Method for balancing states of charge of an electrical energy store having a plurality of battery cells

Abstract

The invention relates to a method for balancing states of charge of an electrical energy store having a plurality of battery cells, wherein the method comprises the following method steps: a) a capacity is measured for each battery cell; b) a target capacity value (cap) is determined for each battery cell; c) a state variable is calculated for each battery cell, specifically by forming the difference between the capacity measured in step a) and the target capacity value (cap) determined in step b) for each battery cell at a previously defined or definable charging voltage; d) a duration for balancing the state of charge of the battery cell is defined for each battery cell; and e) the state of charge of the battery cell is balanced for the duration defined in step d) for each battery cell.

Description

[0001] METHOD FOR BALANCED CHARGE STATES OF AN ELECTRICAL ENERGY STORAGE DEVICE WITH A MULTIPLE BATTERY CELLS

[0002] Description

[0003] The present invention relates to a method for balancing the charge states of an electrical energy storage device comprising a plurality of battery cells. The invention further relates to a corresponding electrical energy storage device.

[0004] Especially in hybrid and electric vehicles, lithium-ion energy storage systems are typically used, consisting of a large number of electrochemical battery cells connected in series. A battery management system usually monitors the energy storage system and, in addition to safety monitoring, is intended to ensure the longest possible lifespan.

[0005] It should be noted that the present invention is not limited to energy storage devices using lithium-ion technology. In general, the invention relates to battery systems that can be based on various chemical types, including lithium-ion, nickel-metal hydride, lead-acid, etc.

[0006] To ensure the longest possible lifespan for such an energy storage device, it is essential to ensure that the state of charge (SOC) of the battery cells is balanced despite differing self-discharge rates. This is typically achieved through appropriate cell balancing, which is usually performed resistively.

[0007] Each battery cell is equipped with a resistor and a switching element to allow for the selective discharge of individual cells via this ohmic resistance. In addition to varying self-discharge rates, the capacities of the individual battery cells also differ, for example, due to manufacturing variations. This effect is usually negligible at the beginning of the battery cells' lifespan, but can increase over time due to differences in cell aging, resulting in capacity differences of several percent between the battery cells.

[0008] Due to manufacturing variations, individual cell capacities, discharge resistance, and other factors, the capacity of each individual battery cell varies to hold the electrical charge. Furthermore, the cell capacity currently used also varies due to the series connection of the battery cells in the battery pack or module. In a series connection, the electric current charges and discharges all cells simultaneously. This results in the cell with the lowest charge determining the usable charge of the battery system, locking up the usable charge in other cells. State-of-charge balancing is one possible approach to obtain additional charge from the other battery cells by redistributing the charge among them, starting from the voltage of one cell.

[0009] A charge level adjustment is performed during the standby phase of the battery management system's control unit for controlling the energy storage system. For this purpose, an electronic unit is activated for a predefined duration to perform the charge level adjustment. This predefined duration is set while the control unit is being switched off. Therefore, it is not possible to adjust the programmed duration or the electrical charge to be adjusted.

[0010] The object of the present invention is to further improve the prior art. This object is achieved in particular by the subject matter of independent claim 1.

[0011] Accordingly, the invention relates in particular to a method for balancing the charge states of an electrical energy storage device with a plurality of battery cells, wherein the method comprises the following steps: a) a capacity is measured for each battery cell of the plurality of battery cells of the electrical energy storage device; b) a most suitable capacity value or target capacity value is determined for each battery cell of the plurality of battery cells of the electrical energy storage device; c) a state variable is calculated for each battery cell of the plurality of battery cells of the electrical energy storage device by forming, for each battery cell at a predetermined or definable charging voltage, the difference between the capacity measured in step a) and the capacity value determined in step b);d) for each battery cell of the plurality of battery cells of the electrical energy storage system, a duration for balancing the state of charge of the battery cell is determined; and e) for each battery cell of the plurality of battery cells of the electrical energy storage system, the state of charge of the battery cell is balanced for the duration determined in step d).

[0012] In this context, it is advisable to measure the capacitance in step a) by relating the integrated current to the measured difference in open-circuit voltages.

[0013] According to implementations of the present invention, it is provided that in step b) the capacity value is determined using an iterative estimation method, in particular using a Kalman filter.

[0014] It is conceivable, in particular, that the state variable calculated in step c) represents an electrical charge of the battery cell that needs to be balanced.

