FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
AND
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See Section 10; rule 13)
TITLE OF THE INVENTION
“HYBRID CELL BALANCING MODULE FOR AN ENERGY STORAGE DEVICE”
APPLICANT(S)
TATA MOTORS LIMITED
Bombay House, 24 Homi Mody Street,
Hutatma Chowk, Mumbai 400 001, Maharashtra, India; an Indian company.
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
[0001] The present invention relates to energy storage device, and more specifically related to hybrid cell balancing module for an energy storage device in which dual sensing of State of Charge-Open Circuit Voltage (SOC-OCV) is used to minimize a voltage sensing error and increase the functional safety of the electric vehicles or any other application (where the battery is used for power storage).
BACKGROUND OF THE INVENTION
[0002] A device powered by rechargeable batteries generally include several battery pack each comprises multiple cells to achieve the voltage and/or current levels used by the device. The battery and Battery Management System (BMS) play a huge role in deciding the cost as well as efficiency of an electric vehicle (EV). The cell balancing technique which is used in most of the conventional EVs relies mostly on just voltage-based balancing that too only for a short duration of its charging process. When there is a huge difference between cell voltages attached in series then the pack will be discharged as per the minimum voltage among all the cell voltages. There can be a cell with capacity left but will not be of any use which reduces the usable capacity of the pack.
[0003] It is very important to bring these cells to almost equal voltage levels to utilize the maximum capacity of the pack and to avoid weakening of any cell. The balancing is done only during charging which cannot ensure that the pack delivers maximum capacity and affects the range of the EV’s. Differences in voltage also lead to inaccurate State of Charge (SoC) determination which further impacts range estimation, depth of discharge, and other strategies related to a Vehicle Control Unit (VCU).
[0004] In the conventional approach used by most BMS suppliers, passive balancing is used to achieve cell balancing as shown in the FIG. 1. A switch (1) is

controlled by the BMS to dissipate extra energy of higher cells1-n through a bleeding resistor. Cells are balanced based on the value of voltage difference during constant voltage mode which occurs for around 10-20% in 1 charge cycle hence giving very little time for cells to achieve balanced conditions.
[0005] FIG. 2 illustrates SoC vs OCV curve of a cell, according to conventional the BMS. The use of SoC-OCV determines whether the cells will be balanced using voltage values or SoC values. A huge imbalance can be observed at the end of balancing with a bandgap-based cell voltage sensor as shown in the FIG. 2.
[0006] To overcome the above mentioned drawback, there is a need of a hybrid cell balancing module for an energy storage device.
OBJECT OF THE INVENTION
[0007] The principal object of the embodiments herein is to provide a hybrid cell balancing module for an energy storage device in which dual sensing of State of Charge-Open Circuit Voltage (SOC-OCV) is used to minimize a voltage sensing error and increase the functional safety of the electric vehicles or any other application (where the battery is used for power storage).
[0008] Another object of the embodiments herein is to provide a new secondary battery management system (BMS) that utilizes SOC-OCV flat band region and accurate voltage sensors to make itself more efficient in terms of achieving minimum voltage imbalance among battery cells.
SUMMARY OF THE INVENTION
[0009] In one aspect the object is satisfied by providing an energy storage device for hybrid cell balancing. The energy storage device includes a plurality of battery packs each comprising a plurality of cells, a primary Battery Management System

