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

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. 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. 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. 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.
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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. 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. 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. 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. 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.