Method for determining the health status of a battery cell using its balancing circuit

Abstract

The invention relates to a method for determining the health status of a battery cell (C) by analyzing the time evolution of an internal cell resistance. The method includes a control procedure comprising the following steps: obtaining a measurement of the open-circuit voltage (VOC) of the cell; controlling a cell balancing circuit such that a balancing resistor (VB) is placed in parallel with the cell and obtaining a measurement of a balanced voltage (VB) of the cell corresponding to a constant voltage across the cell terminals with the balancing resistor placed in parallel with the cell; and determining the internal cell resistance from the open-circuit voltage and the balanced voltage. Figure 4 is shown in the abstract.

Description

Title of the invention: Method for determining the health status of a battery cell using its balancing circuit. Technical field

[0001] The field of the invention is that of accumulator batteries and more particularly that of systems and methods for monitoring the health status of such batteries. Previous technique

[0002] Backup power systems are widely used in data centers, hospitals, and critical industrial facilities. These systems utilize batteries that are kept constantly charged. However, these batteries are susceptible to damage from corrosion, internal short circuits, sulfation, drying out, and seal failures. Therefore, understanding their condition is essential to ensure that these batteries are able to deliver optimal performance and take over in the event of a power outage.

[0003] Current health monitoring methods require a complete battery discharge cycle to assess its actual available capacity. Performing such a discharge cycle impacts the availability of this safety system, which must nevertheless remain constantly charged.

[0004] Other methods perform a lifespan test rather than a capacity test by monitoring the evolution of the battery's internal resistance. This internal resistance remains relatively constant until the battery reaches the end of its life. At that point, the internal resistance increases and the battery's capacity decreases. Monitoring this value helps determine the appropriate time to replace a battery.

[0005] A solution is known from US patent 2021 / 0302502 A1 for raising an alert when the internal capacity of a battery falls below a threshold or when the internal resistance of the battery rises above a threshold. This solution estimates the internal resistance and internal capacity of a battery by solving an optimization problem aimed at minimizing the difference between an equivalent battery model that takes into account changes in the battery's state of charge during cycling and measurements of battery properties such as its voltage, current, and temperature. Description of the invention

[0006] The invention aims to provide a solution for determining the health status of a battery cell at a lower cost, without affecting its quality of service and without having to perform cycling.

[0007] To this end, the invention proposes a method for determining the health status of a battery cell by analyzing the temporal evolution of an internal resistance of the cell, said method including a control procedure which comprises the following steps: obtain a measurement of the cell's open-circuit voltage; control a cell balancing circuit so as to place a balancing resistor in parallel with the cell and obtain a measurement of a balanced cell voltage corresponding to a permanent voltage across the cell terminals with the balancing resistor placed in parallel with the cell; determination of the internal resistance of the cell from the voltage at empty and balanced tension.

[0008] Some preferred, but not exhaustive, aspects of this process are as follows: Determining the internal resistance of the cell includes calculating where Re is the balancing resistance, and is the open-circuit voltage measured and Vmes is the measured balanced voltage; The steps of the control procedure are repeated, and the control procedure provides a calculated value for the internal resistance of the cell. based on the internal resistances of the cell determined at each iteration of the control procedure steps; the value of the internal resistance of the cell provided by the control procedure is an average or a median of the internal resistances of the cell determined at each of the iterations of the steps of the control procedure; the control procedure includes a measurement of the cell temperature; the control procedure includes a measurement of a temporary voltage appearing at the cell terminals following the placement of the balancing resistor in parallel with the cell; the control procedure includes determining a transient regime duration corresponding to the time it takes for the voltage across the cell to reach the permanent voltage after the balancing resistor is placed in parallel with the cell; The control procedure is repeated over time to determine a current value and a previous value of the internal resistance; the analysis of the temporal evolution of the cell's internal resistance includes determining the difference between the current value and the previous value. and the generation of an alert when said difference exceeds a threshold; - it also includes a measurement of the balanced voltage carried out for different control frequencies of the balancing circuit which places the balancing resistance in parallel with the cell. Brief description of the drawings