(BMS) connected to the plurality of battery packs, a State of Charge (SoC) sensor connected to the primary BMS and a secondary battery management system (BMS) connected to the primary BMS. The SoC sensor detect a SoC of each cell of the plurality of cells corresponding to each battery pack of the plurality of battery packs. The secondary BMS includes a voltage sensor connected to the plurality of battery packs and a Equalization controller connected to the voltage sensor. The voltage sensor is configured to detect an Open-circuit voltage (OCV) of each cell of the plurality of cells corresponding to each battery pack of the plurality of battery packs. The equalization controller is configured to receive the current SoC of each cell of the plurality of cells, achieve an optimal SoC of each cell by equalizing at least one cell of the plurality of cells corresponding to each battery pack of the plurality of battery packs based on the current SoC of each cell of the plurality of cells and the OCV of each cell of the plurality of cells, and input the optimal SoC of each of the equalized cells to the primary BMS. The primary BMS perform charging or discharging of the cells based on the optimal SoC of each of the equalized cells and the current SoC of each of the cells.
[0010] In an embodiment, equalizing the at least one cell includes determine the at least one cell of the plurality of cells meeting a equalization criteria, wherein the equalization criteria comprises at least one of detecting of a full charge condition of a cell and detecting the current SoC meets a predetermined equalization threshold, and equalize the at least one meeting the equalization criteria based on the current SoC of each cell of the plurality of cells and the OCV of each cell of the plurality of cells.
[0011] In another aspect the object is satisfied by providing a hybrid cell balancing module for an energy storage device. The hybrid cell balancing module includes a secondary BMS connected to the primary BMS. The secondary BMS includes one or more voltage sensors connected to the battery packs and a equalization controller connected to the voltage sensor. The voltage sensor is configured to detect an OCV of each cell of the plurality of cells corresponding to each battery pack of the

plurality of battery packs. The equalization controller is configured to receive a current SoC of each cell of the plurality of cells corresponding to each battery pack of the plurality of battery packs of the energy storage device, achieve an optimal SoC of each cell by equalizing at least one cell of the plurality of cells corresponding to each battery pack of the plurality of battery packs based on the current SoC of each cell of the plurality of cells and the OCV of each cell of the plurality of cells, and input the optimal SoC of each of the equalized cells to the primary BMS to perform charging or discharging of the cells based on the optimal SoC of each of the equalized cells and the current SoC of each of the cells.
[0012] In yet another aspect the object is satisfied by providing a method for hybrid cell balancing by an energy storage device. The method includes detecting an SoC of each cell of a plurality of cells corresponding to each battery pack of a plurality of battery packs of the energy storage device, detecting an OCV of each cell of the plurality of cells corresponding to each battery pack of the plurality of battery packs using a secondary BMS, achieving an optimal SoC of each cell by equalizing at least one cell of the plurality of cells corresponding to each battery pack of the plurality of battery packs based on the current SoC of each cell of the plurality of cells and the OCV of each cell of the plurality of cells, inputting the optimal SoC of each of the equalized cells to a primary BMS, and charging or discharging of the cells based on the optimal SoC of each of the equalized cells and the current SoC of each of the cells.
[0013] These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the scope thereof, and the embodiments herein include all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The proposed hybrid cell balancing module for an energy storage device are illustrated in the accompanying drawings, throughout which like reference letters indicate corresponding parts in the various figures. The embodiments herein will be better understood from the following description with reference to the drawings, in which:
[0015] FIG. 1 illustrates passive cell balancing circuit to achieve cell balancing, according to prior art;
[0016] FIG. 2 illustrates SoC vs OCV curve of a cell, according to prior art;
[0017] FIG. 3 illustrates hybrid cell balancing module for an energy storage device, according to the embodiment as disclosed herein;
[0018] FIG. 4 illustrates SoC vs OCV curve of a cell, according to embodiment as disclosed herein; and
[0019] FIG. 5 illustrates a method for hybrid cell balancing by an energy storage device, according to the embodiment as disclosed herein.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. Also, the various embodiments described herein are not necessarily mutually exclusive, as

some embodiments can be combined with one or more other embodiments to form new embodiments. The term “or” as used herein, refers to a non-exclusive or, unless otherwise indicated. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein can be practiced and to further enable those skilled in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
[0021] Referring now to the drawings, and more particularly to FIGS. 3 through 5, where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments.
[0022] FIG. 3 illustrates hybrid cell balancing module for an energy storage device, according to the embodiment as disclosed herein. The energy storage device includes a plurality of battery packs (2) each comprising a plurality of cells (3), a primary Battery Management System (BMS) (4) connected to the plurality of battery packs (2), a State of Charge (SoC) sensor (5) connected to the primary BMS (4), and a secondary battery management system (BMS) (6) connected to the primary BMS (4).
[0023] The SoC sensor (5) detects a SoC of each cell of the plurality of cells (3) corresponding to each battery pack of the plurality of battery packs (2).
[0024] The secondary BMS (6) includes one or more voltage sensor (7) connected to the plurality of battery packs (2), and a equalization controller (8) connected to the one or more voltage sensors (7) battery pack of the plurality of battery packs of the energy storage device.
[0025] In an embodiment, the one or more voltage sensor (7) is a Zener based voltage sensor. The voltage sensor (7) is configured to detect an Open-circuit

