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Title:
CONTINUOUS EVALUATION OF HEALTH PARAMETERS FOR HIGHER SAFETY IN BATTERY OPERATION
Document Type and Number:
WIPO Patent Application WO/2015/162259
Kind Code:
A1
Abstract:
The method for operating an energy storage arrangement with a plurality of serially connected battery cells according to the present invention comprises an initialization process as calibration process performed when the storage arrangement is first brought into operation and a recalibration processes performed repeatedly during operation. The internal resistances is evaluated, preferably continuously elalueted, as safety indicator and compared with a predetermined maximum internal resistance trend R i,MAX derived from an expected internal resistance trend Ri,exp. A safety warning is reported if the actual measured internal resistance exceeds this limit.

Inventors:
BLOCHBERGER THOMAS (AT)
HARJUNG HANS (AT)
Application Number:
PCT/EP2015/058937
Publication Date:
October 29, 2015
Filing Date:
April 24, 2015
Export Citation:
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Assignee:
SCHOELLER LOGISTICS TECHNOLOGIES HOLDING GMBH (DE)
International Classes:
G01R31/36
Domestic Patent References:
WO2010088944A22010-08-12
Foreign References:
US20110307202A12011-12-15
GB2461350A2010-01-06
Attorney, Agent or Firm:
LESKE, Thomas (Possartstraße 20, München, DE)
Download PDF:
Claims:
Claims

1. A method for operating an energy storage arrangement with a plurality of serially connected battery cells,

wherein an initialization process as calibration process is performed when the storage arrangement is first brought into operation and a recalibration processes is performed repeatedly during operation,

wherein the internal resistances is evaluated as safety indicator and compared with a predetermined maximum internal resistance trend RI,UAX derived from an expected interna! resistance trend Ι¾ιβΧρ , and

wherein a safety warning is reported if the actual measured internal resistance exceeds this limit.

2. The method as claimed in claim 1 , characterized in that the internal resistance

change between the initialization and the recalibration processes is evaluated as safety indicator and a safety warning is reported, if the internal resistance R, exceeds the predetermined maximum internal resistance trend R/ ΜΧ,·

3. The method as claimed in one of the claims 1 ©r 2, characterized in that the internal resistance change between initialization and the recalibration processes is evaluated as safety indicator, and a safety warning is reported if the change of internal resistance AR, compared to previous measurements exceeds a predetermined maximum change of internal resistance ARIIMAX.

4. The method as claimed in one of the claims 1 to 3, characterized in that the internal resistance RT is measured outside of the initialization and the recalibration processes in a regular cycle and compared to data of the last measured internal resistance and/or to the Internal resistance trend over lifetime, wherein an increase of the internal resistance is evaluated as safety indicator and a safety warning is reported, If the internal resistance R, exceeds the predetermined maximum interna! resistance trend

5. The method as claimed in one of the claims 1 to 4, characterized in that the internal resistance is measured outside of the initialization and the recalibration processes in a regular cycle and compared to data of the last measured internal resistance and/or to the internal resistance trend over lifetime, wherein an increase of the internal resistance is evaluated as safety indicator and a safety warning is reported if the change of internal resistance ARt compared to previous measurements exceeds the predetermined maximum change of internal resistance ARhexp.

6. The method as claimed in one of claims 1 to 5, characterized in that the internal resistance Rt is used as safety indicator.

7. The method as claimed in one of claims 1 to 6, characterized in that the change of internal resistance A compared to an expected value is used as safety indicator.

8. The method as claimed in one of claims 1 to 7, characterized in that the internal resistance over state of charge is used as a safety indicator, wherein a maximum internal resistance trend from an expected or measured trend over the state of charge F¾ Soc is defined and a safety report is triggered if the actual measured internal resistance exceeds this limit.