[0015] For example, the predetermined or definable charging voltage in step c) can correspond to a final charging voltage of the battery cell or to a final charging voltage of the battery cell minus a predetermined or definable voltage value.

[0016] According to implementations of the present invention, it is provided that in step d) the duration for balancing the state of charge of the battery cell is set such that the state variable for each battery cell settles to the same value. For example, it is conceivable that in step e) the state of charge of the battery cell is balanced by activating at least one bypass resistor for the duration set in step d).

[0017] According to implementations of the present invention, the battery cell is a supercapacitor, in particular with a charging voltage of 3.1 volts.

[0018] In this context, it is advantageous that the open-circuit voltage (OCV) and the amount of charged charge (Q) of the battery cell designed as a supercapacitor exhibit at least essentially linear behavior, whereby the following applies to the battery cell designed as a supercapacitor:

[0019] • in fully charged state: Q = cap * 100% and V = Vmax;

[0020] • in fully discharged state: Q = 0 Ah and V = Vmin 0 = V.

[0021] According to implementations of the last-mentioned design variant, it is provided that in step b) the target capacity value (cap) is determined by measuring a first open-circuit voltage (OCV1) of the battery cell designed as a supercapacitor with a first charged charge (Ql) and a second open-circuit voltage (OCV2) of the battery cell designed as a supercapacitor with a second charged charge (Q2) different from the first charged charge (Ql), whereby the target capacity value (cap) is determined using the following formula: cap = Delta Q / (Delta V / Vtotal) where: Delta Q = Ql - Q2 Delta V = V1 - V2 Vtotal = Vmax - Vmin

[0022] Alternatively, in step b) the target capacity value (cap) can be determined by measuring a first open-circuit voltage (OCV1) of the battery cell designed as a supercapacitor at a first charged charge quantity (Ql) and a second open-circuit voltage (OCV2) of the battery cell designed as a supercapacitor at a second charged charge quantity (Q2) different from the first charged charge quantity (Ql), whereby the value pairs (OCV1, OCV2) and (Ql, Q2) are fed into a Caiman filter or similar filter, which is based on a state-of-charge model and has the target capacity (cap) as a parameter, based on the formula:

[0023] Q = V / (Vmax - Vmin) * cap

[0024] Phases of constant discharge current can also be used because this leads to a nearly constant voltage offset. In this case, OCVx would be replaced by Vx according to the formula above, with Vx = OCVx + I * ESR, where ESR is the "equivalent serial resistance," i.e., the internal resistance assumed to be constant. This results in the same delta V as above for the unloaded state.

[0025] In principle, it is advisable for the electrical energy storage system to have several battery cells connected in series, and in particular six battery cells connected in series.

[0026] The invention further relates to an electrical energy storage device comprising a plurality of battery cells, at least one voltage sensor and at least one device, in particular in the form of an electronic battery management control unit, which is configured to carry out the steps of the aforementioned method according to the invention.

[0027] The electrical energy storage device can have discharge resistors for the selective discharge of individual battery cells.

[0028] Furthermore, the invention relates to a machine-readable storage medium on which a computer program is stored, wherein the computer program comprises instructions that cause the aforementioned electrical energy storage device to perform the process steps of the aforementioned method according to the invention.

[0029] The inventive method has the particular advantage that the calculation accuracy of an electrical charge to be balanced is determined and taken into account in a calculation of the duration for balancing the state of charge. This improves the accuracy and effectiveness of the state-of-charge balancing and thus increases the capacity and performance of an electrical energy storage device. For the purposes of this disclosure, an electrical energy storage device is understood to be an energy storage device with a plurality of battery cells from which electrical energy can either be drawn or supplied and drawn. The electrical energy storage device is designed as a charge storage device and / or as a magnetic energy storage device and / or electrochemical energy storage device. The electrochemical energy storage device is, in particular, a rechargeable battery or accumulator.

[0030] The method according to the invention is advantageously used in an electrical energy storage device for electric vehicles, hybrid vehicles, plug-in hybrid vehicles, aircraft, e-bikes, for portable telecommunications or data processing equipment, for electric hand tools or kitchen machines, as well as in stationary storage devices for storing, in particular, regeneratively generated electrical energy.

[0031] The following is a summary of aspects of the present invention:

[0032] The aim is a balancing or equalization method for cells connected in series, especially supercapacitor cells, which leads to a cell state that is as similar as possible, e.g. with regard to the SOC or the open-circuit voltage, at a critical overall state, e.g. at the upper end of the state of charge range.

