DE102014203606A1 - Method for monitoring the thermal load of balancing resistors - Google Patents

Abstract

The invention relates to a method for monitoring the thermal load of balancing resistors in a battery device having a plurality of battery cells. According to the invention, it is provided that a thermal load TBV of balancing resistances is determined, caused by a multiplicity of varying temperature strokes (5) due to a frequency varying in frequency (6 ', 6 ", 6' '') and intensity Use of the balancing resistors for balancing operations, - that a determination of an equivalent number of temperature strokes (5) of a certain predetermined or predeterminable height takes place on the balancing resistors exert a thermal load TBA, which corresponds to the previously determined thermal load TBV and - that a comparison of the equivalent number with a predetermined or predeterminable threshold and the delivery of a signal when reaching or exceeding the threshold takes place. The invention further relates to a battery device with a plurality of battery cells and a balancing device having a control device which is adapted to monitor the thermal load of Balancing- resistors and a battery-powered vehicle with such a battery device.

Description

The invention relates to a method for monitoring the thermal load of balancing resistors in a battery device having a plurality of battery cells.

State of the art

There is a growing demand for electrical energy storage systems that find many uses in both stationary and mobile applications. As such examples, u.a. Emergency power systems or electric / hybrid vehicles are listed.

The electrical energy storage device is designed for such applications mostly as a battery with a plurality of battery cells, which are connected in series and or also in parallel with each other electrically to the battery. In this way, batteries can be provided with power and / or energy data adapted to the respective application.

Thereby u.a. due to production variations in the individual battery cells in terms of electrical properties (such as voltage, capacitance, internal impedance) differences observed. In a battery, the individual battery cells are therefore never identical in terms of their aging state and their electrical power.

Usually, in battery cells that are in the battery in a string, i. are connected in series, the discharging stopped when one of the battery cells has reached its discharge voltage limit. The same applies to the charging of the battery. Charging stops as soon as the charging voltage limit of one of the battery cells is reached. The weakest cell is therefore the first to be fully charged or discharged. Thus, the battery or the battery module is always only as good as its / its weakest cell.

In order to be able to provide a battery with similarly loadable cells and, moreover, to prevent the individual battery cells from aging differently due to different voltage states, methods for so-called cell balancing are known, which aim to balance the voltages of the individual battery cells to have.

Without such voltage and charge balancing, the state of charge of the different cells in operation drifts apart due to slightly different capacitances and slightly different self-discharges of the cells or modules.

With regard to the methods for cell balancing, a distinction is made between active and passive cell balancing, with an active cell balancing by means of a battery management system, a charge equalization taking place under the memory modules in such a way that the energy of the cells that are too highly charged, which is the highest cell voltage, is distributed to low-charged cells with low cell voltage. With passive cell balancing, those cells with the highest battery voltage are deliberately discharged, for example at fixed predetermined intervals, until all cells have the same voltage. The discharge usually takes place via balancing resistors, so that this method is also called "resistive balancing". The electrical energy to be discharged is converted into heat via balancing resistors.

The temperature fluctuations caused by the resulting heat are a thermal load for the entire battery, but especially for the balancing resistors themselves. These are exposed to a large number of temperature changes over the lifetime of the battery. The temperature changes vary with the height of the Temperarturhübe, which depends on whether one or more balancing resistors are used simultaneously.

The multiplicity and the magnitude of the temperature changes represent a not inconsiderable thermal load for the solder joints of the balancing resistors, so that they are subject to the risk of tearing or breakage due to material fatigue. Not only is this risk associated with the risk of a failure of a balancing resistor, but in particular in moving batteries, for example in vehicle batteries, a falling balancing resistance can lead to considerable consequential damage such as short-circuits and fires. It is therefore necessary to recognize such a risk in good time before the occurrence of a corresponding damage.

The documents US 2012/0094151 A1 and JP 2011-155752 A show different measures to Monitoring the thermal load of balancing resistors in a battery device.

Disclosure of the invention

The invention relates to a method for monitoring the thermal load of balancing resistors in a battery device having a plurality of battery cells. The invention further relates to a battery device with a plurality of battery cells and a balancing device having a control device which is adapted to monitor the thermal load of balancing resistors and a battery-powered vehicle with such a battery device. Advantageous embodiments of the invention are specified in the subclaims.

