DE102021205122A1 - Procedure for balancing the state of charge of the battery cells of a lithium-ion battery - Google Patents

Abstract

The invention relates to a method for balancing the state of charge of the battery cells (2) of a lithium-ion battery (4) of an electrically powered motor vehicle (6), with a voltage drop (ΔU _Z,i ) as a result of a load jump for each of the battery cells (2). (ΔP) is detected, with a mean value (ΔU _Z ) being determined from the detected voltage drops (ΔU _Z,i ) of the battery cells (2), with a difference (D _i ) from the respective detected voltage drop ( ΔU _Z,i ) and the mean value (ΔU _Z ) of the voltage drops (ΔU _Z,i ) is determined, and at least one of the battery cells (2) is charged or discharged depending on the respective difference (D _i ). The invention also relates to an electrically driven motor vehicle (6) with a device (8) for carrying out this method.

Description

The invention relates to a method for balancing the state of charge of the battery cells of a lithium-ion battery, in particular an electrically driven motor vehicle, and an electrically driven motor vehicle with a device for carrying out this method.

An electrically driven motor vehicle has a traction battery (high-voltage battery, HV battery) which supplies an electric motor for driving the motor vehicle with energy. An electrically powered motor vehicle is, in particular, an electric vehicle that only stores the energy required for the drive in the traction battery (BEV, battery electric vehicle), an electric vehicle with a range extender (REEV, range extended electric vehicle), a hybrid vehicle (HEV, hybrid electric vehicle), a plug-in hybrid vehicle (PHEV, plug-in hybrid electric vehicle) and/or a fuel cell vehicle (FCEV, fuel cell electric vehicle), which temporarily stores the electrical energy generated by a fuel cell in the traction battery.

Such a traction battery is typically designed as a lithium-ion battery, which in turn includes a number of battery cells. These battery cells can have different self-discharge rates, internal resistances and/or capacities, for example due to signs of aging or tolerances in the production of the battery cells. As a result, the battery cells can have different states of charge (SOC) from one another.

In particular, in the case of unbalanced states of charge, in other words in the event of an imbalance in the state of charge of the battery cells, on the one hand an end of charging of the lithium-ion battery is determined based on the battery cell with the highest voltage (and therefore the highest state of charge) and on the other hand a ready-to-drive limit the battery cell with the lowest voltage (and therefore with the lowest state of charge).

This disadvantageously results in a reduction in the effective capacity of the lithium-ion battery and, associated with this, in a comparatively short range of the motor vehicle.

Methods and devices for balancing the state of charge of the battery cells of a lithium-ion battery are known from the prior art. For example, to detect an imbalance, the state of charge of the battery cells according to the KR 2013 0126085 A and according to the U.S. 2016/0233696 A1 an open-circuit voltage (OCV) of each battery cell is recorded and, based on this, the respective state of charge of the battery cells is determined.

For certain types of lithium-ion battery cells, however, a so-called OCV characteristic, ie a state of charge (open circuit) voltage curve for a comparatively large average state of charge range, is comparatively flat. Such a characteristic is exemplary for a lithium iron phosphate battery cell (LFP battery cell) in the 1 shown. The precise progression of this characteristic depends, for example, on the structure of the battery cell, the materials used for the battery cell components, the electrolyte, the temperature of the battery cell, or the age of the battery cell. At best, however, is in the 1 for example in a range between 20% SOC and 80% SOC (state of charge) - a slope of the characteristic curve in a medium state of charge range is comparatively small. a determination of the state of charge of the battery cell based on its (measured) no-load voltage is therefore not possible for states of charge that correspond to this range, or only with a comparatively large error.

The invention is based on the object of specifying a suitable method for balancing the state of charge of the battery cells of a lithium-ion battery in an electrically powered motor vehicle and/or enabling battery cells designed as lithium-iron-phosphate battery cells to be balanced at an average state of charge . Furthermore, an electrically driven motor vehicle is to be specified which has a device for carrying out this method.

With regard to the method, the object is achieved according to the invention by the features of claim 1. With regard to the electrically driven motor vehicle, the object is achieved by the features of claim 9 according to the invention. Advantageous refinements and developments are the subject of the dependent claims. The statements made in connection with the method also apply analogously to the electrically driven motor vehicle and vice versa.

