Patent Application
20250149904
The invention relates to a method for charging a rechargeable energy storage, the energy storage includes at least one cell block with a number J of series-connected battery cells.
A rechargeable energy storage including a plurality of galvanic cells connected in series and/or in parallel, referred to as battery cells. When these battery cells are discharged, the stored chemical energy is converted into electrical energy. This electrical energy can be used by an off-grid consumer, such as an electric vehicle. In addition, the electrical energy from the rechargeable energy storage can be used by a consumer that is connected to a power grid to bridge an interruption in the power supplied from the power grid. The rechargeable energy storage equipped with rechargeable battery cells is recharged after a discharge to be available for reuse.
In the case of rechargeable energy storages (accumulators) consisting of several rechargeable battery cells connected in series, it is important for the service life of the energy storage that each individual cell is neither overcharged when the rechargeable energy storage is charged, nor discharged too deeply when discharged, and that all cells have the same state of charge as far as possible. This applies in particular to rechargeable energy storages such as of several lithium-ion batteries, lithium-polymer batteries and/or lithium-iron-phosphate batteries connected in series.
When battery cells are connected in series, the same charging current flows through all of the battery cells. Because battery cells may have different internal resistances or efficiencies, the power dissipated by each battery cell during charging varies. This means that after charging, not all battery cells have the same capacity available for subsequent discharging. This capacity is also called usable capacity. The battery cell with the highest internal resistance or poorest efficiency has the lowest usable capacity after the cell block has been charged because it has the highest power dissipation. As a result, this cell will be discharged the furthest during the subsequent discharge. It is therefore the cell with the highest Depth of Discharge (DoD) in the cell block. However, the higher the DoD of a cell, the shorter the life of that cell. This leads to disproportionate aging of that one cell and thus to premature failure of the cell block.
To determine the respective state of charge of a battery cell, its respective cell voltage can be measured and then, if necessary, charge equalization can be performed between the battery cells of the cell block if the battery cells have different states of charge when the cell voltage exceeds or falls below certain cell voltage values. However, the problem is that the cell voltage remains largely constant, at least in sections, during the respective charging process of a battery cell, so that it is difficult to deduce the current state of charge of the corresponding battery cell from the cell voltage. Only shortly before the respective end-of-charge or end-of-discharge voltage is reached, there is a relatively sharp rise or fall in the respective cell voltage, which can be used for corresponding control processes for charge equalization. However, these equalization processes take a long time. During this time, the battery block cannot be used for normal operation.
Such rechargeable energy storages are therefore usually connected to a device, often referred to as a battery management system, which on the one hand constantly monitors the state of charge of the individual battery cells by means of a charge controller and on the other hand attempts to equalize them if the state of charge of the individual battery cells differs. The equalization of the state of charge of the battery cells, also known as balancing, can be achieved by passive or active balancing. In addition, with known battery management systems, charge equalization does not begin until at least one of the battery cells is fully charged, making the entire charging process of a cell block relatively time-consuming.
In passive equalization, the excess energy is converted to heat via a resistor in the battery cell that has reached its end-of-charge voltage first and is therefore lost to the charging process.
In active balancing, the energy extracted from a cell with too high voltage is not converted into thermal energy, but is used to charge the other cells in the rechargeable energy storage. However, even with active balancing, charge equalization does not begin until at least one of the battery cells in the cell block has reached its end-of-charge voltage.
DE 10 2017 009 850 A1 discloses a method for charging and discharging a rechargeable energy storage with at least one cell block including several 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 and the capacity CN of each of the battery cells. During a predetermined time t, which is less than or equal to the reciprocal of the C-factor, all the battery cells are charged simultaneously with the maximum charging currents IN;max assigned to them. The difference between the available charging current I0 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. Discharge takes place accordingly.
DE 10 2019 129 415 B3 discloses a method for charging and/or discharging a rechargeable energy storage with a current I0, wherein the rechargeable energy storage includes 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 nu with 1≤N≤J. The battery cell with the worst efficiency ηmin is determined first. The efficiencies ηN of all other battery cells are then adjusted to this worst efficiency ηmin such that the adjusted efficiency ηN′ of the battery cells is: ηN′=ηmin.
