bq27441-G1A or Design
Capacity × 3.8 if using the bq27441-G1B. Design Energy is used when the gauge is operating in
constant-power model. The bq27441-G1 defaults to constant-power model and this is reflected by the
[LDMD] bit in the Control() register being set.
NOTE: When updating the fuel gauge with an EVM using GaugeStudio, Design Energy is
automatically calculated based on the type of gauge connected and the Design Capacity.
Terminate Voltage should be set to the minimum operating voltage of your system. This is the target
where the gauge typically reports 0% capacity. It is not usually necessary to include a guard band when
selecting this value, because the gauge also learns the level of load spikes in the system and
automatically uses a higher voltage when necessary to ensure that load spikes, aging, and low
temperatures do not allow sudden voltage drops below Terminate Voltage before 0% is reported. The
actual point at which 0% is reported is therefore dynamic, so Terminate Voltage should be set to the
minimum operating voltage supported by the system. If additional reserve capacity is desired between the
0% point and the actual Terminate Voltage, then the optional Reserve Capacity memory parameter can
also be configured. This ensures that a known amount of energy is available for shutdown activities once
0% SOC is reported, but before Terminate Voltage is actually reached.
Taper Rate should be set to the current threshold in mA below which your charger IC is set to stop
charging once it considers the battery to be full. The Taper Rate is stored in units of 0.1-hr rate and can
be derived from the taper current value in mA by the following equation:
Taper Rate = Design Capacity / (0.1 * Taper Current)
This is simply a way to store the taper current value with respect to Design Capacity. The Taper Rate
value allows the gauge to synchronize its full charge detection point with that of the charger. The gauge
Taper Rate should be set to a value slightly higher than the taper current detection threshold of the
charger (including charger tolerances).
For example, if using a 1000-mA battery and the charger is set to stop charging when the voltage is 4.2 V
and the current tapers to less than 100 mA (±15%), then the bq27441-G1 Taper Rate should be set to 87
(Taper Rate = 1000 / (0.1 × 115 mA) where Taper Current = 115 mA) to give a slight guard band. It is
important that the gauge detect full charge (StateOfCharge() = 100%) before the charger shuts off. An
alternative system design to improve synchronization is to have the system read the [FC] (full charge) bit
from the Flags() register and then disable the charger when it is set.
NOTE: When updating the fuel gauge with an EVM using GaugeStudio, Taper Rate is calculated
automatically based on the Taper Current value.
The procedure and commands required to update the configuration parameters are shown in the flowchart
of Figure 1.
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Reading the Gauge Registers
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Steps to compute the
new checksum
Steps to unseal
the fuel gauge
Start
TMP_CHKSUM = 0xFF – OLD_CHKSUM
I2CWriteByte(0x3E, 0x52, 100)
OLD_DC[2] = I2CReadBlock(0x4A, 2, 100)
I2CWriteByte(0x3F, 0x00, 100)
OLD_DE[2] = I2CReadBlock(0x4C, 2, 100)
CHKSUM = I2CReadBlock(0x60, 1, 100)
I2CWriteWord(0x00, 0x8000, 100)
I2CWriteWord(0x00, 0x8000, 100)
I2CWriteWord(0x00, 0x0013, 100)
Steps to place the
gauge into
CONFIG UPDATE
mode
OLD_TV[2] = I2CReadBlock(0x50, 2, 100)
Flags[2] = I2CReadSubCommand(0x06, 2, 100)
False
CHKSUM ==
NEW_CHKSUM
OLD_TR[2] = I2CReadBlock(0x5B, 2, 100)
Steps to verify
RAM update
completed
correctly
True
[CFGUPMODE] == 1
True
TMP_CHKSUM = TMP_CHKSUM – OLD_DC[2] –
OLD_DE[2] – OLD_TV[2] –OLD_TR[2]
False
I2CWriteWord(0x00, 0x0042, 100)
I2CWriteByte(0x61, 0x00, 100)
I2CWriteByte(0x3E, 0x52, 100)
I2CWriteWord(0x4A, NEW_DC[2], 100)
Steps to setup
block RAM
update
Flags[2] = I2CReadSubCommand(0x06, 2, 100)
Steps to exit
CONFIG
UPDATE mode
I2CWriteWord(0x4C, NEW_DE[2], 100)
I2CWriteByte(0x3F, 0x00, 100)
False
I2CWriteWord(0x50, NEW_TV[2], 100)
OLD_CHKSUM = I2CReadBlock(0x60, 1, 100)
I2CWriteWord(0x5B, NEW_TR[2], 100)
[CFGUPMODE] == 0
True
I2CWriteWord(0x00, 0x0020, 100)
TMP_CHKSUM = TMP_CHKSUM + NEW_DC[2] +
NEW_DE[2] + NEW_TV[2] + NEW_TR[2]
End
Step to seal
the fuel gauge
NEW_CHKSUM = 0xFF - TMP_CHKSUM
I2CWriteByte(0x60, NEW_CHKSUM, 100)
Figure 1. Flowchart for Updating the Gauge Configuration Parameters
NOTE: The process for updating the RAM can also be handled through parsing the data contents in
a *.dffs file generated by GaugeStudio. A *.dffs is a series of I2C commands that can be
processed by the host. Using the *.dffs file will allow the host an alternative route to updating
the RAM instead of following the flow outlined in Figure 1.
