Lithium-ion batteries are gradually becoming a common skill for portable use products using rechargeable batteries because of their high energy density, high voltage, low self-discharge rate, and no memory effect.
Common problems of battery handling
In the selection of lithium-ion batteries, it is necessary to be handled correctly to complete safe operation and obtain the highest capacity per cycle and the longest life, and the general choice is to participate in the battery handling unit (BMU). To achieve safe operation, the BMU must be able to ensure that the battery cell is always within its production specifications in terms of voltage, temperature and current. This means that when planning a battery handling system, it is necessary to be able to take into account the worst conditions. As an example, the advocated cell voltage for a standard laptop battery is 4.25V or less. In order to insist that the cell voltage does not exceed the upper limit, it is generally recommended to first obtain the voltage measurement specification deviation in the BMU and subtract 4 times the specification deviation value from the charging end voltage. For example, if the BMU measures the voltage at 4.25V and the norm deviation is 12.5mV, it is immediately instructed to stop charging at 4.2V. However, this is in direct conflict with the purpose of obtaining the maximum capacity of the battery cell. Since the higher the charging voltage, the greater the capacity. Similarly, when the battery exceeds the recommended charging cut-off voltage (EOCV) and discharging cut-off voltage (EODV), the battery wears out the most, so to extend the battery life, it is necessary to try to prevent too high charging voltage and too low discharging voltage.
The importance of accurate measurement
Accurate voltage measurement accuracy can define the required EOCV and EODV safety margin (safety margin) of the battery. The more accurate the measurement, the smaller the safety margin needed to stay within the recommended limits. Thus, the more accurate the voltage measurement, the closer the charging and discharging can be to the recommended EOCV and EODV values without sacrificing safety and without risking premature battery capacity degradation. Therefore, the accuracy of the charge flow measurement is also very critical to ensure the accuracy of charge accounting.
Necessary to take into account temperature offset
The key parameters to achieve high voltage measurement accuracy during temperature changes are ADC gain drift and voltage reference drift. For a voltage of 4200 mV, the offset of the voltage measurement value is generally less than 3 μV, which is negligible in practical planning.
It is not difficult to achieve excellent measurement accuracy at a fixed temperature, and it is even easier if the BMU is calibrated when the battery pack is installed. However, in practice, battery packs generally suffer from various temperature changes, so temperature drift is a key parameter to distinguish a true high performance BMU from an ordinary BMU.
Temperature drift can be compensated to a certain extent by calibrating several temperature points, but this is a costly solution that is not generally used by most battery pack manufacturers. Therefore, a good BMU must have minimal temperature excursions, and battery pack planners must consider the worst-case change in the BMU to ensure safe planning.
To accurately measure the charge flow, many other parameters must be considered to minimize the voltage drop across the sense resistor. The calibrated ADC offset, ADC zero drift, ADC gain drift, reference voltage drift, and time base drift all have a serious impact on accuracy. For small currents, the parameters related to offset are the most important; while in the case of larger currents, gain error, reference voltage and time base start to be the primary influencing factors.
Current measurement: the foundation of power meter accuracy
To accomplish outstanding lithium-ion battery power meter, the most useful way is to accurately track the charge flow in the battery table. To a certain extent, appropriate voltage measurements can be used to compensate for charge flow errors caused by the stable connection between open circuit voltage (OCV) and state of charge (SoC). Some of the most advanced Li-ion batteries have very flat voltage characteristics, which makes it more difficult to use OCV measurements to correct for current measurement errors. Just a small error in voltage measurement can lead to serious deviations in SoC accounting. Therefore, only excellent current measurement and accurate time base are required to achieve the best accuracy.
As mentioned above, the biggest cause of current measurement errors in low-current conditions is the offset in the current measurement ADC, and there are several techniques available to reduce this offset. One of the most common techniques is to measure the offset in a controlled environment and then subtract the offset from each measurement. However, this approach has the disadvantage that it does not take into account the drift of the offset. Figure 1 shows the remaining offset after applying the technique to a certain number of components. Atmel's battery handling unit is a better approach, and the ATmega16HVA compensates for the offset by periodically changing the polarity of the current measurement, for example. Although there is still a very small but stable offset remaining with this approach, this small remaining offset can be removed simply by measuring the maintenance FET before it opens and supplying a known current through the battery pack. As shown in Figure 2, the offset can be significantly reduced using this approach, and the residual error due to offset drift in the Atmel BMU is even lower than the quantum level. The advantage of eliminating the offset is the ability to accurately measure very small currents, while for devices with large offsets, the current measurement has to be discontinued at some point and the current guessing is started instead. Some BMUs choose a 5mΩ sense resistor to supply up to 100mA of locked zero or dead zone. In the case of notebook computers, for example, this is a very significant amount of current, enough to adhere to a certain operating form for a very long time.
Accurate measurement of small currents
For a given size of sense resistor, the offset error of the current measurement ADC often limits the minimum current level it can measure, resulting in a significant trade-off between the low sense resistor value and the required deadband (where the current level is too low to gather the charge flow). Recently, most device manufacturers are looking for ways to reduce power consumption and adhere to low-power forms as much as possible, making it increasingly important to ensure that small currents are accurately measured.
