Method for predicting need for equalizing state of charge of plurality of electrochemical energy storage cells of electrochemical energy storage device

Abstract

The invention relates to a method for predicting the need for equalizing the state of charge of a plurality of electrochemical energy storage cells of an electrochemical energy storage device.

Description

Technical Field

[0001] The invention relates to a method for predicting the need for equalizing the states of charge of a plurality of electrochemical energy storage cells of an electrochemical energy storage device, a device for operating an electrochemical energy storage device, and the use of the method for predicting the need for equalizing the states of charge of a plurality of electrochemical energy storage cells according to the preambles of the independent claims. Background Art

[0002] Hybrid vehicles and electric vehicles use lithium-ion battery packs, which consist of a large number of electrochemical cells connected in series. Battery management systems are used to monitor the batteries and, in addition to safety monitoring, should also ensure the longest possible service life.

[0003] For this purpose, it must be ensured that the states of charge (SoC; state of charge of a cell, cell SOC; state of charge of a battery pack, battery pack SOC, defined, for example, as the minimum value within all cell SOCs) of the individual cells match one another despite different self-discharges. This is achieved by equalizing the states of charge with suitable cell balancing, which is usually carried out resistively. For this purpose, resistors and switching elements are provided for each cell in order to be able to discharge the individual cells in a targeted manner via such (balancing) resistors.

[0004] In addition to the different self-discharge rates of the individual cells, the capacity of the cells also varies from one another due to production variations. This effect is negligibly small at the beginning of the life of the cell (BOL), but increases during the life due to differences in cell aging and leads to capacity differences of several percentage points between the cells.

[0005] The balancing process is performed during the idle phase of the controller (BCU). To this end, for a programmable balancing time t Bal to activate the balancing hardware unit. This programming is usually done during the controller off time. There is no way to adjust the programmed balancing time or the charge to be balanced.

[0006] In the current prior art, the charge ΔQ to be balanced can be determined using the following relationship: i and the duration of the single pool equilibrium t Bal :

[0007] 1. Determine the state of charge SOC relative to a reference rat The charge Q i :

[0008] Q i =C BOL*SOC C,i *(SOC i -SOC rat )

[0009] 2. Determine the balancing requirements for each single pool:

[0010] ΔQ i =Q i -Q min

[0011] 3. Determine the balancing time for each single pool:

[0012]

[0013] Reference state of charge SOC rat is the charge state of all cells in the equilibrium state. min is the state of charge SOC of all cells relative to the reference rat The minimum charge.

[0014] In the current prior art it is assumed that the state of charge can always be determined and therefore the balancing requirement can be determined. However, this is no longer the case when using lithium iron phosphate cells (LFP) in a 12V environment and in a high voltage environment. In the case of this type of cell, the constant voltage characteristic curve (OCV curve) is very flat and the state of charge determination or balancing requirement determination can only be achieved in a few states of charge of the battery, usually only when the battery is fully charged.

[0015] The balancing process is therefore carried out only to a very small extent or only with increased effort. In the case of LFP cells, the battery must be fully charged in addition in order to be able to meet the balancing requirements. However, such a charging process can lead to increased battery aging.

[0016] Document CN 109428356 A discloses a battery balancing method.

[0017] Document DE 102013214817 A1 discloses a method for diagnosing the state of a battery having a plurality of battery cells, which provides for obtaining a charge state equalization requirement value, obtaining a feasible diagnosis option, comparing the obtained charge state equalization requirement value with a requirement threshold value for the obtained feasible diagnosis option, and thereby diagnosing the state of the battery cell. Summary of the invention

[0018] The object of the invention is to further improve the prior art. This object is achieved by the features of the independent claims.

[0019] Advantages of the present invention.

