Method for balancing states of charge of an electrical energy store

Abstract

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

Claims

1. A method for balancing states of charge of an electrical energy store with a plurality of battery cells, the method comprising: a) acquiring a voltage magnitude U.sub.mess,i that represents an electric voltage of a battery cell; b) determining a maximum positive electric charge Q.sub.i,Pos of the battery cell with respect to an electric reference voltage level U.sub.BalLevel of all the battery cells of the electrical energy store with balanced states of charge according to
Q.sub.i,Pos=OCV.sub.PosErr(U.sub.mess,i+U.sub.PosErr)−OCV.sub.PosErr(U.sub.BalLevel), wherein OCV.sub.PosErr represents a no-load voltage curve with a positive capacity estimation error and U.sub.PosErr represents a sensor tolerance of the acquired voltage magnitude U.sub.mess,i and a modeled positive battery cell voltage error; c) determining a maximum negative electric charge Q.sub.i,Neg of the battery cell with respect to the electric reference voltage level U.sub.BalLevel of all the battery cells of the electrical energy store with balanced states of charge according to
Q.sub.i,Neg=OCV.sub.NegErr(U.sub.mess,i−U.sub.NegErr)−OCV.sub.NegErr(U.sub.BalLevel), wherein OCV.sub.NegErr represents a no-load voltage curve with a negative capacity estimation error and U.sub.NegErr represents a sensor tolerance of the acquired voltage magnitude U.sub.mess,i and a modeled negative battery cell voltage error; d) determining an electric charge ΔQ.sub.i to be balanced of the battery cell according to
ΔQ.sub.i=Q.sub.i−Q.sub.min, wherein Q.sub.i represents an electric charge of the battery cell with respect to the reference voltage level U.sub.BalLevel and Q.sub.min represents a minimum electric charge of all the battery cells located in a series circuit with respect to the reference voltage level U.sub.BalLevel; e) determining a charge error ΔQ.sub.i,Err, of the electric charge ΔQ.sub.i to be balanced according to
ΔQ.sub.i,Err=Q.sub.i,Pos−Q.sub.min,Neg, wherein Q.sub.min,Neg represents an electric charge Q.sub.i,Neg of the battery cell with the minimum electric charge Q.sub.min; f) determining an electric charge ΔQ.sub.i,use to be balanced of the battery cell according to
ΔQ.sub.i,use=ΔQ.sub.i−W*ΔQ.sub.i,err, wherein W represents a predefinable weighting factor; g) calculating a duration t.sub.i,bal for balancing the state of charge of the battery cell on the basis of the electric charge ΔQ.sub.i,use that is to be balanced when the electric charge ΔQ.sub.i,use to be balanced exceeds a predefined threshold value, according to $t_{i,bal}=\frac{\Delta Q_i*R_{Bal}}{U_{\mathrm{mess},i}},$ wherein R.sub.Bal represents an ohmic resistance assigned to the battery cell; h) balancing the state of charge of the battery cell for the duration t.sub.i,bal.

2. The method according to claim 1, wherein the full charge error ΔQ.sub.i,err is subtracted from the electric charge ΔQ.sub.i that is to be balanced if the weighting factor W adopts the value 1, and the charge error ΔQ.sub.i,err is not taken into consideration if the weighting factor W adopts the value 0.

3. The method according to claim 1, wherein the no-load voltage curve, the sensor tolerance, and/or the capacity estimation errors are determined statically and/or dynamically in order to determine the maximum positive charge Q.sub.i,Pos and/or the maximum negative charge Q.sub.i,Neg.

4. The method according to claim 1, wherein the electric reference voltage level U.sub.BalLevel of the battery cells of the electrical energy store with balanced states of charge of all the battery cells is 3.7 V.

5. The method according to claim 1, wherein the capacity estimation error of the no-load voltage curve OCV is a maximum of ±4%.

6. An electrical energy store comprising: a plurality of battery cells; at least one voltage sensor; and at least one electronic battery control device, the at least one electronic battery control device configured to carry out the steps of the method according to claim 1.

7. The electrical energy store according to claim 6, further comprising: discharge resistors for the selective discharge of individual battery cells.

