DESC:FIELD OF THE INVENTION
[001] The present invention relates to estimation of temperature of electrochemical cells. More particularly, the present invention relates to a method for estimating temperature of electrochemical cells and a system thereof.

BACKGROUND OF THE INVENTION
[002] Conventionally, in vehicles, electrochemical batteries used for energy storage (for example in electric vehicles). If the electrochemical batteries are operated in an unsafe manner, can result in hazards ranging from cell internal damage which reduces battery life to thermal runaway which may lead to fire. Hence, batteries (especially Lithium batteries) require continuous monitoring for optimum performance and for the safety of the battery as well as the rider and passengers. Most of the battery management systems (BMS) typically measure three quantities from the batteries, namely ‘cell voltages’, ‘cell current’ and ‘cell temperature’ using voltage, current and temperature sensors respectively. The ‘Safe Operating Area’ (SOA) is typically defined based on these three parameters.
[003] The electrochemical phenomena underlying the operation of electrochemical cells depends on internal temperature of the cells. Heat dissipation in electrochemical cells increases with an increase in cell temperature. The cell may get irreversibly damaged because of unsafe temperature of cells and may also result in thermal runaway condition in the battery. Hence, it is critical to know the cell internal temperatures accurately.
[004] To measure cell temperatures, typical battery management systems use temperature sensors such as thermistors or thermocouples attached to the body of plurality of cells in the battery pack. Such an external temperature sensor measures only the surface temperature of a cell. Estimation of the internal temperature of the cell, based on surface temperature, can be done using thermal modelling of the cell. However, thermal modelling of the cell requires accurate knowledge of the internal structure of the cell and an accurate knowledge of the surrounding conditions which maybe unavailable in practice.
[005] Moreover, a battery pack requires large number of temperature sensors for more accurate measurement of cell temperatures. This increases the system cost and complexity due to the additional wiring in the system. The cell temperature can be estimated by thermal modelling of the whole battery pack with a limited number of temperature sensors. This however requires a very accurate understanding of the surrounding conditions as well as the placement of each cell and cannot be applied to any generic battery pack.
[006] To maintain the battery in its safe operating conditions, it is important to continuously monitor the temperature. However, the available set of methods for temperature estimation without using temperature sensors require a cell current to be injected in a fixed form which is not possible when the loads (motor, etc.) are operational. Thus, these methods fail to satisfy the function of continuous temperature monitoring.
[007] Thus, there is a need in the art for a method and system for estimating temperature of electrochemical cells which addresses at least the aforementioned problems.