[0020] In contrast, the procedure according to the invention having the characterizing features of the independent claim has the advantage that the method for predicting the need for equalizing the charge states of a plurality of electrochemical energy storage cells of an electrochemical energy storage device comprises the following steps:

[0021] - Determine the coefficient k, which represents the self-discharge rate according to different state of charge (SOC), temperature (T) and relaxation duration (ti relax )according to The change of S x represents the self-discharge rate of the electrochemical energy storage cell x in the electrochemical energy storage cell, and S norm represents a normalized self-discharge rate at a predetermined state of charge, a predetermined temperature and a relaxation time;

[0022] - Get the charge Q x , which respectively represent the charge quantity ΔQ equalized during the equalization process of the charge state of the electrochemical energy storage cell x x The sum of

[0023] -According to the cond obs =∫k(SOC,T,ti relax )dt to obtain the first state condition of the electrochemical energy storage device;

[0024] - Determine the charge Q bal,x , which represents the requirement of the electrochemical energy store x for equalizing the charge states of the electrochemical energy store cells at a first moment;

[0025] -According to the cond obs,act =∫k(SOC,T,ti relax )dt to obtain the second state condition;

[0026] - Determine the predicted charge ΔQ pred,x , the predicted charge is calculated according to represents the requirement of the electrochemical energy storage cell x for equalizing the charge states of the electrochemical energy storage cells at a second point in time.

[0027] Advantageously, a requirement is detected predictively, which is taken into account for equalizing the states of charge of a plurality of electrochemical energy storage cells of an electrochemical energy storage device, thereby increasing the availability of the state of charge equalization even if the requirement cannot be determined via the constant voltage characteristic curve (OCV curve).

[0028] Furthermore, due to the predictive demand determination, the charge state equalization can be carried out more frequently, which leads to an increase in the effective power of the electrochemical energy store.

[0029] Furthermore, the frequency of the required full charging of the electrochemical energy store, which is necessary for the demand determination by means of the constant voltage characteristic curve (OCV curve), is advantageously minimized.

[0030] The method according to the invention can also be used to verify the demand determination performed with the aid of the constant voltage characteristic curve (OCV curve). The OCV window (steeper OCV gradient) for determining the demand is very small (2-3% state of charge (SOC)). Before the OCV-based demand determination is carried out, a predictive balancing can be carried out. By verifying the determined demand, defects in electrochemical energy storage cells can be detected in advance.

[0031] Further advantageous embodiments are the subject matter of the dependent claims.

[0032] The method according to the invention for predicting the need for equalizing the charge states of a plurality of electrochemical energy storage cells of an electrochemical energy storage device further comprises the following steps:

[0033] - Obtaining the charge state of each electrochemical energy storage cell based on the constant voltage characteristic curve (OCV) of the electrochemical energy storage cell.

[0034] Advantageously, the predicted demand only needs to be verified and, if necessary, corrected in a very short period of time by means of the constant voltage characteristic curve (OCV curve).

[0035] Follow Q bal,x =Q x -Q min To determine the charge Q bal,x , where Q bal,x represents the charge to be equalized for the electrochemical energy storage cell x, Q x represents the balanced charge of the electrochemical energy storage cell x, Q min Indicates the minimum balanced charge of an electrochemical energy storage cell.

[0036] according to To obtain the charge quantity ΔQ equalized during the equalization process of the charge state of the electrochemical energy storage cell x x , where t represents the duration of the equilibrium process, U cell,x represents the voltage of the electrochemical energy storage cell x, R bal represents the resistor for equalizing the state of charge of the electrochemical energy storage cell x.

[0037] The device according to the invention for operating an electrochemical energy store comprising a plurality of electrochemical energy store cells comprises: at least one sensor for detecting a physical variable, in particular a voltage, a current, a temperature; at least one resistor for carrying out resistive state-of-charge equalization; and at least one component, in particular an electronic control unit, which is designed to carry out the steps of the method according to the invention.

[0038] According to an advantageous embodiment of the invention, a computer program is provided which includes commands which cause the device for operating a battery to carry out the method steps according to the invention.

[0039] Furthermore, a machine-readable storage medium is provided, on which the computer program is stored.

[0040] Advantageously, the method according to the invention for predicting the need for equalizing the charge states of a plurality of electrochemical energy storage cells of an electrochemical energy storage device and / or the device according to the invention are used in electric vehicles, fuel cell vehicles, hybrid vehicles, plug-in hybrid vehicles, aircraft, pedelecs or electric bicycles; in portable devices for telecommunication or data processing; in electric hand tools or kitchen appliances; and in stationary storage devices for storing electrical energy, in particular regeneratively obtained electrical energy. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description.