8. A non-transitory, machine-readable storage medium containing instructions that, when executed by a computer, cause the computer to a) acquire a voltage magnitude U.sub.mess,i that represents an electric voltage of a battery cell; b) determine a maximum positive electric charge Q.sub.i,pos of the battery cell with respect to an electric reference voltage level U.sub.BalLevel of all the battery cells of the electrical energy store with balanced states of charge according to
Q.sub.i,Pos=OCV.sub.PosErr(U.sub.mess,i+U.sub.PosErr)−OCV.sub.PosErr(U.sub.BalLevel), wherein OCV.sub.PosErr represents a no-load voltage curve with a positive capacity estimation error and U.sub.PosErr represents a sensor tolerance of the acquired voltage magnitude U.sub.mess,i and a modeled positive battery cell voltage error; c) determine a maximum negative electric charge Q.sub.i,Neg of the battery cell with respect to the electric reference voltage level U.sub.BalLevel of all the battery cells of the electrical energy store with balanced states of charge according to
Q.sub.i,Neg=OCV.sub.NegErr(U.sub.mess,i−U.sub.NegErr)−OCV.sub.NegErr(U.sub.BalLevel), wherein OCV.sub.NegErr represents a no-load voltage curve with a negative capacity estimation error and U.sub.NegErr represents a sensor tolerance of the acquired voltage magnitude U.sub.mess,i and a modeled negative battery cell voltage error; d) determine an electric charge ΔQ.sub.i to be balanced of the battery cell according to
ΔQ.sub.i=Q.sub.i−Q.sub.min, wherein Q.sub.i represents an electric charge of the battery cell with respect to the reference voltage level U.sub.BalLevel and Q.sub.min represents a minimum electric charge of all the battery cells located in a series circuit with respect to the reference voltage level U.sub.BalLevel; e) determine a charge error ΔQ.sub.i,Err of the electric charge ΔQ.sub.i to be balanced according to
ΔQ.sub.i,Err=Q.sub.i,Pos−Q.sub.min,Neg, wherein Q.sub.min,Neg represents an electric charge Q.sub.i,Neg of the battery cell with the minimum electric charge Q.sub.min; f) determine an electric charge ΔQ.sub.i,use to be balanced of the battery cell according to
ΔQ.sub.i,use=ΔQ.sub.i−W*ΔQ.sub.i,err, wherein W represents a predefinable weighting factor; g) calculate a duration t.sub.i,bal for balancing the state of charge of the battery cell on the basis of the electric charge ΔQ.sub.i,use that is to be balanced when the electric charge ΔQ.sub.i,use to be balanced exceeds a predefined threshold value, according to $t_{i,bal}=\frac{\Delta Q_i*R_{Bal}}{U_{\mathrm{mess},i}},$ wherein R.sub.Bal represents an ohmic resistance assigned to the battery cell; and balancing the state of charge of the battery cell for the duration t.sub.i,bal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Exemplary embodiments of the invention are illustrated in the drawing, and explained in more detail in the following description.

(2) Here:

(3) FIG. 1 shows a schematic illustration of no-load voltage curves of an electrical energy store; and

(4) FIG. 2 shows a schematic illustration of a calculation uncertainty of a charge determination for an electric charge that is to be balanced.

(5) The same reference suns identify the same apparatus components in all the figures.

DETAILED DESCRIPTION

(6) FIG. 1 shows a schematic illustration of no-load voltage curves 100 (“OCV”) of an electrical energy store at different ageing states SOH.sub.C (beginning of the lifetime (“BOL”) end of the lifetime (“EOL”)).

(7) TABLE-US-00001 Reference sign Ageing state [%] 101 95 102 90 103 85 104 80 105 75

(8) These curves can, for example, be ascertained through ageing trials in the laboratory depending on the battery cell type and chemistry. All the no-load voltage curves intersect at a 50% state of charge, which corresponds to approximately 3.7 V in this cell type. This state of charge corresponds to a 48 V energy store or energy storage system, with a balanced state of charge of all the battery cells.

(9) In a battery management system, the respective no-load voltage curve is used for the determination of the charge to be balanced, which corresponds largely to the estimated capacity (OCV(SOH.sub.C)). The cell capacity can be ascertained using methods that correspond to the prior art. If the measured cell capacity lies between two no-load voltage curves, then the corresponding no-load voltage curve is interpolated.

(10) If the currently estimated or maximum capacity estimation error is known, for example ±4%, then the enclosing no-load voltage curves can be determined with the maximum capacity estimation error. The real no-load voltage curve of the battery cell lies inside these enclosing no-load voltage curves.

(11) The accuracy of the calculation of the charge to be balanced depends on the accuracy of the modeled no-load voltage, on the measurement accuracy of the battery cell voltage measurement, and on the accuracy of the modeled battery cell voltage. If a battery cell is fully relaxed, the error in the modeled battery cell voltage is equal to 0, otherwise this error is overlaid additively by the accuracy of measurement of the cell voltage measurement.

(12) If all the battery cells are to adopt a state of charge of 3.7 V in a balanced charge state and if, for example, a cell voltage of 3.7 V is measured, then the capacity estimation error has no influence.

(13) As the measured cell voltage deviates increasingly from the reference voltage (U.sub.BalLevel=3.7V), the influence of the capacity estimation error grows. The voltage measurement error of the sensor and the error in the modeled battery cell voltage are combined additively with the no-load voltage error.

(14) The accuracy of the state of charge balancing between the battery cells is increased through the method according to the invention. The charges of the battery cells that are connected electrically in series are balanced better, and the capacity and performance capability of the energy storage are thereby increased.

(15) In a 48 V energy store, the battery cells are balanced at a target state of charge of 50%. This means that battery cells pass through the state of charge of 50% together. By increasing the accuracy of the state of charge balancing, the minimum and maximum voltage limits are reached later, which leads to an increase in the performance of the electrical energy store.

(16) The number of unnecessary state of charge balancing processes (“pseudo-balancing”) is also minimized, which leads to an increase in the lifetime of the energy store and of the discharge resistors.

(17) FIG. 2 shows a schematic illustration of a calculation uncertainty of a charge determination for an electric charge that is to be balanced. A curve 201 shows the Q.sub.Pos and a curve 202 shows the Q.sub.Neg less the correct charge Q that is to be balanced of a battery cell. A capacity estimation error of 3.5%, together with a voltage measurement accuracy of the voltage sensor of 3 mV, are assumed for this calculation.