SUMMARY OF THE INVENTION
[008] In one aspect of the invention, the present invention is directed towards a method for estimating temperature of one or more electrochemical cells in a battery pack. The method has the step of: initialising, by a central processing unit, a first parameter and a set of second parameters; detecting, by a current sensing circuitry, current in the one or more electrochemical cells; and detecting, by a voltage sensing circuity, terminal voltage across the one or more electrochemical cells. Thereafter, the method has the steps of: receiving, by the central processing unit, current in the one or more electrochemical cells from the current sensing circuitry, and terminal voltage across the one or more electrochemical cells from the voltage sensing circuitry; estimating and updating, by the central processing unit, the first parameter based on at least one of the current in the one or more electrochemical cells or the terminal voltage across the one or more electrochemical cells; estimating and updating, by the central processing unit, the set of second parameters based on the updated first parameter, current in the one or more electrochemical cells and the terminal voltage across the one or more electrochemical cells; and determining, by the central processing unit, the temperature of the one or more electrochemical cells based on the first parameter and one or more of the set of second parameters.
[009] In an embodiment of the invention, the first parameter includes a cell capacity of the one or more electrochemical cells.
[010] In a further embodiment of the invention, the set of second parameters includes one or more dynamic time constants of the one or more electrochemical cells, and the one or more dynamic time constants includes voltage time constant of the one or more electrochemical cells.
[011] In a further embodiment of the invention, the method has the step of estimating and updating, by the central processing unit, the set of second parameters based on a Dual Extended Kalman Filter.
[012] In a further embodiment of the invention, the method has the step of dynamically or intermittently, estimating and updating, by the central processing unit, the first parameter based on at least one of the current in the one or more electrochemical cells or the terminal voltage across the one or more electrochemical cells.
[013] In another aspect of the invention, the present invention is directed towards a system for estimating temperature of one or more electrochemical cells in a battery pack. The system has a current sensing circuitry configured to detect current in the one or more electrochemical cells, and a voltage sensing circuity configured to detect terminal voltage across the one or more electrochemical cell. The system further has a central processing unit configured to initialise a first parameter and a set of second parameters. The central processing unit receives current in the one or more electrochemical cells from the current sensing circuitry, and terminal voltage across the one or more electrochemical cells from the voltage sensing circuitry. The central processing unit estimates and updates the first parameter based on at least one of the current in the one or more electrochemical cells or the terminal voltage across the one or more electrochemical cells, and estimates and updates the set of second parameters based on the updated first parameter, current in the one or more electrochemical cells and the terminal voltage across the one or more electrochemical cells. The central processing unit is configured to determine the temperature of the one or more electrochemical cells based on the first parameter and one or more of the set of second parameters.
[014] In an embodiment of the invention, the first parameter includes a cell capacity of the one or more electrochemical cells.
[015] In a further embodiment of the invention, the set of second parameters includes one or more dynamic time constants of the one or more electrochemical cells, and the one or more dynamic time constants includes voltage time constant of the one or more electrochemical cells.
[016] In a further embodiment of the invention, the central processing unit estimates and updates the set of second parameters based on a Dual Extended Kalman Filter.
[017] In a further embodiment of the invention, the central processing unit is configured to dynamically or intermittently, estimate and update the first parameter based on at least one of the current in the one or more electrochemical cells or the terminal voltage across the one or more electrochemical cells.

BRIEF DESCRIPTION OF THE DRAWINGS
[018] Reference will be made to embodiments of the invention, examples of which may be illustrated in accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.
Figure 1 illustrates a system for estimating temperature of electrochemical cells, in accordance with an embodiment of the invention.
Figure 2 illustrates a comparison of terminal voltage of an electrochemical cell when subjected to a rectangular current pulse at different cell temperatures, in accordance with an embodiment of the invention.
Figure 3 illustrates plot of voltage time constant response as a function of cell temperature, in accordance with an embodiment of the invention.
Figure 4 illustrates the method steps involved in a method for estimating temperature of electrochemical cells, in accordance with an embodiment of the invention.