[0042] Further advantages and advantageous embodiments of the subject matter of the invention are shown in the drawings and explained in the following description. It should be noted that the drawings have only descriptive characteristics and are not intended to limit the invention in any way. Furthermore, the features described below can illustrate the subject matter of the invention individually or in any combination, unless the context clearly indicates otherwise.

[0043] in:

[0044] Figure 1a A schematic diagram showing the change in the self-discharge rate in the case of different states of an electrochemical energy store;

[0045] Figure 1b A schematic diagram showing the functional flow of the method according to the invention for predicting the need for equalizing the state of charge; and

[0046] Figure 2 A flow chart of one specific embodiment of the method according to the invention is shown for predicting the need for equalizing the states of charge of a plurality of electrochemical energy storage cells of an electrochemical energy storage device. DETAILED DESCRIPTION

[0047] The same reference numerals denote the same device components in all the figures.

[0048] Figure 1a A schematic diagram shows the course of the self-discharge rate for different states of an electrochemical energy store.

[0049] Self-discharge S of a single cell of electrochemical energy storage device x It can be described with the help of the following formula:

[0050] S x =S norm *fac*k

[0051] S norm : The self-discharge rate of the electrochemical energy storage cell under normal conditions (for example, according to the data sheet of the electrochemical energy storage cell);

[0052] fac: Deviation of the self-discharge rate due to manufacturing tolerances of the electrochemical energy storage cell;

[0053] k: The self-discharge rate is affected by the existing boundary conditions (state of charge (SOC), temperature (T), cell relaxation (ti relax )) caused by acceleration.

[0054] The function for equalizing the state of charge equalizes the different self-discharges of the electrochemical energy storage cells. Bal ) increases over time and must be balanced with the capacitor with the highest self-discharge (S max )’s self-discharge difference of electrochemical energy storage cell:

[0055] ΔQ Bal =(S x -S max )*time.

[0056] Due to the existing boundary conditions (state of charge (SOC), temperature (T) and cell relaxation (ti relax )), the self-discharge of the electrochemical energy storage cell varies and is taken into account with the aid of a factor k (S x =S norm *k):

[0057] ΔQ Bal =(S norm,x -S norm,max )*k*time

[0058] The coefficient k describes the self-discharge rate in different states of the electrochemical energy storage device (SOC, temperature (T) and relaxation duration (ti relaxThe factor k is the same for all electrochemical energy storage cells in the same state of the electrochemical energy storage for all electrochemical energy storage cells and can be determined, for example, by means of laboratory tests. norm is an arbitrarily selected state of the electrochemical energy storage device, for example, SOC = 100%, temperature = 25°C, ti relax = 0 (no relaxation) at the obtained self-discharge rate.

[0059] Figure 1b A schematic diagram of the functional sequence of the method according to the invention for predicting a need for equalizing the state of charge is shown.

[0060] During operation of the electrochemical energy store, information about the self-discharge is measured, which information is later necessary for the prediction. The following steps are required for the measurement.

[0061] At the start time 100 of the self-discharge measurement, the electrochemical energy storage cells must be in a balanced state of charge, or the charge differences between the electrochemical energy storage cells must be known. The charge state of the electrochemical energy storage can therefore be determined using an isostatic voltage characteristic curve (OCV curve).

[0062] The state condition of the electrochemical energy store is measured during operation 101 of the electrochemical energy store. By integrating k over time: cond obj,1 =∫k*dt to achieve this.

[0063] The state of charge of the electrochemical energy store can in turn be determined with the aid of the constant voltage characteristic curve (OCV curve), and the relative charge loss between the individual cells of the electrochemical energy store can then be determined with the aid of a reference measurement.

[0064] Any electrochemical energy storage cell must be balanced with the cell with the highest self-discharge loss Q min Relative self-discharge loss Q between electrochemical energy storage cells x , and assumes that the same amount of charge must be balanced in the future under the same boundary conditions (∫k*dt).

