Abstract
A method for operating an energy storage system, which includes at least one energy store with a plurality of cells and is designed to supply an electric drive system of a vehicle is provided. The method includes identifying a reference cell from among the cells, and carrying out a first symmetrization procedure for the cells at a first point in time, at which the reference cell has a first reference charge state. The method also includes carrying out a second symmetrization procedure for the cells, if the following conditions a) and b) are met at a second point in time following the first point in time: a) the voltage difference between the voltage of the cell with the lowest voltage and the voltage of the cell with the highest voltage is greater than or equal to a specified voltage difference; and b) the reference state of charge of the reference cell at the second point in time lies within a specified state of charge range, the state of charge range being determined in such a way that it includes the first reference state of charge.
Claims
1. A method for operating an energy storage system, which includes at least one energy store with a multiplicity of cells and which is designed to power an electric drive of a vehicle, the method comprising the acts of: carrying out a selection procedure of a reference cell from the cells; carrying out a first balancing procedure of the cells at a first point in time, at which the reference cell has a first reference state of charge; and carrying out a second balancing procedure of the cells if the following conditions a) and b) are met at a second point in time following the first point in time: a) a voltage difference between a voltage of a first cell with the lowest voltage and a voltage of a second cell with the highest voltage is greater or equal to a specified voltage difference; and b) a reference state of charge of the reference cell at the second point in time is within a state-of-charge range, wherein the state-of-charge range is determined in such a way that it contains the first reference state of charge, the second balancing procedure of the cells is carried out up until a predetermined maximum time duration is reached, which extends from the first point in time up to a predetermined maximum point in time if the conditions a) and b) are met, and upon reaching the predetermined maximum point in time, the second balancing procedure of the cells is carried out if the conditions a) and b) are met or if only the condition a) is met.
2. The method according to claim 1, wherein the second balancing procedure of the cells is then carried out if the conditions a) and b) are met at the second point in time or if the conditions a) and c) are met at the second point in time, wherein the condition c) is defined as follows: c) the reference state of charge of the reference cell at the second point in time is greater than the state-of-charge range, and from a balancing assessment procedure, it results that a balancing quality of the second balancing procedure for the reference state of charge of the reference cell at the second point in time increases in comparison to a balancing quality of the second balancing procedure for a reference state of charge within the state-of-charge range.
3. The method according to claim 2, wherein the first balancing procedure and the second balancing procedure are carried out during a resting phase of the energy storage system.
4. The method according to claim 3, wherein the predetermined maximum time duration is defined depending on a number of resting phases.
5. The method according to claim 2, wherein the reference cell is determined from the cells by a cell being selected from the cells that have the lowest state of charge.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 is a flow chart, which represents a method according to the invention for operating an energy storage system.
(2) FIGS. 2 to 6 are schematic illustrations of a method for operating an energy storage system in accordance with an embodiment.
(3) FIGS. 7 to 10 are schematic illustrations of embodiments of the method according to the invention for operating an energy storage system.
(4) FIG. 11 is a schematic illustration of a vehicle with an energy storage system in accordance with an embodiment.
DETAILED DESCRIPTION OF THE DRAWINGS
(5) FIG. 1 shows the procedure of the method according to the invention for operating an energy storage system. Initially, a reference cell is determined from the cells of the energy storage system; this procedure is identified with the reference symbol R. Preferably, this procedure is carried out at point in time t0 (basis point in time). A first balancing procedure S1 of the cells is carried out at a point in time t1. At a point in time t2, it is checked if conditions a) and b) have been met. This procedure is identified with the reference symbol B. If both conditions a) and b) are met, a second balancing procedure S2 is carried out. If one of the conditions a) and b) is not met, initially no further balancing procedure is carried out. Only when the conditions a) and b) are met is a new balancing procedure carried out.
(6) In FIGS. 2 to 5, as an example, cells Z1 to Z6 of an energy store of an energy storage system are depicted. In the following, an embodiment of the method according to the invention for operating an energy storage system shall be described based on this.