9. The method as claimed in one of the claims 1 to 8, characterized in that the internal resistance change over state of charge ¾,Soc between initialization and the recalibration processes is evaluated as safety indicator and a safety warning is reported if the Internal resistance exceeds the predetermined maximum internal resistance Rj,soc_MAx-

10. The method as claimed in one of the claims 1 to 9, characterized in that the internal resistance change over state of charge AR^soc between initialization and the recalibration processes is evaluated as safety indicator and a safety warning is reported if the change of internal resistance compared to data of previous

measurements exceeds the predetermined maximum change of internal resistance

1 1. The method as claimed in one of the claims 1 to 10, characterized in that the internal resistance of an specific state of charge point is measured outside of the initialization and the recalibration processes in a regular cycle () and compared to the last measured data of internal resistance and/or to the internal resistance trend over state of charge, wherein an increase of the internal resistance is evaluated as safety indicator and a safety warning is reported if the interna! resistance RiSoc compared to previous measurements exceeds the predetermined maximum internal resistance

12. The method as claimed in one of the claims 1 to 11 , characterized in that the internal resistance of an specific state of charge point is measured outeide of the initialization and the recalibration processes in a regular cycle and compared to the last measured data of internal resistance and/or to the internal resistance trend over state of charge, wherein a increase of the internal resistance is evaluated as safety indicator and a safety warning is reported if the change of internal resistance ARiiSoc compared to previous measurements exceeds the predetermined maximum change of internal resistance ARIISOC_MAX,-

13. The method as claimed in one of the claims 1 to 12, characterized in that a lithium ion cell electrolyte deterioration either alone or in combination with other state of health parameters is used for the determination of the safety indicator.

14. The method as claimed in one of the claims 1 to 13, characterized in that a pressure of the lithium ion cell either alone or in combination with other state of health parameters is used for the determination of the safety indicator.

15. The method as claimed in one of the claims 1 to 14, characterized in that an outer or inner temperature of the lithium ion cell either alone or in combination with other state of health parameters is used for the determination of the safety indicator.

Description:
CONTINUOUS EVALUATION OF HEALTH PARAMETERS FOR HIGHER SAFETY IN

BATTERY OPERATION

The invention relates to a method for operating an energy storage arrangement.

Electrical energy stores, for example for electric vehicles, generally comprise high-power rechargeable batteries, which comprise a large number of galvanic cells connected to one another in order thus to meet the requirements placed on the energy store as regards supply voltage, power and capacity.

Such an interconnection of cells is also referred to as a battery system or rechargeable battery pack.

Various electrochemical methods which are named referring to materials used are available as the technological basis for the galvanic cells. These include nickel-metal hydride (NiMH), lead acid (PbA) and especially all variations of lithium chemistry technologies (e.g. NMC, LFP, LiPo, Li-Si, Li-Air).

A common feature to all available technologies is that cells which are identical per se with the same operating age have slightly different properties, for example owing to production tolerances, temperature influences or mechanical Influences.

As a result, they have different charge and discharge characteristics, which means that individual cells are subject to a greater load and are therefore destroyed prematurely, as a result of which the entire rechargeable battery then fails.

The properties of galvanic cells also change over the course of their existence. This change is dependent on many influencing factors and takes place irrespective of whether the cell is in the quiescent state, or is being discharged or charged.

The extent of the change is dependent on the nature, size and the combined effect of the influencing factors and on the temporal duration of this change.

When connecting galvanic cells to one another to form a storage arrangement, the performance of the entire arrangement is critically determined by the weakest cells, i.e. the cells with the comparatively lowest performance. Without any control interventions, these weakest ceSSs are the first to reach their end-of-charge voltage during a charge operation and to reach the end-of-discharge voltage during a discharge operation and thus limit the usable capacity of the entire rechargeable battery arrangement.

In order to avoid the described problems, it is known to produce, where possible, cells with equaling properties, but this is subject to physical and economic limits. In addition to this, therefore, after production the cells are measured individually in accordance with specific properties, usually capacity and internal resistance, and sorted into groups such that in each case cells with very similar properties, i.e. properties which are within a narrow tolerance range, are used for one arrangement. This operation is referred to as

"matching".

The process is complex and accounts for approximate!y 10% of the manufacturing costs of a rechargeable battery pack. However, this expenditure does not prevent cell drift, but merely delays it and therefore extends the life of the rechargeable battery pack slightly.

Recent battery systems are moreover controlled by battery management systems which monitor the celts and regulate the charge or discharge current, in this case, a distinction is drawn between passive and active balancing.

