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Texas Instruments Enhanced Resolution Gauging for Low Current Applications Using Scaling Application notes
Application Report
SLUA792 – October 2016
Enhanced Resolution Gauging for Low Current
Applications Using Scaling
...................................................................................... Battery Management Solutions/Battery Gauges
ABSTRACT
This application report aims to address key concerns and design limitations associated with using large (>
20 mΩ) current sense resistors to achieve greater current resolution on Texas Instruments battery gauge
products. Using this method, it is possible to achieve < 1 mA current resolution and improve accuracy for
systems with small battery capacities which experience very low charge and discharge currents. This
document details the enhanced resolution process and highlights primary concerns associated with large
sense resistors in battery gauging applications. An implementation example with a bq27426 Impedance
Track gauge is included.
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Contents
Introduction ...................................................................................................................
Choosing A Calibration Ratio ..............................................................................................
Hardware Considerations ...................................................................................................
Current Calibration...........................................................................................................
Gauge Configuration Considerations......................................................................................
Enhanced Resolution Example ............................................................................................
References ...................................................................................................................
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6
List of Figures
1
Gauge Reported Current vs Applied Current ............................................................................ 5
2
Charging Profile .............................................................................................................. 6
1
System Design Parameters ................................................................................................ 3
2
Equipment List ............................................................................................................... 3
3
bq27426 Parameter Settings ............................................................................................... 4
List of Tables
1
Introduction
Enhancing current resolution relies on increasing the input to the coulomb counter and adjusting the
device configuration to accommodate this change. The coulomb counter determines the amount of passed
charge by measuring the voltage across the sense resistor. Using a larger sense resistor results in a
larger voltage for the coulomb counter to measure. By scaling the current during calibration, it is possible
to decrease the least significant bit value to less than 1 mA. This process allows the gauge to measure
currents less than 1 mA and enables gauging functionality for systems with < 1 mA discharge current.
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1
Choosing A Calibration Ratio
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Choosing A Calibration Ratio
The calibration ratio refers to the factor by which the reported gauge current will be scaled. This ratio is
chosen by the user in order to meet their application’s necessary resolution. The default resolution is 1 mA
per LSB. The equation below is used to calculate the calibration ratio, K, where LSBvalue is the user’s
desired resolution. For example, if the desired resolution is 100 µA, a calibration ratio of 10x should be
used.
K
3
1 mA
LSB value
(1)
Hardware Considerations
In order to improve resolution below 1 mA, it is recommended to use a larger sense resistor in order to
create a larger voltage for the coulomb counter to measure. A general rule is to increase the resistance by
the same factor as the calibration ratio. For example, if the default sense resistor value is 10 mΩ and a
calibration ratio of 10x is needed, a 100 mΩ sense resistor is recommended.
3.1
Maximum Charge and Discharge Rate
A larger sense resistor will limit the maximum charge/discharge current rate that the fuel gauge can
measure without saturating the coulomb counter. For proper operation, the maximum voltage range of the
gauge coulomb counter, which is described in the device data sheet, must not be exceeded. This limit can
be calculated using the equation shown in Equation 2.
I m ax
V range
R sense
(2)
For example, if a 200 mΩ current sense resistor is used and the voltage range of the gauge coulomb
counter is ±125 mV, the maximum measurable charge/discharge rate is 625 mA.
3.2
System Headroom Reduction
Using a large sense resistor will result in a larger I x R drop across the sense resistor for a given current.
The headroom available to the system would ultimately be less due to this increased voltage drop.
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Current Calibration
The gain and offset of the gauge coulomb counter must be adjusted appropriately in order to measure the
desired current range and achieve greater resolution. This is accomplished through current calibration
using evaluation software such as bqStudio or bqEVSW. When calibrating, the reported current in the
evaluation software should be equal to K x Irate where Irate is the applied current and K is the user’s chosen
calibration ratio. For example, if calibration is performed at an actual current of 100 mA and the reported
calibration current is 1000 mA, the calibration ratio is 10x and all current values will be reported as Iactual
multiplied by the calibration ratio. The least significant bit is equal to 100 µA in this example.
I GAUGE
2
K
I rate
(3)
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Gauge Configuration Considerations
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5
Gauge Configuration Considerations
The gauge uses several parameters including FCC, Qstart and Passed Charge to calculate the current
state of charge for the battery. If the current is multiplied by some scaling factor, these other parameters
must also be multiplied by the same ratio. As such, all current and capacity related data flash parameters
need to be multiplied by the calibration ratio to maintain proper operation and accuracy of the gauge. As
an example, if a 200 mAh battery with a nominal voltage of 3.7 V is used with a calibration ratio of 10x, the
user should set design energy to 2000 mAh and design energy to 7400 mWh. This does present a
limitation on the maximum allowable calibration ratio in order to maintain all data flash parameters within
their respective limits.
