METHOD FOR RECHARGING A RECHARGEABLE ENERGY STORAGE

Description

[0001] The invention relates to a method for charging a rechargeable energy storage device, wherein the energy storage device has at least one cell block with a number J of battery cells connected in series.

[0002] An energy storage device comprises several galvanic cells connected in series and / or parallel, known as battery cells. When the battery cells are discharged, the stored chemical energy is converted into electrical energy. This electrical energy can be used by a consumer that is independent of the electrical grid, such as an electric vehicle. Furthermore, the electrical energy from the energy storage device can be used by a consumer connected to a power grid to bridge an interruption in the power supply from the power grid. The energy storage device, equipped with rechargeable battery cells, is recharged after a discharge so that it is available for further use.

[0003] For energy storage devices (accumulators) consisting of several serially connected rechargeable battery cells, it is important, among other things, for the service life of the energy storage device that each individual cell is neither overcharged during charging nor overdischarged during discharging, and that all cells have as similar a charge level as possible. This applies particularly to energy storage devices consisting of several serially connected lithium-ion batteries, lithium polymer batteries, and / or lithium iron phosphate batteries.

[0004] In a series connection of battery cells, the same charging current flows through all the battery cells. Since battery cells can have different internal resistances or efficiencies, the power losses of the individual battery cells vary during charging. This means that not all battery cells have the same capacity after charging, which can be used for later discharging. This capacity is also referred to as the usable capacity. The battery cell with the highest internal resistance or the worst efficiency has the lowest usable capacity after the cell block has been charged, as it has the greatest power loss. This means that this cell is discharged the most during subsequent discharging. It is therefore the battery cell with the highest Depth of Discharge (DoD) in the cell block. However, the higher the DoD of a cell, the shorter the service life of that cell.This leads to disproportionate aging of this single cell and thus to premature failure of the cell block.

[0005] To determine the current state of charge of a battery cell, its respective cell voltage can be measured. If the cell voltage exceeds or falls below specified values, charge equalization can then be carried out between the battery cells in the cell block, provided the battery cells have different states of charge. However, this presents the problem that the cell voltage remains largely constant, at least in sections, during the respective charging process of a battery cell, making it difficult to draw conclusions about the current state of charge of the corresponding battery cell from the cell voltage. Only shortly before the respective final charge or discharge voltage is reached does a relatively sharp rise or fall in the respective cell voltage occur, which can be used for corresponding control processes for charge equalization.

[0006] However, these balancing processes take a lot of time. InDuring this time the battery block cannot be used for normal operation.

[0007] In As a rule, such energy storage devices are therefore connected to a device, often referred to as a battery management system. This system, on the one hand, continuously monitors the charge level of the individual battery cells using a charge control device and, on the other hand, attempts to equalize any differing charge levels of the individual battery cells. This balancing of the charge levels of the battery cells, also known as "balancing," can be achieved through passive or active balancing. Furthermore, with existing battery management systems, charge equalization only begins when at least one of the battery cells is fully charged, making the entire charging process of a cell block relatively time-consuming.

[0008] With passive balancing, the excess energy of the battery cell that reaches its final charging voltage first is converted into heat via a resistor and is thus lost for the charging process.

[0009] With active balancing, however, the energy extracted from a battery cell with excessively high cell voltage is not converted into thermal energy, but rather used to charge the other cells in the energy storage system. However, even with active balancing, charge balancing only begins when at least one of the battery cells in the cell block has reached its final charging voltage.

[0010] DE 10 2017 009 850 A1 discloses a method for charging and discharging an energy storage device with at least one cell block comprising a plurality of battery cells connected in series, without active or passive balancing. In this known method, all battery cells reach their end-of-charge voltage or end-of-discharge voltage simultaneously. For this purpose, the characteristic maximum charging current IN;max is determined from the capacity CN of each battery cell, taking into account a predetermined C-factor, which corresponds to the quotient of the maximum charging current IN;max to the capacity CN of each of the battery cells. During a predetermined time t, which is less than or equal to the inverse of the C-factor, all battery cells are charged simultaneously with their respective assigned maximum charging currents IN;max.The difference between the available charging current I 0 and the maximum charging current IN;max of a battery cell is taken from or supplied to the cell block as auxiliary charging current via auxiliary charging / discharging devices. Discharging occurs accordingly.