The object of the invention is to provide a method for charging and discharging a rechargeable energy storage with battery cells connected in series without active or passive balancing, in which all battery cells are charged simultaneously and all battery cells have their predetermined end-of-charge voltage at the end of the charging process, in particular even if individual battery cells have different charge states, capacities, internal resistances, efficiencies or states of health, whereby auxiliary charging currents and auxiliary discharging currents as well as active equalization of the efficiencies of the battery cells can be dispensed with. This object is achieved by a method for charging a rechargeable energy storage including a rechargeable energy storage with a charging current, wherein the rechargeable energy storage includes at least one cell block including a number J of series-connected battery cells. At least some of the battery cells have different capacities Cn, different internal resistances and/or different efficiencies, wherein 1≤n≤ J. The method includes a) charging all J battery cells with a charging current I0, b) recording cell voltage Un of all J battery cells wherein the recording is carried out continuously or at predetermined time intervals, c) comparing the recorded cell voltage Un of each battery cell with the predetermined end-of-charge voltage Un,L specified for this battery cell, d) as soon as a battery cell i with 1≤i≤J reaches a predetermined end-of-charge voltage Ui,L for this battery cell i, the charging current for all battery cells is reduced to a value Ii for which the following applies 0A<Ii<I0, whereby Ii is a predetermined charging current for this battery cell i at which the cell voltage Ui of battery cell i does not rise above the end-of-charge voltage Ui,L, while all battery cells continue to be charged with the charging current Ii, e) as soon as during charging of the battery cells 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 Ii<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 and the cell voltage Un is determined for each battery cell with 1≤n≤J, g) the battery cell m with the lowest cell voltage Um=Umin is determined and the battery cell I with the highest cell voltage UI=Umax is determined, h) with exception of the battery cell m, all other battery cells are discharged via resistors which are connected in parallel with the battery cells until for the cell voltage UI the following applies: UI=Umin, wherein 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.
The method is characterized by a rechargeable energy storage comprising at least one cell block with a number J of series-connected battery cells which may have different capacities Cn, different internal resistances and/or different efficiencies, where 1≤n≤J, whereby all J battery cells are charged with a charging current I0 until a battery cell i, where 1≤i≤J, reaches an end-of-charge voltage Ui,L which is predetermined for this battery cell i. The charging current is then reduced for all battery cells to a value Ii for which the following applies 0A<Ii<I0. Ii is a predetermined charging current for this battery cell i at which the cell voltage Ui of the battery cell i does not rise above the end-of-charge voltage Ui,L of this battery cell i, while all battery cells continue to be charged with the charging current Ii.
All reduced charging currents Ii do not result in an increase in the cell voltage Ui,L of battery cell i. Nevertheless, the charging current Ii ensures that the remaining battery cells in the cell block continue to be charged and their cell voltage continues to increase.
The cell voltage of all J battery cells is measured continuously or at specific times during the charge. For each battery cell, the recorded cell voltage is periodically compared with the end-of-charge voltage predetermined for the battery cell. For example, the cell voltage can be determined at regular intervals. In addition, the cell voltage can be determined whenever one of the battery cells reaches its predetermined end-of-charge voltage.
As soon as charging a battery cell i reaches its end-of-charge voltage, the charging current is reduced from I0 to Ii. The charging is switched from constant current CC to constant cell voltage CCV so that the cell voltage Ui in this battery cell i does not increase any further. In all other battery cells, charging with the reduced charging current Ii will result in an increase in the cell voltage Un with 1≤n≤J and n≠i and an increase in the state of charge.
The charging current Ii is reduced to a charging current Ij with 1≤j≤J and j≠i and Ii<Ii, when during charging the battery cells with the charging current Ii another battery cell j reaches its predetermined end-of-charge voltage before charging is completed. This applies to the battery cell which is the next to reach its predetermined end-of-charge voltage after battery cell i, and also to each additional battery cell which reaches its end-of-charge voltage thereafter, as long as charging continues. Charging is terminated only when the reduced charging current is less than a specified charging current threshold value ISW.