voltage (OCV) of each cell of the plurality of cells (3) corresponding to each battery pack of the plurality of battery packs (2).
[0026] The equalization controller (8) is configured to receive the current SoC of each cell of the plurality of cells (3), achieve an optimal SoC of each cell by equalizing at least one cell of the plurality of cells (3) corresponding to each battery pack of the plurality of battery packs (2) based on the current SoC of each cell of the plurality of cells (3) and the OCV of each cell of the plurality of cells (3), and input the optimal SoC of each of the equalized cells (3) to the primary BMS (4).
[0027] In an embodiment, the primary BMS (4) perform charging or discharging of the cells (3) based on the optimal SoC of each of the equalized cells (3) and the current SoC of each of the cells (3). In order to equalize the cells, the voltage sensor (7) determine the at least one cell of the plurality of cells (3) meeting a equalization criteria, wherein the equalization criteria comprises at least one of detecting of a full charge condition of a cell and detecting the current SoC meets a predetermined equalization threshold, and equalize the at least one meeting the equalization criteria based on the current SoC of each cell of the plurality of cells (3) and the OCV of each cell of the plurality of cells (3).
[0028] In an another embodiment, a hybrid cell balancing module for the energy storage device, The hybrid cell balancing module includes a secondary battery management system (BMS) (6) connected to the primary BMS (4). The secondary BMS (6) includes the one or more voltage sensors (7) connected to the plurality of battery packs (2) and a equalization controller (8) connected to the one or more voltage sensors (7) battery pack of the plurality of battery packs of the energy storage device.
[0029] The voltage sensor (7) is configured to detect an Open-circuit voltage (OCV) of each cell of the plurality of cells (3) corresponding to each battery pack of the plurality of battery packs (2). Further, the equalization controller (8) is

configured to receive the current SoC of each cell of the plurality of cells (3), achieve an optimal SoC of each cell by equalizing at least one cell of the plurality of cells (3) corresponding to each battery pack of the plurality of battery packs (2) based on the current SoC of each cell of the plurality of cells (3) and the OCV of each cell of the plurality of cells (3), and input the optimal SoC of each of the equalized cells (3) to the primary BMS (4). Further, the primary BMS (4) perform charging or discharging of the cells based on the optimal SoC of each of the equalized cells and the current SoC of each of the cells using a hybrid charging and discharging controller (9).
[0030] FIG. 4 illustrates SoC vs OCV curve of a cell, according to embodiment as disclosed herein. As shown in the FIG. 4, regions (10 and 12) indicates voltage based balancing using the one or more voltage sensor (7) and region (11) indicates SoC based balancing. In the proposed mechanism, 100% charge region coverage is provided for each cell using the one or more voltage sensor (7).
[0031] The proposed hybrid cell balancing module performs cell balancing even in constant current mode during charging by utilizing the SoC results instead of using cell voltage hence providing an accurate way and longer duration for cells to get balanced. This proposed hybrid cell balancing module uses the SoC vs OCV curve of a cell to determine whether cells will be balanced using voltage values or SoC values as shown in the FIG. 4. The equalization points reset the cell SoC values and make them more accurate during the process of charging and discharging. An improved voltage sensing using Zener-based reference helps in improvement in the cell balancing during this mode. Also dual sensing of voltage provides another estimate of the error in voltage reading and a flag is raised using strategy whenever voltage error crosses its band limits. The proposed hybrid cell balancing gives a significant reduction in delta voltage of cells and more accurate SoC estimation using accurate the Zener-based voltage sensors.