In the case of passive balancing, a switchable load resistance is connected in parallel with each battery cell. During charging, this load resistance is switched on when a

predetermined end-of-charge voltage is reached, and therefore the current is guided past the battery cell in question. The charge operation Ss continued until all of the cells in the battery system have reached the predetermined end-of-charge voltage.

In a discharge operation, the predetermined end-of-discharge voltage of the cells is monitored, and the discharge operation is terminated when this voltage through the first (weakest) cell has been reached. The stronger cells at this time still have residual energy, but this cannot be used in systems with passive balancing.

In the case of so-called active balancing methods, a switchable voltage transformer is used in place of a load resistance, said voltage transformer having the capacity to transfer the energy to be guided past a ceil into an adjacent cell, for example, or a cell group. As a result, in contrast to the passive balancing methods, the excess energy is returned to the system and is not converted into heat. The common asm of the known methods is always to avoid overloading individual cells and therefore prevent an excessive reduction in the life, whilst taking into consideration charge discharge current intensity, end-of-charge voltage and depth of discharge.

These methods of passive and active balancing are state of the art. The invention goes beyond these two methods.

The method for operating an energy storage arrangement with a plurality of serially connected battery cells according to the present invention comprises an initialization process as calibration process performed when the storage arrangement is first brought into operation and a recalibration processes performed repeatedly during operation. The internal resistances is evaluated, preferably continuously evalueted, as safety indicator and compared with a predetermined maximum internal resistance trend derived from an expected internal resistance trend R^. A safety warning is reported if the actual measured internal resistance exceeds this limit.

According to the invention, the charge and discharge operations of the individual cells in a battery system are configured such that over the different aging process of the individual cells in the group, the cells adjust such that their properties are harmonized over the course of time.

The aging process of the individual cells is in this case controlled via corresponding, cell- individual limit values for aging-influencing cell parameters.

Thus, for example, a cell ages considerably slower when it is charged during the charge operations in each case only to 95%, instead of to the full, predetermined end-of-charge voltage.

Precisely the end-of-charge voltage represents a considerable stress factor, especially for Li-ion cells with virtually any chemistry. In particular in the case of electric vehicles, the battery system is usually charged to the maximum capacity (i.e. the respective cell voltage is at the level of the end-of-charge voltage) since, where possible, it is desirable to always have the full operating range available owing to the limited operating range in the case of electric vehicles.

Essential further influencing factors on the change in the cell properties and therefore the aging of the cells are as follows: storage voltage, operating voltage, charge and discharge current intensity, state of charge, level of defined end-of-charge and end-of-discharge voltage, calendrical age of the cell, number of previous charge and discharge cycles, speed of charge/discharge changeovers and temperature during all quiescent and operating states, i.e. during storage, in the quiescent state, during charging and during discharging. The temporal duration for which one or more of the influencing factors take effect also substantially influences the cell properties.

The invention will be explained in more detail with reference to an exemplary method illustrated in the figures, as examples:

Figure 1 shows the ratio of charge voltage to state of charge in a typical cell in an energy storage arrangement,

Figure 2 shows the profiles of charge voltage, current and state of charge in typical charge and discharge operations,

Figure 3 shows a comparison of the aging processes of cells when using various battery management methods,

Figure 4 shows an arrangement for implementing the methods.

Figure 5 shows the typical trend of the internal resistance over the state of charge.

Figure 6 shows the typical trend of the internal resistance over lifetime.

The exemplary method comprises three subprocesses: an initialization process, an operating process and at least one calibration process.

The initialization process is performed when the storage arrangement is first brought into operation. It serves the purpose of precisely detecting the different properties of the individual cells in the storage arrangement and then deriving the bases for the further control method from these properties.

In this case, first all of the cells in the storage arrangement are charged completely, i.e. in each case until the end-of-charge voltage CVL predetermined by the manufacturer is reached. Then, the storage arrangement is completely discharged, with the result that each individual cell reaches the end-of-discharge voltage DVL predetermined by the manufacturer.

For this purpose, for example during discharge, all of the cells are discharged with a constant, equal absolute current value until a first cell has reached the end-of-discharge voltage DVL. Then, this first cell is not loaded any further, i.e. the further current flow is guided past said cell by means of the energy transfer units in a battery management system, as is described in WO 2010/088944, for example.