According to the invention, a method is provided for monitoring the thermal loading of balancing resistors in a battery device having a plurality of battery cells, which is characterized by the steps of determining a thermal load TBV of balancing resistors, caused by a multiplicity of varying temperature strokes due to a frequency and intensity varying use of the balancing resistors for balancing operations, a determination of an equivalent number of temperature strokes of a certain predetermined or predeterminable magnitude exerting on the balancing resistors a thermal load TBA corresponding to the previously determined thermal load TBV and a comparison of the equivalent number with a predetermined or predefinable threshold and output of a signal when reaching or exceeding the threshold. In other words, the thermal load TBV caused by a plurality of varying temperature strokes is represented by the number of temperature strokes of a given height and the thermal load caused by the single temperature stroke of this predetermined height.

The advantage of this method is that it provides a method for the measurement of a successful or already performed thermal load, so that the determined measured value comparable to a permissible limit or threshold value and the occurrence of a damage caused by thermal load case is averted. For example, in the case of monitoring a vehicle battery, the method is suitable for informing a driver that, for example, a permissible maintenance interval or a permissible aging limit of a control unit containing the balancing resistors has been reached and must be serviced or replaced.

It is an advantageous feature of this method that such signaling is not only due to, for example, the expiration of a fixed time interval or a fixed number of charging cycles, but in dependence on the actual thermal load.

The thermal load is caused by a variety in their height varying temperature changes. Much of this is due to temperature swings that arise due to the use of balancing resistors for balancing operations. Here, the amount of Temperarturhübe that are caused by the balancing, depends on how many balancing resistors, the balancing is performed simultaneously.

This number in turn can scatter significantly depending on the battery or battery module and depends strongly on an equal distribution of battery cell characteristics of the operated within a string battery cells.

If, for example, a single battery cell has a high self-discharge rate in a battery module having six battery cells, the remaining five battery cells must be discharged relatively frequently via balancing to the same discharge state. Thus, it is relatively common to expect the use of five balancing resistors, combined with a relatively high temperature swing.

On the other hand, if the voltage, capacity and self-discharge values are almost identical in all battery cells, then balancing is comparatively rare and probably only required for isolated battery cells. The resulting temperature strokes are therefore comparatively small and comparatively rare.

Since the battery characteristics vary due to aging processes, it is difficult to predict an expected, balancing-related thermal load. Accordingly advantageous is the statement of the method according to the invention, which specifies a method for monitoring the thermal load of the balancing resistors, which takes into account the actual thermal load due to the use of balancing resistors for balancing operations.

In this case, the thermal load TBV determined in a first step, which results on the basis of a multiplicity of varying temperature strokes, is converted in a second step into an equivalent number of temperature strokes of a specific predetermined or predefinable, thus non-varying, but fixed height. In other words, a number of temperature strokes of a fixed height are determined, which leads to a thermal load TBA of the balancing resistors, which corresponds to the thermal load TBV caused by the multiplicity of varying temperature strokes.

The advantage of this second step is that a wide variety of histories can be represented in terms of thermal loading by a single value, the equivalent number.

This value is in a third step with a predetermined or predefinable threshold compared, when reaching or exceeding a signal is delivered. In this case, the threshold value can be selected, for example, such that the signal is emitted at a thermal load TBA of 90% of a thermal load TBmax, at which cracking or breaking of the solder joint is to be expected. In the example of using the method for monitoring a vehicle battery, this may mean that the driver is informed, for example via an indicator in the dashboard, that a permitted balance load of the control unit is 90% consumed and that it must be replaced.

Advantageously, the determination of the thermal load TBV of the balancing resistors is carried out in such a way that a determination of a distribution function takes place on the height of the temperature strokes, which occur due to a use of the balancing resistors, so that the thermal load TBV can be described by the distribution function.

By means of such a distribution function, the frequency of temperature strokes of a respective height can be described; cumulatively, it describes the thermal load TBV which has arisen due to the balancing.

Advantageously, the determination of the distribution function on the height of the temperature strokes takes place in such a way that a determination of the number and frequency of use of the balancing resistors takes place and calculation of a distribution function on the height of the temperature strokes is based on a predetermined heating function which determines the dependence of the temperature strokes describes the number of simultaneously used balancing resistors.

Such an approach has the advantage that no temperature data must be collected for the determination of the distribution function over the temperature strokes. Rather, the temperature strokes are calculated on the basis of mostly available data on the successful balancing operations, including the number of balancing Vörgänge and the number of involved battery cells and the associated balancing resistors is required. This data may be available, for example, from a memory of the control device, also called field data acquisition device.

