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# Method of setting initial value of SOC of battery using OCV hysteresis depending on temperatures

Method of setting initial value of SOC of battery using OCV hysteresis depending on temperatures

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### Method of setting initial value of SOC of battery using OCV hysteresis depending on temperatures

1. 1. Method of setting initial value of SOC of battery using OCV hysteresis depending on temperatures 1. A method of setting an initial value of a SOC of a battery comprising steps of: experimentally measuring open circuit voltage (OCV) values under various temperatures; structuring a table correlating the measured OCV values and SOC values of the battery classified by the temperatures; storing the table in a battery management system (BMS); measuring current temperature and OCV value with the BMS; obtaining a SOC value of the battery corresponding to the measured values by referring to the table; and setting the obtained value as an initial SOC value of the battery. 2. The method according to claim 1, further comprising a step of resetting the SOC of the battery using the OCV values according to the various temperatures. 3. The method according to claim 1, wherein the table has a horizontal axis in which the temperatures are divided in a unit of 5° C. between −30° C. and +45° C. and a vertical axis in which the SOC is divided in a unit of 1% between 0 and 100%. 4. The method according to claim 2, wherein the table has a horizontal axis in which the temperatures are divided in a unit of 5° C. between −30° C. and +45° C. and a vertical axis in which the SOC is divided in a unit of 1% between 0 and 100%. 5. The method according to claim 1, further comprising a step of resetting approximating the obtained value by a bilinear interpolation. 6. The method according to claim 2, wherein the table has a horizontal axis in which the temperatures are divided in a unit of 5° C. between −30° C. and +45° C. and a vertical axis in which the SOC is divided in a unit of 1% between 0 and 100%. SUMMARY OF THE INVENTION Accordingly, the invention has been made to solve the above problems. An object of the invention is to provide a method of setting an initial value of a SOC of a battery more accurately in consideration of an open circuit voltage (OCV) hysteresis depending on temperatures. In order to achieve the above object, according to the invention, there is provided a method of setting an initial value of a SOC of a battery comprising steps of: experimentally measuring open circuit voltage (OCV) values under various temperatures; structuring a table correlating the measured OCV values and SOC values of the battery classified by the temperatures; storing the table in a battery management system (BMS); measuring current temperature and OCV value with the BMS; obtaining a SOC value of the battery corresponding to the measured values by
2. 2. referring to the table; and setting the obtained value as an initial SOC value of the battery. According to a preferred embodiment of the invention, the method may further comprise a step of re-setting the SOC of the battery using the OCV values depending on the various temperatures. According to an embodiment of the invention, the table may have a horizontal axis in which the temperatures are divided in a unit of 5° C. between −30° C. and +45° C. and a vertical axis in which the SOC is divided in a unit of 1% between 0 and 100%. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: FIG. 1 is a flow chart showing a process of carrying out a method according to an embodiment of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a preferred embodiment of the present invention will be described with reference to the accompanying drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear. As described above, an open circuit voltage (OCV) which is referred to set an initial value of a SOC of a battery is changed depending on temperatures and aging, instead of having a fixed value irrespective of the environments. Contrary to the prior art of setting the initial value of the SOC on the assumption that the OCV has a fixed value, according to the present invention, an OCV hysteresis, which is changed depending on the temperatures, is considered and used to set the initial value of the SOC, so that it is possible to reduce a general error of an algorithm for estimating the SOC. In the followings, it is more specifically described a method according to an embodiment of the invention, with reference to FIG. 1. First, the OCV values are experimentally measured under various temperatures in which the battery is operated (S 10 ), contrary to the prior art.