[0009] Other aspects, objectives, advantages and features of the invention will become clearer upon reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the accompanying drawings in which:

[0010] - [Fig. 1] is a diagram of a battery illustrating the battery management system associated with each of the battery modules and the balancing circuit associated with each of the cells of a module;

[0011] - [Fig. 2] is a diagram of a balancing circuit associated with a cell of the battery ;

[0012] - [Fig. 3] is a schematic diagram of an equivalent circuit model of a cell of the battery ;

[0013] - [Fig. 4] is a diagram illustrating an equivalent circuit model representing the cell and its balancing circuit respectively without (left) and with (right) the connection of the balancing resistor in parallel with the cell;

[0014] - [Fig. 5] is a diagram illustrating a hashing of the circuit connection balancing at the cell.

[0015] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0016] Figure 1 is a diagram of a battery in a stationary storage system or backup system. This battery comprises a set of BM modules and an Energy Management System (EMS). Each module comprises a plurality of Cl, C2, ..., Cn cells and a Battery Management System (BMS).

[0017] The BMS system continuously monitors the state of each cell of the module. It collects data such as voltage, current and temperature (in particular by means of a temperature sensor T).

[0018] The BMS system is also responsible for balancing the cells so that they are at the same state of charge at rest, which is crucial for optimizing battery performance and extending its lifespan. To do this, the BMS system controls CBU balancing circuits, each connected in parallel to a cell.

[0019] Figure 2 illustrates such a connection of a cell C with a balancing circuit CBU, in this case a (semi-)passive balancing circuit with a switching shunt resistor. The balancing circuit comprises a voltage sensor V in parallel with the series combination of a balancing resistor Re and a semiconductor switch Q. Controlling the switching of switch Q by the BMS system allows the balancing resistor Re to be connected or disconnected in parallel with cell C.

[0020] Figure 3 shows a schematic of an equivalent circuit model of a battery cell. This model includes a voltage source Vo representing the open-circuit voltage of the cell in series with an internal ohmic resistance Ro. This series combination may further include an internal inductance Lo and one or more R / C circuits (commonly known as "parallel RC") with different time constants to model the transient voltage response of the cell. In the example in Figure 3, the equivalent circuit model comprises two R / C circuits, each composed of the parallel combination of an internal bias resistance RbR2 and an internal bias capacitance CbC2. The internal ohmic resistance Ro and the internal bias resistances RbR2 together form an equivalent series resistance Ro+Ri+R2, which models the internal resistance of the cell Rint.

[0021] The invention relates to a method for determining the health status of a battery cell by analyzing the time evolution of the cell's internal resistance Rint. This method can, of course, be implemented for all or part of the cells composing the battery.

[0022] The invention more particularly proposes to use the CBU balancing circuit associated with a cell to estimate the value of the cell's internal resistance Rint. As described in more detail below, this method measures, on the one hand, the open-circuit voltage Vo of the cell and, on the other hand, the equivalent series resistance Ro+ Ri+R2 by exploiting the balancing resistance Re of the CBU balancing circuit. The invention thus repurposes the balancing function to estimate the value of the internal resistance Rint. It makes it possible to monitor the cell's health status by using hardware (CBU circuit and BMS system) already present in the battery, without requiring additional equipment and therefore at a lower cost.

[0023] The method according to the invention includes a control procedure which can be initiated when no current is flowing through the battery and which comprises the following steps: - obtain a measurement of the open-circuit voltage Vo of the cell; - check the cell's CBU balancing circuit to position the balancing resistor Re in parallel with the cell and obtain a measurement of a balanced cell voltage corresponding to a permanent voltage across the cell terminals with the balancing resistor placed in parallel with the cell; - determine the internal resistance Rint of the cell from the open-circuit voltage Vo and the balanced voltage.