Since the charging current is reduced from I0 to Ii when the battery cell i has reached its predetermined end-of-charge voltage Ui,L and the charging current Ii is less than I0, during the first charging of a cell block after charging according to the invention, it may take some time for all battery cells to reach their predetermined end-of-charge voltage. Since all battery cells start at their predetermined end-of-charge voltage during subsequent discharging, the battery cells are not discharged to a DoD that leads to disproportionate aging and premature failure of the cell block. It is assumed that in all further charging processes of this cell block the battery cells will reach their predetermined end-of-charge voltage almost simultaneously or shortly one after the other, so that rapid charging will take place in the further charging processes.
The method according to the invention differs from known methods in which the entire block of cells is charged to a block end-of-charge voltage. This block end-of-charge voltage is generally significantly lower than an 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 of the cell block have reached a predetermined end-of-charge voltage for them, taking into account a tolerance which is small compared to the predetermined end-of-charge voltage of the cells. The predetermined end-of-charge voltage for the J battery cells in the cell block may be the same for all the battery cells. However, it is also possible to specify different end-of-charge voltages for different battery cells. Again, the end-of-charge voltage of the cell block is the sum of the end-of-charge voltages of the individual battery cells.
The charging current is switched off for all battery cells if the reduced charging current Ii or Ii is less than a predetermined charging current threshold value ISW. Then the cell voltage Un is determined for each battery cell with 1≤n≤J. Then the battery cell m with the lowest cell voltage Um=Umin and the battery cell I with the highest cell voltage UI=Umax are determined. With the exception of battery cell m, all other battery cells in the cell block are then discharged via resistors connected in parallel with the battery cells until UI=Umin applies to the cell voltage Ui of battery cell I. The method according to the invention is then repeated and all J battery cells are charged with the charging current I0 until battery cell i is the first to reach its predetermined end-of-charge voltage Ui,L and the charging current is reduced to Ii. Lowering 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 then discharging with the charging current turned 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 move closer together. After several cycles the difference between the end-of-charge voltages of the individual cells becomes smaller and smaller. The process is terminated when the difference Umax−Umin is less than or equal to a predefined limit ΔU1. This limit can be, for example, 5 mV.
According to an advantageous embodiment of the invention, when the predetermined end-of-charge voltage UiL of the battery cell i is reached, the reduction of the charging current to Ii during the charging of the rechargeable energy storage switches from constant current, also referred to as constant current CC, to constant cell voltage of the battery cell i, also referred to as constant cell voltage CCV. In contrast to known charging methods, it is therefore important to reach the end-of-charge voltage of a battery cell instead of the block end-of-charge voltage, and when this goal is reached, the block voltage is not 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.
According to a further advantageous embodiment of the invention, for the predetermined voltage value ΔU1 the following applies: 0.01 Un,L≤ΔU1≤0.02*Un,L where Un,L is the predetermined end-of-charge voltage of at least one of the J battery cells.
According to a further advantageous embodiment of the invention, for the predetermined voltage value ΔU1 the following applies: 5 mV≤ΔU1≤10 mV.
According to a further advantageous embodiment of the invention, the predetermined end-of-charge voltage Un,L is the same for all J battery cells, and the following applies Un,L=Un+1,L for 1≤n≤J−1 or in other words U1,L=U2,L=U3,L= . . . =UJ,L.
According to a further advantageous embodiment of the invention, the charging current threshold value ISW is the same for all battery cells.
According to a further advantageous embodiment of the invention, the reduced charging current ISW,n is predetermined for each battery cell as a function of a maximum charging current In,max assigned to each battery cell, where the following applies to the reduced charging current ISW,n: 0.01 In, max≤ISW,n≤0.02 In,max with 1≤n≤J.
The maximum charging current In;max of a battery cell is determined taking into account a predetermined C-factor. The C-factor is the quotient of the maximum charging current In;max and the capacity Cn of the battery cell in question. The maximum charging current of a battery cell and the C-factor are determined, for example, by a monitoring and control device.
According to a further advantageous embodiment of the invention, the reduced charging current In for each battery cell is between 1% and 2% of the charging current I0, and the following applies 0.01 I0≤In≤0.02 I0. The reduced charging current In can be the same for all battery cells. Alternatively, the reduced charging current may be different.