The method is intended for balancing the state of charge of the battery cells of a lithium-ion battery. In other words, the method is used to match the charge states of the battery cells of the lithium-ion battery, referred to below as battery for short, to one another.

The battery is, for example, the battery of a laptop or a mobile phone. In particular, however, the method is provided for a battery of an electrically driven motor vehicle, preferably for its traction battery, which provides the electrical energy required to drive the motor vehicle.

According to the method, a voltage drop in the cell voltage provided by the respective battery cell is first detected for each of the battery cells, which voltage drop occurs as a result of a load jump of an electrical load connected to the battery. In particular, a sudden load change is to be understood as meaning the sudden increase in the power provided by the lithium-ion battery. Such a load jump results from a corresponding change in the load requirement on the battery, for example in the event of comparatively large acceleration, the starting of a HV air conditioning system, or the discharging of the battery if it is connected to a charging station of a power grid that is intended for bidirectional charging is, so the charging and discharging of the battery is possible.

According to an expedient embodiment, the method is only continued if the sudden change in load is sufficiently large. For this purpose, for example, a power provided by the lithium-ion battery, a current provided by it, a change in the power provided by the battery based on a predefined period of time, and/or a change in the current provided by the battery based on a predefined period of time compared with a respective predefined threshold value and the method is only continued if the threshold value or the respective threshold values are exceeded. As an alternative or in addition to this, a threshold value for the voltage drops, in particular for its amount, is specified and the method, ie the following steps, is only continued if this threshold value is exceeded. For example, the threshold for the voltage drops is 100 mV. In this way, erroneous or comparatively imprecise results due to a voltage drop that is too low or a load jump that is too small are avoided.

A voltage drop is the difference between the voltage provided by the battery cell immediately before the load step and the minimum of the voltage reduced due to the load step, or alternatively the difference between the no-load voltage and the minimum of the voltage reduced due to the load step.

According to the method, in a next step, a mean value, expediently the arithmetic mean value or the median, is determined from the detected voltage drops. Then, in a further method step, a difference between the respective recorded voltage drop and the mean value of the voltage drops is determined for each battery cell. For example, the differences are additionally normalized. The sign and the amount of the respective difference is a measure of a deviation of the state of charge from a mean value of the state of charge of the battery cells.

At least one of the battery cells is then charged or discharged depending on the respective difference in order to balance the charge states of the battery cells. For example, using the difference, expediently using a predefined function, using a constant factor or a factor dependent on the difference, or using a conversion table, a discharge quantity or a discharge duration or a charging quantity or charging duration is determined and the respective battery cell is charged or discharged accordingly. In particular, the respective difference or the differences are used as an input variable for a regulation and, for example, a value of zero (0) is used as the desired value for the difference. In summary, the balancing process takes place depending on the difference assigned to the respective battery cell.

For example, those battery cells are discharged that have a comparatively high state of charge according to the difference. In this way, those battery cells are discharged that show a comparatively small voltage drop, i.e. whose difference is positive.

Alternatively, control or balancing methods and devices known from the prior art are used for this purpose. For this purpose, for example and if necessary, the differences are converted into an input variable using a specified function, using a specified factor, or using a specified conversion table, which uses the respective method known from the prior art. In particular, the respective difference is converted into a state of charge.

In summary, an imbalance is first determined, i.e. the determination of an inequality, of the state of charge of the battery cells based on the recorded voltage drops in the (cell) voltage provided by the battery cells, which occur as a result of a load jump. So this determination of the imbalance is not based on an open circuit voltage (OCV), but under load, ie while power is being provided by the lithium-ion battery, in particular while the motor vehicle is being driven.

In principle, the method is possible for battery cells connected in series and/or parallel to one another. The battery cells are expediently connected in series with one another. If a (partial) number of battery cells are connected in parallel, an additional switching effort, for example the disconnection of battery cells, is necessary to detect the voltage drops. However, in the case of a sudden change in load and/or a comparatively high load, this switching off can be undesirable in terms of time directly after the sudden change in load.