DETAILED DESCRIPTION OF THE INVENTION
[019] The present invention relates to estimation of temperature of electrochemical cells. More particularly, the present invention relates to a method for estimating temperature of electrochemical cells and a system thereof. The system and method of the present invention is typically used for electrochemical cells in a battery pack of an electric vehicle such as a two wheeled vehicle, or a three wheeled vehicle, or a four wheeled vehicle, or other multi-wheeled vehicles as required. However, it should be understood that the system and method of the present invention as illustrated may find its application in any other automotive or non-automotive application using electrochemical cells as required. The present invention is capable of continually estimating temperature of cells even during normal battery operation, and without using temperature sensors.
[020] Figure 1 illustrates a system 100 for estimating temperature of one or more electrochemical cells 12 in a battery pack 10. Correspondingly, Figure 4 illustrated method steps involved in a method 200 for estimating temperature of one or more electrochemical cells 12 in the battery pack 10. As illustrated in Figure 1, the battery pack 10 comprises of one or more electrochemical cells 12 connected to each other in a series and parallel connection combination.
[021] As illustrated in Figure 1, the system 100 has a current sensing circuitry 110 configured to detect current in the one or more electrochemical cells 12. In an embodiment, the battery pack 10 comprises one or more current sensors 12 that are configured to measure the current in the one or more electrochemical cells 12, and the one or more current sensors 112 are communicatively coupled with the current sensing circuitry 110. The system 100 further has a voltage sensing circuity 120 that is configured to detect terminal voltage across the one or more electrochemical cell 12.
[022] Further, the system 100 has a central processing unit 130. The central processing unit 130 is configured to initialise a first parameter and a set of second parameters. The first parameter and the set of second parameters are parameters linked to the one or more electrochemical cells 12 in the battery pack 10, that are utilised for estimation of temperature of the one or more electrochemical cells 12. Thereafter, the central processing unit 130 receives current in the one or more electrochemical cells 12 from the current sensing circuitry 110, and terminal voltage across the one or more electrochemical cells 12 from the voltage sensing circuitry 120.
[023] Thereafter, the central processing unit 130 is configured to estimate and update the first parameter based on at least one of the current in the one or more electrochemical cells 12 or the terminal voltage across the one or more electrochemical cells 12. Thus, the first parameter is updated based on the detected current in the one or more electrochemical cells 12 or terminal voltage across the one or more electrochemical cells 12 or both. Further, the central processing unit 130 is configured to estimate and update the set of second parameters based on the updated first parameter, current in the one or more electrochemical cells 12 and the terminal voltage across the one or more electrochemical cells 12. Thus, the set of second parameters are updated based on a combination of the updated first parameter, detected current in the one or more electrochemical cells 12 and terminal voltage across the one or more electrochemical cells 12. Thereafter, the central processing unit 130 is configured to determine the temperature of the one or more electrochemical cells 12 based on the updated first parameter and one or more of the updated set of second parameters. Thus, out of the updated set of second parameters, one or more are used for estimation of temperature of the one or more electrochemical cells 12.
[024] In line with the aforementioned configuration of the central processing unit 130, the method steps involved in the method 200 for estimating the temperature of one or more electrochemical cells 12 have been illustrated in Figure 4. At step 202, the first parameter and the set of second parameters are initialised by the central processing unit 130. In an embodiment, the first parameter includes a cell capacity of the one or more electrochemical cells 12. Cell capacity also referred to as Q is defined as the quantity of electricity that is removed from the electrochemical cell at a given rate of discharge under specified conditions of voltage and temperature. Cell capacity (Q) measured in ampere-hours. Further, in an embodiment, the second set of parameters comprise one or more dynamic time constants of the one or more electrochemical cells 12, and the one or more dynamic time constants comprise voltage time constant of the one or more electrochemical cells 12. Time constant is understood as a time which represents the speed with which a particular system, such as the one or more electrochemical cells 12 can respond to change, typically equal to the time taken for a specified parameter to vary by a factor of 1- 1/ e (approximately 0.6321). Accordingly, the voltage time constant is representative of the time taken by the terminal voltage across the one or more electrochemical cells 12 to vary by a factor of 1-1/e in response to a change in input.
[025] Thereafter, at step 204, current in the one or more electrochemical cells 12 is detected by the current sensing circuitry 110, and terminal voltage across the one or more electrochemical cells 12 is detected by the voltage sensing circuity 120. Thereafter, at step 206, the central processing unit 130 receives current in the one or more electrochemical cells 12 from the current sensing circuitry 110, and terminal voltage across the one or more electrochemical cells 12 from the voltage sensing circuitry 120.
[026] Further, at step 208, the first parameter is estimated and updated by the central processing unit 130. The first parameter is updated based on at least one of the current in the one or more electrochemical cells 12 or the terminal voltage across the one or more electrochemical cells 12. Thereafter, at step 210, the set of second parameters are estimated and updated by the central processing unit 130 based on the updated first parameter, current in the one or more electrochemical cells 12 and the terminal voltage across the one or more electrochemical cells 12. Finally, at step 212, the temperature of the one or more electrochemical cells 12 is determined by the central processing unit 130 based on the first parameter and one or more of the set of second parameters.
[027] As explained hereinbefore, in an embodiment, the first parameter includes the cell capacity Q of the one or more electrochemical cells 12, and the set of second parameters include one or more dynamic time constants of the one or more electrochemical cells 12. Specifically, the one or more dynamic time constants comprise voltage time constant of the one or more electrochemical cells 12.
[028] The relationship between the set of second parameters such as the dynamic time constants, the first parameter such as cell capacity Q and the temperature of the one or more electrochemical cells 12 is explained in Figure 2 and Figure 3. Figure 2 illustrates the typical effect of temperature (T) on terminal voltage (Vt) of the electrochemical cell 12 when subjected to a rectangular discharging current pulse. Terminal voltage (Vt) across the electrochemical cells 12 is seen as the voltage that is applied by the electrochemical cell 12 across a load or circuit that the electrochemical cell 12 is connected to. In Figure 2, the said circuit is represented as a combination of resistors R0, R1 and R2 along with capacitors C1 and C2. The terminal voltage (Vt) is measured corresponding to the open circuit voltage (VOC) representative of the state of charge (SOC) of the electrochemical cell 12. As is seen in Figure 2, the terminal voltage (Vt) for the electrochemical cell 12 is found to be higher for a higher temperature (T) of the electrochemical cell 12 when all other conditions are the same. It is observed that the voltage of the electrochemical cell 12 also takes higher time to relax for a lower temperature as compared to that for a higher temperature.
[029] The time taken by the voltage to relax can be explained using the dynamic time constants (t) of the electrochemical cell 12 as explained above. The dynamic time constants (t) arise mainly out of the diffusion phenomenon taking place inside the electrochemical cell 12. It is an established suggestion under Kinetic theory that diffusion process is faster at higher temperature than at the lower temperature, thus suggesting that the dynamic time constants (t) have a direct relationship with the temperature of the electrochemical cell 12. Moreover, aging of the electrochemical cell 12 affects the length of time over which this diffusion takes place, and thus the dynamic time constants(t) are also dependant on the cell capacity (Q). This dependence of dynamic time constants (t) on temperature (T) of electrochemical cells 12 and cell capacity (Q) is utilised for estimation of temperature of electrochemical cells 12. One example of such relationships between the dynamic time constants (t), temperature (T) and cell capacity (Q) is given in the following equations:

Herein, ‘K’ stands for the ratio of the activation energy to the gas constant, and ‘ß’ is a constant of value ‘2/3’.
[030] From the above equations, it is understood that the dynamic time constant (t) as a function of temperature (T) of the electrochemical cell 12 is dependent on the difference between inverse of temperature (T) of the electrochemical cell 12 and the inverse of initial temperature (T0) of the electrochemical cell 12. It is also understood that the dynamic time constant (t) as a function of cell capacity (Q) of the electrochemical cell 12 is dependent on the ratio of the cell capacity (Q) of the electrochemical cell 12 to the initial cell capacity (Q0) of the electrochemical cell 12. Accordingly, the system 100 and method 200 in the present invention involve both the initialisation of the first parameter and set of second parameters for initial values of the first parameter, say cell capacity and the set of second parameters, say dynamic time constants, as well as updating of the first parameter and the set of second parameters.
[031] A typical plot of variation of time constant (t) due to temperature variation given by the above set of equations is shown in Figure 3. More specifically, Figure 3 illustrates variation in time constant (t), specifically voltage time constant with respect to temperature at different cell capacities. As can be seen in Figure 3, for a larger cell capacity (Q), the variation in time constant (t) with respect to temperature change is higher as compared to lower cell capacity (Q). Combining the above two equations, the temperature (T) of the electrochemical cell is estimated as a function of cell capacity (Q) and the dynamic time constants (t), exemplified by the below equation:

[032] In the above equation, Tref stands for reference temperature of the electrochemical cell 12, and t0 stands for initial value of the time constant. It is understood that the above equation is an exemplary relationship between the temperature of the electrochemical cell 12, the first parameter such as a cell capacity (Q) as the dynamic time constants (t), and there exist other mathematical models relating time constants and cell temperature which result in different, but valid formulations.
[033] Thus, in an exemplary embodiment, in operation, the central processing unit 130 initializes the first parameter such as the cell capacity, and the set of second parameters such as the voltage time constant either from initial default values or from memory where they were stored prior to the previous system shutdown.
[034] Thereafter, the central processing unit 130 receives the individual terminal voltages of the one or more electrochemical cells 12 and current of the one or more electrochemical cells 12 and stores them for processing. Thereafter, the central processing unit 130 estimates and updates the first parameter, say cell capacity based on at least one of the current in the one or more electrochemical cells 12 or the terminal voltage across the one or more electrochemical cells 12. The first parameter is updated using techniques like Coulomb counting or Kalman filter. Coulomb counting is a technique in which current drawn from or supplied to the electrochemical cells 12 is integrated over time to receive the value of cell capacity. Similarly, using a Kalman filter is technique which uses a model based estimator for estimation of cell capacity of the electrochemical cells 12. In an embodiment, the first parameter is dynamically estimated and updated based on at least one of the current in the one or more electrochemical cells 12 or the terminal voltage across the one or more electrochemical cells 12. In an alternative embodiment, the first parameter is intermittently estimated and updated based on at least one of the current in the one or more electrochemical cells 12 or the terminal voltage across the one or more electrochemical cells 12. Accordingly, the central processing unit 130 is configured to dynamically or intermittently, estimate and update the first parameter based on at least one of the current in the one or more electrochemical cells 12 or the terminal voltage across the one or more electrochemical cells 12. The first parameter is intermittently updated since the updating of the first parameter is a slower step than other steps such as temperature estimation.
[035] Thereafter, the central processing unit 130 estimates and updates the set of second parameters including the dynamic time constants using the terminal voltage and current of the electrochemical cells 12, and first parameter, say cell capacity estimated and updated in the previous step. The method for time constant estimation can be based on Dual Kalman filter, curve fitting or similar techniques. In an embodiment, the central processing unit 130 estimates and updates the set of second parameters based on a Dual Extended Kalman Filter. Accordingly, the set of second parameters are estimated based on a Dual Extended Kalman Filter. An extended Kalman Filter is the nonlinear version of the Kalman filter which linearizes about an estimate of the current mean and covariance. Accordingly, a Dual Extended Kalman Filter includes two extended Kalman filters including a state filter and a weight filter that run concurrently and interoperate to estimate the set of second parameters, specifically the dynamic time constants. In an embodiment, the Extended Kalman filters are to be further realized as a Predictor step and a Corrector step. Both the extended Kalman filters are capable of exchanging information to provide an optimal output for estimation of the set of second parameters. In an embodiment, the Dual Extended Kalman filter assumes an ageing model such as “Null model” to predict or estimate the set of second parameters. Thereafter, the central processing unit 130 determines the temperature of the one or more electrochemical cells 12 based on the combination the set of second parameters, namely one or more of the dynamic time constants and the first parameter, namely cell capacity.
[036] Advantageously, the present invention provides a system and method for estimating temperature of electrochemical cells that estimates the temperature of the electrochemical cells during normal operation of the battery pack, and without the usage of temperature sensors. This allows for the battery pack to be accurately operated within safe operating area of the temperature.
[037] The estimation of temperature of the cells ensures that overheating of the battery pack is prevented, and any irreversible damage of thermal runaway condition of the battery pack is prevented. Further, the limitation of using temperature sensors such as thermistors, which only measure surface temperature of the cell, and thereafter requirement of thermal modelling is also eliminated in the present invention.
[038] The elimination of requirement of temperature sensors for estimation of temperature of the electrochemical cells reduces costs and complexity, since not only are the sensors eliminated, but their associated wiring and hardware is also eliminated. Further, the present invention eliminates the requirement of cell current being injected in fixed forms, thus making the temperature estimation of the electrochemical cells possible even when the load that is attached to the battery is operational.
[039] While the present invention has been described with respect to certain embodiments, it will be apparent to those skilled in the art that various changes and modification may be made without departing from the scope of the invention as defined in the following claims.
,CLAIMS:WE CLAIM:
1. A method (200) for estimating temperature of one or more electrochemical cells (12) in a battery pack (10), comprising the steps of:
initialising, by a central processing unit (130), a first parameter and a set of second parameters;
detecting, by a current sensing circuitry (110), current in the one or more electrochemical cells (12);
detecting, by a voltage sensing circuity (120), terminal voltage across the one or more electrochemical cells (12);
receiving, by the central processing unit (130), current in the one or more electrochemical cells (12) from the current sensing circuitry (110), and terminal voltage across the one or more electrochemical cells (12) from the voltage sensing circuitry (120);
estimating and updating, by the central processing unit (130), the first parameter based on at least one of the current in the one or more electrochemical cells (12) or the terminal voltage across the one or more electrochemical cells (12);
estimating and updating, by the central processing unit (130), the set of second parameters based on the updated first parameter, current in the one or more electrochemical cells (12) and the terminal voltage across the one or more electrochemical cells (12); and
determining, by the central processing unit (130), the temperature of the one or more electrochemical cells (12) based on the first parameter and one or more of the set of second parameters.