[0011] From DE 10 2019 129 415 B3 a method for charging and / or discharging a rechargeable energy storage device with a current I 0 is known, wherein the energy storage device comprises at least one cell block with a number J of battery cells connected in series and at least some of the battery cells have different efficiencies η N with 1 ≤ N ≤ J. Firstly, the battery cell with the worst efficiency η min is determined. Then the efficiencies η N of all other battery cells are adjusted to this worst efficiency η min, so that the following applies to the adjusted efficiency η N' of the battery cells: η N' = η min .

[0012] The invention is based on the object of providing a method for charging and discharging an energy storage device with battery cells connected in series without active or passive balancing, in which all battery cells are charged simultaneously and at the end of the charging process all battery cells have their predetermined final charging voltage, in particular even if individual battery cells have different states of charge, capacities, internal resistances, efficiencies or states of health, whereby auxiliary charging currents and auxiliary discharging currents as well as an active equalization of the efficiencies of the battery cells can be dispensed with.

[0013] This object is achieved by a method for charging an energy storage device according to claim 1. The method is characterized in that, in an energy storage device with at least one cell block with a number J of battery cells connected in series, which can have different capacities C n , different internal resistances and / or different efficiencies with 1 ≤ n ≤ J, all J battery cells are charged with a charging current I 0 until a battery cell i with 1 ≤ i ≤ J reaches a final charging voltage U i,L predetermined for this battery cell i, and that then the charging current for all battery cells is reduced to a value I i for which the following applies: 0 A < I i < l 0 . .

[0014] Here, I i is a charging current specified for this battery cell i, at which the cell voltage Ui of the battery cell i does not rise above the final charging voltage U i,L of this battery cell i, while all battery cells continue to be charged with the charging current I i.

[0015] All reduced charging currents I i do not lead to an increase in the cell voltage U i,L of battery cell i. Nevertheless, the charging current I i ensures that the remaining battery cells in the cell block continue to be charged and their cell voltage continues to increase.

[0016] The cell voltage of all J battery cells is measured continuously during charging or at specific times. For each battery cell, the measured cell voltage is regularly compared with the specified end-of-charge voltage for that battery cell. For example, the cell voltage can be measured at regular intervals. Furthermore, the cell voltage can be measured whenever one of the battery cells has reached its specified end-of-charge voltage.

[0017] As soon as a battery cell i reaches its final charging voltage during the charging process, the charging current is reduced from I 0 to I i . The charging process is switched from constant current CC to constant cell voltage CCV, so that the cell voltage U i in this battery cell i no longer increases. In all other battery cells, charging with the reduced charging current I i leads to an increase in the cell voltage U n with 1≤n≤J and n≠i and to an increase in the state of charge.

[0018] The charging current of is reduced to a charging current I j with 1 ≤ j ≤ J and j ≠ i and I j < I i if, while charging the battery cells with the charging current I i, another battery cell j reaches its specified end-of-charge voltage before charging is terminated. This applies to the battery cell that reaches its specified end-of-charge voltage next after battery cell i and also to each further battery cell that reaches its end-of-charge voltage thereafter, as long as charging is still ongoing. Charging is only terminated when the reduced charging current is less than a specified charging current threshold value Isw.

[0019] Since the charging current is reduced from I 0 to I i when the battery cell i has reached its predetermined end-of-charge voltage U i,L , and the charging current I i is less than I 0 , it can take some time during the first charging of a cell block after charging according to the invention until all battery cells have reached their predetermined end-of-charge voltage. Since all battery cells start at their predetermined end-of-charge voltage during the subsequent discharging, the battery cells are not discharged to a DoD, which leads to disproportionate aging and premature failure of the cell block. It is assumed that during all further charging processes of this cell block the battery cells reach their predetermined end-of-charge voltage almost simultaneously or shortly after one another, so that rapid charging occurs during the further charging processes.