Via a heating function, an assignment of temperature strokes to a predetermined number of balancing resistances involved in the balancing can be specified, so that the distribution function can be formed via the temperature strokes on the basis of the data of the control device.

Advantageously, as an intermediate step during or after the determination of the number and frequency of use of the balancing resistors, a classification of the number of simultaneously used balancing resistors is advantageously carried out. Such class division advantageously takes place in such a way that classes for the simultaneous use of up to 5, from 6 to 10, from 11 to 20 and from over 20 balancing resistors are formed. Such a classification helps to reduce the complexity of the process.

Advantageously, the heating function is performed by the control device, mostly in two parts as Cell Supervision Circuit CSC and Battery Control Unit BCU, measured and / or stored in the battery device during use of the balancing resistors. In this case, a measurement of the heating function can be done once during a first commissioning. Alternatively, the heating function can also be preprogrammed or fed via interfaces usually present in the control device.

Further advantageously, the method is designed such that before the determination of an equivalent number of predetermined or predeterminable temperature strokes to the thermal load TBV, caused by the use of balancing resistors, a base load is added. This is advantageously done by adding a distribution function representing the base load before the conversion of the aforementioned distribution function to an equivalent number of predetermined or predefinable temperature strokes to the distribution function. Advantageously, this base load corresponds to the number of temperature strokes due to the change of the ambient temperature and / or of the control device switch-on periods. In this way, the entire load spectrum of temperature changes is taken into account when determining the thermal load TBV.

In an advantageous embodiment of the method, this is characterized in that the determination of an equivalent number of temperature strokes of a certain predetermined or predeterminable height, which exert on the balancing resistors a thermal load TBA, which corresponds to the previously determined thermal load TBV, via suitable aging and / or acceleration laws, in particular Coffin-Manson, takes place.

Such an embodiment of the method allows a good description and conversion of occurring due to the temperature changes material fatigue in the solder joints, which is subject to no linear but exponential dependence.

The invention furthermore relates to a battery device having a plurality of battery cells and a balancing device, which has a control device and balancing resistors for balancing the battery cells, and wherein monitoring by the control device of the thermal load of the balancing resistors according to a method of the preceding claims , As well as a battery-powered vehicle with such a battery device.

Due to the monitoring, such a battery device offers a high level of operational safety, as required in particular in the automotive industry.

The invention will be explained in more detail below with reference to the figures of exemplary embodiments. Show it:

1 Examples of distribution functions of the balancing number n over the number N of balancing resistors

2 an example of a heating function

3 Examples of distribution functions of the number n 'of temperature strokes over their height .DELTA.T

4 Examples of an equivalent number A determined by transformation of the distribution function TBV

1 shows three examples of distribution functions 11 . 21 . 31 the number of balancing n over the number N of balancing resistors. In other words, the three distribution functions describe 11 . 21 . 31 for three battery devices (battery device 1, battery device 2, battery device 3) how often a balancing with a number of N balancing resistors has been performed. The three distribution functions characterize this 11 . 21 . 31 three identical battery devices, hereinafter also referred to as batteries, but the battery cell characteristics of the battery cells connected in series within the battery differ production and or due to aging. This manifests itself in different charging / discharging efficiencies of the battery cells, which require a more or less frequent balancing with more or less involved battery cells. Decisive for the frequency of balancing with many battery cells is the maximum Selbstentladedifferenz between the battery cells of a battery.

This is how the distribution function belongs 11 to a battery whose battery cells have low self-discharge differences of 0.15 mV / day. The low Selbstentladedifferenz requires only minor corrections to the state of charge of the individual battery cells, the most common therefore only a single battery cell is involved. Balancing with many battery cells is rare or never necessary.

The distribution function 21 belongs to a battery whose battery cells have average self-discharge differences of 0.3mV / day. Due to the larger deviations of the states of charge among each other, two battery cells with their respective balancing resistors are most often involved in the balancing.

The distribution function 31 belongs to a battery whose battery cells have high self-discharge differences of 0.5 mV / day. This means that due to, for example, a single very bad battery cell all the others have to be discharged relatively frequently via their balancing resistors. The most common value 6''' results in the distribution function 31 for the simultaneous use of a high number 4 involved balancing resistors.

The use of a high number 4 of balancing resistors is done with the associated frequency values 6' and 6 '' for the distribution functions 11 and 21 correspondingly rarer. The data for creating the distribution functions can be taken from a field data acquisition device of a control device, which consists of a BCU and a CSC.