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Enhanced Resolution Example
In this example, a bq27426 gauge is used with a 400 mAh battery with nominal voltage of 3.7 V. The
desired resolution is 50 μA. The maximum current the battery is expected to deliver is 400 mA with a
termination voltage of 3.3 V. A CCCV charger is used with a charging current of 400 mA and charging
voltage of 4.05 V.
Table 1. System Design Parameters
6.1
DESIGN PARAMETER
VALUE
Capacity
400 mAh
Nominal Voltage
3.7 V
Resolution
50 µA
Maximum System Current
± 400 mA
Charging Voltage
4.05 V
Charging Current
400 mA
Termination Voltage
3.30 V
Equipment
Table 2 summarizes the equipment used in this example to perform current scaling.
Table 2. Equipment List
6.2
MODEL NUMBER
DESCRIPTION
Description
Evaluation module for bq27426 Impedance Track gas gauge
bq27GDK000EVM
TI Gauge Development Kit
Keithley 2400
TI Gauge Development Kit
bqStudio 1.3.52
Gauge development software
Sense Resistor Selection
To achieve the desired 50 µA resolution, a calibration ratio of 20x is needed. As the default value of the
sense resistor is 10 mΩ, a 200 mΩ, 1% resistor is installed at R1 on the EVM. The maximum measurable
current is 400 mA.
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Enhanced Resolution Example
6.3
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Calibration and Configuration
The gauge development kit was connected to Load+ and Load- on the EVM in order to perform the
calibration. Using bqStudio, a board offset calibration is performed while there is no current flowing in the
system. Once the offset calibration is completed, a charging current of 200 mA is applied to the EVM. On
the calibration tab In bqStudio, type in the calibration current as 4000 mA. This sets the calibration ratio to
20x.
Table 3 summarizes the necessary configuration changes to implement the design parameters in Table 1
and to properly scale capacity and current parameters on the bq27426. Parameters with units of mA, mW
or mAh should be multiplied by the calibration ratio accordingly. Parameters defer from gas gauge to gas
gauge so great care must be taken when making adjustments. The technical reference manual should
always be referenced. For the bq27426 gas gauge, current thresholds are in units of 0.1 Hr rate, meaning
a value of 100 is equivalent to C/10, 200 is equivalent to C/20, and so on. As the design capacity is
already scaled, the threshold rates should not be scaled.
Table 3. bq27426 Parameter Settings
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PARAMETER
CLASS
SUBCLASS
STANDARD VALUE
SCALED VALUE
Taper Capacity (mAh)
Configuration
Charge Termination
50
1000
Taper Rate (0.1 Hr Rate)
Gas Gauging
State
100
100
Chg Current Threshold
(0.1 Hr Rate)
Gas Gauging
Current Thresholds
167
167
Dsg Current Threshold
(0.1 Hr Rate)
Gas Gauging
Current Thresholds
100
100
Quit Current Threshold
(0,1 Hr Rate
Gas Gauging
Current Thresholds
250
250
Design Capacity (mAh)
Gas Gauging
State
400
8000
Design Energy (mWh)
Gas Gauging
State
1480
29600
V at Chg Term (mV)
Chemistry Info
Chem Data
4050
4050
Taper Voltage (mV)
Chemistry Info
Chem Data
3950
3950
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Enhanced Resolution Example
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6.4
Results and Discussion
To verify the resolution, the Keithley source meter was connected to Load+ and Load- on the EVM. Using
the source meter, current ranging from -1 mA (discharging) to 1 mA (charging) were applied to the EVM
while reading the gauge Current() output.
As shown in Figure 1, the gauge reports the measured current with the expected 20X scale factor. The
data readings obtained for current in the range -200 µA to +200 µA is expected to be zero from the gauge
Deadband setting of 200 µA, which limits the gauge from reporting any current below this level.
Figure 1. Gauge Reported Current vs Applied Current
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References
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Figure 2 shows the charging profile for the implemented gauge configuration. The current and voltage
waveforms are consistent with a constant current, constant voltage profile (CCCV). The GDK charges the
battery at a constant current of 400 mA until the battery voltage is greater than the taper voltage threshold
set in data memory. The charger then provides a constant voltage until the charge current has fallen
below 50 mA.
Figure 2. Charging Profile
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References
bq34z100-G1 High Cell Count and High Capacity Applications (SLUA760)
bq27426 System-Side Impedance Track™ Fuel Gauge, data sheet (SLUSC91)
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