[0020] The method according to the invention differs from known methods in which the entire cell block is charged to a block end-of-charge voltage. This block end-of-charge voltage is generally well below a end-of-charge voltage resulting from the sum of the individual end-of-charge voltages of all cells. In the method according to the invention, the cell block is charged until all battery cells in the cell block have reached a specified end-of-charge voltage, taking into account a tolerance that is small compared to the specified end-of-charge voltage of the cells. The specified end-of-charge voltage for the J battery cells in the cell block can be the same for all battery cells. However, it is also possible to specify different end-of-charge voltages for different battery cells. The end-of-charge voltage of the cell block is also the sum of the end-of-charge voltages of the individual battery cells.

[0021] The charging current is switched off for all battery cells if the reduced charging current I i or I j is smaller than a predetermined charging current threshold value Isw. The cell voltage U n is then determined for each battery cell with 1 ≤ n ≤ J. Battery cell m with the lowest cell voltage U m = U min and battery cell I with the highest cell voltage UI = U max are then determined. With the exception of battery cell m, all other battery cells in the cell block are then discharged via resistors connected in parallel to the battery cells until the cell voltage UI of battery cell I is UI = U min . The method according to the invention is then repeated and all J battery cells are charged with the charging current I 0 until battery cell i is the first to reach its predetermined end-of-charge voltage U i,L and the charging current is reduced to I i.Reducing the cell voltage of all battery cells ensures that all battery cells start with the same or similar cell voltages during subsequent charging. This process of charging and subsequent discharging with the charging current switched off can be repeated several times. If necessary, this can be done at increasingly shorter intervals. With each cycle, the cell voltages of the cells become closer to one another. The deviation of the final charging voltage of the individual cells deviates less and less between the cells after several cycles. The process is terminated when the difference U max -U min is less than or equal to a predetermined limit ΔU 1. This limit can be, for example, 5 mV.

[0022] According to an advantageous embodiment of the invention, when the predetermined end-of-charge voltage U IL of the battery cell i is reached, the charging current is reduced to I i when charging the energy storage device, switching from constant current, also called constant current CC, to constant cell voltage of the battery cell i, also called constant cell voltage CCV. In contrast to known charging methods, the important thing is therefore to reach the end-of-charge voltage of a battery cell rather than the block end-of-charge voltage. Furthermore, when this goal is reached, it is not the block voltage that is set to constant voltage CV, but only the cell voltage of the battery cell that has already reached its predetermined end-of-charge voltage. All other battery cells continue to be charged.

[0023] According to a further advantageous embodiment of the invention, the following applies to the predetermined voltage value ΔU 1 : 0 , 01 * U n , L ≤ ΔU 1 ≤ 0 , 02 * U n , L where U n,L is the specified final charging voltage of at least one of the J battery cells.

[0024] According to a further advantageous embodiment of the invention, the following applies to the predetermined voltage value ΔU 1 : 5 mV ≤ ΔU 1 ≤ 10 mV.

[0025] According to a further advantageous embodiment of the invention, the predetermined final charging voltage U n,L is the same for all J battery cells and the following applies: U n , L = U n + 1 , L für 1 ≤ n ≤ J − 1 or in other words: U 1 , L = U 2 , L = U 3 , L = … . . = U J , L .

[0026] According to a further advantageous embodiment of the invention, the charging current threshold value I SW is the same for all battery cells.

[0027] According to a further advantageous embodiment of the invention, the reduced charging current I SW,n is predetermined for each battery cell as a function of a maximum charging current I n,max assigned to each battery cell. The following applies to the reduced charging current I SW,n : 0 , 01 I n , max ≤ I SW , n ≤ 0 , 02 I n , max mit 1 ≤ n ≤ J .

[0028] The maximum charging current I n;max of a battery cell is determined taking into account a specified C-factor. The C-factor corresponds to the quotient of the maximum charging current I n;max and the capacity C n of the battery cell in question. The maximum charging current of a battery cell and the C-factor are determined, for example, by a control and monitoring device.

[0029] According to a further advantageous embodiment of the invention, the reduced charging current I n for each battery cell corresponds to between 1% and 2% of the charging current I 0 , and the following applies: 0.01 I 0 ≤ I n ≤ 0.02 I 0 . The reduced charging current I n can be the same for all battery cells. Alternatively, the reduced charging current can be different.

