DE102017009448A1 - Method for determining a self-discharge of a battery with at least one battery cell - Google Patents

Abstract

The invention relates to a method for determining a self-discharge of a battery having at least one battery cell (10), in which a voltage of the at least one battery cell (10) by means of a measuring device (12) is detected. The voltage is taken into account when determining the self-discharge of the battery. By means of the measuring device (12), a plurality of measured values of the voltage are detected in chronological succession. The measured values are fed to a computing device (16), which uses the measured values to determine an at least partially non-linear time profile of the voltage of the at least one battery cell (10). Based on the non-linear time course of the voltage, a self-discharge of the battery expected for at least one future point in time is predicted as a function of time.

Description

The invention relates to a method for determining a self-discharge of a battery having at least one battery cell, in which a voltage of the at least one battery cell is detected by means of a measuring device. The voltage is taken into account when determining the self-discharge of the battery.

The DE 10 2010 003 421 A1 describes a method for operating an energy store, in which an energy storage voltage is determined during a current operating phase of the energy store. Depending on the energy storage voltage, a first state of charge of the energy storage device is determined. Furthermore, a second state of charge of the energy store is determined. An average self-discharge current may be determined depending on the state of charge loss and a resting phase duration, wherein the resting phase duration is representative of a period of time between two operating phases. By means of the mean self-discharge current, it is intended to estimate how long a possible resting phase of the energy store may last at most until the energy store falls below a critical state of charge.

A disadvantage here is the fact that a prediction of the self-discharge of the energy store based on the mean self-discharge current is not reliably possible.

Batteries based on lithium-ion cells are currently being used in electric-only electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs) and other electric vehicles. For this purpose, several battery cells are installed in modules, wherein in turn one or more of these modules, the entire battery is composed. To ensure the functionality of the cells and batteries regular monitoring of the cells is necessary, both during operation (ie during power consumption / current delivery) and when not in use (zero current).

The battery cells (not exclusively those based on lithium ions) undergo a continuous self-discharge. This means that the voltage level and the state of charge (SOC) of each cell drop over time, although the cell is not discharged from the outside via the arresters (plus pole and minus pole). The amount of self-discharge is different for each battery cell and depends on a number of factors.

For batteries or electric vehicles can be provided due to the self-discharge fixed-defined Nachladeintervalle. To avoid a deep discharge, therefore, the battery or battery cell (inside or outside a vehicle) at fixed intervals, for example, every 2 or every 3 months, recharged. This is expensive and in no way flexible. The work required to prevent over-discharge is carried out more often than necessary because the worst case scenario is assumed. In addition, the average SOC of the cell (s) is kept unnecessarily high due to overcharging, resulting in premature calendar aging of the battery cell (s).

If a battery user wants to prevent the battery from dropping into the total discharge due to self-discharge, the battery user must always recharge the battery. However, this reloading should preferably be done just as often as necessary.

In order to ensure the functionality of battery cells and batteries, it is also necessary to provide for regular monitoring of the cells in the uninstalled state.

Two states of batteries and battery cells can be distinguished: The installed cells are regularly monitored by an active or self-activating or externally activated battery management system (BMS). In a vehicle or stationary storage, such a BMS is typically present. If the BMS is active or can activate itself, this can be referred to as a use of the battery. During storage, however, the cells are not monitored for a long time. Either because the cells (or the battery) are not installed, or because the battery is not connected to an external power supply.

When storing batteries or battery cells, therefore, a fixed inspection interval is regularly defined. At fixed intervals, for example, the battery voltage is checked by manually connecting to a voltmeter, and the cell (s) are recharged to prevent over-discharge. When determining the test intervals, a constant, linear decrease in the cell voltage or state of charge is usually assumed over time.

Disadvantage is therefore also in this application that the self-discharge is not predicted correctly. Therefore, often a very short check interval is set for security. Furthermore, the cells or batteries are not distinguished according to their Nachladebedarf.