In particular, the detection of the voltage drops takes place at different times in order to balance the states of charge by charging and/or discharging the corresponding battery cell or battery cells. In this way, discharging or charging of the respective battery cell is avoided in the event of a comparatively high load following the sudden change in load.

In particular, the method takes place automatically. In particular, the mean value is determined, the differences are determined and the control signal is generated using a control unit.

For example, the method is carried out every time a corresponding voltage drop is detected as a result of a sudden change in load, a predefined threshold value for the sudden change in load having to be exceeded, for example, in order to carry out the method. Alternatively, a minimum period of time is specified, after which the method is carried out again as soon as a voltage drop as a result of a sudden change in load is detected for each of the battery cells.

In principle, the method is suitable for a lithium-ion battery whose battery cells have any cell chemistry. In particular, however, the method is also suitable for a lithium-ion battery whose battery cells each have a comparatively flat OCV characteristic. An example of such battery cells are lithium iron phosphate battery cells. In comparison to methods in which the state of charge of the respective battery cell is determined directly based on the open circuit voltage (or based on the OCV characteristic curve), a voltage drop as a result of a load step is used here to determine whether a battery cell is lithium-ion compared to the other battery cells -Battery cell has a higher or lower state of charge. In particular, this determination is independent of a course of the OCV characteristic curve of the battery cells, so that balancing of the charge states of the battery cells is made possible even with comparatively flat characteristic curves.

In areas of the OCV characteristic for comparatively small states of charge or for comparatively large states of charge, i.e. for areas with a comparatively large slope of the OCV characteristic, in other words in areas in which a change in the state of charge of the battery cell is accompanied by a comparatively large change in the open-circuit voltage goes hand in hand, a method known from the prior art for determining the state of charge using the OCV characteristic curve is also made possible. For example, these methods can be combined with the method according to the invention presented above. For example, if a detected voltage of the battery cell, in particular its open-circuit voltage, is between a predetermined first and a predetermined second threshold value, the method according to the invention is used, otherwise a method known from the prior art is preferably used. In this case, the first and the second threshold value expediently represent the limits of that region of the OCV characteristic curve which has a comparatively flat slope. For example, the slope of the OCV characteristic curve in this area is less than 2 mV / (1%), i.e. less than 2 millivolts per 1 percent state of charge, in particular less than 1 mV / (1%) or less than 0.5 mV / ( 1%) or less than 0.2mV/(1%).

For example, the method according to the invention can be combined with other balancing methods known from the prior art. For this purpose, the voltage provided by one or all of the battery cells can be compared with a third threshold value and/or with a fourth threshold value before the load jump, the fourth threshold value being greater than the third threshold value. If the cell voltage of the respective battery cell (or alternatively its no-load voltage) that is provided before the load jump falls below the third threshold value or exceeds the fourth threshold value, the method according to the invention is carried out. Otherwise, the method known from the prior art is used. This is also known as hysteresis. As an alternative to this, the method according to the invention is only used if the cell voltage of the respective battery cell (or alternatively its open-circuit voltage) provided before the load jump falls below the third threshold value or exceeds the fourth threshold value. Otherwise there is no balancing. In a manner analogous to the first and the second threshold value, the third and the fourth threshold value expediently represent the limits of that area of the OCV characteristic curve that has a comparatively flat slope.

According to an expedient refinement of the method, the charging or discharging of the at least one battery cell is carried out while the electrically powered motor vehicle is being driven. The charging states are thus also balanced during operation of the motor vehicle, ie while the motor vehicle is being operated by a user. Advantageously, the balancing does not only take place while the motor vehicle, and thus the battery, is connected to a charging station or this is in a relaxed state/idle state.

According to a suitable embodiment of the method, the voltage drops, in particular their amounts, are changed, ie increased or reduced, in particular weighted or scaled, before the determination of the mean value depending on the current strength of a (battery) current provided by the lithium-ion battery. As an alternative to this, the differences are changed as a function of this current intensity.

In particular, the voltage drop is influenced by a (battery) current flowing through them, or its current intensity, or is dependent on this. For example, with a comparatively high battery current, the voltage drop in the respective battery cell is comparatively large. To correct or compensate for this influence, the voltage drops, in particular their amounts, or the differences are multiplied, for example, by a constant factor or by a factor that is dependent on the current intensity. Alternatively, the voltage drops are (re)calculated or corrected or normalized using a specified function dependent on the current intensity, or changed according to a specified table.