[0030] According to a further advantageous embodiment of the invention, the capacity C n is determined for all J battery cells after charging the cell block with 1 ≤ n ≤ J. In this case, all J battery cells initially have the same end-of-charge voltage taking into account the tolerance ΔU1. The end-of-charge voltage for all battery cells either corresponds to their predetermined end-of-charge voltage U n,L or deviates from it by a maximum of ΔU 1 and the following applies: U n,L - ΔU 1 ≤ U n ≤ U n,L. . Starting from this end-of-charge voltage of the battery cells, all J battery cells are discharged with the discharge current I 0 '. As soon as a battery cell p with 1 ≤ p ≤ J reaches its predetermined end-of-discharge voltage U p,E, the discharge of all battery cells is stopped and the time t E from the start of the discharge to the end of the discharge is determined. Subsequently, the cell voltage U n with 1≤ n ≤ J is determined for each battery cell at time t E .For the battery cell p, which was the first to reach its specified end-of-discharge voltage U p,E, the capacity C p is determined from the time t E , the discharge current I 0 ' , the end-of-charge voltage U p,L and the cell voltage U p = U p,E. For the battery cell p, the cell voltage between the start of the discharge and the end of the discharge is specified as a voltage-time curve as a function of time. On this voltage-time curve of the battery cell p, times tn are assigned to the cell voltages U n ; which the battery cells have at time t E. From the times tn and the capacity C p, the capacity C n of all remaining J-1 battery cells is determined, which they have when they reach their specified end-of-discharge voltage.

[0031] According to a further advantageous embodiment of the invention, the cell voltage of all battery cells is recorded as a function of time during discharging. The cell voltage is stored as a function of time for all battery cells as a voltage-time curve.

[0032] According to a further advantageous embodiment of the invention, the capacity C n of the J battery cells is determined at specific intervals and stored in a memory. It is thus available for comparison or to determine the aging state of the battery cells and can be retrieved as needed.

[0033] According to a further advantageous embodiment of the invention, the initial capacity C n,initial is specified for each battery cell with 1 ≤ n ≤ J, where the initial capacity C n,initial is the capacity that the battery cell has before initial commissioning. For each battery cell, the state of health SoH of the battery cell is determined at specific time intervals from the capacity C n and the initial capacity C n,initial using the following equation: SoH = C n / C n,initial * 100. The initial capacity C n,initial can be determined using the method according to the invention immediately upon initial commissioning of the cell block or using a different method, or it can be specified by a manufacturer. The aging state of the battery cells can be determined in this way at specific time intervals or upon specific request and is therefore available at any time.The SoH state of the battery cells can be stored in a memory so that it can be retrieved at any time.

[0034] Further advantages and advantageous embodiments of the invention can be found in the following description, the drawings and the claims. drawing

[0035] The drawing shows an embodiment of the invention. It shows: Figure 1Circuit diagram of an energy storage device, Figure 2Cell voltage as a function of time for three battery cells of the cell block of the energy storage device according to Figure 1 Figure 3Voltage-time diagram of the battery cells of one of Figure 1 various energy storage systems. Description of the embodiment

[0036] In Figure 1A circuit diagram of an energy storage device 1 is shown, which serves, for example, to supply energy to a building's power grid and can be charged and discharged by a system for generating renewable energy (photovoltaic system, wind power system, biogas plant, etc.), for example, via a bidirectional AC / DC converter 100. In the illustrated embodiment, the energy storage device 1 comprises a cell block 2 with five serially connected, rechargeable battery cells 3, 4, 5, 6, 7. The number J of battery cells is thus 5: J=5. the battery cell with n=1 has the reference number 3, the battery cell with n=2 has the reference number 4, the battery cell with n=3 has the reference number 5, the battery cell with n=4 has the reference number 6, the battery cell with n=5 has the reference number 7.

[0037] Each of the battery cells 3 to 7 is equipped with a switchable resistor 8, 9, 10, 11, 12. The switchable resistor 8 of battery cell 3 is connected in parallel. The same applies to resistors 9, 10, 11, 12 and battery cells 4, 5, 6, 7. Switchable means that the resistors are connected in parallel to the battery cells for a limited period of time during charging or discharging of the cell block.

