EP3864734A1

EP3864734A1 - Detection of abnormal self-discharge of lithium ion cells, and battery system

Info

Publication number: EP3864734A1
Application number: EP19756176.4A
Authority: EP
Inventors: Sebastian Scharner; Sonia Dandl; Jan Philipp Schmidt
Original assignee: Bayerische Motoren Werke AG
Current assignee: Bayerische Motoren Werke AG
Priority date: 2018-09-25
Filing date: 2019-08-19
Publication date: 2021-08-18
Also published as: US20210396817A1; KR20210040123A; KR102533462B1; DE102018216356A1; CN112740504A; WO2020064221A1; JP2022502990A; CN112740504B; JP7389797B2

Abstract

The present invention relates to a method for detecting abnormal self-discharge in a battery system by monitoring the balancing charge for each cell and to a battery system which is configured to use this method. The method according to the invention makes it possible to predict the probability of the occurrence of a safety-critical state, for example an internal short circuit, with the result that suitable countermeasures can be taken in good time.

Description

Detection of abnormal self-discharge from lithium ion cells and battery system

Technical field

The present invention relates to a method for detecting abnormal self-discharge of lithium ion cells in a battery system, and to a battery system which uses this method.

Technical background

Battery system

Battery systems for electric or hybrid-electric

operated vehicles include a plurality of individual ones

secondary cells connected to one another in parallel and in series, typically lithium ion cells, which are separated by a

Battery management system (BMS) are checked.

Among other things, the BMS has the function of operating data such as cell voltage, state of charge (SoC, State of Charge),

Monitoring the degree of aging (SoH, State of Health), electricity, temperature and charging or discharging the cells

Taxes. Other tasks of the BMS are thermal

Management of the battery system, protection of the cells, as well as the prediction of the remaining life of the cells based on the recorded operating data.

Balancing

An important function of the BMS is the so-called balancing, the balancing of the charge status of the individual cells. It can happen that the state of charge (SoC) of individual cells is due to increased self-discharge, for example

uneven temperature distribution or Manufacturing fluctuations from the SoC of the remaining cells of one

Cell network deviates. Such an imbalance manifests itself in a drifting apart of the cell voltages and can lead to a shortening of the lifespan and increased wear of the cells. When balancing the

The state of charge of the cells is aligned to restore the balance.

A general distinction is made between active and passive balancing processes. With active balancing processes, charge is transferred from a cell with an increased SOC to a cell with a lower SOC. This can be done via a capacitor, a coil and / or a voltage converter. In the case of passive balancing methods, on the other hand, the excess charge is simply transferred via a cell with an increased SOC

Resistance (shunt) dissipates until the state of charge

is balanced.

Safety-critical operating states

A main reason for the widespread use of

Lithium ion cells is their high energy density. However, the high energy density also holds the potential

catastrophic damage in the event of a fault, for example a thermal runaway of the cell ("thermal runaway"). It is therefore important that the BMS detects safety-critical states of a lithium-ion cell in good time and can take suitable countermeasures.

A "thermal runaway" can occur because an abnormally high current flow, for example as a result of an internal or external short circuit, causes the temperature of the cell to rise to such an extent that the integrity of the separator is no longer guaranteed, and local defects initially occur, for example by local deformation or local melting of the separator, so that the anode (reducing agent) and cathode (oxidizing agent) come into direct or at least electrically conductive contact at these points locally enables a strongly exothermic redox reaction, which leads to a further rise in temperature and further destruction of the separator. The initially local redox reaction can spread further, evaporation and decomposition of the electrolyte can occur, which then leads to a

Bursting of the housing, contact with atmospheric oxygen and finally fire or explosion can result.

The risk of an external short circuit can be countered, for example, by attaching a fuse. An internal short circuit occurs when there is an electrically conductive connection between the anode and cathode inside the cell. On the one hand, such internal short circuits can result from catastrophic external influences, for example due to a mechanical deformation of the cell or

Penetration with a metallic object as a result of an accident.

Another class of internal short circuits can be caused by

Operations in the cell itself are caused, for example, by being trapped in the cell due to a manufacturing defect

Metal particles and / or by depositing metallic lithium in the form of dendrites on the anode, which "grow" through the separator and can form a conductive connection to the cathode. It is believed that these types of errors do not immediately lead to a "thermal runaway" leads. It can be assumed that the dendrites are initially very thin and have a comparatively low current carrying capacity, so that they dissolve automatically in the event of a short circuit.