In summary, the voltage drops or the differences are adjusted directly to the current strength of the battery current. Thus, an excessively strong or too weak balancing of the states of charge, that is to say excessive charging or excessive discharging of at least one of the battery cells, is advantageously avoided.

Furthermore, the amount of the voltage drop depends on the temperature of the respective battery cell. According to a suitable embodiment of the method, the voltage drops, in particular their magnitudes, are changed, in particular weighted or scaled, before the mean value is determined—or alternatively the differences to this—as a function of a temperature spread of the battery. The temperature spread here means the spatial distribution of the temperature in the battery. In particular, the difference or the voltage drop of the respective battery cell is changed or adjusted differently for predetermined temperature ranges. These temperature ranges can in turn depend on other variables, for example on an average battery temperature. Alternatively or additionally, the respective battery cell is charged or discharged only when its temperature is within a predetermined temperature range.

In order to reduce a disruptive influence of the temperature spread on the voltage drops, for example, the amount of the voltage drop or the amount of the difference is reduced for a battery cell with a comparatively high temperature, and/or the amount of the voltage drop or the amount of the difference is increased for a battery cell with a comparatively low temperature .

For example, the voltage drops or the differences are multiplied by a factor that is expediently dependent on the temperature of the respective battery cell, calculated using a predefined function that is dependent on the temperature, or changed according to a predefined table.

In this way, temperature differences between battery cells are taken into account and comparatively precise balancing is achieved.

In addition or as an alternative to this, according to a suitable embodiment of the method, the voltage drops, in particular their magnitudes, are changed in an analogous manner before the mean value is determined, or alternatively the differences are changed as a function of a temperature difference between the battery temperature, for example an average battery temperature, and its ambient temperature , especially weighted.

According to a suitable embodiment of the method, the voltage drops, in particular their amounts, are changed, ie increased or reduced, in particular weighted or scaled, before the determination of the mean value as a function of a period of time since a preceding balancing process. As an alternative to this, the differences in the mean value are changed as a function of a period of time since a preceding balancing process.

The amounts of the voltage drops or the amounts of the differences are preferably reduced for comparatively short periods of time. at For example, the amounts of the voltage drops or the amounts of the differences are scaled in such a way that they are reduced for shorter periods of time. In other words, the at least one battery cell that is charged or discharged for balancing purposes is charged or discharged to a lesser extent for shorter periods of time.

In this way, an undesirable tendency to oscillate when controlling, and thus when balancing, the state of charge of the battery cells is advantageously avoided.

In summary, an influence of variables that influence the voltage drop and/or the difference is weighted by means of the change(s) described above, and thus an adaptation, of the voltage drops, in particular their amounts, or alternatively by means of the change in the differences, which is advantageously weakened or even corrected. A comparatively precise determination of the differences in the states of charge of the battery cells is thus advantageously achieved.

A further aspect of the invention relates to an electrically driven motor vehicle which has a lithium-ion battery, expediently designed as a traction battery, with battery cells expediently connected in series. In particular, the battery cells are designed as lithium iron phosphate battery cells.

The motor vehicle also includes a device for balancing the states of charge of the battery cells, ie for adjusting the states of charge of the battery cells to one another. In particular, this device is provided and set up to carry out the method in one of the variants presented above.

The device for balancing the state of charge of the battery cells, also referred to as a balancing device for short, has a voltage sensor for each of the battery cells for this purpose. The voltage sensors are used to record the (cell) voltage provided by the respective battery cell or a time profile of the cell voltage. In particular, the voltage sensors are used to detect a voltage drop in the event of a sudden change in load. The balancing device also includes a control unit to which the voltage sensors are coupled in terms of signal transmission.

The control unit is designed in such a way that, in accordance with the method described above, it determines an average value from voltage drops, which are detected using the voltage sensors in the event of a sudden change in load. Furthermore, the control unit is designed in such a way that, for each of the battery cells, it determines a difference between the mean value and the associated voltage drop across the respective battery cell, and controls a charging or discharging device in such a way that at least one of the battery cells depends on the respective, i.e Battery cell associated difference is charged or discharged. The charging or discharging process is also possible while the motor vehicle is in motion.

