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Texas Instruments Impedance Track Gauge Configuration For Dynamic Loads (EPOS) Application notes
Application Report
SLUA948 – September 2019
Impedance Track™ Gauge Configuration For Dynamic
Loads (EPOS)
Dominik Hartl
ABSTRACT
TI’s Impedance Track algorithm for battery gauging uses load predictions to determine capacity. This
application note discusses various options to tune the algorithm for dynamic loads. It is intended to help
an engineer configure a TI Impedance Tracking gauge.
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Contents
Impedance Tracking ......................................................................................................... 2
Load Prediction .............................................................................................................. 5
Highly Dynamic Load Example (EPOS) .................................................................................. 9
List of Figures
1
Discharge Example .......................................................................................................... 3
2
Capacity Simulation ......................................................................................................... 4
3
Coulomb Counting ........................................................................................................... 5
4
Constant Current Simulation ............................................................................................... 6
5
Constant Power Simulation................................................................................................. 7
6
Avg Load Previous Discharge
7
Avg Load Since Start of Discharge ........................................................................................ 8
8
Low Pass Filtered Average Load .......................................................................................... 9
9
C/5 Load ...................................................................................................................... 9
10
SOC Jump
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12
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14
15
16
.............................................................................................
..................................................................................................................
Flat Zone Simulation Termination ........................................................................................
Simulated Voltage Mismatch .............................................................................................
User Defined Load .........................................................................................................
Reserve Capacity and Lower Terminate Voltage ......................................................................
Capacity Simulation ........................................................................................................
Before and After Optimization ............................................................................................
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15
Trademarks
Impedance Track is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
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Impedance Tracking
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Impedance Tracking
A battery gauge running TI’s Impedance Tracking algorithm reports the Remaining Capacity (and State Of
Charge) of a battery for an assumed load current. This is based on a prediction, which uses past load
characteristics. The algorithm has access to three periodic measurements:
• Current
• Voltage
• Temperature
The gauge uses a model of the cell to gauge how much capacity is still available to the system until the
cell’s voltage reaches the system’s minimum operating voltage (Terminate Voltage). While capacity is still
available, this voltage will be reached sometimes in the future and as the cell’s voltage is a function of
load, the gauge must make a prediction based on load estimation. The capacity prediction based on load
estimation is performed at various times during a discharge. The gauge will run discharge simulations
from the actual depth of discharge to a predicted time when the simulated voltage will cross the Terminate
Voltage threshold. The result of this simulation is a predicted Remaining Capacity. Applications with a very
dynamic load profile need special considerations to achieve good gauging performance because load
prediction will need to account for dynamic loads.
1.1
RM, FCC and SOC
The three main gauging results are:
• Remaining Capacity (RM)
• Full Charge Capacity (FCC)
• State Of Charge (SOC)
Remaining Capacity (RM) is the amount of charge (coulombs) that can be drawn from the battery until it is
deemed empty.
Full Charge Capacity is the total amount of charge that the battery can deliver to a system for a specific
set of conditions (average load, temperature).
State Of Charge (SOC) is a unit-less representation of RC relative to Full Charge Capacity. SOC [%] =
100 * RC/FCC.
A battery is deemed empty when its voltage drops below a threshold (Terminate Voltage).
1.2
Capacity Simulation
The Impedance Tracking algorithm determines FCC, RM and SOC through simulations.
FCC: This is established before discharge starts and is based on a simulation from full to empty for a
specific temperature and anticipated load.
RM: The algorithm triggers simulations at various points during a discharge. Triggers are significant
changes in discharge level and changes in temperature (among others). Simulations become more
frequent closer to Terminate Voltage. Between simulations, the algorithm uses the coulomb counter to
adjust RM.
SOC: A simple equation: SOC [%] = 100 * RM/FCC
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Figure 1. Discharge Example
This picture shows a typical progression of cell voltage and SOC over time during a discharge.
The grey vertical lines denote when the gauge ran a capacity simulation during discharge.
The capacity simulation is the core of the Impedance Tracking algorithm. Every gauging result that the
gauge reports is based on capacity simulations. As these capacity simulations assume a specific load, the
load prediction is extremely important for accurate gauging results.