3. 3. For example, instead of obtaining only a relationship between the SOC and the OCV which is a reference value of the SOC, the OCV values are experimentally measured in advance under various temperatures in which the battery is actually mounted and operated and then it is structured a table correlating the OCV values and the SOC depending on the temperatures (S 20 ). According to an embodiment of the invention, the table has a horizontal axis in which the temperatures are divided in a unit of 5° C. between −30° C. and +45° C. in consideration of the actual operating temperatures of the battery and a vertical axis in which the SOC is divided in a unit of 1% between 0 and 100%. An example of the table is shown as follows. TABLE 1 OCV and SOC depending on the temperatures temperature (° C.) SOC −30 . . . 25 30 . . . 0.01 (1%) 2.845 . . . 2.90 2.95 . . . 0.02 (2%) 2.855 . . . 2.92 2.96 . . . . . . . . . . . . . . . . . . . . . Next, the above table is stored in a battery management system (BMS) (S 30 ) and then current temperature and OCV value are measured in the BMS (S 40 ). In the mean time, in general, the current temperature and OCV value which are measured at real time in the BMS do not accurately correspond to the temperature and the OCV in the table, but belong to between the values before and after the measured value. Accordingly, in order to find out a SOC value corresponding to the current temperature and OCV measured with reference to the table, the most approximate 2 values are read out from the table and then applied to a bilinear interpolation to approximate a middle value (S 50 ). For example, when the BMS measures the current temperature 27° C. and the OCV 2.93, the corresponding SOC is between 0.01 (1%) and 0.02 (2%) in the table 1. Accordingly, a middle value of the SOC is found out by applying a universal bilinear interpolation and the found SOC value is set as an initial SOC value of the battery (S 50 ). The initial SOC value estimated and set through the procedures is transmitted to a vehicle control device of the hybrid electric vehicle via the BMS to control the charge/discharge output of the battery.
4. 4. Like this, according to the invention, contrary to the prior art of setting an initial SOC value with reference to the fixed OCV, it is structured a table in which the OCV values which are changed depending on the temperatures are previously correlated with the SOC values depending on the temperatures. Then, the OCV is measured at a temperature at which it is desired to set the initial SOC value and an approximate SOC value corresponding to the measured OCV is found out from the table and set as the initial value. Accordingly, it is possible to estimate the initial SOC value depending on the temperatures. In the mean time, according to a preferred embodiment of the invention, the method may further comprise a step of re-setting the SOC of the battery using the OCV values depending on the various temperatures, so that it is possible to carry out a setting of an initial SOC value at each of the temperatures, as necessary. As described above, according to the invention, it is set the initial value of the SOC in consideration that the open circuit voltage is changed depending on the temperatures. Accordingly, it is possible to correct the error resulting from no consideration of the OCV change depending on the temperatures, so that the initial value of the SOC can be more accurately set. While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. ****************************************************************************** 100% SOC is meant by the 1C capacity itself, if the δSOC is 20%, the 1C capacity is δSOC×5. For example, if the calculated δSOC is 20% and the Ah-counting value is 1.5 Ah, the Ah- counting value 1.5 Ah corresponds to the 20% SOC. The 20% SOC is one-fifth of 100%, the 1C-rated capacity is 1.5 Ah×5, i.e., 7.5 Ah.
5. 5. Operation principle of impedance track gauging As shown in Figure 3, the Impedance Track fuel gauge ICs allow for accurate measurement of the following key parameters: • Open circuit voltage (OCV) of a battery when it is in relaxation mode • Battery impedance: OCV-battery voltage under load/average load current, measured during discharge only • PassedCharge: integrated charge or coulombs during battery discharge or charge • QMAX: Maximum battery chemical capacity • State-of-charge (SOC) at any moment, defined as SOC=QD/QMAX, where QD is the PassedCharge counted from the full discharge state • Remaining capacity (RM) • Full charge capacity (FCC), the amount of charge passed from the full charge state to the Termination Voltage SOC—Because of a strong correlation between SOC and OCV for particular Li-ion battery chemistry, the SOC can be estimated from the OCV of the battery. The OCV I measured when the cells are in relaxation mode, which is defined as the state of the battery when its current is below a small threshold (such as 10mA) and when the cell voltage is stabilized. The SOC is then determined using the predefined OCV-SOC relationship. This serves to mark an initial battery state for subsequent discharge or charge cycles, and is done when the system is in low-power mode, such