Such a charging or discharging device is already known from the prior art. For example, the charging or discharging device includes a switching term for each of the battery cells which is connected in series with a discharging resistor between the two electrical poles of the respective battery cell, the switch being controllable by the control unit. Closing the switch, ie when the switch is electrically conductive, causes the corresponding battery cell to be discharged. The discharging process takes place depending on the difference assigned to the respective battery cell. For example, based on the difference, expediently based on a predefined function or based on a conversion table, a discharge amount or a discharge duration is determined and the switch is switched accordingly.

Alternatively, the charging or discharging device includes another vehicle-internal battery, for example a 12 V battery or a recuperation unit, wherein the additional battery or the recuperation unit can be connected to each of the battery cells so that they can be powered by the additional battery or by the Recuperation unit can be loaded. For example, only those battery cells are charged whose difference is within a predetermined value range, or alternatively these battery cells are charged for a comparatively long time.

The balancing device expediently also includes a current sensor coupled to the control unit, by means of which a (battery) current provided by the lithium-ion battery can be detected. For example, the balancing device also includes a further voltage sensor for detecting the (battery) voltage provided by the lithium-ion battery. A time profile of the power provided by the lithium-ion battery and/or a time profile of the current provided by the lithium-ion battery can be detected using the current sensor and using the additional voltage sensor.

As already described in connection with methods, the charging states of the battery cells are equalized by charging or discharging at least one of the battery cells, in particular more by means of a corresponding control of the charging or discharging device, only if the sudden change in load, which causes the voltage drop, is sufficiently large.

As an alternative to the motor vehicle shown above, the control unit is not part of it. The control unit is then arranged externally to the vehicle and is expediently coupled via a wireless radio link to the voltage sensors or to another control unit internal to the vehicle, which in turn is coupled to the voltage sensors in terms of signal transmission.

As an alternative or in addition to this, the charging or discharging device can comprise a charging station (external to the vehicle) for charging the lithium-ion battery of the motor vehicle, which is in particular designed as a traction battery.

• 1 eine OCV-Kennlinie einer Lithium-Eisenphosphat-Batteriezelle, wobei die diese in einem mittleren Ladezustandsbereich eine vergleichsweise geringe Steigung aufweist,
• 2 einen Spannungsverlauf der von einer Batteriezelle bereitgestellten Zellspannung, mit einem Spannungsabfall aufgrund eines Lastsprungs,
• 3 in einem Flussdiagramm einen Verfahrensablauf zum Balancieren des Ladezustands der Batteriezellen einer Lithium-Ionen-Batterie, und
• 4 ein elektrisch angetriebenes Kraftfahrzeug mit einer als Lithium-Ionen-Batterie ausgebildeten Traktionsbatterie sowie mit einer Einrichtung zum Balancieren des Ladezustands der Batteriezellen der Traktionsbatterie.

Exemplary embodiments of the invention are explained in more detail below with reference to a drawing. Show in it:

• 1 an OCV characteristic curve of a lithium iron phosphate battery cell, which has a comparatively low gradient in a medium state of charge range,
• 2 a voltage curve of the cell voltage provided by a battery cell, with a voltage drop due to a load jump,
• 3 in a flowchart, a method sequence for balancing the state of charge of the battery cells of a lithium-ion battery, and
• 4 an electrically powered motor vehicle with a traction battery designed as a lithium-ion battery and with a device for balancing the state of charge of the battery cells of the traction battery.

Corresponding parts and sizes are always provided with the same reference symbols in all figures.

In the 2 a time profile of the (cell) voltage U _Z,i (t) provided by one of the battery cells 2 of a lithium-ion battery 4 is shown. The lithium-ion battery 4 is also referred to as battery 4 below.

At time t ₁ the load demand on the battery increases suddenly. In other words, at this point in time there is a sudden change in load ΔP and the power provided by the lithium-ion battery 4 increases. Such a sudden change in load ΔP results here, for example, from a comparatively large acceleration if the lithium-ion battery 4 is a traction battery of an electrically driven motor vehicle 6 .