(7) FIG. 2 shows six cells Z1 to Z6, which all have different states of charge. Initially, a reference cell Zref is determined from this. In the case of this embodiment, the very cell is defined as a reference cell Zref, which has the lowest state of charge. In FIG. 3, a first balancing procedure S1 is schematically shown at the point in time t1. Thereby, the states of charge L1.sub.−t1, L2.sub.−t1, L3.sub.−t1, L4.sub.−t1 and L5.sub.−t1 of the cells Z1 to Z5 are balanced to the state of charge Lref.sub.−t1 of the reference cell Zref. Due to the balancing procedure S1, all cells Z1 to Z5 and Zref have the same state of charge Lref.sub.−t1. The state of charge Lref.sub.−t1 furthermore serves to determine the state-of-charge range ΔL. The state-of-charge range ΔL contains the state of charge Lref.sub.−t1 and a tolerance range, which is formed starting from the state of charge Lref.sub.−t1 all the way to higher and lower state-of-charge values. That means that the state-of-charge range ΔL contains states of charge that are greater than or equal to the state of charge Lref.sub.−t1 and contains states of charge that are lower than or equal to the state of charge Lref.sub.−t1. In FIG. 4, meeting condition a) is schematically shown. Condition a) is met when, at point in time t2, the difference, meaning the voltage difference ΔU.sub.t2 between the voltage of the cell with the lowest voltage Up.sub.min meaning cell Z1, and the voltage of the cell with the highest voltage U.sub.max, meaning cell Zref, is greater than a specified voltage difference ΔU.sub.pre or equal to a specified voltage difference ΔU.sub.pre. Since, in the depicted case, the voltage difference ΔU.sub.t2 is equal to the specified voltage difference ΔU.sub.pre, condition a) is met. Since the reference state of charge L.sub.ref-t2 of the reference cell Zref is not within the specified state-of-charge range ΔL at the second point in time t2, condition b) is not met and, in the case of the constellation shown in FIG. 4, the second balancing procedure S2 is not carried out. In the constellation shown in FIG. 5, the reference state of charge Lref.sub.−t2 of the reference cell Zref is decreased to the extent that it lies within the specified state-of-charge range ΔL. Thereby, in addition to condition a), also condition b) is met and the second balancing procedure S2 is carried out. If the assessment of the balancing quality (condition c) is included into the conditions for the second balancing procedure, in the case of the constellation shown in FIG. 4, the second balancing procedure S2 would be carried out, nevertheless.
(8) In FIG. 6, the cells Z1 to Z5 and Zref are shown after carrying out the second balancing procedure S2. The states of charge Lsymm of all cells Z1 to Z5 and Zref are identical. After the second balancing procedure S2, the process described above starts from the beginning. The cells Z1 to Z5 and Zref are charged or discharged depending on the load of the energy store or a new reference cell Zref is determined from the cells Z1 to Z6. Then, after determining the reference cell Zref, a third balancing procedure is then carried out, which is carried out according to the process described above of the first balancing procedure. Also, the successive fourth balancing procedure is carried out according to the process of the second balancing procedure described above.
(9) In FIGS. 7 to 10, conditions a) and b) are described depending on the switch-off behavior of the vehicle, in particular, depending on the switch-off procedures of the vehicle, meaning the resting phases of the energy storage system, wherein it is respectively required that condition a) is met at each point in time. In FIGS. 7 to 10, the individual switch-off procedures A.sub.s, A of the vehicle are identified with a star that is filled in or not filled in. In the following, embodiments of the method for operating an energy storage system shall be described as an example based on the reference cell. The respective state of charge (SOC) of this cell is plotted on the y-axis. The x-axis represents a time axis.
(10) In FIGS. 7 to 10, the switch-off procedures of the vehicle where a balancing is performed are identified with A.sub.s or with a filled-in star. The switch-off procedures of the vehicle where no balancing is performed are identified with A or with a star that is not filled in. For the depicted balancing procedures A.sub.s, it applies that these are then carried out if the state of charge of the reference cell is within the previously determined state-of-charge range ΔL at a determined point in time before the respective balancing procedure.
(11) In FIG. 7, a switch-off behavior of the vehicle is shown where the cells of the energy store are balanced at regular intervals. Thereby, the duration ΔT between two successive balancing procedures A.sub.s is smaller than the specified maximum time duration ΔTmax. In the situation shown in FIG. 7, the state-of-charge range ΔL is the same for all balancing procedures. That means that the state-of-charge range ΔL for the second to the fifth balancing procedure is respectively determined in such a way that it contains the previous reference state of charge of the reference cell.