as shutdown. Impedance—As shown in Figure 3, when the portable device is in normal operation, the load current shapes the discharge curve of the battery and leads to a deviation from the OCV characteristics. When a load is applied, the impedance of each cell is measured by finding the difference between the measured voltage under load and the OCV specific to the cell chemistry at the present SOC. This difference, divided by the applied load current, yields the impedance R. Additionally, impedance is correlated with the temperature at the time of measurement to fit in a model that accounts for temperature effects. RM—With the impedance information, the remaining capacity (RM) is calculated using a voltage simulation implemented in the firmware. The simulation starts from the present SOCFINAL and calculates future voltage profile under the same load with a four percent SOC increment, consecutively. Once the future voltage profile is obtained, the impedance track algorithm determines the value of SOC that corresponds to the system termination voltage, SOCFINAL The remaining capacity may then be calculated using the following formula:
6. 6. FCC—This is defined to represent the actual usable capacity of a fully-charged battery under a specific load. It can be calculated using the following formula, where QSTART is the initial capacity of the battery: From time to time, the chemical capacity of the battery (QMAX) needs to be updated to account for the aging effects. This update happens less frequently than that of the impedance because of the much smaller changing rate of QMAX. The method is to take two OCV values before and after a charge (can also be a discharge) period. These two OCV values are first converted to SOC values using the OCV-SOC characteristics. Then the following equation leads to the new QMAX. The above equation can be easily derived from the definition of SOC. Apparently, the algorithm does not require a complete discharge cycle to learn the battery chemical capacity. However, accuracy of the calculated QMAX is ensured only with relatively high PassedCharge and accurate SOC values. Design impedance track fuel gauge Impedance Track technology relieves designers from the burden of learning extensive knowledge of battery chemistry. Moreover, this new gauging technology does not require cycling of every production pack. Once the typical characteristics of the battery for a specific model are learned, the same configuration can be applied to all production packs without the lengthy cycling time, thanks to the learning capability of the algorithm.
7. 7. Figure 3. OCV characteristics and battery discharge curve under load. As an example, the design of a multicell fuel gauge solution using the bq20z90 is presented. It is assumed that the application is a three-series, two-parallel pack, with a capacity of 2,200=mAh for each of the Panasonic CGR18650C cells. The fast charging current is 4A, and maximum discharging current is 4A. The termination voltage is 3V per cell. At both charge and discharge, the maximum allowable temperature is 60°C. The application operates at a constant power load most of the time. The pack is removable and does not require pre-charge. An independent, secondary voltage protector is needed to blow the fuse, if any of the cells goes above 4.45V. Hardware design example The application requires a chipset of three ICs: a) the bq20z90 fuel gauge IC; b) the bq29330 analog front end (AFE) IC; and c) the secondary voltage protector IC bq29412 that has an activation voltage of 4.45V. Figure 4 shows a functional diagram of the circuit. The AFE powers the fuel gauge directly from its 2.5V, 16mA low-dropout regulator (LDO). The AFE derives power from the battery voltage or the charger voltage. The main functions of the AFE are to condition the cell voltages for the 16bit voltage ADC in the fuel gauge and to provide hardware-level over current protection features. The fuel gauge IC, running gauging and protection firmware, is the master of the chipset. The AFE is configured by the fuel gauge IC for how it should respond to handle fuel-gauging situations. These include when and which cell voltage information to provide to the fuel gauge IC, and what overload and short-circuit threshold and delay value should be used.
8. 8. While the fuel gauge and its associated AFE provide over-voltage protection, the sampled nature of voltage monitoring limits the response time of this protection system. The bq29412 provides a fast-response, real-time and independent over-voltage monitor that operates in conjunction with the fuel gauge and the AFE. When any of the cells exceeds a voltage of 4.45V, after a capacitor - programmed delay, the bq29412 outputs a triggering signal to blow the fuse.

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Method of setting initial value of SOC of battery using OCV hysteresis depending on temperatures

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