Immediately before, i.e. directly before time t ₁ , the battery cell 2 provides an output voltage U ₀ which is, for example, the open circuit voltage (OCV) of the battery cell 2 or the voltage of the battery cell 2 provided under a comparatively low load. As a result of the sudden change in load, the voltage provided by the battery cell 2 drops from time t ₁ . The absolute value of the minimum of the cell voltage U _Z,i , which is reached in the course of the sudden change in load, is denoted by U _Z,i,min . Correspondingly, ΔU _Z,i designates the difference between the output voltage U ₀ and the amount of the minimum of the cell voltage U _Z,i . $$\text{So valid}:\mathrm{\Delta }{\text{u}}_{\text{Z},\text{i}}={\text{<figure-callout id="0" label="u" filenames="DE102021205122A1\_0003.png" state="{{state}}">u</figure-callout>}}_0-{\text{u}}_{\text{Z},\text{i},\text{at least}}$$

At time t ₂ the load requirement on the battery 4 falls and the voltage U _Z,i provided by the battery cell 2 rises.

In the 3 a flowchart is shown that represents a process flow for balancing the state of charge SOC of the battery cells of a lithium-ion battery. In particular, the battery cells 2 are designed as lithium-iron phosphate battery cells whose OCV characteristic, for example, according to the 1 , has a region with a comparatively low gradient, in particular a gradient of less than 0.5 mv/(1%). Balancing or balancing is understood here as matching the states of charge SOC of the battery cells 2 of the battery 4 to one another.

In a first step I., a voltage drop ΔU _Z,i as a result of a sudden change in load ΔP is recorded for each of the battery cells 2 . As an alternative to the difference between the output value U ₀ and the amount of the minimum U _Z,i,min shown above, the difference between the no-load voltage and the amount of the minimum U _Z,i,min is used as a voltage drop ΔU _Z,i for the respective battery cell 2 used.

In an optional second step II 3 shown in dashed lines, a threshold value S for the amounts of the voltage drops Δ _UZ,i is specified and compared with these. If at least one battery cell 2 - or in an alternative embodiment of the second step the voltage drops ΔU _Z,i at all battery cells 2 - exceed the threshold value S, the method is continued. For example, the threshold S for the voltage drops is 100 mV. This ensures that the load jump ΔP is sufficiently large and that balancing can take place with sufficient accuracy. For example, can or, additionally or alternatively, a threshold value for a power provided by the lithium-ion battery 4, a threshold value for a current intensity I provided by this, a threshold value for a change in the power provided by the battery related to a predefined period of time, and /or a threshold value for a change in the current intensity I provided by the battery that is related to a predefined period of time can be specified, with the method only being carried out further if this threshold value or these threshold values are exceeded.

If the optional second step II. is not carried out, the first step I. is followed directly by the third step III.

In the third step III. a mean value ΔU _z , in particular the arithmetic mean value or the median, is determined from the detected voltage drops ΔU _z,i . In a subsequent fourth step IV., an associated difference D _i is determined for each of the battery cells 2 from the respective detected voltage drop ΔU _Z,i and the mean value ΔU _z of the voltage drops ΔU _Z,i . $$\text{So valid}:{\text{D}}_{\text{i}}=\mathrm{\Delta }{\text{u}}_{\text{Z},\text{i}}-{\underset{\_}{\mathrm{\Delta }\text{u}}}_{\text{Z}}$$

The sign and the amount of the respective difference D _i is a measure of a deviation of the state of charge of the associated battery cell 2 from a mean value of the states of charge of the battery cells 2.

Optionally (in terms of time) before the determination of the mean value ΔU _z in step III. the voltage drops ΔU _Z,i, in particular their amounts, as a function of the current I of a (battery) current provided by the lithium-ion battery, as a function of a temperature spread T _x of the lithium-ion battery 4, as a function of a temperature difference ΔT between the battery temperature and the ambient temperature, and / or depending on a period of time t _v changed since a previous balancing process. This is in the 3 represented by dashed arrows. In this case, the mean value ΔU _Z is determined in step III. determined based on these changed voltage drops.