[0038] To control the charge or discharge state of the individual battery cells 3 to 7, a control and storage device 13 is provided, which is connected via corresponding data lines 14 both to the switchable resistors 8 to 12 and to the bidirectional AC / DC converter 100.

[0039] The charging of cell block 2 is described below: All battery cells 3 to 7 in cell block 2 are first charged with the charging current I 0. The cell voltage of the battery cells is continuously recorded. For each battery cell, the recorded cell voltage U n is compared with the specified end-of-charge voltage U n,L of this battery cell. The specified end-of-charge voltage U n,L is the same for all battery cells 3 to 7 and amounts to 4.2 V. The following applies: U 1,L = U 2,L = U 3,L = U 4,L = U 5,L = 4.2 V. The charging of all battery cells with the charging current I 0 continues until one of the battery cells 3 to 7 is the first to reach its specified end-of-charge voltage. In the present exemplary embodiment, this is the battery cell with n=2 and reference number 4. The battery cell i, which is the first to reach its specified end-of-charge voltage, is therefore battery cell 4 and i=2 applies. Now the charging current is reduced to I 2, where: 0 A < I 2 < I 0 .

[0040] Subsequently, all battery cells 3 to 7 are charged with the charging current I 2 until the next battery cell reaches its specified end-of-charge voltage. In the present embodiment, this is the battery cell with n = 1 and reference number 3. The second battery cell to reach its specified end-of-charge voltage is battery cell 3. The charging current is now reduced to I 1 , where: 0 A < I 1 ≪ I 2 < I 0 .

[0041] Subsequently, by recording the cell voltages U n and comparing them with the specified end-of-charge voltages U n,L of the battery cells, it is determined that the remaining battery cells with reference numbers 5, 6, and 7 already have a cell voltage that differs from their specified end-of-charge voltage by a maximum of the tolerance ΔU 1 = 5 mV. Charging is terminated. The block end-of-charge voltage of the cell block is 5*4.2 V = 21 V. This also applies when taking into account the tolerance ΔU 1 = 5 mV for the end-of-charge voltage of each of the battery cells.

[0042] If it is determined that the reduced charging current I 1 is less than a predefined charging current threshold value Isw, the charging current is switched off. Then, the battery cell with the lowest cell voltage is determined. In this case, this is battery cell 6 with n=4. Furthermore, the battery cell with the highest cell voltage is determined. In this case, this is battery cell 4 with n=2. U 4 < U 5 < U 3 < U 1 < U 2 .

[0043] Battery cells 3, 4, 5, and 7 are then discharged via their assigned switchable resistors 8, 9, 10, and 12 with the charging current switched off until their cell voltage matches cell voltage U4. The charging process is then repeated with I0 as described above. Since all battery cells now start at the same cell voltage when charging is resumed, it is assumed that the specified final charging voltages are reached as described above.

[0044] The capacities Cn of battery cells 3 to 7 are determined as follows: Initially, cell block 2 is charged, with all battery cells 3 to 7 having their specified end-of-charge voltage U n = 4.2 V with 1≤n≤5. Cell block 2 is then discharged with the discharge current I 0 '. The discharge current I 0 ' flows through the series-connected battery cells 3 to 7. During discharging, the cell voltage U n of battery cells 3 to 7 is determined continuously or at specified intervals and compared with the end-of-discharge voltage U n,E specified for each of the battery cells. If it is determined that at least one of the battery cells 3 to 7 has reached its specified end-of-discharge voltage, the following occurs: the discharge of cell block 2 is terminated so that no more discharge current flows, the time t E is determined which has elapsed from the start of the discharge to the end of the discharge, for each battery cell the cell voltage U n (t E ) is determined which the battery cells 3 to 7 have at the time t E, for the battery cell 5 with n=3, which is the first to reach its predetermined end-of-discharge voltage, the capacity C 3 of the battery cell 5 is determined from the time t E, the discharge current I 0 ', the end-of-charge voltage U 3,L of the battery cell 5 and the end-of-discharge voltage U 3 (t E ) = U 3,E, for the battery cell 5 with n=3 a voltage-time curve is created by plotting the cell voltage as a function of time during the discharge from t=0 sec to t= t E, on this voltage-time curve the cell voltages U n (t E ) of all Battery cells except battery cell 5 are assigned times tn, for which U n (t E ) = U 3 (tn ),From the times tn and the capacity C 3, the capacity of the battery cells 3, 4, 6, 7 is determined when their specified final discharge voltage is reached.