[0032] FIG. 5 illustrates a method for hybrid cell balancing by an energy storage device, according to the embodiment as disclosed herein. At S1, the method includes detecting a SoC of each cell of a plurality of cells (3) corresponding to each battery pack of a plurality of battery packs (2) of the energy storage device.
[0033] At S2, the method includes detecting an Open-circuit voltage (OCV) of each cell of the plurality of cells (3) corresponding to each battery pack of the plurality of battery packs (2) using a secondary BMS (7).
[0034] At S3, the method includes determining an optimal SoC of each cell by equalizing one or more cells of the plurality of cells (3) corresponding to each battery pack of the plurality of battery packs (2) based on the current SoC of each cell of the plurality of cells (3) and the OCV of each cell of the plurality of cells (3). In an embodiment, in order to equalize the cell the method includes determine the at least one cell of the plurality of cells (3) meeting a equalization criteria, wherein the equalization criteria comprises at least one of detecting of a full charge condition of a cell and detecting the current SoC meets a predetermined equalization threshold and equalize the at least one meeting the equalization criteria based on the current SoC of each cell of the plurality of cells (3) and the OCV of each cell of the plurality of cells (3).
[0035] At S4, the method includes inputting the optimal SoC of each of the equalized cells (3) to a primary BMS (4).
[0036] At S5, the method includes charging or discharging of the cells (3) based on the optimal SoC of each of the equalized cells (3) and the current SoC of each of the cells (3).
[0037] In the conventional BMS, a huge imbalance was observed at the end of balancing with a bandgap-based cell voltage sensor. Whereas in the proposed hybrid cell balancing module, an imbalance among the cells is reduced by

implementing the Zener-based sensor. The presented strategy at the proposed BMS (4 and 6) suggests a way to perform cell balancing throughout the charging process using other accurate means (such as voltage sensors) which provides much better cell balancing results.
[0038] The proposed hybrid balancing takes cell voltage as well as each cell SoC into consideration so that balancing can be done throughout the charging process. This provides a longer duration for balancing which reduces cell voltage difference to a larger extent as compared to the earlier technique and also ensures longer life and better health of the battery pack.
[0039] The proposed hybrid cell balancing module also proposes balancing during static conditions using very little discharge current which will almost eliminate any voltage difference. This will allow the energy storage devices to use the maximum capacity of the pack and thus increase the usable capacity of the pack. The hybrid cell balancing module ensures that cells are brought to almost equal voltage levels so that the weakening of cells with minimum voltage can be avoided during every discharging cycle. Since the usable capacity of the pack is increased hence the range of the EV can also be improved.
[0040] The hybrid cell balancing module uses SOC and voltage equalization points for calculation. The one or more Zener-based voltage sensors can be used for very precise voltage sensing up to +/-1mV which improves the SoC accuracy and balancing trigger points also. An accurate SoC ensures that correct input is fed to other strategies like range estimation and VCU strategies which improves the overall performance of the vehicle. A weak cell can lead to the sudden failure of the battery pack. BMS can detect weak cell strings which could lead to sudden pack failure & provide warning beforehand.
[0041] Further, the proposed design can be applied for balancing of cells inside battery pack used in electric vehicle segments or any other application where the

battery is used for power storage and source to achieve better operating range of electric vehicle or hybrid vehicle battery pack.
[0042] The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
[0043] List to reference numerals:

Sr. No. Description
1 Switch
2 Battery packs
3 Battery Cells
4 Primary BMS
5 SoC Sensor
6 Secondary BMS
7 Voltage sensor
8 equalization controller
9 Hybrid Charging and discharging controller
10-12 Regions

We Claim:
1. An energy storage device with hybrid cell balancing, wherein the energy storage device comprises:
a plurality of battery packs (2) each comprising a plurality of cells (3); a primary Battery Management System (BMS) (4) connected to the plurality of battery packs (2);
characterized in that
a State of Charge (SoC) sensor (5) connected to the primary BMS (4) to detect an SoC of each cell of the plurality of cells (3) corresponding to each battery pack of the plurality of battery packs (2); and
a secondary battery management system (BMS) (6) connected to the primary BMS (4), wherein the secondary BMS (6) comprises:
at least one voltage sensor (7) connected to the plurality of battery packs (2) and configured to detect an Open-circuit voltage (OCV) of each cell of the plurality of cells (3) corresponding to each battery pack of the plurality of battery packs (2), and
a Equalization controller (8) connected to, the at least one voltage sensor, configured to: estimate the current SoC of each cell of the plurality of cells (3),
achieve an optimal SoC of each cell by equalizing at least one cell
of the plurality of cells (3) corresponding to each battery pack of the
plurality of battery packs (2) based on the current SoC of each cell of the
plurality of cells (3) and the OCV of each cell of the plurality of cells (3),
and
input the optimal SoC of each of the equalized cells (3) to the primary BMS (4),
wherein the primary BMS (4) perform charging or discharging of the cells
(3) based on the optimal SoC of each of the equalized cells (3) and the
current SoC of each of the cells (3).