The influence of the variables I, T _x , ΔT and t _v on the voltage drops ΔU _Z,i is already known and can be represented, for example, as a factor dependent on this variable, a corresponding adaptation function or as a conversion table. If these relationships are not previously known, they can either be recorded experimentally or estimated by calculations/simulations.

To change the voltage drops ΔU _Z,i , these are multiplied for each of these influencing variables I, T _x , ΔT and t _v by the factor that depends on the respective variable, used as an independent variable for the adaptation function, or according to the specified one table changed.

In this way, an influence of these variables I, T _x , ΔT and t _v on the voltage drops ΔU _Z,i is compensated for or corrected. In summary, the voltage drops ΔU _Z,i are adapted to variables that influence (disrupt) them.

According to an alternative that is not shown in more detail, instead of changing the voltage drops ΔU _Z,i in an analogous manner to this, a change in the differences D _i from step IV. made as a function of a temperature spread T _x of the lithium-ion battery 4, as a function of a temperature difference ΔT between the battery temperature and the ambient temperature, and/or as a function of a time period t _v since a previous balancing process.

In a fifth step V., at least one of the battery cells 2 is charged or discharged depending on the respective difference D _i in order to balance the states of charge SOC of the battery cells 2 . For this purpose, a discharge quantity, a discharge duration, a charge quantity or charging duration is determined from the respective difference D _i , expediently using a predefined function, using a constant factor or a factor dependent on the difference D _i , or using a conversion table, and the respective battery cell 2 accordingly loaded or unloaded. If the battery 4 is the traction battery of an electrically powered motor vehicle 6 , the respective battery cell 2 is preferably charged or discharged while the electrically powered motor vehicle 6 is in operation.

In 4 an electrically driven motor vehicle 6 is shown. This has a traction battery 4 which is designed as a lithium-ion battery 4 . The traction battery 4 comprises a number of battery cells 2 connected to one another in series, which are configured here as lithium iron phosphate battery cells 2 by way of example.

Furthermore, the electrically powered motor vehicle 6 includes a device 8 for balancing the state of charge of the battery cells 2. This device 8 is provided and set up for this purpose 3 carry out the procedures shown.

The device 8 in turn has a voltage sensor 10 for each of the battery cells 2 and a control unit 12 to which the voltage sensors 10 are coupled in terms of signal transmission. The voltage sensors 10 serve to detect the (cell) voltage provided by the respective battery cell or a time profile of the cell voltage. It is thus possible to detect the associated voltage drops ΔU _Z,i at the respective battery cell 2 in the event of a sudden change in load ΔP of an electrical load (not shown in any more detail) connected to the battery 4 .

The control unit 12 is designed such that it corresponds to the method described above according to the 3 to determine the mean value ΔU _Z from the detected voltage drops ΔU _Z,i and for each of the battery cells 2 the respective difference D _i .

The control unit 12 is also coupled to a charging or discharging device 14 in terms of signal transmission. The control device 12 activates the charging or discharging device 14 in such a way that at least one of the battery cells 2 is charged or discharged depending on the respective difference D _i according to step V of the method.

For example, the charging or discharging device 14 includes a switch 16 for each of the battery cells, which is connected in series with a discharging resistor 18 between the two electrical poles of the respective battery cell 2 . In this case, the switches 16 can be controlled by the control unit 12 . When the respective switch is closed, ie when the switch 16 is electrically conductive, the corresponding battery cell 2 is discharged via the discharge resistor 18 in accordance with step V of the method—and thus as a function of the respective difference D _i . So based on the difference, expediently based on a predefined function or based on a conversion table, a discharge quantity or a discharge duration is determined and the switch 16 is switched accordingly.

The device 8 optionally includes, in particular for carrying out step II, a current sensor 20 coupled to the control unit, by means of which the (battery) current I provided by the lithium-ion battery 4 can be detected.

According to the embodiment of 4 the charging or discharging device 14 is part of the device 8 for balancing the state of charge of the battery cells 2.