The cell voltage of the installed battery cells in the battery, for example in the vehicle is also monitored. If the vehicle or the battery is not used, but is connected to the on-board power supply, the BMS activates at regular intervals and checks the cell voltages. If the voltage of the cells differs greatly, for example, more than 100 mV, a so-called balancing process is activated. In particular, when the cells are installed in a battery module, so a difference of the cell voltage by the balancing, that is a suitable (de) charge of cells, balanced. Either the higher-voltage cell is discharged by means of a resistor (passive balancing), or charge is transferred from the cell or cells with a higher voltage level to the cell or cells with a lower voltage level (active balancing).

Disadvantage here is that the BMS must wake up regularly after a few hours to check the voltage of the cells. This energy is used (quiescent current of the cell monitoring unit and the BMS), which is withdrawn from the battery itself or a 12V on-board power supply. At the same time, the balancing is not anticipatory. This means that only a current need is recognized and the balancing process started, but the future development of the cell voltage is not taken into account. This may lead to unnecessary balancing operations and an unnecessarily low-impedance design of the balancing resistors because the acute need is always taken into account. This leads to a high balancing current, which leads to a high heat generation.

On both, the electricity and the heat, electronics must be designed. In addition, there may be an unnecessary loss of energy.

Object of the present invention is therefore to provide an improved method of the type mentioned.

This object is achieved by a method having the features of patent claim 1. Advantageous embodiments with expedient developments of the invention are specified in the dependent claims.

In the method according to the invention for determining a self-discharge of a battery having at least one battery cell, a voltage of the at least one battery cell is detected by means of a measuring device. The voltage is taken into account when determining the self-discharge of the battery. For this purpose, a plurality of measured values of the voltage are detected in chronological succession by means of the measuring device. The measured values are fed to a computing device which, on the basis of the measured values, determines an at least partially non-linear time profile of the voltage of the at least one battery cell. Based on the non-linear time course of the voltage, a self-discharge of the battery expected for at least one future point in time is predicted as a function of time. The expected self-discharge is thus predicted time-dependent, and the at least one battery cell can be recharged on time on the basis of this calculation. Consequently, an improved method is provided.

A recharging of the at least one battery cell or the battery can be made so exactly when it is necessary and not for a fixed and for safety often very short-sized reload interval. This reduces the number of reloads, resulting in reduced effort. Furthermore, the average state of charge can be reduced, in particular during storage of the battery, so that a reduced aging of the battery is achieved. In addition, a battery user knows which state of charge his battery will have after a longer service life. This allows the battery user to more easily ensure that the battery always has a desired minimum charge level.

A battery user should be understood to mean a person who has to do with the battery and wants to maintain its functionality. This can be either the driver of an electric vehicle, as well as an employee in a factory, a user of consumer electronics, such as a cordless screwdriver, or a driver of an e-bike. The application is not limited to designed as lithium-ion cells battery cells, but applies to battery cells in general.

Preferably, for at least one future time within a first time period of a predetermined duration, in particular a duration of up to 30 days, the expected self-discharge is determined based on a logarithmic dependence of the voltage on time. In the context of a short-term model, a logarithmic dependence can thus be determined by means of the computing device as a functional course.

It has further been found to be advantageous if, for at least one future time within a second period of a predetermined duration, in particular a duration of about 30 days to about 160 days, the expected self-discharge is determined based on an exponential dependence of the voltage on time , In order to describe the long-term behavior of the self-discharge, an exponential approximation can thus be determined by means of the computing device as a functional course.

Further advantages, features and details of the invention will become apparent from the following description of preferred embodiments and from the drawing (s). The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of the figures and / or in the figures alone can be used not only in the respectively specified combination but also in other combinations or in isolation, without the scope of To leave invention.