[0065] In an alternative embodiment, the relative self-discharge loss can be measured and detected within a predetermined, longer period of time t1. For this purpose, the equalized charges of each electrochemical energy storage cell are added together and stored within the predetermined period of time t1. The charges equalized during the charge state equalization are calculated by is obtained and accumulated in time period t1.

[0066] During the entire time period t1, the condition cond1 of the electrochemical energy storage cell is also recorded as ∫k(SOC, T, ti relax )dt.

[0067] After the charge state recognition (OCV curve), all detected self-discharge losses and conditions (cond) can alternatively also be stored 102 in a ring memory of any size. Thus, the entire history 105, 106 of the relative self-discharge and conditions of the electrochemical energy storage cells can be advantageously retraced.

[0068] An assessment of the relative self-discharge loss can thus be made possible and a possible increase in the self-discharge rate of the second electrochemical energy storage cell can be detected.

[0069] Defects of an electrochemical energy storage cell can thus be advantageously detected at an early stage. An abnormal increase in the self-discharge rate can indicate an internal short circuit.

[0070] If the relative self-discharge losses and the conditions of the electrochemical energy storage cells are observed over a sufficient period of time t1 , the need for equalizing the states of charge can be predicted.

[0071] In an alternative embodiment, the condition cond can also be used instead of the time period t1. obs,1 =∫k(SOC,T,ti relax )dt as the activation criterion.

[0072] During the observation period t1, it is assumed that the self-discharge rate of the individual electrochemical energy storage cells does not change or changes only slightly. The self-discharge rate may change due to aging of the electrochemical energy storage cells. Therefore, the period t1 is limited to a maximum period.

[0073] After the relative self-discharge of each electrochemical energy storage cell has been determined and the observation period t1 is sufficiently long, the need for equalizing the states of charge can be predicted.

[0074] Divide the obtained relative self-discharge by the single cell condition cond obtained in the time period t1 obs,1 =∫k(SOC,T,ti relax )dt.

[0075] After the carried out charge state equalization, ie after the charges of the electrochemical energy storage cells have been equalized again, in a further time period t2 according to condition obs,2 =∫k(SOC,T,ti relax )dt and then re-record the conditions of the electrochemical energy storage cell.

[0076] according to This results in a predicted need for state of charge equalization which occurs during this time period t2 .

[0077] The predicted requirement can then be used to carry out a state of charge equalization 104. Advantageously, the predicted requirement for the state of charge equalization can be equalized at any time.

[0078] Advantageously, the state of charge equalization should be carried out before the targeted charging of the electrochemical energy store in order to determine the need for the state of charge equalization by means of the constant voltage characteristic curve (OCV curve). The need for the state of charge equalization can thus be fully determined, since, for example, in the case of lithium iron phosphate cells (LFP), the observation window is only very small, for example, in the case of a state of charge (SOC) <2-3%.

[0079] State-of-charge equalization with a predictively determined need can be carried out continuously and advantageously independently of measurements.

[0080] By continuous balancing, the hardware used can be relieved. For example, larger resistors can be used for charge state equalization. In addition, the thermal heating of the hardware is reduced and the service life of the circuit is thus extended.

[0081] Figure 2 A flow chart of one specific embodiment of the method according to the invention is shown for predicting the need for equalizing the states of charge of a plurality of electrochemical energy storage cells of an electrochemical energy storage device.

[0082] In step 205 , the state of charge of each electrochemical energy storage cell is acquired based on the constant voltage characteristic curve (OCV) of the electrochemical energy storage cell.

[0083] In step 210, the charge Q is obtained. x , which respectively represent the charge quantity ΔQ equalized during the equalization process of the charge state of the electrochemical energy storage cell x x The sum of .

[0084] In step 220, according to cond obs =∫k(SOC,T,ti relax )dt to obtain the first state condition of the electrochemical energy storage device.

[0085] The method according to the invention is not limited to the sequence shown in the specific embodiment. Rather, steps 210 and 220 can be performed repeatedly in any sequence, sequentially and / or simultaneously.

[0086] In step 225, it is checked whether a sufficiently long time period has been observed. If the time period is not yet sufficiently long, the method is continued after step 205.

[0087] In step 230, the charge Q is determined bal,x, which represents the requirement of the electrochemical energy store x for equalizing the charge states of the electrochemical energy store cells at a first point in time.