According to a further advantageous embodiment of the invention, the capacity Cn for all J battery cells is determined after the cell block has been charged to be 1≤n≤J. Initially, all J battery cells have the same end-of-charge voltage, taking into account the tolerance ΔU1. The end-of-charge voltage of all battery cells is either equal to their predetermined end-of-charge voltage Un,L or differs from it by a maximum of ΔU1, and the following applies Un,L-ΔU1≤Un≤Un,L. Starting from this end-of-charge voltage of the battery cells, all J battery cells are discharged with the discharging current I0′. As soon as a battery cell p with 1≤p≤J reaches its specified end-of-discharge voltage Up,E, the discharge of all battery cells is stopped and the time tE from the start of the discharge to the end of the discharge is determined. The cell voltage Un is then determined for each battery cell with 1≤n≤J at time tE. For the battery cell p which was the first to reach its predetermined end-of-discharge voltage Up,E, the time tE, the discharging current I0′, the end-of-charge voltage Up,L, the cell voltage Up=Up,E and the capacity Cp is determined. For the battery cell p, 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 of the battery cell p, the times tn are assigned to the cell voltages Un; which the battery cells have at the time tE. From the times tn and the capacity Cp, the capacity Cn of all remaining J−1 battery cells is determined, which they will have when they reach their predetermined end-of-discharge voltage.
According to another advantageous embodiment of the invention, the cell voltage is recorded as a function of time for all battery cells during discharge. For all battery cells, the cell voltage as a function of time is stored as a voltage-time curve.
According to a further advantageous embodiment of the invention, the capacity Cn of the J battery cells is determined at specific time intervals and stored in a memory. It is thus available for comparisons or for determining the aging state of the battery cells and can be retrieved as required.
According to a further advantageous embodiment of the invention, the initial capacity Cn,initial for each battery cell is predetermined as 1≤n≤J, where the initial capacity Cn,initial is the capacity that the battery cell has before the initial commissioning. 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 using the following equation SoH=Cn/Cn,initial*100. The initial capacity Cn,initial can be determined by the method according to the invention directly during the initial commissioning of the cell block or by another method, or it can be predetermined by a manufacturer. The aging state of the battery cells can thus be determined at specific intervals or on specific request and is thus 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.
Further advantages and advantageous embodiments of the invention are set forth in the following description, the drawing and the claims.
The invention is now described based on an advantageous embodiment with reference to drawing figures, wherein:
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, and the battery cell with n=5 has the reference number 7.
Each of the battery cells 3 to 7 is provided with a switchable resistor 8, 9, 10, 11, 12. The 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 with the battery cells for a limited period of time during charging or discharging of the cell block.
A control and memory device 13 is provided for monitoring the charge or discharge status of the individual battery cells 3 to 7, which is connected via corresponding data lines 14 both to the switchable resistors 8 to 12 and to the bidirectional AC/DC converter 100.
The charging of cell block 2 is described below:
All battery cells 3 to 7 in the cell block 2 are initially charged with the charging current I0. The cell voltage of the battery cells is continuously recorded. For each battery cell, the recorded cell voltage Un is compared with the predetermined end-of-charge voltage Un,L of this battery cell. The predetermined end-of-charge voltage Un,L is the same for all battery cells 3 to 7 and is 4.2 V. The following applies: U1,L=U2,L=U3,L=U4,L=U5,L=4.2 V. Charging of all battery cells with the charging current I0 will continue until one of the battery cells 3 to 7 is the first to reach its predetermined end-of-charge voltage. In this example this is the cell with n=2 and reference number 4. The first cell i to reach its predetermined end-of-charge voltage is therefore cell 4 and i=2. The charging current is now reduced to 12, where 0 A<12<I0. Then, all cells 3 through 7 are charged with the charging current 12 until the next cell reaches its predetermined end-of-charge voltage. In this example, this is the cell with n=1 and reference number 3. Battery cell j, the second to reach its predetermined end-of-charge voltage, is battery cell 3. The charging current is now reduced to I1, where 0A<11<<12<I0.
