How to perform the TI Impedance TrackingTM battery fuel gauge used in lithium iron phosphate (LiFePO4) batteries in shallow discharge applications…

TI’s Impedance TrackingTM battery fuel gauge technology is a powerful adaptive algorithm that remembers the changes in battery characteristics over time. Combining this algorithm with the specific chemical properties of the battery pack can know the state of charge (SOC) of the battery very accurately, thereby prolonging the service life of the battery pack.

TI’s Impedance TrackingTM battery fuel gauge technology is a powerful adaptive algorithm that remembers the changes in battery characteristics over time. Combining this algorithm with the specific chemical properties of the battery pack can know the state of charge (SOC) of the battery very accurately, thereby prolonging the service life of the battery pack.

Figure 1 Battery OCV measurement based on DOD

How to perform the TI Impedance TrackingTM battery fuel gauge used in lithium iron phosphate (LiFePO4) batteries in shallow discharge applications…

TI recommends that all lithium iron phosphate batteries use the impedance tracking 3 (IT3) algorithm. IT3’s improvements to the early impedance tracking algorithm include:

  • Achieve better low temperature performance through better temperature compensation
  • More filtering to prevent SOC capacity jumps
  • Higher accuracy for non-ideal OCV reading of lithium iron phosphate batteries
  • Conservative estimation of remaining capacity, and additional load selection configuration

IT3 is included in TI’s bq20z4x, bq20z6x and bq27541-V200 fuel gauges (not all listed).

Typical conditions for Qmax update

The impedance tracking algorithm defines Qmax as the total chemical capacity of the battery, which is calculated in milliampere hours (mAh). For a correct Qmax update, the following two conditions must be met:

1. The two OCV measurements must be performed outside the unqualified voltage range, based on the battery chemical identity (ID) code determined by TI. OCV can only be measured on an unused battery (without charging or discharging for several hours).

Reference 3 lists some unqualified voltage ranges, some of which are shown in Table 1. We can see that as far as chemical ID code 100 is concerned, if any battery voltage exceeds 3737mV or is lower than 3800mV, OCV measurement is not allowed. In fact, this is the “forbidden” range for the best accuracy of OCV measurements. Although the SOC percentage is given in this article, the fuel gauge only determines the unqualified range based on the voltage.

Table 1 is an excerpt from Reference 3, which lists the unqualified voltage ranges according to the updated chemical properties of Qmax

How to perform the TI Impedance TrackingTM battery fuel gauge used in lithium iron phosphate (LiFePO4) batteries in shallow discharge applications…

2. The minimum passing charge must be integrated by the fuel gauge. By default, it is 37% of the total battery capacity. In order to update the shallow discharge Qmax, this passing charge percentage can be reduced to 10%. The price of this reduction is the loss of SOC accuracy, but it is allowable in other systems where he cannot update Qmax.

Now that we understand the requirements of the shallow discharge Qmax update, let us look at an example of data flash parameters, we need to modify it in a lower capacity battery pack configuration. The default impedance tracking algorithm is based on a typical laptop battery pack. The battery pack has 2 parallel groups, each with 3 batteries in series, which is a 3s2p configuration structure. Each group has a capacity of 2200-mAh, so the total capacity is 4400hAh. Lithium iron phosphate batteries have about half the capacity, so if they are used in a 3s1p configuration, the total battery pack capacity is 1100mAh. If you use a smaller capacity battery pack like this, you need to fine-tune the specific data flash parameters in TI’s fuel gauge evaluation software to get the best performance. The rest of this article will cover this process.

Example calculation

Let’s take a look at a 3s1p configuration battery pack using A123 system TM1100-mAh 18650 lithium iron phosphate/carbon rod battery. The TI chemical ID code for this battery type is 404. This battery will be used in a storage system with a normal temperature around 50°C. The discharge rate is 1C, and a 5-mΩ sense resistor is used for the fuel gauge for the purpose of coulomb counting.

As shown in Table 1, the unqualified voltage range for the OCV measurement of Chemical ID 404 is 3274mV (minimum, ~34% SOC) to 3351mV (maximum, ~93% SOC). Most lithium iron phosphate batteries have a very wide unqualified voltage range (see Chemical ID 409 for comparison). However, depending on the specific battery characteristics, it is possible to find a higher minimum reject voltage for the shallow discharge Qmax update. When the chemical ID is 404, it is possible to increase this value to 3322 mV, allowing a shallow discharge Qmax update window of 3309 to 3322 mV (see Figure 2). Designers can use this mid-range low error window to implement data flash modification. Since only the high and low unqualified voltage ranges can be set, the main system must ensure that lower OCV measurements will not be made below 3309mV. (As the correlation error grows, the OCV measurement error increases sharply between 3274 and 3309mV.) Although there is only one 13-mV window that works for lower OCV measurements (3322 – 3309 mV = 13 mV), it corresponds to In a SOC range of 70% to 64%.

Lithium iron phosphate battery has a very long relaxation time, so we can increase the data flash parameter “OCV waiting time” to 18000 seconds (5 hours). Since the normal operating temperature of the battery has been increased, the parameter “Q invalid maximum temperature” should be modified to 55°C. In addition, the “Qmax maximum time” should be modified to 21600 seconds (6 hours).

