Research on the Application of Impedance Tracking Measurement Technology in Power Management System

Many mobile applications such as wireless access account management, data processing, and medical monitoring have high requirements for remaining battery capacity measurement to avoid sudden shutdown due to battery exhaustion. However, it is difficult to ensure the measurement of the remaining power during the battery's entire life cycle, over-temperature state, or when the load is used. End users and even some system designers underestimate this. The main reason is that the available electric energy of the battery has a functional relationship with its discharge speed, working temperature, aging degree and self-discharge characteristics. It is almost impossible to develop an algorithm to define the influence of battery self-discharge characteristics and the degree of aging on battery capacity. Furthermore, the traditional battery fuel gauge requires the battery to be fully charged and fully discharged to update the battery capacity, which rarely occurs in real applications, which causes greater measurement errors. Therefore, it is difficult to predict the remaining battery capacity and working time during the battery operating cycle.

This article will introduce how to use the battery fuel measurement technology-impedance tracking measurement technology to solve the above problems, the article will also list the simple design of a single-cell lithium-ion battery pack solution.

Problems with existing electricity measurement technology

The decrease in the capacity of lithium-ion batteries is the main reason for the shortening of battery running time. This misunderstanding is widespread. In fact, the continuous increase in battery impedance (rather than the decrease in battery capacity) is a key factor leading to shortened battery runtime and premature shutdown of the system. In about 100 cycles of charging and discharging the battery, the battery capacity drops only by 5%, while the DC impedance of the battery increases by a factor of one or two times. The direct result of the increase in the impedance of the aging battery is the increase in the internal voltage drop caused by the load current. As a result, the time for the aging battery to reach the system operating voltage (or called the termination voltage) is much earlier than that for the new battery.

The traditional battery power measurement technology is mainly developed based on the voltage and coulomb counting algorithm, which has obvious limitations in measurement performance. Due to low cost and simple implementation, voltage-based measurement methods are widely used in handheld devices such as mobile phones, but the battery impedance will change after a period of use, which affects the measurement of this method. The battery voltage can be obtained by the following formula:

VBAT=VOCV-I×RBAT

Among them, VOCV is the open circuit voltage of the battery, and RBAT is the internal DC impedance of the battery. It can be seen from Figure 1 that the voltage of the aging battery is lower than that of the new battery, which will make the system shutdown time earlier.

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Figure 1 Battery cycle discharge characteristics

Changes in load conditions and temperature will reduce the usable capacity of the battery by 50%. Most end users have experienced sudden shutdowns caused by battery depletion when using portable devices that are not equipped with real fuel gauges. On the other hand, the Coulomb counting method adopts another method: by continuously performing Coulomb integration to calculate the amount of charge consumed and the state of charge (SOC), and the total capacity is known, therefore, the remaining capacity can be obtained value. The disadvantage of this method is that it is difficult to quantify (model) the self-discharged power, and because this method does not perform periodic full cycle correction, the measurement error becomes larger and larger over time. None of these algorithms solve the problem of battery impedance changes. In order to prevent a sudden shutdown, designers must terminate system operation early and reserve more energy, which results in a lot of power being wasted.

Dynamic monitoring of battery impedance and chemical capacity

Impedance tracking (IT) technology is very unique, more than existing solutions, because the technology has a self-learning mechanism that can solve the aging problem that leads to changes in battery impedance and chemical full capacity (QMAX) under no-load conditions. Impedance tracking technology uses dynamic simulation algorithms to learn and track battery characteristics, that is, in the actual battery use process, first measure the impedance and capacity values, and then track their changes. The use of this algorithm eliminates the need for periodic full cycle capacity correction.

Using the knowledge of battery impedance, the load and temperature compensation can be realized. What is important is that through dynamic learning of battery parameters, this measurement method can measure the amount of electricity during the entire service life of the battery. Compared with the coulomb counting method or the battery voltage correlation method alone, the impedance tracking technology is more excellent in measuring the remaining capacity of the battery.

During the operation of IT, it is necessary to continuously maintain a table database that maintains the functional relationship between battery impedance (RBAT), depth of discharge (DOD) and temperature. Understanding what happens in different states can help determine when these tables need to be updated or used. In the meter, the non-volatile memory stores multiple current thresholds that define charging, discharging, relaxation after charging, and relaxation after discharging. After stopping charging or discharging, the "relaxation time" can stabilize the battery voltage.

Before turning on the handheld device, measure the battery open circuit voltage (OCV) and then compare it with the OCV (DOD, T) meter to determine the battery's state of charge. When the handheld device is active and connected to the load, it starts to execute the coulomb counting algorithm based on current integration. The coulomb counter measures and integrates the amount of charge passed to calculate the SOC value without interruption.

The total capacity QMAX can be calculated from two OCV readings when the battery voltage changes before and after charging or discharging is small enough and in a fully relaxed state. For example, before the battery is discharged, the SOC can be obtained by the following formula:

When the battery is discharged and the passing charge is 芉, the SOC can be obtained by the following formula:

subtract the two formulas to get:

, where 芉=Q1-Q2

It can be seen from the equation that the total battery capacity can be determined without going through a full charge and discharge cycle. This also eliminates the time-consuming battery learning cycle in the battery pack production process.

RBAT (DOD, T) table is continuously updated during the discharge process. IT uses this table to calculate when the cut-off voltage is reached under the current load and temperature conditions. The overall impedance of the battery increases as the battery ages and the charge-discharge cycle increases. The impedance can be obtained by the following formula:

RBAT (DOD, T)=

With the battery impedance information, the remaining power (RM) can be determined using the voltage simulation algorithm contained in the program instructions in the read-only memory (inthefirmware). The simulation algorithm first calculates the current SOCSTART value, and then calculates the future battery voltage value when the load current is the same and the SOC value continues to decrease. When the simulated battery voltage VBAT (SOCI, T) reaches the battery termination voltage (typical value is 3.0V), the SOC value corresponding to this voltage is obtained and recorded as SOCFINAL. The remaining power RM can be obtained by the following formula:

RM=(SOCSTART-SOCFINAL)×QMAX

Impedance tracking single-cell battery fuel gauge test results

Impedance tracking lithium-ion single-cell battery pack circuit is shown in Figure 2. The battery voltage is measured through the BAT2 pin input terminal, and the current is monitored through the differential signal input terminal (SRP and SRN) of the coulomb counter. The system uses the fuel gauge to obtain SOC and Run-Time-to-Empty information from the single-wire SDQ communication port.

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Figure 2 Typical impedance tracking single battery fuel gauge circuit

Even when the load changes, the IT fuel gauge can accurately predict the remaining battery power. For example, when the digital camera is in different working modes, the battery load is also different. Figure 3 shows how the IT fuel gauge predicts the remaining battery power. The error rate of the remaining power prediction can be less than 1%. In addition, since the battery impedance and aging effect used to predict the remaining power can be updated in real time, such a small error can be maintained throughout the life of the battery.

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Figure 3 Measure SOC and its degree based on real SOC and battery fuel gauge

Conclusion

Impedance tracking battery fuel gauge combines the advantages of Coulomb counting algorithm and voltage-related algorithm to achieve higher battery fuel monitoring. An accurate SOC value can be obtained by measuring OCV in a relaxed state. Since all self-discharge activities are reflected in the battery OCV reduction process, there is no need to perform self-discharge correction. When the device is in active mode and connected to the load, it starts to execute the current integration-based coulomb counting algorithm. The battery impedance is updated through real-time measurement.