[0033] The currently common balancing method aims to balance the cells in their current state, typically in the middle of the state-of-charge (SOC) range. This can have the disadvantage that, with unequal cell capacities, the cell state in the upper SOC range is uneven, and one cell may reach its upper voltage limit sooner than the others. This limits the effective operating range of the system to the uppermost cell.

[0034] Unequal capacities can be caused by the manufacturing process or by uneven aging, e.g. due to temperature gradients in the system, over the product's lifetime.

[0035] According to the invention, the capacity of each individual cell, or of parallel-connected cell arrays, is determined by the system. For this purpose, the relationship between the amount of charged charge Q (in As or Ah) and the capacity can be used.

[0036] For a fully charged supercapacitor cell, Q = Cap * 100% and V = V_max, while a fully discharged cell has Q = 0 Ah and V = V_min = 0V.

[0037] Typically, supercapacitors exhibit a nearly linear behavior of unloaded voltage OCV and charged charge Q, so that differences can also be used, meaning that full charging and discharging is not required.

[0038] One can, for example, measure at two points: 1 and 2 with OCV1 and OCV2, and ΔV = OCV2 - OCV1, and Q1, Q2, and ΔQ = Q2 - Q1. The capacitance is then calculated as cap = ΔQ / (ΔV / Vtotal) with Vtotal = V_max - V_min

[0039] Alternatively, the value pairs OCVx and Qx can be fed into a Kaiman filter or similar filter, which is based on a state-of-charge model and has the capacity cap as a parameter, based on the formula: Q = V / (Vmax- V_min) * cap

[0040] Phases of constant discharge current can also be used because this leads to a nearly constant voltage offset. In this case, OCVx would be replaced by Vx according to the formula above, with Vx = OCVx + I * ESR, where ESR is the equivalent serial resistance, i.e., the internal resistance assumed to be constant. This results in the same delta V as above in the unloaded state.

[0041] Below, with reference to the accompanying drawings, an exemplary embodiment of the balancing method according to the invention is compared with a conventional balancing method. The conventional balancing method is referred to as "Scenario 1" and the balancing method according to the invention as "Scenario 2". The conventional balancing method involves an approach of equal voltage: at the current voltage, all cells are brought to a minimum cell voltage in order to maximize the discharge capacity.

[0042] The inventive method employs the so-called DELTA Q 3V strategy. In this strategy, all cells are brought to an equal voltage of 3.0 V to reduce the risk of overcharging. The equalization time per cell to reach the target voltage is then calculated. For example, a equalization time of 11 minutes is obtained for a difference of 0.2 V using a 188 F capacitor with a 65 mA equalization current.

[0043] The balancing can take place during a parking phase, a standby phase, or a driving phase of the vehicle.

[0044] The cells are typically operated in the 10 to 15 V range. If the cells are balanced to the same open-circuit voltage within this range and exhibit capacity variations, charging to 18.0 V will cause cell voltage fluctuations that can exceed 3.1 V in one or more cells, potentially leading to safety-critical conditions or premature aging.

[0045] Therefore, according to the invention, the capacity of each cell is measured by relating the electrical capacity of the cell (in Ah or As) to the measured difference in open-circuit voltages. Subsequently, a Kalman filter or another suitable filter is used to determine the most suitable capacity values ​​for each cell. The difference in ampere-seconds (As) for each cell from its projected state of charge at 3.0 V is calculated, and this value is used as a state variable for each cell: “DELTA Q 3\T.

[0046] Subsequently, compensation takes place via bypass resistors, which are activated for an appropriate time depending on the measured state of charge, particularly during the standby or driving phase.

[0047] The drawings show:

[0048] FIG. 1A shows the measured values ​​of an electrical energy storage system with six battery cells during balancing or balancing the charge states of the battery cells according to the conventional method;

[0049] FIG. 1B shows a corresponding table with the values ​​for the battery cells of the energy storage system according to FIG. 1A during or for balancing the charge states according to the invention;

[0050] FIG. 2A shows the cell voltage of the individual cells at different operating voltages of the electrical energy storage system balanced according to FIG. 1A; and

[0051] FIG. 2B shows the cell voltages of the individual cells at different operating voltages of the energy storage system according to FIG. 1B.

[0052] The diagrams show that the conventional balancing method (Scenario 1) ensures maximum effective capacity upon complete discharge. However, this method carries the risk of cell overcharging if cell capacities vary significantly.