2. The energy storage device as claimed in claim 1, wherein equalizing the at least
one cell of the plurality of cells (3) based on the current SoC of each cell of the
plurality of cells (3) and the OCV of each cell of the plurality of cells (3)
comprises:
determine the at least one cell of the plurality of cells (3) meeting a equalization criteria, wherein the equalization criteria comprises at least one of detecting of a full charge condition of a cell and detecting the current SoC meets a predetermined equalization threshold; and
equalize the at least one meeting the equalization criteria based on the current SoC of each cell of the plurality of cells (3) and the OCV of each cell of the plurality of cells (3).
3. A hybrid cell balancing module for an energy storage device, wherein the hybrid
cell balancing module comprises:
a secondary battery management system (BMS) (6) connected to the primary BMS (4), wherein the secondary BMS (6) comprises:
at least one voltage sensor (7) connected to the plurality of battery packs (2)
and configured to detect an Open-circuit voltage (OCV) of each cell of the
plurality of cells (3) corresponding to each battery pack of the plurality of
battery packs (2); and
a equalization controller (8) connected to, the at least one voltage sensor,
configured to:
estimate a current SoC of each cell of the plurality of cells (3) corresponding
to each battery pack of the plurality of battery packs (2) of the energy storage
device, achieve an optimal SoC of each cell by equalizing at least one cell of the plurality of cells (3) corresponding to each battery pack of the plurality of battery packs (2) based on the current SoC of each cell of the plurality of cells (3) and the OCV of each cell of the plurality of cells (3), and

input the optimal SoC of each of the equalized cells (3) to the primary BMS (4) to perform charging or discharging of the cells (3) based on the optimal SoC of each of the equalized cells (3) and the current SoC of each of the cells (3).
4. The hybrid cell balancing module as claimed in claim 3, wherein equalize the at
least one cell of the plurality of cells (3) based on the current SoC of each cell of
the plurality of cells (3) and the OCV of each cell of the plurality of cells (3)
comprises:
determine the at least one cell of the plurality of cells (3) meeting a equalization criteria, wherein the equalization criteria comprises at least one of detecting of a full charge condition of a cell and detecting the current SoC meets a predetermined equalization threshold; and
equalize the at least one meeting the equalization criteria based on the current SoC of each cell of the plurality of cells (3) and the OCV of each cell of the plurality of cells (3).
5. A method for hybrid cell balancing by an energy storage device, wherein the
method comprises:
estimating a SoC of each cell of a plurality of cells (3) corresponding to each battery pack of a plurality of battery packs (2) of the energy storage device;
detecting an Open-circuit voltage (OCV) of each cell of the plurality of cells (3) corresponding to each battery pack of the plurality of battery packs (2) using a secondary BMS (7);
achieving an optimal SoC of each cell by equalizing at least one cell of the plurality of cells (3) corresponding to each battery pack of the plurality of battery packs (2) based on the current SoC of each cell of the plurality of cells (3) and the OCV of each cell of the plurality of cells (3);
inputting the optimal SoC of each of the equalized cells (3) to a primary BMS (4); and
charging or discharging of the cells (3) based on the optimal SoC of each of the equalized cells (3) and the current SoC of each of the cells (3).

6. The method as claimed in claim 5, wherein equalizing the at least one cell of the plurality of cells (3) based on the current SoC of each cell of the plurality of cells (3) and the OCV of each cell of the plurality of cells (3) comprises:
determine the at least one cell of the plurality of cells (3) meeting a equalization criteria, wherein the equalization criteria comprises at least one of detecting of a full charge condition of a cell and detecting the current SoC meets a predetermined equalization threshold; and
equalize the at least one cell meeting the equalization criteria based on the current SoC of each cell of the plurality of cells (3) and the OCV of each cell of the plurality of cells (3).