• 1 ein Ablaufdiagramm, gemäß welchem Nachladeintervalle von Batteriezellen bestimmt werden, indem ein Verfahren zur Vorhersage der Selbstentladung der Batteriezellen genutzt wird;
• 2 eine zeitliche Darstellung der Abnahme des Ladezustands der Batterie, wobei eine Kurve berechnete Werte des Ladezustands angibt;
• 3 ein Ablaufdiagramm, gemäß welchem basierend auf der vorhergesagten Selbstentladung von Batteriezellen die Batteriezellen sortiert werden;
• 4 ein Ablaufdiagramm, gemäß welchem die mittels einer Recheneinrichtung vorhergesagte Selbstentladung der Batterie auf einer Anzeige in einem Fahrzeug oder in einer App dargestellt wird, wobei auch eine Meldung an ein Serviceteam ausgegeben werden kann; und
• 5 ein Ablaufdiagramm, gemäß welchem basierend auf der mittels der Recheneinrichtung vorhergesagten Selbstentladung ein Ladungsausgleich von elektrisch miteinander verbundenen Batteriezellen der Batterie durchgeführt wird.

Showing:

• 1 a flowchart, according to which recharging intervals of battery cells are determined by a method for predicting the self-discharge of the battery cells is used;
• 2 a time chart of the decrease of the state of charge of the battery, wherein a curve indicates calculated values of the state of charge;
• 3 a flowchart according to which the battery cells are sorted based on the predicted self-discharge of battery cells;
• 4 a flowchart according to which the predicted by means of a calculator self-discharge of the battery is displayed on a display in a vehicle or in an app, wherein a message can be issued to a service team; and
• 5 a flowchart, according to which, based on the predicted by the calculator self-discharge, a charge equalization of electrically interconnected battery cells of the battery is performed.

In 1 schematically is a battery with a plurality of battery cells 10 represented, wherein the respective cell voltage of the respective battery cells 10 by means of a measuring device 12 is detected. For example, therefore, the cell voltage of the battery cells 10 The battery that is at rest is measured for a few days a few times (for example, once, twice, or three times a day, or every other, third, or even longer intervals). For this purpose, the measuring device 12 used. A storage or recording of the measured values takes place in a memory device or a memory 14 which is present in a computing device 16 is integrated. The computing device 16 performs a calculation based on the measurements 18 the expected time course of the state of charge of the battery cells 10 or the battery through.

An appropriate fit determines the most likely future course of the stress. For this purpose, parameters in a model are adjusted such that a functional dependency maps the course of the previous recorded voltage values as well as possible in the sense of a least squares approximation (or another criterion).

The voltage drop is, however, not assumed to be linear in the present case, but instead different models are used as a function of the idle time of the battery or of the battery cells 10 used.

und eine exponentielle Annäherung für das Langzeitverhalten: $$\text{V}\left(\text{t}\right)={\text{r}}_1+{\text{r}}_{\text{2}}\text{exp}\left(\mathrm{-}{\text{r}}_{\text{3}}\text{ t}\right)$$

vorgeschlagen. Beide ergeben sich als Spezialfall des Gesamtmodells: $$\text{V}\left(\text{t}\right)={\text{p}}_1-{\text{p}}_{\text{2}}\text{ln}\left(1-\text{exp}\left({\text{p}}_{\text{3}}-{\text{p}}_{\text{4}}\text{ t}\right)\right)$$

In particular, a logarithmic dependency as a short-term model becomes a functional course: $$\text{V}\left(\text{t}\right)={\text{q}}_1-{\text{q}}_{\text{2}}\text{ln}\left(1+{\text{q}}_{\text{3}}\text{t}\right)$$

and an exponential approach to long-term behavior: $$\text{V}\left(\text{t}\right)={\text{r}}_1+{\text{r}}_{\text{2}}\text{exp}\left(\mathrm{-}{\text{r}}_{\text{3}}\text{t}\right)$$

proposed. Both arise as a special case of the overall model: $$\text{V}\left(\text{t}\right)={\text{p}}_1-{\text{p}}_{\text{2}}\text{ln}\left(1-\text{exp}\left({\text{p}}_{\text{3}}-{\text{p}}_{\text{4}}\text{t}\right)\right)$$

Here, the parameters q, r and p stand for numbers which result from the result of the fit. The actual values of the voltage curves become the values from the memory 14 and determine, for example, a least-squares solution using a common algorithm (for example, by iterative zero-finding with QR decomposition in each step).