However, if such "soft" short circuits (hereinafter also referred to as "soft shorts") occur repeatedly, in particular at the same point (where, for example, a metal particle or a separator weak point can be), it can be assumed that there is one forms an increasingly more resilient bridge between anode and cathode and finally "thermal runaway" occurs. Before this is the case, however, the "soft shorts" should be noticeable increased self-charging of the cell. Monitoring the self-discharge is therefore a possible measure to prevent the occurrence of a safety-critical state

predict and take appropriate countermeasures at an early stage.

As a method for monitoring the self-charging, it is proposed in US 2012/182021 to monitor the differential current between two cells connected in parallel, which is zero if the self-discharge is the same and otherwise from the cell with low self-discharge to the cell with higher ones

Self-discharge flows.

Task

In view of the above problems, the present invention is based on the task, a method by monitoring the self-discharge of lithium ion cells

To provide, which can be integrated into an existing battery system with minimal equipment.

Brief description of the drawings

Figure 1 is a sketch of a cell network with passive

Balancing circuit. The balancing charge that flows when the balancing circuit of cell i is actuated can be determined as Qi = / Ii (t) dt = JUi (t) / R dt.

FIG. 2 schematically shows the structure of a battery system in which the method according to the invention can be implemented.

FIG. 3 shows the course of the accumulated total balancing charge for a group of 10 cells, plotted against time. Cell 5 shows an increase in self-discharge by a factor of 2.5 after the 10th cycle. Description of the invention

The invention is based on the idea of solving this problem by using the balancing function which is present in the battery system anyway.

The invention thus relates to a method for detecting abnormal self-discharge in a battery system by monitoring the balancing charge per cell, and to a

Battery system necessary for the use of this method

is configured.

The method according to the invention enables a prediction for the probability of occurrence of a

safety-critical condition such as an inner

Short circuit, so that suitable countermeasures can be taken at an early stage.

Battery system and balancing

The battery system in which the method according to the invention is used comprises a plurality of lithium ion cells and a battery management device (hereinafter also referred to as a battery management system, BMS), in which the cells are each provided with a balancing circuit individually or in groups. The battery management device is set up to carry out charge equalization at predetermined times, i.e. a balancing

perform. For this purpose, in the case of a cell or cell group whose cell voltage is higher than at least one other cell or cell group, the balancing circuit is actuated in order to remove charge from this cell or cell group and, optionally, the charge removed from another cell or

Feed the cell group with a lower cell voltage until the cell voltages are equalized.

The balancing is typically carried out during a rest phase, for example after charging and at one Time when the battery system is not loaded.

If the battery system is installed in an electric vehicle, the balancing can be carried out at any time outside of the ferry operation, preferably directly after the storage device has been charged. In a hybrid-electric vehicle or plug-in-hybrid-electric vehicle, ferry operation with an internal combustion engine can also be considered. According to the invention, the time and the exact method of balancing are not particularly limited, as long as the charge converted for each cell can be determined by the BMS.

In the simplest case of passive balancing, the cell with increased cell voltage (and thus increased SOC) is only

Charge removed and connected to a load resistor (shunt)

dissipates. A simplified schematic representation of such a passive balancing circuit for the case of N cells connected in series is shown in FIG. The cell voltage Ui is monitored by the BMS for each cell i. In addition, each cell is provided with a shunt circuit, which includes at least one switch Si (e.g. a MOSFET) controlled by the BMS and the actual parallel resistance (shunt) Ri.

In practice, a possibility for direct measurement of the balancing current Ii, as shown in the figure for illustration, is typically omitted in order to increase the expenditure on equipment

minimize. The balancing stream is instead generated from the

Resistance value Ri and the voltage curve Ui (t) measured during balancing are calculated as Ii (t) = Ui (t) / R.

Integration over time provides the flow of cargo.

When an excessive SOC or an increased voltage compared to the other cells is detected, the switch Si is closed by the BMS for balancing, and the balancing current Ii flows, which is converted into heat at the resistor Ri. In this way, the cell is discharged in a controlled manner via the shunt circuit until the voltage Ui reaches the setpoint. Then switch Si is opened again and the shunt circuit is separated from the cell. With active balancing, charge is removed from one cell and fed to another cell. The charge is transferred via a switched component, which as

Energy buffer acts. This can be a capacitor, a coil, a transformer or a voltage converter

(Switching regulator) are used.