When a simulation is kicked off (by one of the triggers during discharge or when the gauge determines
initial capacity), the gauge will use the built in model of the battery (ChemID) to calculate the voltage drop
across the internal impedance for a predicted load. As the predicted load discharges the simulated
battery, the voltage will start to drop. The simulation runs until the voltage drops below Terminate Voltage.
The accumulated charge (by integrating the predicted load current) equals the capacity.
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Impedance Tracking
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Figure 2. Capacity Simulation
This picture shows the actual progression and result of a capacity simulation. Note that this shows a
predicted discharge, not a physically existing discharge. The horizontal axis is the simulation time. The
vertical axis is the simulated cell voltage (based on ChemID voltage and impedance tables). Capacity
result is the accumulation of simulated charges (simulated current) from starting depth of discharge all the
way when the simulated voltage crosses Terminate Voltage.
The initial capacity estimate is performed with a load prediction based on parameters stored in data
memory / configuration (average I/P from the previous discharge). These parameters are part of the
golden image.
Initial Capacity Simulation:
FCC = Simulation Result
Remaining Capacity Simulation:
RM = Latest Simulation Result + Coulomb count
1.3
Coulomb Counting
The gauge doesn’t run simulations continuously. In between simulations, the gauge uses coulomb
counting to adjust RM.
Any abrupt changes in RM (for example, if the load changed significantly) after a simulation may cause
jumps in reported SOC.
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Load Prediction
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The gauge uses an (optional) smoothing algorithm if jumps are not acceptable (even though they reflect
the true RM and true SOC).
Figure 3. Coulomb Counting
The zoomed detail shows when the gauge uses the coulomb counter (delta sigma ADC) to adjust RM
between simulations.
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Load Prediction
Remaining capacity simulations use load prediction (future load) and therefore it is very import to predict
the load accurately.
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Load Prediction
2.1
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Load Mode
The gauge uses two main load prediction models (“Load Mode”) that are available in data memory /
configuration:
1. Constant Current: Capacity simulations use a constant current, regardless of simulated battery voltage.
Figure 4. Constant Current Simulation
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Load Prediction
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2. Constant Power: Capacity simulations use a variable current as a function of simulated battery voltage.
The current is adjusted automatically so that the simulated power drawn from the battery is constant.
Figure 5. Constant Power Simulation
Besides choosing the Load Mode (Constant Current or Constant Power), one must also select how the
gauge will determine the load for simulations.
2.2
Load Select
The gauge uses the Load Select parameter from the data memory / configuration to predict current (or
power) for simulations.
The algorithm has the following options:
• Average discharge load from the previous discharge cycle:
The gauge calculates the average load from the previous discharge cycle and saves this in data
memory (Avg I/P Last Run – this is also part of the golden image). Capacity simulations use this data
(regardless of present load) for load prediction.
Use this if the load profile is not known but not expected to change much between discharge cycles.
• Present average load from the beginning of the current discharge cycle:
The gauge calculates the average load from the current discharge cycle and uses this for capacity
simulations.
Use this if the load profile not known but expected to change significantly between discharge cycles.
This is the default setting.
• Low pass filtered average load:
The gauge calculates a low pass filtered version of the average load and uses this for capacity
simulations.
Use this if the load profile is dynamic. This can lead to underestimation of capacity if the low pass
filtered load is larger than the average load over the discharge cycle and lead to accelerated decrease
of SOC.
• Design Capacity / 5:
The gauge uses a C/5 load value for capacity simulations.
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Load Prediction
•
2.2.1
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Use this if the load profile is not known.
User Rate:
The gauge uses a fixed load value for capacity simulations.
Use this if the load profile is very dynamic (choose the max load) and if underestimation of capacity is
preferred over fast changes in State Of Charge.
Load Select Examples, Dynamic Load
The following examples show the impact of Load Select for an application with a highly dynamic load
(EPOS, where the printer is drawing a lot of current. The printer is on for short times only.):
Terminate Voltage is in a steep zone of the cell’s voltage curve so capacity simulations will terminate
cleanly and stable:
Figure 6. Avg Load Previous Discharge
Figure 7. Avg Load Since Start of Discharge
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Figure 8. Low Pass Filtered Average Load
Figure 9. C/5 Load
3
Highly Dynamic Load Example (EPOS)
Some applications have a load profile that is very challenging for battery gauging.