The discharge operation is continued in this way until a second cell has also reached the end-of-discharge voltage DVL. This cell is also not loaded any further and the discharge operation is only continued with the cells which have not yet reached their end-of- discharge voltage DVL. This continues until all of the cells have been discharged, i.e. until the end-of-discharge voltage DVL predetermined by the manufacturer has been reached.

During the discharge operation, the total current and the individual currents guided past the respective cell are measured. From this, the critical properties of the cells, such as the cell capacity, the internal resistance and so-called state-of-health parameters (SOH), are determined.

The health condition is determining the safety of the further energy store operation. It is well documented from daily practice in mobile applications like laptops that external influences such as change of temperature or humidity, vibrations, crushing,

electromagnetic disturbances and the unlikely case of chemical reactions with the battery cells can dramatically change the health condition of the individual cells. Immediate fire in accidents with electric vehicles is recognized as a huge potential safety risk and big efforts have been made to reduce this risk to a minimum. The risk of minor accidents has been fairly under estimated in the past, but is potentially resulting in thermal runaways several days after the accident. Another widely unrecognized factor is vibration in vehicle applications where this influence can cause minor short circuits in the energy store. An increased number of internal short circuits in the cell can lead to a thermal runaway - a rapid unregulated decomposition of the energy in the cell.

The health condition and the safe operation of the energy store can be determined via several factors, including the internal resistance. Continuous tracking of the internal resistance makes it possible to Indicate If a unsafe condition for an individual cell is approaching and allows to warn and take measures far in advance.

Hence, especially the change in internal resistance provides information of the aging process within the cells. It is known that with increased age of an energy store, the internal resistance is increasing. Atypical changes of the internal resistance signalize a potential damage of the individual cell and further to a safety issue. These data form the basis for the determination of cell-individual regulation parameters and their limit values.

This will be described below using the example of the end-of-charge voltage CVL in a possible variant for a storage arrangement comprising 5 cells connected in series:

The determined cell capacities are illustrated in Table 1 :

Table 1

The greatest capacity C M AX is defined as reference capacity C RE F: CREF = CMAX

Then, the cells are classified and ordered in accordance with their capacities C , or the difference in the individual cell capacity AC ¾ with respect to the reference capacity

AC; = C JJEP - C, , as illustrated in Table 2:

Table 2

The end-of-charge voltage CVL predetermined by the manufacturer is set as maximum end-of-charge voltage CVLMAX.

Conservation of the weaker cells is achieved according to the invention by a cell-individual reduction in the end-of-charge voltage CVL< in accordance with

CVL, = f jAC, ). The relationship function f C vL is derived in the exemplary embodiment from the open circuit voltage characteristic OCV predetermined by the manufacturer for the cells, as is illustrated in figure 1.

In this case, it is advantageous if the minimum end-of-charge voltage C LMIN, i.e. the end- of-charge voltage of the weakest cell, is still fixed in the steep end section of the open circuit voltage characteristic OCV.

The cell-individual end-of-charge voltages CVLj of the remaining cells are then distributed between CVLMAX and CVLMIN in accordance with the ordering of the cells using the differences in capacity. This distribution can in the simplest case take place linearly, but also with any other desired form (for example exponentially, logarithmically).

As a variant, the absolute values of the cell-individual end-of-charge voltage CVLi can also be fixed in such a way that only a group of the weaker cells, for example half of them, are given an end-of-charge voltage CVLi which is reduced individually in accordance with their ordering and all of the other cells are given the end-of-charge voltage CVL predetermined by the manufacturer. Thus, the reduction in capacity as a result of a reduced end-of- charge voltage CVLi is less.

Assuming that the end-of-charge voltage CVL predetermined by the manufacturer is 4.2 V and the gradient of the open circuit voltage characteristic OCV reaches a value of 100% at a cell voltage of 4.0 V and the call-individual end-of-charge voltage CVLi is distributed linearly, the cell-individual end-of-charge voltages C Li shown in Table 3 result:

Table 3

Once cell-individual end-of-charge voltages CVLi have been defined on the basis of the determined capacities Q of the cells, all of the cells are charged to their individual end-of- charge voltage CVLi. In this case, the charge quantities per cell are measured and the value of the available amount of energy in the entire energy store is determined.