In the simplest case, it is a switched capacitor that is cyclically charged on a high SOC (and high voltage) cell, switched, and discharged on a low SOC (and low voltage) cell. The balancing charge can be determined from the respective voltage, the capacitance of the capacitor and the switching frequency. This principle applies analogously to other and / or more complex active balancing systems, including those with

inductive components (coils, transformers) or

Switching regulators as transmission elements. In any case, the balancing charge can be made up of the voltage at the cells involved, the switching frequency and the characteristics of each

Transmission element can be determined. The characteristics of the active balancing circuit are typically already known and stored in the BMS, so that those taken from a cell i

Balancing charge and the balancing charge supplied to a cell j can be determined from the voltage curve and the actuation duration of the balancing circuit.

method

The inventive method includes the following

Steps (1), (2) and / or (3) and (4):

(1) Determination of the charge qi that occurs when the

Balancing circuit of each cell or cell group is removed or supplied by the balancing (with i = 1 ... N, where N represents the number of cells or cell groups), (2) Determination of the total balancing charge Q _ge s, i for each cell or cell group by accumulation of the in 1

certain cargo:

Qges, i - Sc qi, x where x = l ... n is the count variable for the xth actuation of the balancing circuit and n is the total number of previous actuations for the respective cell or

Represents cell group i;

(3) Determination of the balancing rate dqi / dt as the quotient of the charge qi measured in FIG. 1 and the time interval Ät that has occurred since the balancing circuit was actuated previously

has passed;

(4) diagnosis of abnormally high self-discharge for the

Cell or cell group i if Q _ge s, i or dqi / dt are too low with regard to the criteria described below.

Step (1) comprises the determination of each cell or

Cell group i removed during a balancing process or, in the case of active balancing, possibly also supplied charge Qi by the BMS, as described above. In case of a

the charge is reversed. The

The following description speaks unified of the removed charge, whereby a supplied charge is regarded as a removed charge with the opposite sign.

According to step (2), the charge qi converted during a single balancing process can be accumulated for each cell or cell group i. In the simplest case, it is sufficient to save the current accumulated value Q _tot , i in each case. The age of the cell t _ge s, ί is also known, so that a quotient Qges, i / t _g es, i can be determined, which represents the total balancing charge per time. Alternatively, the respective

accumulated values Q _ge s, i are also recorded in time for each balancing process. According to step (3), as an alternative or in addition to the total balancing charge, the balancing rate dqi / dt can also be determined. This is the quotient of the charge implemented during balancing and the time interval At between two balancing processes. Since the balancing rate is subject to comparatively large fluctuations in practice, an average value is preferably used for the diagnosis, for example in the form of an overall mean value over the age of the cell or a moving average value, over which the values of a previous period (e.g. 2nd up to 10 days) or the values of the last 2 to 10 balancing processes, for example, are averaged.

On the basis of the total balancing charge and / or balancing rate determined in this way, it is subsequently determined in step (4) whether there is an abnormally high self-discharge. The criterion for this is ultimately a charge that is too low compared to the other cells or to the total balancing charge. If less charge had to be taken from the cell for balancing than the other cells, which were otherwise subject to the same conditions, this indicates that the self-discharge must have been higher. In addition, an abnormally high total balancing charge can in itself give an indication of excessive self-discharge, although no assignment to individual cells is possible here.

Diagnostic criteria

In step (4), the previously in step (2) or

(3) determined total balancing charge and / or balancing rate determined those cells in which the removed balancing charge is abnormally low. Come to this

According to the invention one or more of the following criteria are used: (a) Q _ge s, i / t _tot , i falls below a predetermined one

Threshold Qref / t _ge s, where t _{ges, i represents} the age of the cell or cell group i;

(b) Q _ge s, i / t _g es, ί falls below one across all cells or

Cell groups or one over all cells or

Cell groups except i averaged by one

predetermined threshold value AQ;

(c) dQg _es , i / dt, obtainable by chronological recording of the values determined in (2) and derivation over time, falls below a predetermined threshold value Q'ref;

(d) The mean value of dqi / dt, obtainable by averaging over all values determined over time in (3), falls below a predetermined threshold value q 'ges, ref}

(e) The moving average of dqi / dt, obtainable by averaging those of the values determined in (3) that are at most a predetermined interval below, falls below a predetermined threshold q '_ref;

(f) The change in balancing rate dq _± / dt, available

by chronological recording of the values determined in (3) and derivation over time, falls below a predetermined threshold q ″ _ref .

Since the accumulated total balancing charge increases with the age of the cell t _tot , the values in criteria (a) and (b) must be divided by t _tot ; Here t _tot can be used for the absolute age of the cell (e.g. in days since

Commissioning) or for the total number of already

performed charging cycles are available.

