Cell stabilizing method and system of energy storage system (ESS)
A method and a system for stabilizing battery cells of an energy storage system (ESS) to enable uniform charge/discharge without the generation of a state of charge (SOC) deviation among the battery cells by charging/discharging the plurality of battery cells included in a battery rack differently by module unit to minimize the deterioration and impact caused by intermittent charge/discharge.
1. A method for controlling charge/discharge of a plurality of battery cells constituting each battery module included in an energy storage system (ESS), the method comprising:
a charge/discharge step of charging/discharging the plurality of battery cells constituting each battery module;
a data measurement step of measuring state of charge (SOC) measurement data of each battery cell at a predetermined interval while the plurality of battery cells are simultaneously charged/discharged by the charge/discharge step;
an SOC calculation step of calculating a SOC using the measured SOC measurement data of each battery cell;
a maximum/minimum value extraction step of extracting a maximum SOC, which is a largest SOC value for each battery module, and a minimum SOC, which is a smallest SOC value for each battery module, for a same time point using the calculated SOC of each battery cell;
a charge/discharge operation determination step of determining a charge/discharge operation of the plurality of battery cells included in each battery module, based on the extracted maximum SOC and the extracted minimum SOC; and
a charge/discharge operation control step of controlling the charge/discharge operation of the plurality of battery cells included in each battery module, according to the determination result,
wherein the charge/discharge operation determination step comprises:
a first determination step of a first determining to stop the charge/discharge operation of the plurality of battery cells included in each battery module, when the extracted maximum SOC reaches a maximum limit value in a predetermined operation limit range or the extracted minimum SOC reaches a predetermined minimum limit value in the predetermined operation limit range, and
if the first determination step determines to stop the charge/discharge operation, performing a second determination step of performing a second determination based on the first determination, according to a comparison result of comparing whether a calculated difference between the maximum SOC and the minimum SOC exceeds a predetermined reference range, the second determination being the determination result.
2. The method of claim 1 , wherein the calculating of the SOC in the SOC calculation step is performed by the following Equation (1):
[
SOC
d
+
1
V
1
,
d
+
1
]
=
[
1
0
0
exp
(
-
Δ
t
R
d
C
d
)
]
[
SOC
d
V
1
,
d
]
+
[
Δ
t
Capacity
R
d
(
1
-
exp
(
-
Δ
t
R
d
C
d
)
)
]
I
k
Equation
(
1
)
Cd: Capacitor, Rd: Resistance connected in parallel with the capacitor in Voltage Model, V1,d: Voltage in Rd and Cd regions in Voltage Model during an interval, V1,d+1: calculated voltage, Δt: Time variation, capacity: Capacity of capacitor, d=1.d: Interval, Ik: Current value flowing through a resistor.
3. The method of claim 1 , wherein the charge/discharge operation determination step further comprises:
a deviation calculation step of calculating the difference between the maximum SOC and the minimum SOC of each battery module, when the first determination step determines to stop the charge/discharge operation.
4. The method of claim 3 , wherein the second determination step determines to continue the charge/discharge operation of the plurality of battery cells included in each battery module, even though the maximum SOC reaches the maximum limit value or even though the minimum SOC reaches the minimum limit value, when the difference between the maximum SOC and the minimum SOC exceeds the predetermined reference range.
5. The method of claim 3 , wherein the second determination step determines to stop the charge/discharge operation of the plurality of battery cells included in each battery module, when the difference between the maximum SOC and the minimum SOC is within the predetermined reference range.