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Reading the Gauge Registers
There are a total of 15 registers available for the system to read from the bq27441. The registers most
commonly used are Voltage( ), Temperature( ), AverageCurrent( ), and StateOfCharge( ). The commands
to read these registers are shown in Figure 2. Other useful registers include Control(), Flags(),
RemainingCapacity(), FullChargeCapacity(), and StateOfHealth().
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Quickstart Guide for bq27441-G1
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Reading the Gauge Registers
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Start
Details of the I2CReadSubCommand() function
Note that the fuel gauge
address is 0x55
Voltage[2] = I2CReadSubCommand(0x04, 2, 100)
I2CWrite(0x04, 100)
Buffer[2] = I2CRead(2, 100)
Temperature[2] = I2CReadSubCommand(0x02, 2, 100)
AvgCurrent[2] = I2CReadSubCommand(0x10, 2, 100)
SOC[2] = I2CReadSubCommand(0x1C, 2, 100)
ControlStatus[2] = I2CReadSubCommand(0x00, 2, 100)
Flags[2] = I2CReadSubCommand(0x06, 2, 100)
Additional recommended commands to read from
the fuel gauge. However, the basic set of
Voltage(), Temperature(), AverageCurrent() and
StateOfCharge() will suffice for most applications.
SOH[2] = I2CReadSubCommand(0x20, 2, 100)
RemCap[2] = I2CReadSubCommand(0x0C, 2, 100)
FullChgCap[2] = I2CReadSubCommand(0x0E, 2, 100)
End
Figure 2. Gauge Register Commands
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Gauge Learning
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Gauge Learning
The gauge makes initial capacity predictions using the value entered in Design Capacity. However, as
the battery is charged and discharged, the gauge is learning the true battery capacity which is usually
slightly different from the nominal capacity. It stores the learned maximum battery chemical capacity in
Qmax, as well as the learned resistance profile in the Ra table. As long as the gauge has power, Qmax
and the Ra profile will update once the proper conditions for an update have been met.
The [QMAX_UP] bit in the Control() register can be checked by the host to see if there has been a Qmax
update. The [QMAX_UP] bit is set on the first Qmax update after a reset or battery insertion event. Once
the [QMAX_UP] bit has set, resistance updates are allowed. The [RES_UP] bit in the Control() register is
set once there has been an update to the Ra profile during discharge. Both the [QMAX_UP] and
[RES_UP] bits remain set until the gauge is reset or a battery insertion event occurs.
It is recommended that the host save the Qmax and Ra profile in the system NVM once the [QMAX_UP]
and [RES_UP] bits have set. After the initial Qmax update, updates to Qmax most likely occur upon:
• Entering discharge mode
• Exiting discharge mode
• Being in relaxation mode for a while after a certain amount of charge has been removed or added to
the battery.
Subsequent resistance profile updates occur only during discharge as long as a Qmax update has
previously occurred which can be verified by the [QMAX_UP] bit in the Control() register being set.
Ideally, the system should be designed so that the bq27441-G1 is always powered by the battery, even
during system shutdown. The gauge maintains the learned values in RAM as long as it is powered and
operates in NORMAL, SLEEP, or HIBERNATE mode. The gauge automatically transitions to SLEEP
mode when the system current is low to minimize power consumption.
If the battery is removed or the gauge is put into SHUTDOWN mode, the learned values stored in RAM
are lost. Once powered up again, the [ITPOR] bit in the Flags() register is set indicating that all values are
initialized to the defaults, including Design Capacity, Design Energy, Terminate Voltage, and Taper
Current. The [ITPOR] bit indicates one of the following:
• The fuel gauge has been reset due to loss in power.
• A full RESET (0x0041) Control() subcommand has been sent to the gauge.
• The gauge has exited the SHUTDOWN mode.