The threshold Q _ref according to criterion (a) indicates the permissible lower limit for the total balancing charge per cell i.

As indicated, this value should be scaled with the age of the cell t _ge s. The threshold value can be determined, for example, by subjecting a battery system of the same type as that in which the method according to the invention is to be carried out to field tests and recording the balancing charge.

Because anomalies of the balancing charge, which increased

Display self-discharge, which is expected to be rare, it is also possible to determine the threshold value by recording the balancing charge of the battery system itself. The values thus obtained for a specific battery system may have to be verified by comparison with other battery systems of the same type. The mean value and the distribution of the balancing charge over the individual cells can then be calculated. The threshold can be determined by taking a specific one from the mean

Multiple of the distribution width is subtracted.

In addition, other external factors known to the battery management system (BMS) or based on other measured variables can also be included in the definition of the threshold value

can be estimated. For example, a natural increase in self-discharge due to aging processes such as e.g. SEI construction

are taken into account by modifying the threshold value depending on the expected state of aging. Due to the finite input impedance of the connected measuring electronics, discharge via leakage currents can still occur, which can also be taken into account for the formation of the threshold value.

According to criterion (b), Q _ge s, i / t _{tot, i is} compared with the average of all cells or all other cells except cell i.

If the value for cell i falls below this average by a predetermined threshold value ÄQ, this can also indicate an excessive self-discharge. Dz) can be determined, for example, from the width or the standard deviation s of the distribution of Q _tot , i / t _ge s, i over all cells. In terms of avoiding false positives

The results in this case are preferably at least 1.5 s, more preferably at least 2s, especially at least 3s.

In order to avoid false-negative results, it is further preferred that 5 s is not exceeded. The determination of AQ as a multiple of the distribution range has the advantage that it can be done in situ on the basis of the determined data and no reference data is required.

According to (c) a further criterion is the change rate of total balancing charge dQ _tot, i / dt. This can be obtained by recording the total balancing charge values, Q _ge s, i, x, determined for each balancing in step (2), where x is the counting index for the actuations of the balancing system. By applying an analytical curve fit (such as a polynomial), and forming the time derivative dQ to be _saturated, determine i / dt. If the rate of change of the total balancing charge decreases over time, this is also an indication of an increased self-discharge. The criterion again falling below a threshold value serves Q'ref consisting of the above-described Q f _re again by the formation of the derivative

can be determined.

Taking the rate of change into account has the advantage that static effects, e.g. Different high measuring currents due to a slight difference in the input impedance of the measuring electronics, which can also lead to a different balancing charge, but are constant over time, can be eliminated and only the change (increase) in the self-discharge of the cells is taken into account.

As an alternative or in addition to the total balancing load, the balancing rate dqi / dt can also be used as a criterion. This can be calculated as the quotient from the step (1)

determined charge and the time interval between two

Actuations of the balancing system can be determined. Since the individual values obtained in this way can typically fluctuate relatively strongly, an average is first calculated (criterion (d)). This in turn is compared with a threshold value q 'ref, _g es, which is obtained in a manner analogous to Q _re f, for example can be, or from Q _re t by forming the temporal

Derivative is calculated.

As an alternative to the overall mean, a moving average can also be determined according to criterion (e), for example as the average of the last 2 to 10 operations of the

Balancing systems or as an average of the balancing processes that occurred less than a certain time interval, eg 1 to 20 days. The moving average is again compared to a threshold q ' _ref .

Finally, according to criterion (f), the time

The course of the balancing rate can be considered, which corresponds to the second time derivative of the balancing charge qi. A decrease in the balancing rate over time that one

_falls below a predetermined threshold value q ″ _ref also indicates an increased self-discharge.

implementation

The measurement, recording and evaluation of the data in steps (1) to (4) is, as described, by the BMS

carried out, the exact type of implementation and the control units involved are not particularly limited.

These functions can preferably be implemented by the

Battery system already existing control units are perceived.

In the simplest case, the BMS is implemented in a single control unit (BCU, Battery Control Unit) that the

Operating data of all cells monitored simultaneously or sequentially in a multiplex process. The cells can also be connected in groups to form modules. In this case, all monitoring and control functions are carried out by the BCU, so that the method according to the invention

is expediently also carried out in the BCU. Alternatively, a separate control unit (cell monitoring circuit, CSC) can be provided for each module or for each cell, which takes over the monitoring of the cells at the module level and the recorded data via a communication system (for example a CAN bus) to the BCU

transmitted. Such an arrangement is shown in FIG. 3.