[0024] It is possible to connect the cells one by one to their balancing resistance in order to determine the state of health of each of the cells.

[0025] Figure 4 on the left illustrates the situation in which the CBU balancing circuit is controlled so as not to place the balancing resistor Re in parallel with the cell. Since no current flows through the cell, the measurement Vmes of the voltage across the cell using the voltage sensor V of the CBU balancing circuit provides a measurement of the open-circuit voltage Vo of the cell.

[0026] Figure 4 shows on the right the situation in which the balancing circuit CBU is controlled so as to place the balancing resistor Re in parallel with the cell. After a transient response period of the cell (corresponding, for example, to a period of 10 seconds with the balancing resistor placed in parallel with the cell), it is possible to access the current I flowing through the cell by means of the measurement Vmes of the balanced voltage and to deduce the equivalent series resistance Zmeasured according to the following equations:

[0027] r _ Vmes ; and 1 ~ Re+Rq 100281 ^^=^+^+^ = (^+^) , with Rq the resistance in the on state of switch Q.

[0029] To obtain a more reliable determination of the cell's internal resistance, the steps of the control procedure can be repeated (for example, five times consecutively), and the control procedure provides a value for the cell's internal resistance calculated from the internal resistances of the cell determined at each iteration of the steps of the control procedure. This value for the internal resistance can be an average or a median of the internal resistances of the cell determined at each iteration of the steps of the control procedure.

[0030] It should be noted that the balancing resistance must not vary. To this end, the balancing resistance can be maintained within a suitable temperature range. Alternatively, it can be made with one or more resistors whose assembly (for example, nature of the components, manufacturing materials, quality of the electrical connections) results in the balancing resistance not varying with temperature.

[0031] The control procedure may also include a measurement of the cell temperature. This makes it possible to correlate the internal resistance measurements with the temperature. In particular, if the temperature of the balancing resistor is known, it is possible to estimate its value or its derivative quite reliably. Furthermore, knowing the series resistance as a function of the cell temperature can provide additional information on its health status.

[0032] The monitoring procedure is preferably repeated over time, for example, periodically. This repetition makes it possible, in particular, to determine a current value and a previous value of the internal resistance. The previous value can be an original value of the internal resistance or a value determined on a given date before the date of determination of the current value (for example, one month before). The analysis of the temporal evolution of the cell's internal resistance then includes determining the difference between the current value and the previous value and generating an alert when this difference exceeds a threshold. The alert thus generated triggers a maintenance or battery replacement action. In one possible embodiment, the threshold value is adapted to the state of charge and the temperature of the cell.

[0033] Analyzing the temporal evolution of the cell's internal resistance makes it possible to detect an excessive variation relative to the initial value (for example, a variation greater than 10% of the initial value) or an excessively rapid variation over time (for example, a variation greater than 5% in one month). An excessive variation relative to the initial value indicates an approach to a break-point (known as the "knee point") in the growth of internal resistance as a function of usage time, with this growth accelerating after exceeding this break-point. An excessively rapid variation over time, on the other hand, indicates that this break-point has been exceeded. This excessive variation or this excessively rapid variation is interpreted as resulting from a significant deterioration in the cell's health.

[0034] In one possible embodiment, the control procedure includes measuring a transient voltage appearing across the cell terminals after the balancing resistor is placed in parallel with the cell. The control procedure then provides information about the transient response of the cell and, in so doing, provides information about other elements of the model. For example, the control procedure may include determining a transient response time corresponding to the time it takes for the voltage across the cell terminals to reach the steady-state voltage after the balancing resistor is placed in parallel with the cell.