2. The method (200) as claimed in claim 1, wherein the first parameter comprises a cell capacity of the one or more electrochemical cells (12).

3. The method (200) as claimed in claim 1, wherein the set of second parameters comprises one or more dynamic time constants of the one or more electrochemical cells (12), and the one or more dynamic time constants comprise voltage time constant of the one or more electrochemical cells (12).

4. The method (200) as claimed in claim 1, comprising the step of:
estimating and updating, by the central processing unit (130), the set of second parameters based on a Dual Extended Kalman Filter.

5. The method (200) as claimed in claim 1, comprising the step of:
dynamically or intermittently, estimating and updating, by the central processing unit (130), the first parameter based on at least one of the current in the one or more electrochemical cells (12) or the terminal voltage across the one or more electrochemical cells (12).

6. A system (100) for estimating temperature of one or more electrochemical cells (12) in a battery pack (10), the system (100) comprising:
a current sensing circuitry (110) configured to detect current in the one or more electrochemical cells (12);
a voltage sensing circuity (120) configured to detect terminal voltage across the one or more electrochemical cell (12); and
a central processing unit (130) configured to:
initialise a first parameter and a set of second parameters;
receive current in the one or more electrochemical cells (12) from the current sensing circuitry (110), and terminal voltage across the one or more electrochemical cells (12) from the voltage sensing circuitry (120);
estimate and update the first parameter based on at least one of the current in the one or more electrochemical cells (12) or the terminal voltage across the one or more electrochemical cells (12);
estimate and update the set of second parameters based on the updated first parameter, current in the one or more electrochemical cells (12) and the terminal voltage across the one or more electrochemical cells (12); and
determine the temperature of the one or more electrochemical cells (12) based on the first parameter and one or more of the set of second parameters.

7. The system (100) as claimed in claim 6, wherein the first parameter comprises a cell capacity of the one or more electrochemical cells (12).

8. The system (100) as claimed in claim 6, wherein the set of second parameters comprises one or more dynamic time constants of the one or more electrochemical cells (12), and the one or more dynamic time constants comprise voltage time constant of the one or more electrochemical cells (12).

9. The system (100) as claimed in claim 6, wherein the central processing unit (130) estimates and updates the set of second parameters based on a Dual Extended Kalman Filter.

10. The system (100) as claimed in claim 6, wherein the central processing unit (130) is configured to dynamically or intermittently, estimate and update the first parameter based on at least one of the current in the one or more electrochemical cells (12) or the terminal voltage across the one or more electrochemical cells (12).

Dated this 19th day of April 2024

SEDEMAC Mechatronics Pvt Ltd
By their Agent & Attorney

(Nikhil Ranjan)
of Khaitan & Co
Reg No IN/PA-1471