The invention is not limited to the exemplary embodiments described above. On the contrary, other variants of the invention can also be derived from this by a person skilled in the art without departing from the subject matter of the invention. In particular, all of the individual features described in connection with the exemplary embodiments can also be combined with one another in other ways without departing from the subject matter of the invention.

reference list

2

battery cell

4

Lithium Ion Battery

6

electrically powered motor vehicle

8th

Device for balancing the state of charge of the battery cells

10

voltage sensor

12

control unit

14

loading or unloading device

16

Switch

18

discharge resistance

20

current sensor

ΔP

load jump

ΔT

temperature difference

ΔUZ,i

Voltage drop in one of the battery cells of the lithium-ion battery

ΔUZ

Mean value of the recorded voltage drops

Tue

Difference between the voltage drop of the respective battery cell and the mean value

I

amperage

S

threshold

SOC

state of charge

t1

time

t2

time

television

duration

tx

temperature spread

I

Detection of voltage drops

II.

Comparison with threshold

III.

Determination of the mean value

IV

determination of the differences

V

Charging and / or discharging at least one battery cell

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was 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.

Patent Literature Cited

• KR 20130126085 A [0006]
• US 2016/0233696 A1 [0006]

Claims (9)

Method for balancing the state of charge (SOC) of the battery cells (2) of a lithium-ion battery (4), in particular an electrically powered motor vehicle (6), - wherein for each of the battery cells (2) a voltage drop (ΔU _Z,i ) as a result a load step (ΔP) is detected, - wherein a mean value (ΔU _Z ) from the detected voltage drops (ΔU _Z,i ) of the battery cells (2) is determined, - wherein for each battery cell (2) a difference (D _i ) from the respective detected voltage drop (ΔU _Z,i ) and the mean value (ΔU _Z ) of the voltage drops (ΔU _Z,i ) is determined, and - wherein at least one of the battery cells (2) is charged or discharged depending on the respective difference (D _i ). . procedure after claim 1 , characterized in that the charging or discharging of the at least one battery cell (2) is carried out while the electrically powered motor vehicle (6) is in motion. procedure after claim 1 or 2 , characterized in that the voltage drops (ΔU _Z,i ) before the determination of the mean value (ΔU _Z ) or the differences (D _i ) depending on the current (I) of a lithium-ion battery (4) provided current changed will. Procedure according to one of Claims 1 until 3 , characterized in that the voltage drops (ΔU _Z,i ) before determining the mean value (ΔU _Z ) or the differences (D _i ) depending on a temperature spread (T _x ) of the lithium-ion battery (4) are changed. Procedure according to one of Claims 1 until 4 , characterized in that the voltage drops (ΔU _Z,i ) before determining the mean value (ΔU _Z ) or the differences (D _i ) depending on a temperature difference (ΔT) between the battery temperature and the ambient temperature are changed. Procedure according to one of Claims 1 until 5 , characterized in that the voltage drops (.DELTA.U _Z,i ) before the determination of the mean value (.DELTA.U _Z ) or the differences (D _i ) depending on a period of time (t _v ) since a previous balancing process are changed. Procedure according to one of Claims 1 until 6 , characterized in that a threshold value (S) for the voltage drops (ΔU _Z,i ) is specified, the mean value (ΔU _Z ) being formed only if at least one of the voltage drops (ΔU _Z,i ) exceeds this threshold value (S) exceeds. Procedure according to one of Claims 1 until 7 , characterized in that lithium iron phosphate battery cells are used as battery cells (2). Electrically driven motor vehicle (6), having - a lithium-ion battery (4), in particular designed as a traction battery, with battery cells (2), - a device (8) for balancing the charge states of the battery cells (2), in particular according to the method after Claims 1 until 8th , wherein the device (8) has a voltage sensor (10) for each of the battery cells (2) as well as a control unit (12) coupled to the voltage sensors (10), - wherein the control unit (12) is designed such that - this one Mean value (ΔU _Z ) determined from voltage drops (ΔU _Z,i ), which are detected using the voltage sensors (10) in the event of a sudden change in load (ΔP), - that these have a difference (D _i ) between the mean value for each of the battery cells (2). (ΔU _Z ) and the associated voltage drop (ΔU _Z,i ) at the respective battery cell (2) is determined, and - that this controls a charging or discharging device (14) in such a way that at least one of the battery cells (2) depending on the respective Difference (D _i ) is charged or discharged.

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