[0088] In step 240, according to cond obs,act =∫k(SOC,T,ti relax )dt to obtain the second state condition.

[0089] In step 245, it is checked whether a sufficiently long time period has been observed. If the time period is not yet sufficiently long, the method is continued after step 230.

[0090] In step 250, the predicted charge ΔQ is determined pred,x , the predicted charge is calculated according to represents the requirement of the electrochemical energy storage cell x for equalizing the charge states of the electrochemical energy storage cells at a second point in time.

Claims

1. A method for predicting a requirement for equalizing the states of charge of a plurality of electrochemical energy storage cells of an electrochemical energy storage device, the method comprising the following steps: -(200) determines the coefficient k, which represents the self-discharge rate according to different state of charge (SOC), temperature (T) and relaxation duration (ti relax )according to The change of S x represents the self-discharge rate of the electrochemical energy storage cell x in the electrochemical energy storage cell, and S norm represents a normalized self-discharge rate at a predetermined state of charge, a predetermined temperature and a relaxation time; -(210) Get the charge Q x , which respectively represent the charge quantity ΔQ equalized during the equalization process of the charge state of the electrochemical energy storage cell x x The sum of -(220) According to cond obs =∫k(SOC,T,ti relax )dt to obtain a first state condition of the electrochemical energy storage device; -(230) Determine the charge Q bal,x , which charge quantity represents the requirement of the electrochemical energy store x for equalizing the charge states of the electrochemical energy store cells at a first moment; -(240) According to cond obs,act =∫k(SOC,T,ti relax )dt to obtain the second state condition; -(250) Determine the predicted charge ΔQ pred,x , the predicted charge is calculated according to represents the requirement of the electrochemical energy storage cell x for equalizing the charge state of the electrochemical energy storage cell at a second point in time.

2. The method for predicting a requirement for equalizing the charge states of a plurality of electrochemical energy storage cells of an electrochemical energy storage device according to claim 1 , further comprising the following steps: -(205) Obtaining the charging state of each electrochemical energy storage cell according to the constant voltage characteristic curve (OCV) of the electrochemical energy storage cell.

3. The method for predicting the need for equalizing the charge states of a plurality of electrochemical energy storage cells of an electrochemical energy storage device according to claim 1 , wherein: Follow Q bal,x =Q x -Q min To determine the charge Q bal,x , where Q bal,x represents the charge quantity to be equalized for the electrochemical energy storage cell x, Q x represents the balanced charge of the electrochemical energy storage cell x, Q min denotes the minimum balanced charge of the electrochemical energy storage cell.

4. The method for predicting the need for equalizing the charge states of a plurality of electrochemical energy storage cells of an electrochemical energy storage device according to claim 1, wherein: according to To obtain the charge ΔQ x , which is equalized during an equalization process of the charge state of the electrochemical energy storage cell x, wherein t denotes the duration of the equalization process, U cell,x represents the voltage of the electrochemical energy storage cell x, R bal y represents the resistor for equalizing the state of charge of the electrochemical energy storage cell x.

5. A device for operating an electrochemical energy store comprising a plurality of electrochemical energy store cells, comprising: at least one sensor for detecting a physical variable, in particular a voltage, a current, a temperature; at least one resistor for performing resistive state-of-charge equalization; and at least one component, in particular an electronic control unit, which is designed to carry out the steps of the method according to any one of claims 1 to 4 .

6. Computer program comprising instructions which cause the device according to claim 5 to carry out the method steps according to any one of claims 1 to 4.

7. A machine-readable storage medium having stored thereon the computer program according to claim 6.

8. Use of the method for predicting the need for equalizing the charge states of a plurality of electrochemical energy storage cells of an electrochemical energy storage device according to any one of claims 1 to 4 and / or the device according to claim 5 for electric vehicles, fuel cell vehicles, hybrid vehicles, plug-in hybrid vehicles, aircraft, electric scooters or electric bicycles, for portable devices for telecommunications or data processing, for electric hand tools or kitchen appliances, and in stationary storage devices for storing electrical energy, in particular regeneratively obtained electrical energy.

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