The time-dependent assumed voltage curves represent a concrete innovation, because previously assumed a constant drop in the voltage or the SOC, resulting in a fixed recharging time, for example, a recharge, which is carried out every three months. By contrast, the time-dependent relationship described here makes the entire calculation meaningful.

By converting to the state of charge (SOC) via a quiescent voltage characteristic, the future course of the self-discharge can be predicted. It preferably lies in the memory as expected SOC for the future time (days, weeks or months) in the form of a table of values 14 the computing device 16 , Furthermore, the quiescent voltage characteristic is preferably also in the memory 14 filed and there is a (linear) interpolation between the nodes made. For example, the lookup table might look like this: Time from today / d 0 30 60 90 120 150 180 SOC trend /% 76 72 69 66 64 62 60

The user preferably sets a minimum permissible SOC, up to which the battery (or the battery cells 10 ) may unload. On the basis of the calculations, it can be read by looking up in the value table, when the SOC of the battery or the battery cells 10 expected to fall below the minimum allowable SOC. If the table is too coarse, a linear interpolation is used again. Based on this, a reload will occur at the specified time 20 initiated (cf. 1 ). This can be done by a notice to the user, who manually initiates the charging process at the specified time, or automatically.

Alternatively, first the (rest) voltage in the associated SOC can be converted by means of the rest voltage characteristic and interpolation, and the fit is then carried out directly for the SOC values.

The proposed storage and calculations can be done by batteries with a battery management system (BMS). Since the battery is at rest, the necessary computing capacity is free. Alternatively, the storage of the data and the calculations may be performed by a computing device connected to the battery cells 10 not permanently installed.

In 2 is a concrete fit to readings 22 shown. circles 24 show the SOC values used for the fit, which were calculated from measured voltage values via the open-circuit voltage characteristic. Further crosses are further measured values 22 illustrated. Their course is drawn by the function for the long-term behavior (solid drawn and only from those as circles 24 calculated values) predicted which in 2 through a curve 26 is shown. Simultaneous Confidence Bands 28 and prediction bands have been calculated additionally and are in 2 shown in dashed lines. In the present case, the confidence bands overlap 28 the prediction bands. A first time span 46 during which the course of the curve 26 corresponds to the short-term model, and a second time span 48 in which the curve 26 the long-term behavior of the battery cells 10 indicates are in 2 shown schematically.

In the presently described method, some functional values of the solid curve became 26 together with the associated days in the table above. Is in concrete, in 2 In the case illustrated, a minimum permissible SOC of, for example, 86% is deposited and the procedure is carried out on day 45, so determines the computing device 16 day 106 as the day for the next reload. This agrees with the later acquired measured values 22 match, which in 2 represented by the crosses.

The recharging of the battery is done as needed. Too frequent reloading is omitted. This results in a saving of effort and an extension of the life of the battery, because the average SOC is lowered. The battery user will also be informed early on the expected self-discharge of the battery. The charge loss of the battery is no longer surprising and at an unexpected height.

The measuring device 12 and the memory 14 for the voltage values thus enable in combination with the computing device 16 the prediction of the voltage decrease in the battery cells 10 at rest (ie the prediction of so-called self-discharge).

The memory 14 for state of charge values can therefore in combination with the computing device 16 for predicting the state of charge decrease in the battery cells 10 be used in peace.

Furthermore, a representation of the calculated self-discharge in a user display or such a display device 38 be made (see 4 ). By means of the display device 38 It is also possible to make a representation of the calculated expected time until the state of charge due to self-discharge falls below a given value. The means of the computing device 16 Calculated expected time until self-discharge state of charge drops below a given value can also be used to communicate with a service team or maintenance team. In addition, early prediction of a deep discharge battery failure using the predicted self-discharge is possible and a warning of such a deep discharge by issuing a warning message. The known future course of the self-discharge can be communicated in the event of impending deep discharge so in the form of the warning message about a maintenance service. The maintenance service can then respond with appropriate measures (such as reloading, warning the holder, and the like).

In addition to the prediction of the cell voltage drop, it is possible for a battery control unit, which has the computing device 16 has, the confidence of the model are calculated. Based on this, not only the most probable future course of the voltage, but also the area can be specified, in which the cell voltage is likely to be located.