Sending a SOFT_RESET (0x0042) Control() subcommand to the gauge clears the [ITPOR] bit. The host
can use the [ITPOR] bit to determine if the gauge memory parameters need to be reloaded. If the [ITPOR]
bit is set, at a minimum the Design Capacity, Design Energy, Terminate Voltage, and Taper Current
values should be reloaded to the gauge. Additionally, the host has the option of saving the learned Qmax
and Ra values into the system NVM periodically.
NOTE: The SOFT_RESET subcommand is used to exit the RAM update process. Therefore, the
[ITPOR] bit clears after the RAM has been updated.
See Figure 1 for more details on the RAM update process.
6
Quickstart Guide for bq27441-G1
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Recommended Flowchart
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7
Recommended Flowchart
Figure 3 incorporates all of the best practices described above and describes the recommendation for
system software implementation in programming and interacting with the bq27441-G1 fuel gauge. As seen
in Figure 3, the host system can decide to check the bq27441-G1 for updating the RAM or storing some of
the memory parameters into system NVM based on when the gauge has entered or exited discharge
mode. By default, the bq27441-G1 issues an interrupt on the GPOUT pin when the device enters into
discharge mode, the host can then check to see if the [DSG] bit in the Flags() register is set to verify that
the gauge is in discharge mode. Using interrupts from the gauge allow the system to reduce power. The
alternative approach to when the host system can decide to check the bq27441-G1 is after a certain
amount of time between checks has passed. The time limit of 12 hours is only a suggested amount of time
between checks and this value can be reduced or increase based upon the needs of the system.
Fuel gauge initialization from reset
False
[INITCOMP] == 1
True
Update fuel gauge memory parameters
(see Figure 1)
The host can check to see if the
fuel gauge RAM should be
updated or if values from the
fuel gauge should be stored in
the system NVM at the beginning
and end of discharge
False
[QMAX_UP] == 1
Normal system operation
True
Store Qmax in system NVM
False
System has
entered/exited
discharge
True
True
[ITPOR] == 1
False
[RES_UP] == 1
False
True
True
False
Run time > 12 hours
Store resistance profile in system NVM
Store other learned parameters
The host has the option to store other learned
parameters from the fuel gauge as well
Alternatively an amount of time
for the host to wait between
checking can be used to see if the
fuel gauge RAM should be
updated or if values from the fuel
gauge should be stored into the
system NVM. Note that 12 hours
is only a suggestion, this value can
be altered according to system
needs
Figure 3. Recommended Flowchart
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Other Configuration Options
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Other Configuration Options
The previous instructions outlined the minimum registers to update for accurate fuel gauging. Other
registers are available for configuration to enable different options (such as interrupt conditions) as well as
to pre-optimize the battery profile.
When configured with the nominal battery capacity, the fuel gauge automatically begins learning the true
battery capacity and resistance profile during operation in the end-equipment. This means accuracy
automatically optimizes itself as the user charges and discharges the battery. If desired, the system
designer can perform a simple one-day optimization cycle before production to determine the best settings
for the fuel gauge. This refinement is easiest to perform using the bq27441EVM, but the actual system
can also be used. Once determined, these optimized parameters can be included in the configuration
update outlined above and require no additional overhead. This pre-optimized golden file allows the end
user to experience higher initial accuracy out of the box without having to wait for the gauge to learn
during operation.
Whether the optimization is performed during R&D or in the field operation of the end-equipment, the
patented Impedance Track™ algorithm continually learns and updates the battery capacity and resistance
profile as the battery ages. This enables it to be the only gauging algorithm available in the market that
can maintain the same accuracy for aged batteries as for new ones. Even if the user swaps between
batteries of different ages, the fuel gauge quickly learns and regains optimum accuracy.
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Summary
By requiring no battery characterization and only needing a minimum of four registers to be updated on
power-on reset (POR), the bq27441-G1 fuel gauge allows system designers to quickly incorporate fuel
gauging functionality into their design with minimal effort. Using the patented features of the Impedance
Track™ algorithm allows the gauge to continually optimize its predictions during end-equipment operation
to account for battery variations and aging.
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Related Documentation from Texas Instruments
To obtain a copy of any of the following TI documents, call the Texas Instruments Literature Response
Center at (800) 477-8924 or the Product Information Center (PIC) at (972) 644-5580. When ordering,
identify this document by its title and literature number. Updated documents also can be obtained through
the TI Web site at www.ti.com.
1. bq27441-G1, System-Side Impedance Track™ Fuel Gauge Data Sheet (SLUSBH1)
2. bq27441-G1, Technical Reference Manual (SLUUAC9)
3. bq27441-G1, EVM: Single-Cell Impedance Track™ Technology User's Guide (SLUUAP4)
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Revision History
Version
Change Date
—
December 2013
Description
Initial Release
Quickstart Guide for bq27441-G1
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