When the cell voltages Un are recorded and the cell voltages Un are compared with the predetermined end-of-charge voltages Un,L of the battery cells, it is found that the remaining battery cells with reference numbers 5, 6, 7 already have a cell voltage which differs from their predetermined end-of-charge voltage by a maximum tolerance of ΔU1=5 mV. Charging is terminated. The block end-of-charge voltage of the cell block is 5*4.2 V=21 V. This is also true taking into account the tolerance ΔU1=5 mV for the end-of-charge voltage of each of the battery cells.
If it is determined that the reduced charging current I1 is less than a predetermined charging current threshold value ISW, the charging current is turned off. The battery cell with the lowest cell voltage is then determined. In this case it is cell 6 with n=4. The battery cell with the highest cell voltage is also determined. In this case it is cell 4 with n=2. The following applies: U4<U5<U3<U1<U2.
Then, battery cells 3, 4, 5 and 7 are discharged with the charging current switched off via the switchable resistors 8, 9, 10, 12 assigned to them, until their cell voltage is equal to the cell voltage U4. The charging process is then repeated with I0 as described above. Since all the battery cells now start with the same cell voltage when charging is resumed, it is assumed that the predetermined end-of-charge voltages have been reached, as described above.
The capacities Cn of cells 3 to 7 are determined as follows: First, cell block 2 is charged, with all battery cells 3 to 7 having their predetermined end-of-charge voltage Un=4.2 V with 1≤n≤5. Cell block 2 is then discharged with the discharging current I0′. The discharging current I0′ flows through the battery cells 3 to 7 connected in series. During the discharge, the cell voltage Un of the battery cells 3 to 7 is determined continuously or at predetermined intervals and compared with the end-of-discharge voltage Un,E predetermined for each of the battery cells. If it is determined that at least one of the battery cells 3 to 7 has reached its predetermined end-of-discharge voltage, the following occurs: the discharge of the cell block 2 is terminated so that no more discharging current flows, the time tE elapsing from the start of the discharge to the end of the discharge is determined, the cell voltage Un(tE) which the battery cells 3 to 7 have at the time tE is determined for each battery cell, the capacity C3 of the battery cell 5 is determined from the time tE, the discharging current I0′, the end-of-charge voltage U3,L of the battery cell 5 and the end-of-discharge voltage U3(tE)=U3,E for the battery cell 5 with n=3 which was the first to reach its predetermined end-of-discharge voltage, a voltage-time curve is generated for battery cell 5 with n=3 by plotting the cell voltage as a function of time during the discharge from t=0 sec to t=tE, on this voltage-time curve, the cell voltages Un(tE) of all battery cells except battery cell 5 are assigned times tn, for which Un(tE)=U3(tn), the times tn and the capacity C3 are used to determine the capacity for the battery cells 3, 4, 6, 7 which the battery cells 3, 4, 6, 7 will have when they reach their predetermined end-of-discharge voltage.
The voltages U3(t1) and U3(t2), the times t1, t2 and tE and the capacity C3 already determined are then used to calculate the capacities C1 and C2 that the battery cells 3, 4 will have with n=1 and n=2 when their predetermined end-of-discharge voltage U1,E and U2,E is reached. It is assumed that the voltage-time curves of battery cells 3, 4 are qualitatively identical to the voltage-time curve of battery cell 5.
The capacities of battery cells 6, 7 are determined accordingly.
The capacities Cn determined in this way, where 1≤n≤5, are also referred to as useable capacities.
The capacities C1, C2, C3, C4 and C5 are stored in a memory not shown in the drawing. Also stored in this memory are the capacities Cn,initial, which each of the five battery cells had before initial commissioning. This initial capacity is predetermined for each of the battery cells. Using the equation SoH=Cn/Cn,initial*100, the state of health SoH is determined and stored for each of the five battery cells.
Although several embodiments of the present invention and its advantages have been described in detail, it should be understood that changes, substitutions, transformations, modifications, variations, permutations and alterations may be made therein without departing from the teachings of the present invention, the spirit and the scope of the invention being set forth by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| DE102022109869.9 | Apr 2022 | DE | national |
This application is a continuation of international patent application PCT/DE2023/100298 filed on Apr. 25, 2023, claiming priority from German patent application DE 10 2022 869.9 filed on Apr. 25, 2022, both of which are incorporated in their entirety by this reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/DE2023/100298 | Apr 2023 | WO |
| Child | 18925478 | US |