[0045] In Figure 2the voltage-time curves U n (t) of the battery cells 3, 4 and 5 with n = 1, 2 and 3 are shown during the discharging of the cell block 2 with the discharge current I 0 '. The voltage-time curves of the battery cells 6 and 7 with n = 4 and n = 5 are not shown for the sake of simplicity and clarity. The battery cell 5 with n = 3 is the first to reach its specified final discharge voltage U 3,E at time t = t E . At this time, the battery cell 3 with n = 1 has the cell voltage U 1 (t E ) and the battery cell 4 with n = 2 has the cell voltage U 2 (t E ), where U 3,E < U 1 (t E ) < U 2 (t E ). The two battery cells 3, 4 with n=1 and n=2 have not yet reached their final discharge voltages U 1,E and U 2,E at time t E . Using the voltage-time curve U 3 (t), times t 1 and t 2 are assigned to the cell voltages U 1 (t E ) and U 2 (t E ), so that: U 1 t E = U 3 t 1 and U 2 t E = U 3 t 2 .

[0046] From the voltages U 3 (t 1 ) and U 3 (t 2 ), the times t 1 , t 2 and t E and the already determined capacity C 3 , the capacities C 1 and C 2 are then calculated which the battery cells 3, 4 with n=1 and n=2 have when reaching their predetermined final discharge voltage U 1,E and U 2,E. It is assumed that the voltage-time curves of the battery cells 3, 4 qualitatively correspond to the voltage-time curve of the battery cell 5.

[0047] The capacities of the battery cells 6, 7 are determined accordingly.

[0048] The capacities C n determined in this way with 1 ≤ n ≤ 5 are also called useful capacities.

[0049] The capacities C 1 , C 2 , C 3 , C 4 , and C 5 are stored in a memory not shown in the drawing. This memory also stores the capacities C n,initial that each of the five battery cells had before initial commissioning. This initial capacity is specified for each of the battery cells. Using the equation SoH = C n / C n,initial * 100, the aging state SoH is determined and stored for each of the five battery cells.

[0050] Figure 3 shows two cycles of discharging and subsequent charging of a cell block with several cells connected in series. The number of cells is larger than in the Figure 1 shown cell block 2. However, the structure of the cell block is otherwise the same as in Figure 1 shown cell block 2. In the diagram according to Figure 3the cell voltage U is plotted for the individual cells of the cell block as a function of time t. At time t0, charging is stopped in a first cycle and the charging current I0 is switched off: I0 =0A. It is assumed that at least one cell has previously reached its specified end-of-charge voltage Un,L, that the charging current was then reduced at least once and that the reduced charging current Ii or Ij is less than a specified charging current threshold. At time t0, cell m with the lowest cell voltage has the voltage Um = Umin and cell I with the highest cell voltage has the voltage UI = Umax. Since this is the lowest cell voltage in the first cycle, it is referred to as Umin,1. From time t0, all other cells with the exception of cell m are discharged via resistors connected in parallel with the cells with the charging current switched off.The resistors connected in parallel to the cells are as shown in . Figure 1. The cells are discharged from t 0 to t 1 . During this time, the cell voltage drops. Discharge ends at time t 1 when the cell voltage of the cell corresponds to IU min,1: UI = U min,1 . At this time, the cell voltage U n of the cells is in a range U n ≤ U min,1 . At time t 1 , the charging current I 0 is switched on again and the cells are charged again until time t 2 , at which at least one cell has again reached its end-of-charge voltage, the charging current has been reduced at least once, and the reduced charging current I i or I j is less than a predetermined charging current threshold value Isw. At time t 2 , the charging current is switched off again. The first cycle, which lasts from t 0 to t 2 , is complete. At time t 2 , the cell with the lowest cell voltage has the voltage U min,2 . This voltage is greater than the cell voltage U min,1 at the beginning of the first cycle. U min,2 > U min,1 .From time t 2 to time t 3 , all the cells except for the cell with the lowest cell voltage are discharged again via the resistors connected in parallel with the charging current switched off, until time t 3 , when the cell voltage of the cell corresponds to IU min,2: UI = U min,2 . The charging current is then switched on again and the cells are charged again until time t 4 . At time t 4 , the cell with the lowest cell voltage has the voltage U min,3 . This voltage is greater than the cell voltages U min,2 and U min,1 at the beginning of the first and second cycles. U min,3 > U min,2 > U min,1 . The second cycle lasts from t 2 to t 4 . The representation according to . Figure 3 shows the following: The cell voltage U min increases from cycle to cycle, so that the difference between the final charge voltage U max of the cell with the highest cell voltage and U min becomes smaller and smaller. The cycle repetition can be stopped when the cell voltages U max and U min differ by less than a specified value within a cycle. The duration of the cycles becomes shorter because U min increases.