This calculation can be used for all considered battery cells 10 regardless of whether the battery cells 10 be stored or in use in a vehicle. The calculation is easily possible during storage, because appropriate computing capacity can be easily provided. When using the battery, the calculation is also easily possible because in the idle phase control devices such as the battery control unit are not used otherwise. However, it is also possible for a cell monitoring unit of the battery to be equipped with sufficient computing capacity in order to carry out the above calculations.

Since it is known with the calculation carried out how the cell voltage will develop, intervention can be made early both in cases of storage of the battery and in cases of use of the battery.

In the case of storage can be based on the results of the battery cells 10 be grouped. If a certain voltage level is to be maintained, it is preferable to use battery cells 10 reloaded more frequently with increased need for recharge, or even more heavily charged before. Conversely, battery cells can 10 , which are predicted to be less self-discharging, can be exploited by depriving them of energy for a particular purpose, such as the operation of the voltage check, lighting, or the like.

In case of using the battery cells 10 For example, unwanted differences in voltage can be preemptively balanced with a conventional balancing device (which uses, for example, a fixed resistor). Preventive balancing compensates for voltage differences at an early stage and at the same time takes into account a possible adjustment of the voltage levels in the future. So there is a longer period available for the balancing than when, immediately on a just noted charge difference of the battery cells 10 must be reacted. Therefore, larger resistors can be installed than they are used in the usual, not anticipatory balancing. This results in lower heat losses, so that the balancing device must be designed for lower temperature strokes, and a lower balancing current, to which a connection technique is to be interpreted.

Due to the earlier knowledge of the expected voltage curve but other devices or consumers can be installed to compensate for the expected cell voltage differences. For example, the battery control unit could be targeted from the battery cells 10 provide a higher voltage level.

The knowledge about the future course of the self-discharge can also be passed on to a user of the vehicle (such as the driver). For example, the vehicle itself or an app intended for use of the vehicle may determine an expected self-discharge. Based on this, the user may be presented with a time that the vehicle is likely to be able to remain unused at most, if the user has previously specified a desired remaining state of charge. This is advantageous, for example, if the driver wants to leave his vehicle at the airport during a stay abroad lasting several months and wants to ensure that he can still drive a certain distance with the vehicle after returning.

According to 3 The procedure can be used when storing the battery cells 10 be used. The battery cells 10 are contacted on several days for a short time automatically and the respective voltage is measured. The to the measuring device 12 connected computing device 16 calculates the expected self-discharge and the confidence interval for the future. This results in a time when the battery cell 10 should be recharged, because a certain state of charge, for example, 50% or 75% should not be exceeded. Another machine sorting device 30 sorts and stores the battery cells 10 accordingly. This is also advantageous for the later installation of the battery cells 10 because of the battery cells 10 behave the same way if they are installed in the battery. Furthermore, by means of a recharging device 32 (see 1 ) the reloading process 20 be performed.

In a further variant, according to 4 the algorithm can be used in a battery. At rest, the computing device 16 which is part of a suitably designed battery control unit 34 can be one voltage value or several (for example 2, 3, 4 or 5) voltage values recorded per day. From this, in the manner described above, the expected self-discharge, in particular together with the confidence interval, is calculated for the future. This can be done on the battery control unit 34 , a suitably equipped cell monitoring unit or another control device in the vehicle done. However, the data can also be transmitted, for example via a mobile radio connection, to a computer arranged outside the vehicle 36 be transmitted.

The data about the expected self-discharge of the battery cells 10 can also be provided to the driver. This can be done, for example, in the form that the display device 38 , such as a display of the vehicle, indicates to the vehicle user in how many days the state of charge falls below a critical, adjustable value of, for example, 10% or 20%. The user can define this critical value in advance. The display can be done both as a numeric number, as well as by a bar graph or an LED display.

Alternatively, the user can on a smartphone 40 or the like in an app of the value calculated as described above. This can be designed interactively. For example, the user defines a critical state of charge or range that he or she requires for a ride before having a charging station available. The system or the app on the smartphone 40 or tablet or such a mobile terminal then indicates until which day the user can leave the vehicle, without the state of charge falls below the defined value. Alternatively, via an output device 42 A service team will be contacted, which recharges the battery in case of imminent underrun of the value.