2 shows an example of a heating function in a graph, in which the number N of balancing resistors is plotted against a temperature rise. This heating function is for a number 4 of associated balancing resistors an associated increase in temperature 5 refer to. The characteristic is not linear, the temperature increase increases disproportionately with increasing number of simultaneously used balancing resistors.

3 shows three examples of a distribution function 12 . 22 . 32 that the frequency 6' . 6 '' . 6''' a temperature swing 5 describe. With increasing self-discharge difference 7 increases the frequency (or number n ') of high temperature strokes. The distribution functions 12 . 22 . 32 for describing the frequencies of temperature changes are about the heating function in 2 from the distribution functions 11 . 21 . 31 in 1 determined. They indicate which thermal load TBV the balancing resistors are subject to due to their use.

4 shows three examples of an equivalent number A. 13 . 23 . 33 which about Coffin-Manson from the three in 3 shown distribution functions 12 . 22 . 32 be derived. These are the thermal stresses at each temperature according to the equations of Coffin-Manson transformed into a corresponding load at a predetermined temperature and summed. A division into temperature classes or a classification of the number of resistors involved in the balancing reduces the associated calculation effort. The equivalent number A 13 . 23 . 33 , which represents the thermal load of the balancing resistors TBV, is for monitoring them with a threshold value 8th comparable.

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

• US 2012/0094151 A1 [0011]
• JP 2011-155752 A [0011]

Claims (10)

Method for monitoring the thermal load of balancing resistors in a battery device having a plurality of battery cells, characterized by the steps - determining a thermal load TBV of balancing resistors, caused by a multiplicity of varying temperature strokes ( 5 ) due to a frequency ( 6' . 6 '' . 6''' ) and intensity varying use of balancing resistors for balancing operations, - determination of an equivalent number ( 13 . 23 . 33 ) of temperature strokes ( 5 ) of a certain predetermined or predeterminable height, which exert on the balancing resistors a thermal load TBA which corresponds to the previously determined thermal load TBV, - comparison of the equivalent number ( 13 . 23 . 33 ) with a predetermined or predefinable threshold value ( 8th ) and delivery of a signal upon reaching or exceeding the threshold ( 8th ). A method according to claim 1, characterized in that for determining the thermal load TBV of the balancing resistors a - determination of a distribution function ( 12 . 22 . 32 ) about the height of the temperature strokes ( 5 ), which take place on the basis of a use of the balancing resistors, the thermal load TBV being determined by the distribution function ( 12 . 22 . 32 ) is writable. A method according to claim 2, characterized in that for determining the distribution function ( 12 . 22 . 32 ) about the height of the temperature strokes ( 5 ) first one - determination of number and frequency ( 11 . 21 . 31 ) the use of the balancing resistors and a - calculation of a distribution function ( 12 . 22 . 32 ) about the height of the temperature strokes ( 5 ) is carried out on the basis of a predetermined heating function which determines the dependence of the temperature strokes ( 5 ) of the number ( 4 ) describes the simultaneously used balancing resistors. A method according to claim 3, characterized in that as an intermediate step during or after the determination of number and frequency ( 11 . 21 . 31 ) the use of the balancing resistors first a classification of the number ( 4 ) simultaneously used balancing resistors. Method according to one of claims 3 or 4, characterized in that the heating function is measured and / or stored by a control device in the battery device during use of the balancing resistors. Method according to one of the preceding claims, characterized in that before the determination of an equivalent number ( 13 . 23 . 33 ) predetermined or predefinable temperature strokes ( 5 ) to the thermal load TBV, caused by the use of balancing resistors, a base load is added. A method according to claim 6, characterized in that the base load the number of temperature strokes ( 5 ) due to the change in ambient temperature and / or controller turn-on periods. Method according to one of the preceding claims, characterized in that the determination of an equivalent number ( 13 . 23 . 33 ) of temperature strokes ( 5 ) of a certain predetermined or predeterminable height, which exert on the balancing resistors a thermal load TBA, which corresponds to the previously determined thermal load TBV, via suitable aging and / or acceleration laws, in particular Coffin-Manson. Battery device with a plurality of battery cells and a balancing device having a control device and balancing resistors for balancing the battery cells, characterized in that the control device is configured to perform a monitoring of the thermal load of the balancing resistors according to a method of the preceding claims. Battery-powered vehicle with a battery device according to claim 9.

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