One example is handheld Electronic Point Of Sale (EPOS) devices that have a printer which operates in
short bursts during the day but long periods at the end of the day.
The load from the EPOS electronics except for the printer is fairly low (for example, C/10) with the load
from the (thermo) printer exceeding this significantly (for example, 2C).
Here is the gauging result with poor capacity prediction due to a configuration that is not adequate for the
dynamic load of the application:
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Highly Dynamic Load Example (EPOS)
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Figure 10. SOC Jump
During discharge, the load changed dynamically between around C/10 (EPOS display and keypad) to 2C
(short bursts of printer activity). At the end of discharge, the printer is turned on continuously, which
drastically changes the load profile right when load prediction is the most critical for gauging performance.
Because the average load was a lot smaller than the final load, the gauge over-estimates capacity, which
results in a sudden drop in SOC when the cell voltage drops below Terminate Voltage.
There are two problems with this particular example:
1. Terminate Voltage should be in the steep zone of the loaded cell voltage. This will allow the simulation
to terminate accurately and stable. If the Terminate Voltage shifts into the flat zone of the loaded cell
voltage, a small change in simulated voltage will cause a big change in simulated capacity.
2. Inadequate Load Select for this application.
3.1
Terminate Voltage and Flat Zone
It is very important that the Terminate Voltage that the gauge uses for capacity simulations is in the steep
zone of the simulated, loaded cell voltage.
Very high loads, like 2C in this example, may shift the Terminate Voltage before the drop off of the
simulated, loaded cell voltage at high depth of discharge. This can lead to large changes in simulated
capacity for small changes in simulated voltage (and hence it can impact the quality of the simulation
result):
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Figure 11. Flat Zone Simulation Termination
3.2
Inadequate Load Select
The default setting for Load Select can be inadequate if the load changes significantly which can cause
the predicted load to be incorrect:
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Highly Dynamic Load Example (EPOS)
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Figure 12. Simulated Voltage Mismatch
The gauge uses the average load since start of discharge in this example. While this is a good choice for
most applications, it will not work well for this example and cause a mismatch in predicted capacity vs.
true capacity. The simulation shows that the gauge overestimates simulated voltage because it
underestimates the load, hence there is a gap between actual voltage and simulated voltage as indicated
by the red ellipse in this picture.
3.3
Solutions
Changing Load Select will eliminate the jump in SOC for this application at the expense of
underestimating capacity in possible other uses cases where the load is not significantly increased close
to end of discharge.
The simplest method is to change the Load Select from the default (average load from start of discharge)
to a static, user defined load that is close to the maximum load.
Here is an example for a user defined load (1.5C):
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Figure 13. User Defined Load
The gauge will underestimate capacity during the earlier parts of the discharge because the actual
average load is significantly less than the user defined load (C/2 vs. 1.5C). It will eliminate the jump in
SOC at the end of discharge but gauging performance in general is poor due to the severe
underestimation of capacity for most of the discharge.
An alternative approach is to improve simulation stability by shifting the Terminate Voltage threshold into
the steep part of the simulated, loaded voltage and subtracting excess capacity with the Reserve Capacity
parameter.
This application requires the gauge to report 0% SOC at a cell voltage of 3500 mV. This is high enough
for the simulated, loaded voltage to shift into the flat zone. Changing this to 3200 mV and subtracting
excess capacity (500 mAh) allows for better simulation and capacity predictions:
Terminate Voltage = 3200 mV
Reserve Capacity = 500 mAh
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Highly Dynamic Load Example (EPOS)
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Figure 14. Reserve Capacity and Lower Terminate Voltage
Note that the effective voltage where the gauge hits 0% SOC is 3500 mV
Figure 15. Capacity Simulation
Comparison between original configuration and optimized configuration:
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Figure 16. Before and After Optimization
A good approximation of adequate Reserve Capacity can be determined by analyzing a constant current
discharge with a typical high load for the application:
Integrate the charges passed between the desired Terminate Voltage (for example, 3500 mV) and the
lowered Terminate Voltage (for example, 3200 mV, or any voltage in the steep zone and greater than
3000 mV) and use the result for Reserve Capacity.
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