[0053] The method according to the invention (scenario 2) provides a lower discharge capacity, but eliminates the risk of overcharging the cells.

Claims

Patent claims 1. A method for balancing the charge states of an electrical energy storage device with a plurality of battery cells, the method comprising the following steps: a) a capacity is measured for each battery cell of the plurality of battery cells of the electrical energy storage device; b) a target capacity value (cap) is determined for each battery cell of the plurality of battery cells of the electrical energy storage device, wherein the target capacity value (cap) is preferably a capacity value most suitable for the battery cell; c) a state variable is calculated for each battery cell of the plurality of battery cells of the electrical energy storage device by calculating, for each battery cell at a predetermined or definable charging voltage, the difference between the capacity measured in step a) and the target capacity value (cap) determined in step b);d) for each battery cell of the plurality of battery cells of the electrical energy storage system, a duration for balancing the state of charge of the battery cell is determined; and e) for each battery cell of the plurality of battery cells of the electrical energy storage system, the state of charge of the battery cell is balanced for the duration determined in step d).

2. Method according to claim 1, wherein in step a) the capacitance is measured by relating the integrated current to the measured difference of the open-circuit voltages.

3. Method according to claim 1 or 2, wherein in step b) the target capacity value (cap) is determined using an iterative estimation method, in particular using a Kalman filter.

4. Method according to one of claims 1 to 3, wherein the state variable calculated in step c) represents an electrical charge of the battery cell to be balanced.

5. Method according to any one of claims 1 to 4, wherein the predetermined or definable charging voltage in step c) corresponds to a final charging voltage of the battery cell or to a final charging voltage of the battery cell less a predetermined or definable voltage value.

6. Method according to any one of claims 1 to 5, wherein in step d) the duration for balancing the state of charge of the battery cell is set such that the state variable for each battery cell settles to the same value.

7. Method according to any one of claims 1 to 6, wherein in step e) the state of charge of the battery cell is balanced by activating at least one bypass resistor for the duration specified in step d).

8. Method according to any one of claims 1 to 7, wherein the battery cell is a supercapacitor, in particular with a charging voltage of 3.1 volts.

9. Method according to claim 8, wherein an open-circuit voltage (OCV) and an charged charge quantity (Q) of the battery cell designed as a supercapacitor exhibit at least substantially linear behavior, wherein the following applies to the battery cell designed as a supercapacitor: in the fully charged state: Q = cap * 100% and V = Vmax; in the fully discharged state: Q = 0 Ah and V = Vmin 0 = V.

10. The method of claim 9, wherein in step b) the target capacitance value (cap) is determined by measuring a first open-circuit voltage (OCV1) of the battery cell designed as a supercapacitor at a first charged charge quantity (Ql) and a second open-circuit voltage (OCV2) of the battery cell designed as a supercapacitor at a second charged charge quantity (Q2) different from the first charged charge quantity (Ql), wherein the target capacitance value (cap) is determined using the following formula: cap = Delta Q / (Delta V / Vtotal) with: Delta Q = Ql - Q2 Delta V = VI - V2 Total V = Maximum Vmax - Minimum Vmin 11. Method according to claim 9, wherein in step b) the target capacity value (cap) is determined by measuring a first open-circuit voltage (OCV1) of the battery cell designed as a supercapacitor at a first charged charge quantity (Ql) and a second open-circuit voltage (OCV2) of the battery cell designed as a supercapacitor at a second charged charge quantity (Q2) different from the first charged charge quantity (Ql), wherein the pairs of values ​​(OCV1, OCV2) and (Ql, Q2) are fed into a Caiman filter or similar filter, which is based on a state-of-charge model and has the target capacity (cap) as a parameter, based on the formula: Q = V / (Vmax - Vmin) * cap 12. Method according to any one of claims 1 to 11, wherein the electrical energy storage device comprises several battery cells connected in series and in particular six battery cells connected in series.

13. Electrical energy storage device comprising a plurality of battery cells, at least one voltage sensor and at least one device, in particular in the form of an electronic battery management control unit configured to perform the steps of the method according to any one of claims 1 to 12.

14. Electrical energy storage device according to claim 13, wherein the electrical energy storage device comprises discharge resistors for the selective discharge of individual battery cells.

15. Machine-readable storage medium on which a computer program is stored, wherein the computer program comprises instructions that cause the electrical energy storage device according to claim 13 or 14 to perform the method steps according to one of claims 1 to 12.

Images