If the CSCs are appropriately equipped with regard to the available memory and computing capacity, all process steps (1) to (4),

including the optional chronological recording of the data on which CSCs are carried out, which then only transmit the presence or absence of cells with increased self-discharge to the BCU. Alternatively, the calculation of the charge (1) and possibly also the accumulation (2) per cell can be carried out on the CSCs, and the BCU records the time, forms the derivative (3) and evaluates (4). In another alternative

The CSCs merely transmit the time profile of the voltage during balancing to the BCU, on which the actual process steps (1) to (4) are then carried out.

If cells with abnormal self-discharge are detected, depending on the previous history and / or intensity, the

Deviation from the expected value different reactions

possible. In the event of one-off or sporadic deviations, only an entry can be made in the BCU's fault memory, which marks the cell for checking during the next maintenance of the battery system. In the case of frequent deviations, the cell can e.g. for soon

Exchange are marked, and the BMS transmits a corresponding message via a communication channel (e.g. a CAN bus) to the system in which the battery system is installed (e.g. electrically powered vehicle).

In the worst case, in the case of very large deviations from the setpoint and / or deviations that become increasingly larger, the BMS can also the cell or the module in which the cell is installed, shut down to prevent further deterioration and internal short circuit.

Claims

Expectations

1. A method of detecting abnormal self-discharge in a battery system that includes a plurality of lithium ion cells and a battery management device.

in which the cells are individually or in groups provided with a balancing circuit,

and the battery management device is set up to operate at a predetermined time in a cell or

Cell group, the cell voltage of which is higher than at least one other cell or cell group, actuate the balancing circuit in order to remove charge from this cell or cell group, and optionally supply the removed charge to another cell or cell group with a lower cell voltage until the Cell voltages are equalized,

the method comprising the following steps:

(1) Determination of the charge Qi that occurs when the

Balancing circuit of each cell or cell group is removed or supplied by the balancing (with i = 1 ... N, where N represents the number of cells or cell groups),

(2) Determine the total balancing charge Q _ge s, i for each

Cell or cell group by accumulation of the in 1

certain cargo:

Qges, i - Sc Qi, x where x = l ... n is the count variable for the xth actuation of the balancing circuit and n is the total number of

previous operations for the respective cell or

Represents cell group i;

(3) Determination of the balancing rate dQi / dt as the quotient of the charge Qi measured in FIG. 1 and the time interval At that has elapsed since the previous actuation of the balancing circuit

has passed; (4) diagnosis of abnormally high self-discharge for the

Cell or group of cells i, when Q _tot, i and dQ i / dt of at least one of the following criteria met:

(a) Qges, i / tge _S , i falls below a predetermined value

Threshold Qref / t _ge s, where t _ges , i represents the age of the cell or cell group i;

(b) Qges, i / t _ges, falls below a i over all cells or

Cell groups or one over all cells or

Cell groups except i averaged by one

predetermined threshold AQ;

(c) dQ _g es, i / dt, obtainable by chronological recording of the values determined in (2) and derivation over time, falls below a predetermined threshold value Q ' _r ef,

(f) The change in balancing rate dqi / dt, available

by chronological recording of the values determined in (3) and derivation over time, falls below a predetermined threshold q ″ ref.

2. The method of claim 1, wherein the balancing circuit is a passive balancing circuit comprising a switch and a load resistor Ri, and the charge is determined as Qi = JUi (ti) / Ri dti, wherein Ui (ti) die Cell voltage while operating the balancing circuit and ti die

Actuation time is.

3. The method according to claim 1, wherein the balancing circuit is an active balancing circuit which has a previously known charge extraction or delivery characteristic Qentn (Ui (ti), ti) or Qentn (U _{j (} t _j) , t _j) , where i for the cell or

Cell group from which charge was taken, j represents the one to which charge was supplied, and Ui, _j and ti, _{j represent} the cell voltage and the actuation time.

4. The method of claim 3, wherein the active balancing circuit comprises at least one of a capacitor, a coil, a transformer and a voltage converter.

5. The method according to at least one of claims 1 to 4, wherein each cell is provided with a balancing circuit.

6. The method according to at least one of claims 1 to 5, wherein both the total balancing charge Qges, i and the balancing rate dQi / dt are determined.

7. The method according to at least one of claims 1 to 6, wherein the values determined in (2) and / or (3) for Qges, i and dQi / dt are recorded over time.

8. battery system comprising a plurality of lithium ion cells and a battery management device,

characterized in that the battery management device for performing the method according to

at least one of claims 1-7 is configured.