[0051] All features of the invention can be essential to the invention both individually and in any combination with one another. Reference numbers

[0052] 1Energy storage 2Cell block 3Battery cell 4Battery cell 5Battery cell 6Battery cell 7Battery cell 8Switchable resistor 9Switchable resistor 10Switchable resistor 11Switchable resistor 12Switchable resistor 13Control and storage device 14Data line 100Bidirectional AC / DC converter

Claims

1. Method for charging an energy store (1) with a charging current, wherein the energy store (1) comprises at least one cell block (2) having a number J of series-connected battery cells (3, 4, 5, 6, 7), of which at least some of the battery cells (3, 4, 5, 6, 7) may have different capacities Cn, different internal resistances and / or different efficiencies, where 1 ≤ n ≤ J, with the following method steps: a) charging all J battery cells (3, 4, 5, 6, 7) with a charging current I0, b) recording the cell voltage Un of all J battery cells (3, 4, 5, 6, 7), whereby the recording is carried out continuously or at predetermined time intervals, c) comparing the recorded cell voltage Un of each battery cell (3, 4, 5, 6, 7) with the predetermined end-of-charge voltage Un,L specified for this battery cell, d) as soon as a battery cell i (3, 4, 5, 6, 7) with 1≤ i ≤ J reaches a predetermined end-of-charge voltage Ui,L for this battery cell i (3, 4, 5, 6, 7), the charging current for all battery cells (3, 4, 5, 6, 7) is reduced to a value Ii for which the following applies 0 A < I i < I 0 whereby Ii is a predetermined charging current for this battery cell i (3, 4, 5, 6, 7), at which the cell voltage Ui of battery cell i (3, 4, 5, 6, 7) does not rise above the end-of-charge voltage Ui,L, while all battery cells (3, 4, 5, 6, 7) continue to be charged with the charging current Ii, e) as soon as during charging of the battery cells (3, 4, 5, 6, 7) with the charging current Ii, another battery cell j reaches its predetermined end-of-charge voltage, the charging current is further reduced to a charging current Ij with 1 ≤ j ≤ J and j ≠ i and Ij < Ii, f) as soon as the reduced charging current Ii or Ij is less than a specified charging current threshold value Isw, the charging current is switched off for all battery cells (3, 4, 5, 6, 7) and the cell voltage Un is determined for each battery cell (3, 4, 5, 6, 7) with 1 ≤ n ≤ J, g) the battery cell m (3, 4, 5, 6, 7) with the lowest cell voltage Um =Umin is determined and the battery cell I (3, 4, 5, 6, 7) with the highest cell voltage UI =Umax is determined, h) with exception of the battery cell m (3, 4, 5, 6, 7), all other battery cells (3, 4, 5, 6, 7) are discharged via resistors (8, 9, 10, 11, 12) which are connected in parallel with the battery cells (3, 4, 5, 6, 7) until for the cell voltage UI the following applies: UI = Umin whereby the steps a) to h) are repeated in cycles until the difference Umax - Umin is less than or equal to a predetermined limit value ΔU1 and Umax - Umin ≤ ΔU1 applies.