Figure 2 SOC correlation error of 1-mV voltage error

How to perform the TI Impedance TrackingTM battery fuel gauge used in lithium iron phosphate (LiFePO4) batteries in shallow discharge applications…

To reduce the Qmax pass charge from 37% to 10%, you need to modify the “DOD maximum capacity error”, “Maximum capacity error” and “Qmax filter”, because they all affect the failure time between OCV1 and OCV2 measurements. “Qmax filter” is a compensation factor that changes Qmax based on the passing charge.

The purpose of setting these parameters is to obtain a “maximum capacity error” of less than 1% based on the measured passing charge, including the ADC maximum compensation error (“CC Dead Band”). However, some modifications to these values ​​are required to allow the shallow discharge Qmax to be updated.

Example 1 Qmax update timeout period

To obtain the cumulative error of 1000-mAh battery 10-mΩ detection resistor less than 1%, and the hardware setting of the “CC deadband” with a fixed value of 10μV, the Qmax update timeout period is determined by the following conditions:

10 μV/10 mΩ = 1-mA compensation current.

1000-mAh capacity × 1% allowable error = 10-mAh capacity error.

10-mAh capacitance error/1-mA compensation current=10 hours.

Therefore, from start to finish, including rest time, only 10 hours can be used to complete a Qmax update. After the 10-hour timeout, once the fuel gauge performs its next correct OCV reading, the timer will restart.

Example 2 Data Flash Parameter Modification

In a design scheme using a 1100-mAh battery with a 5-mΩ detection resistor, the same method can be used to calculate the timeout period for Qmax update:

10 μV/5 mΩ = 2-mA compensation current.

1100 mAh × 1% = 11 mAh.

11 mAh/2-mA compensation current = 5.5 hours.

In this case, the percentage of capacity error needs to be relaxed to increase the Qmax timeout. Modify the “Maximum Capacity Error” (from the default value of 1%) to 3%, and get:

1.1 Ah × 3% = 33 mAh

It will increase the Qmax failure time to:

33 mAh/2-mA capacity error = 16.5 hours.

The “DOD capacity error” needs to be set to twice the “maximum capacity error”, so it can be changed to 6% (the default value is 2%).

According to the percentage of passing charge, the default value of “Qmax filter” needs to be reduced proportionally to 96:

“Qmax filter”=96/(37%/10%) = 96/3.7 = 26

Table 2 shows the typical data flash parameters in the fuel gauge evaluation software, which must be modified to achieve the shallow discharge Qmax update. These special parameters are protected (classified as “hidden”), but can be unlocked by TI’s application staff. The example battery pack used in this table is the aforementioned battery pack, which is a 3s1p battery pack using A123 1100-mAh 18650 LiFePO4/carbon rod batteries (chemical ID 404).

Table 2 Some protected data flash memory parameters that can be modified by TI application personnel based on system usage

How to perform the TI Impedance TrackingTM battery fuel gauge used in lithium iron phosphate (LiFePO4) batteries in shallow discharge applications…

  1. This parameter is important during the golden image process. If you are using a standard 4.2-V lithium-ion battery and only charge it to the 4.1V system level, it is still necessary to perform the first Qmax update after the battery is charged to 4.2V, in order to meet the 90% capacity change requirement. According to the chemical ID code set by the fuel gauge, check the start and end points of the specified battery capacity, that is, the “design capacity” and the capacity change of the estimated DOD.
  2. When calculating Qmax, a wide range of temperature changes will cause errors. In a system that works normally at high or low temperatures, it is necessary to modify this parameter.

Qmax update event

The following events describe a practical method for realizing a Qmax update after the data flash parameters described in Examples 1 and 2 are changed.

  1. A Qmax update should start when the battery voltage is within the low correlation error window shown in Figure 2. The designer’s own algorithm can be used to discharge/charge the battery to this range.
  2. In this example, in order to enter the effective measurement range (chemical ID is 404), all battery voltages must be greater than or equal to 3309mV and less than or equal to 3322mV. If the battery voltage happens to be outside the valid range during regular discharge, another discharge or charge cycle must be started before setting the “OCV waiting time” for 18000 seconds. If after 6 hours and 10 minutes, all battery voltages are in the range of 3309 to 3322mV, then a correct OCV measurement has been made.
  3. The next step is to fully discharge the battery. Once the battery is fully charged (ie 100% SOC), it should rest for another 6 hours and 10 minutes before taking the second OCV measurement. After that, the Qmax value is updated. If charging takes about 2 hours, the timeout period will take at least 8 hours. From the calculation of the 16.5 hour timeout period in Example 2, we know that the time is more than enough, and there is an extra 8.5 hours of buffer time.
  4. When the fuel gauge is in the on mode, a ResetCommand (0x41) is issued to the fuel gauge to reset the OCV timer.

Table 3 shows the results of cycling the battery as described when using the example battery pack configuration.

Table 3 Results of full cycle and shallow charge Qmax update

From depleted charge to full charge

in conclusion

TI’s impedance tracking technology is a very accurate algorithm used to determine battery SOC based on battery usage time. In some lithium iron phosphate battery applications, it is impossible to fully discharge the battery after a period of inactivity. Therefore, it is necessary to study a shallow discharge method for Qmax update. This article introduces the factors and data flash programming configuration that need to be considered to implement a shallow discharge Qmax update. The modification of these parameters must be approved by TI application personnel according to the system configuration and requirements.

The Links:   G190ETN013 NL10276BC20-06Y

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