In another, based on 5 illustrated variant, the method is used again in a battery in the vehicle. As described above, for example, on a control unit, which is the computing device 16 which calculates the expected self-discharge and confidence interval for the future. This shows the differences between the battery cells 10 be in the tension. Accordingly, early by means of a balancing device 44 started a balancing process. The time to be provided for the balancing and the next automatic wake-up time of the controller are also preferably determined by the described method. Thus, an unnecessary quiescent current consumption and an unnecessary loss of energy can be reduced. Balancing resistors are preferably designed according to the expected demand.

The calculated future voltage development can therefore be used to sort the batteries and battery cells 10 be used, in which case the battery cells 10 assigned to different groups and loaded depending on the affiliation to a group and / or installed in a battery. Furthermore, check intervals of the battery cells 10 depending on the assignment to a respective group.

The calculated expected self-discharge, in particular together with the confidence level, can also be used for the design of balancing resistors and / or for the display in the vehicle display for the driver. Furthermore, the expected time until the state of charge drops by self-discharge below a given value can be displayed in the vehicle display for the driver. Additionally or alternatively, the expected self-discharge, in particular together with the confidence zone, can be used to display in the app or to communicate with the service team or maintenance team. Also, the expected time until the state of charge drops by self-discharge or by the balancing below a given value can be displayed in the app or in the vehicle display.

In addition, an alternating power supply of the cell monitoring or the battery control device 34 from the battery cell 10 or the battery cells 10 be made with the least expected charge loss. And it is an early prediction of a deep discharge caused by failure of the battery using the expected self-discharge possible. In addition, the method allows dynamic adjustment of the wake-up interval, such as the battery control device 34 for voltage checking of the battery cells 10 according to the predicted voltage behavior.

LIST OF REFERENCE NUMBERS

10

battery cells

12

measuring device

14

Storage

16

computing device

18

calculation

20

reloading

22

readings

24

circle

26

Curve

28

confidence band

30

sorter

32

reloading

34

Battery control unit

36

computer

38

display

40

Smartphone

42

output device

44

balancer

46

Period of time

48

Period of time

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102010003421 A1 [0002]

Claims (7)

Method for determining a self-discharge of a battery having at least one battery cell (10), in which a voltage of the at least one battery cell (10) by means of a measuring device (12) is detected, and in which the voltage is taken into account in determining the self-discharge of the battery, characterized characterized in that a plurality of measured values (22) of the voltage are detected in chronological succession by the measuring device (12), the measured values (22) being fed to a computing device (16) which, based on the measured values (22), is at least partially non-linear Determines the time course of the voltage of the at least one battery cell (10), based on the non-linear time course of the voltage is predicted for at least one time in the future expected self-discharge of the battery as a function of time. Method according to Claim 1 characterized in that for at least one future time within a first time period (46) of a predetermined duration, in particular a duration of up to about 30 days, the expected self-discharge is determined based on a logarithmic dependence of the voltage on time. Method according to Claim 1 or 2 characterized in that for at least one future time within a second time period (48) of a predetermined duration, in particular a duration of about 30 days to about 160 days, the expected self-discharge is determined based on an exponential dependence of the voltage on time. Method according to one of Claims 1 to 3 , characterized in that by means of the computing means (16) predicted self-discharge of the battery is displayed on a display device (38). Method according to one of Claims 1 to 4 , characterized in that based on the by means of the computing means (16) predicted self-discharge of the battery performed a maintenance of the battery and / or a warning is issued before a deep discharge of the battery. Method according to one of Claims 1 to 5 , characterized in that based on the by means of the computing means (16) predicted self-discharge battery cells (10) assigned to each other groups and loaded depending on the membership of a group and / or installed in a battery. Method according to one of Claims 1 to 6 , characterized in that based on the by means of the computing means (16) predicted self-discharge, a charge equalization of electrically interconnected battery cells (10) of the battery is performed.

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