2. Method according to claim 1, characterized in that when the predetermined end-of-charge voltage UiL of the battery cell i (3, 4, 5, 6, 7) is reached through the reduction of the charging current to Ii during charging of the energy store (1), a switchover from constant current, also referred to as constant currant CC, to constant cell voltage of the battery cell i (3, 4, 5, 6, 7), also referred to as constant cell voltage CCV, occurs.

3. Method according to one of the preceding claims, characterized in that for the predetermined voltage value ΔU1 the following applies: 5mV ≤ ΔU1 ≤ 10mV.

4. Method according to one of the preceding claims, characterized in that the predetermined end-of-charge voltage Un,L is the same for all J battery cells (3, 4, 5, 6, 7) and the following applies: Un,L = Un+1,L for 1≤ n ≤ J-1.

5. Method according to one of the preceding claims, characterized in that the charging current threshold Isw is the same for all battery cells.

6. Method according to one of claims 1 to 4, characterized in that for each battery cell (3, 4, 5, 6, 7) the charging current threshold value ISW,n is predetermined as a function of a maximum charging current In,max assigned to each battery cell, and that for the charging current threshold value ISW,n the following applies: 0.01 In,max ≤ ISW,n ≤ 0.02 In,max with 1 ≤ n ≤ J.

7. Method according to one of the preceding claims, characterized in that for each battery cell (3, 4, 5, 6, 7) the reduced charging current In is between 1% and 2% of the charging current I0 the following applies: 0.01 I0 ≤ In ≤ 0.02 I0.

8. Method according to one of the preceding claims, characterized in that for all J battery cells (3, 4, 5, 6, 7) after charging the cell block (2), all J battery cells (3, 4, 5, 6, 7) having the same end-of-charge voltage Un,L, the capacity Cn with 1 ≤ n ≤ J is determined as follows: - all J battery cells (3, 4, 5, 6, 7) are discharged with the discharge current I0', - as soon as a battery cell p (3, 4, 5, 6, 7) with 1 ≤ p ≤ J reaches its specified end-of-discharge voltage Up,E, the discharge of all battery cells (3, 4, 5, 6, 7) is stopped and the time tE from the start of discharge to the end of discharge is determined, - then the cell voltage Un (tE) is determined for each battery cell (3, 4, 5, 6, 7) with 1 ≤ n ≤ J at time tE, - for the battery cell p (3, 4, 5, 6, 7), which is the first to reach its specified end-of-discharge voltage Up,E, the capacity Cp is determined from the time tE, the discharge current I0', the end-of-charge voltage Up,L and the cell voltage Up (tE) =Up,E, - for the battery cell p (3, 4, 5, 6, 7), the cell voltage between the start of discharge and the end of discharge is given as a voltage-time curve as a function of time, - on this voltage-time curve, the cell voltages Un (tE) of all battery cells are assigned times tn, for which Un (tE) =Up (tn) applies, - from the times tn and the capacity Cp, the capacity Cn of all the remaining J-1 battery cells (3, 4, 5, 6, 7) is determined, which they have when their predetermined end-of-discharge voltage Un,E is reached.

9. Method according to claim 8, characterized in that the cell voltage Un of all J battery cells (3, 4, 5, 6, 7) is recorded during discharge, whereby the recording is carried out continuously or at predetermined time intervals, and in that the recorded cell voltage Un of each battery cell (3, 4, 5, 6, 7) is compared with the predetermined end-of-discharge voltage Un,E for this battery cell.

10. Method according to claim 8 or 9, characterized in that the cell voltage is recorded as a function of time for all battery cells (3, 4, 5, 6, 7) during discharging, and in that the cell voltage is stored as a function of time as a voltage-time curve for all battery cells (3, 4, 5, 6, 7).

11. Method according to one of claims 8 to 10, characterized in that the capacity Cn of the J battery cells is determined at specific time intervals and is stored in a memory.

12. Method according to one of claims 8 to 11, characterized in that for each battery cell its initial capacity Cn,initial is predetermined as 1 ≤ n ≤ J, whereby the initial capacity Cn,initial is the capacity which the battery cell has before initial commissioning, and that for each battery cell the state of health SoH of the battery cell is determined at specific time intervals from the capacity Cn and the initial capacity Cn,initial with SoH = Cn / Cn,initial * 100.