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[0042] In another possible embodiment, which can be implemented independently and / or in addition to the previously described control procedure, a balanced voltage measurement can be performed for different control frequencies of the balancing circuit that places the balancing resistor in parallel with the cell. In other words, the connection of the balancing circuit to the cell is chopped, and the chopping frequency is modulated to measure the internal resistance at different frequencies (e.g., 1 Hz, 100 Hz, 1 kHz, 10 kHz, and 100 kHz). In this embodiment, as shown in Figure 5, the switch Q is therefore controlled by a square wave signal S Cm, which alternates between a high and a low level according to a modulated frequency ω. With Vmes(w) being the RMS value of the voltage across the cell at angular frequency ω, the previous equations become: ;And — z Ke"rKq z ^measured 4'0" 4 With I 60 ) I the magnitude of the measured impedance and I measures' ' | r arg(Z , ( ûJ ) ) 'c phase shift, it is then possible to go back to the equivalent model by correlation with a model presenting the same characteristics as the Bode diagram. The invention is not limited to the method as previously described but also extends to a battery management system comprising a controller configured to implement this method. The controller is thus configured, in particular, to: - to obtain from a voltage sensor capable of measuring a voltage across the terminals of a battery cell a measurement of an open-circuit voltage of the cell; - control a cell balancing circuit so as to place a balancing resistor in parallel with the cell; - obtain from the voltage sensor a measurement of a balanced cell voltage corresponding to a permanent voltage across the cell terminals with the balancing resistor placed in parallel with the cell; - determine an internal resistance of the cell from the open-circuit voltage and the balanced voltage. The controller can also be configured to verify that no current is flowing through the cells using a current sensor from the BMS system. The invention also extends to a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to implement the process as previously described.

Claims

Demands

1. A method for determining the health status of a cell (C) of a battery by analyzing the time evolution of an internal resistance of the cell, said method including a control procedure which comprises the following steps: - obtaining a measurement of an open-circuit voltage (Vo) of the cell; - controlling a balancing circuit (CBU) of the cell so as to place a balancing resistor (Re) in parallel with the cell and obtaining a measurement of a balanced voltage (Vmes) of the cell corresponding to a permanent voltage across the cell with the balancing resistor placed in parallel with the cell; - determining the internal resistance of the cell from the open-circuit voltage and the balanced voltage;in which the balancing circuit (CBU) includes a switch (Q) allowing the balancing resistance (Re) to be placed in parallel with the cell and in which the internal resistance is determined according to the formula 7 _ ( p । p 1 vovœes, where Re is the ^measured ^e' Vmes balancing resistance, Vo is the measured open-circuit voltage, Vmes is the measured balanced voltage and Rq is the resistance in the on-state of the switch (Q).;

2. A method according to claim 1, wherein the steps of the control procedure are reiterated and wherein the control procedure provides a value of the internal resistance of the cell calculated from the internal resistances of the cell determined at each of the iterations of the steps of the control procedure.

3. A method according to claim 2, wherein the value of the internal resistance of the cell provided by the control procedure is an average or a median of the internal resistances of the cell determined at each of the iterations of the steps of the control procedure.

4. A method according to any one of claims 1 to 3, wherein the control procedure includes a measurement of the cell temperature.

5. A method according to any one of claims 1 to 4, wherein the control procedure includes a measurement of a temporary voltage appearing across the cell terminals following the placement of the balancing resistor in parallel with the cell.

6. A method according to claim 5, wherein the control procedure includes determining a transient regime duration corresponding to the time it takes for the voltage across the cell to reach the permanent voltage after the balancing resistor has been placed in parallel with the cell.

7. A method according to any one of claims 1 to 6, wherein the control procedure is repeated over time so as to determine a current value and a previous value of the internal resistance and wherein the analysis of the time evolution of the internal resistance of the cell includes determining a difference between the current value and the previous value and generating an alert when said difference is greater than a threshold.

8. A method according to any one of claims 1 to 7, further comprising a balanced voltage measurement carried out for different control frequencies of the balancing circuit placing the balancing resistance in parallel with the cell.

9. Battery management system (BMS) comprising a controller configured to put the process according to any one of claims 1 to 8.

10. Product computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according to any one of claims 1 to 8.

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