The measured internal resistances and especially the changes in internal resistance is an important predictor for the age and health of each individual cell. It is known that the internal resistance over the state of charge must have a specific trend as shown in figure 5. The internal resistance trend is derived either from predetermined studies or from the previous initialization or calibration process.

This trend is dependent on the type and use of the energy store, i.e. chemistry, temperature, current, etc. This information makes it possible to define a maximum internal resistance for a specific state of charge RI,S O C_UAX- i the measured internal resistance exceeds the defined range R irSa c > RI,S O C_UAX, the energy store has an increasing possibility of malfunction in the future. This safety issue is reported to the energy store user interface or to a automatic maintenance server.

The initialization process is thus concluded.

In the operating process, as illustrated in figure 2, the individual cells are discharged and recharged on the basis of the now individually predetermined end-of-charge voltages. In the example, the cells have a matching end-of-discharge voltage DVL, but it may also be expedient for the end-of-discharge voltages DVL to be fixed individually for each cell.

Figure 2 shows the profiles of the charge voltage, current and the state of charge SoC, i.e. in the specified example shown in Table 3 the curve profile of the strongest cell CUA corresponds to cell number 5 with an end-of-charge voltage CVL 5 of 4.2 V and the curve profile of the weakest cell C M IN corresponds to ceil number 4 with an end-of-charge voltage CVU of 4.0 V.

By virtue of the conservation according to the invention of the weaker cells, said cells age more slowly, the cell properties harmonize more and more, the ceil drift is reduced, and there is automatic matching of the cells.

As is apparent from figure 2, the current How through the cells connected in series differs merely in a limited region between times t 2 and ¾ or between and ¾, i.e. in phases in which the cells are already partially discharged. In these phases, stronger cells are loaded with increased current flow to a greater extent as well. This is achieved by suitable driving of the energy transfer units in a battery management system.

Limiting the interventions of the battery management system to regions in which the cells are already partially discharged has the effect of limiting the losses owing to the energy transfer units in a battery management system since the operating state of the extensive discharge of the cells occurs comparatively seldom since, in particular in the case of use in electric vehicles, the aim is to keep the energy store as charged as possible in order thus to allow a maximum operating range.

The start points , for the beginning of the additional energy transfer are firstly dependent on the performance of the energy transfer units in the battery management system, i.e. on the technical implementation thereof, and second!y also on the sum of the differences in capacity of the cells in the energy store. In any case, the start point should be selected such that the maximum capacity of all of the cells is exhausted when said cells have reached their end-of-discharge voltage DVL.

The start points ti , for the beginning of the additional energy transfer are

advantageously defined as a specific state of charge of the weakest cell.

Cell aging brings about a change in the cell properties. This relates in particular to the celi capacity, which does not reach a relatively stable value until approximately 100 charge/discharge cycles after the cell has first been brought into operation and then constantly decreases as the age increases.

It Is therefore necessary to recalibrate the control system at regular time intervals.

In this case, it is expedient to provide different calibration processes, namely a first calibration process which is performed, for example, in each case after 10-20 charge cycles and is used for calibrating essential control parameters.

For example, in this case ail of the cells can be charged to their individual end-of-charge voltage CVLj and the respective charge quantity meters are reset to the value determined in the initialization process. Thus, any measurement errors which are integrated by frequent charge and discharge operations are compensated for. A second calibration process can be performed in each case after 100-200 charge cycles. In this case, complete recalibration of the system and renewed determination of the actual capacities of each individual ceil, as well as of the internal resistances and other SOH parameters is performed.

The internal resistance is recorded with each calibration cycle and data is stored in the battery management system. Over lifetime of the individual battery cells, the internal resistance is increasing. This trend R ITEXP is known from predetermined studies or other prediction systems, shown in Figure 6. The following equation is a example for an internal resistance trend.

Ri,»xp = O = A » exp(B · t) + C

The definition of the maximum allowed internal resistance trend R,,MAX is advantageously selected as a k = 50% increased value compared to the expected trend.