(12) FIG. 8 describes a switch-off behavior of the vehicle where the cells are balanced at regular intervals up to a point in time tbal1, wherein the duration ΔT between two successive balancing procedures is smaller than the specified maximum time duration ΔTmax. Thereby, the situation corresponds to the situation shown in FIG. 7 up to the point in time tbal1. As of the point in time tbal1, the situation changes as follows: the duration between the points in time tbal1 and tmax is greater than the specified maximum time duration ΔTmax, which is why the balancing procedure is no longer dependent upon meeting conditions a) and b) at the point in time tmax but is carried out directly. As is shown in FIG. 8, in the case of exceeding the time duration ΔTmax, it is not waited for until the reference cell enters into the range of the reference state of charge of the previous balancing procedure, but balancing takes place directly at the point in time tmax. The reference cell is newly determined at the point in time tmax for the subsequent balancing procedure. The newly selected reference cell then corresponds to the cell with the lowest state of charge. In other words, by way of this, the state-of-charge range ΔL for the subsequent balancing procedure is shifted.
(13) FIG. 9 basically shows the same switch-off behavior of the vehicle as is shown in FIG. 8, however, the number of switch-off procedures differs within the time period from tbal1 to tbal2. While in the situation shown in FIG. 8, a total of ten switch-off procedures occur in the period between tbal1 and tmax, which corresponds to the point in time tbal2 in FIG. 9, in the situation shown in FIG. 9, only six switch-off procedures take place within the period between tbal1 and tbal2. A situation shown in FIG. 9 could, for example, occur if the vehicle is only seldom moved during the holiday period and, thereby, also few switch-off procedures occur. Although the duration between the points in time tbal1 and tbal2 are greater than the specified maximum time duration ΔTmax, in the case of the situation depicted in FIG. 9, the state-of-charge range ΔL is determined in such a way that it contains the reference state of charge of the previous balancing procedure. That means that, in this case, it is waited with the balancing procedure long enough until the state of charge of the reference cell is near the reference state of charge of the previous balancing procedure.
(14) In FIG. 10, a situation is depicted where the state-of-charge range ΔL is determined depending on the assessed balancing quality of the subsequent balancing procedure. In the case of the situation depicted in FIG. 10, for the third switch-off procedure, no balancing procedure would be carried out because the reference state of charge for the third switch-off procedure is not within the state-of-charge range ΔL determined by the previous balancing procedure. However, due to an assessment procedure, for the third switch-off procedure, it results that the balancing quality when carrying out the balancing procedure in the case of the current reference state of charge for the third switch-off procedure is higher than the balancing quality of a balancing procedure for the state-of-charge range ΔL determined by the state of charge of the second switch-off procedure. Since, in addition, the current reference state of charge for the third switch-off procedure is higher than the determined state-of-charge range ΔL, the balancing procedure is carried out for the current reference state of charge for the third switch-off procedure. For the successive balancing procedures, the state-of-charge range ΔL is determined again depending on the assessed balancing quality. Since a balancing quality is assessed for the reference state of charge prevailing for the fourth switch-off procedure, which is lower than the balancing quality of the balancing procedure during the third switch-off procedure, the state-of-charge range ΔL remains at the previously determined level. In other words, the shifting of the state-of-charge range ΔL is only permitted if the reference state of charge is higher than is the case with the last balancing. Even if the balancing quality for the fourth switch-off procedure would be higher than is the case with the third switch-off procedure, no balancing would therefore be started. In the case of the ninth switch-off procedure, the situation occurs again that the assessed balancing quality for the reference state of charge prevailing during the ninth switch-off procedure is higher than the balancing quality of the previous balancing procedure and the reference state of charge prevailing during the ninth switch-off procedure is greater than the state-of-charge range ΔL, whereby the balancing procedure is carried out for the current reference state of charge for the ninth switch-off procedure. That means that the state-of-charge range ΔL is shifted in comparison to the previous balancing procedure.
(15) FIG. 11 shows a vehicle 100, which includes an energy storage system 1 and an electric drive 10. The energy storage system 1 is designed to feed the electric drive 10 of the vehicle 100. The energy storage system 1 includes a multiplicity of cells Z1 to Z6 and a control unit 3.
(16) The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.