Characteristics of bandgap reference voltages and their impact on voltage measurement
The bandgap reference voltage is a key factor in achieving high accuracy results. The deviation of the actual reference voltage value from the expected value of the firmware translates into gain error in the measurement results, which in most cases is the most important source of error in battery voltage measurement and high current measurement.
The standard bandgap reference voltage is composed of a current proportional to the absolute temperature (PTAT) and a current complementary to the absolute temperature (CTAT), which can supply a relatively stable current independent of temperature changes. This current flows through the resistor to form a relatively stable voltage that is not affected by temperature changes. However, since the shape of the CTAT is curved and the PTAT is linear, the resulting voltage-temperature link is also curved.
Temperature offset of current measurement
It is challenging to accurately measure voltages on the order of μV, and even more difficult to do so when the chip is subjected to temperature changes, because even a laptop computer that operates primarily indoors will still experience temperature changes. For example, during the battery equalization process, a FET inside the BMU consumes the battery's energy at maximum power, causing the chip temperature to rise significantly. Many parameters related to the offset have large temperature shifts, which will affect the accuracy of the measurement if these effects are not eliminated. Atmel's offset calibration approach has proven to be very useful when temperature effects are taken into account. As shown in Figure 2, the temperature effect is completely eliminated, thus ensuring that the offset no longer affects the accuracy of the measurement.
The current stage in the bandgap reference source has a certain production variation, which makes the reference rating at 25°C, the curvature shape and the orientation of the flattest part of the curve are subject to various changes, and therefore factory calibration is required to minimize the impact of such changes, and Figure 3 shows an example of the changes brought about by an uncalibrated reference source. In the temperature range of -20 to +85°C, the maximum variation is -0.9 to 0.20%. Figure 3 shows that there are two outlier curves that are appropriately different from the curves of most other devices.
The standard bandgap reference source commonly used in BM equipment is calibrated for nominal changes with very high accuracy at 25°C. However, the curvature shape and orientation changes are not as good as the curvature. However, compensation for changes in curvature shape and orientation is also common, and this results in large changes associated with temperature changes, making cell voltage measurements inaccurate at high and low temperatures. In addition, it is not possible to detect and reveal outliers with significantly different curve shapes.
Accuracy of voltage measurement with temperature offset
When the battery reaches a fully discharged or fully charged condition, the voltage measurement determines when to shut down the application or stop charging the battery. Since safety considerations for both maximum and minimum battery voltage cannot be compromised, a guard band must be built in to ensure safe operation under all conditions. The higher the accuracy of the voltage measurement, the smaller the maintenance band required, and the higher the actual battery capacity utilization rate will be. The voltage measurement can be calibrated at a given voltage and temperature, and the voltage measurement error at that condition will be minimal. When temperature offset is taken into account, the primary source of measurement error is reference voltage drift. Figure 5 shows the uncertainty introduced by using the normative reference voltage compared to the curvature compensated reference voltage. As shown in Figure 5, curvature compensation can significantly improve accuracy.
Novel approach to reference voltage calibration
In order to achieve better functionality under various temperature changes, Atmel has added a nominal reference voltage calibration mechanism to adjust the temperature coefficient of the bandgap reference source. This calibration process will tune the shape and orientation of the curvature and significantly improve the stability with temperature changes, as shown in Figure 4, with a maximum change of 0.5% over the -20 to +85°C temperature range. Note that the second calibration process was able to detect and reveal outlier points with very different curve shapes.
In general, BM devices do not perform the second calibration process based on the cost factor of the production test. Since the industry norm is to test the package device at one temperature, and the second calibration guideline requires accurate imitation of the package device at two temperatures, participation in a second test process with high imitation accuracy requirements generally increases the cost significantly.
Atmel has developed a novel approach to perform the second test process at a minimal nominal cost. Traditionally, the second step of the test requires high-precision measurement equipment and messy accounting operations. In addition, for each device to be tested, it is necessary to store the data from the first step of the test and then recover it in the second step of the test. All of these requirements increase the cost of testing. Atmel's proprietary technology takes full advantage of the BM unit's own features to minimize test equipment requirements: onboard ADCs to perform measurements with accurate external reference voltages; CPUs to perform the necessary accounting tasks; and flash memory to store the measurement data from the first step. As a result, very accurate results can be achieved using only very low cost test equipment. In this way, Atmel is able to provide industry-leading functionality at a very low nominal test cost.
Conclusion
High measurement accuracy is essential to maximize the use of energy from each battery charge and to maximize the life of the battery pack without sacrificing the safety of the pack. In order to prevent adding calibration costs, it is necessary that the inherent accuracy of the BMU be as high as possible. In addition, with sensitive and novel calibration techniques that make full use of MCU on-board resources, excellent benchmarking can be accomplished at minimal cost, eliminating the effects of temperature.
The discharge cycles of a 10Ah battery are 3h/1.5A, 7h/0.6A, and 22h/60mA. the temperature change is ±10°C, and a 5mΩ sense resistor is used. The error in charge accumulation is greater than 400mAh with the selection of a normative BMU with a common calibration approach, which in this example is appropriate for more than 4% of a 10Ah battery. Atmel's solution is able to greatly improve the accuracy due to the selection of a sensitive imitation plan integrated with a proprietary calibration method. Based on these improvements, the error can be reduced to less than 20 mAh, which is equivalent to 0.2%.
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