Ri,MAx = (1+k)* A · exp(B · t) * ((1+k) - C)

Further the known trend R exp defines specific internal resistance changes AR T between the recalibration processes. An exceeding internal resistance change indicates a high possibility of malfunction.

The same measurement is performed in regular use (cycle), apart of an initialization or calibration process. In the case of a long period of time without discharging or charging the energy store, and a sudden increase of a discharge or charge current, the internal resistance can be determined. However this internal resistance might be with Sower accuracy. The measured internal resistance is be compared to the previous

measurements and to the expected trend range.

In a vehicle application this is a typical situation in the transition of the vehicle being parked for several hours and afterwards operation by the user, if the measured internal resistance exceeds the defined range of safe operation, this condition is reported to the user or even the vehicle is stopped automatically.

The second calibration process largely corresponds to the initialization process. The different properties of the individual cells in the storage arrangement which have changed as a result of aging are redetected in order to derive from these properties the bases for the further control method.

In the case of an electric vehicle, the second calibration process Is required approximately once a year and can therefore be performed in the course of the standard yearly service.

The method according to the invention therefore includes constant adaptation and therefore optimization of the battery management system to the changing cell properties.

As illustrated in figure 3, an extension of the life of energy stores in comparison with conventional battery management approaches is therefore achieved.

The life of a rechargeable battery is defined differently in application-specific fashion. For use in electric vehicles, for example, the life is fixed as the time at which the rechargeable battery now only has 75% of its original capacity. After this, it can still be used for a few more years in other applications with less stringent requirements until the life limit for this other application is reached as well.

Figure 3 shows the ratio of the capacity of cells to their rated capacity C/CN over time in each case for the strongest cell CMAXI , CMAX2, CMAX3 and the weakest cell CM 1N I ,CMIN2. C M IN3 in an arrangement with passive balancing CMAX3. Cum, an arrangement with active balancing Gmx 2 , C M IN2, and an arrangement which is controlled by the method according to the invention CMAXL C M INI- The characteristics show that, in the conventional balancing methods, the respectively weakest cells age more quickly than the strongest cells, the curve profiles of the capacity ratios C/CN of the weaker cells C M | 2. Cu fall away more quickly than the strongest cells, corresponding to curve profiles CMAX2, C A 3. Since, however, the life of the entire storage arrangement is determined by the respective weakest cell, this rapid aging process also results in a shorter life of the storage arrangement.

In contrast, when using the method according to the invention, the aging processes of the cells CMINL CMAX harmonize with one another, whereby an extension of the life of the storage arrangement is achieved.

Figure 3 shows the respective end of life 1 , 2, 3 by virtue of the point of intersection of the curve profiles of the capacity ratios C/CN of the weaker cells C U im , Mm , C U m with the 75% value LG. The exemplary energy storage arrangement shown in figure 4 for implementing the method according to the invention comprises storage celts C1 , C2, C3, ... CN, connected in series.

Each storage cell C1 , C2, C3, ... CN is connected in parallel with an energy transfer unit ET1 , ET2, ET3, ... ETn. The energy transfer units are controlled by a central control unit SE.

The energy storage arrangement depicted can be combined, as a battery module

(rechargeable battery pack) with further battery modules by being connected in series to form high-voltage energy stores. In this case, it is expedient if the control unit SE comprises, in addition to the control electronics, means for energy transfer to other battery modules. It is then possible to construct large energy stores by cascading battery modules.

List of reference symbols

V Voltage

I Current

CVL End-of-charge voltage

CVLMAX End-of-charge voltage of strongest cell

CVLMIN End-of-charge voltage of weakest cell

CVL, End-of-charge voltage of eel! No. i

SoC State of charge

DVL End-of-discharge voltage

t1 ,t2...t7 Notable times in the operating process

CMAX1 , C AX2T CMAX3 Capacity of the strongest cell in a storage arrangement

CMNI . CMIN2> C I 3 Capacity of the weakest cell in a storage arrangement

1 End of life of an arrangement with the method according to the invention

2 End of life of an arrangement with the conventional active balancing method

3 End of life of an arrangement with the conventional passive balancing method

Ci, C2, C3, ... CN Storage cells

ET t , ET 2 , ET 3 ,... ET n Energy transfer units

SE Control unit