Method for determining the capacity of an electrical energy storage unit

Abstract

A method is described for determining the capacity of an electrical energy storage unit, which method comprises the steps: a) determining a first capacity value of the electrical energy storage unit using a first mathematical model based at least on an operating time and/or on a charging throughput of the electrical energy storage unit; b) determining a second capacity value of the electrical energy storage unit using a second mathematical model based at least on a second charging throughput value of the electrical energy storage unit and on a state-of-charge difference, which is obtained from the states of charge at two different time instants; c) obtaining a correction factor for determining a capacity value using the first mathematical model on the basis of the determined first capacity value and the determined second capacity value; d) determining a third capacity value of the electrical energy storage unit using the first mathematical model based at least on the obtained correction factor and on the operating time of the electrical energy storage unit and/or on a third charging throughput value of the electrical energy storage unit.

Claims

1. A method for determining the capacity of an electrical energy storage unit (41), the method comprising the steps of: a) determining, via a computer, a first capacity value of the electrical energy storage unit (41) using a first mathematical model based at least on an operating time, on a charging throughput of the electrical energy storage unit (41), or both; b) determining, via the computer, a second capacity value of the electrical energy storage unit (41) using a second mathematical model based at least on a second charging throughput value of the electrical energy storage unit (41) and on a state-of-charge difference, which is obtained from the states of charge at two different time instants; c) obtaining, via the computer, a correction factor (32) for determining a capacity value using the first mathematical model on the basis of the determined first capacity value and the determined second capacity value; d) determining, via a computer, a third capacity value of the electrical energy storage unit (41) using the first mathematical model based at least on the obtained correction factor (32) and on the operating time of the electrical energy storage unit (41), on a third charging throughput value of the electrical energy storage unit (41), or both.

2. The method according to claim 1, wherein in step b), open-circuit voltage values from a modelled open-circuit voltage characteristic curve (21) are used to determine the states of charge.

3. The method according to claim 2, wherein the modelled open-circuit voltage characteristic curve (21) is divided into different regions, wherein there is at least one first region (Δ1, Δ2), the open-circuit voltage values of which are used to determine the states of charge, and wherein there is at least one second region, the open-circuit voltage values of which are not used to determine the states of charge, but instead measured values of the voltage of the electrical energy storage unit (41) are used to determine the states of charge.

4. The method according to claim 3, wherein the at least one first region (Δ1, Δ2) includes the open-circuit voltage range of 4 Volts to 3.8 Volts and the open-circuit voltage range of 3.7 Volts to 3.6 Volts.

5. The method according to claim 1, wherein the method is performed continuously.

6. A non-transitory, computer-readable storage medium containing instructions which when executed by a computer cause the computer to determine a first capacity value of the electrical energy storage unit (41) using a first mathematical model based at least on an operating time, on a charging throughput of the electrical energy storage unit (41), or both; determine a second capacity value of the electrical energy storage unit (41) using a second mathematical model based at least on a second charging throughput value of the electrical energy storage unit (41) and on a state-of-charge difference, which is obtained from the states of charge at two different time instants; obtain a correction factor (32) for determining a capacity value using the first mathematical model on the basis of the determined first capacity value and the determined second capacity value; determine a third capacity value of the electrical energy storage unit (41) using the first mathematical model based at least on the obtained correction factor (32) and on the operating time of the electrical energy storage unit (41), on a third charging throughput value of the electrical energy storage unit (41), or both.

7. A device (42) for determining the capacity of an electrical energy storage unit (41), comprising at least one means (44), an electronic control unit configured to determine a first capacity value of the electrical energy storage unit (41) using a first mathematical model based at least on an operating time, on a charging throughput of the electrical energy storage unit (41), or both; determine a second capacity value of the electrical energy storage unit (41) using a second mathematical model based at least on a second charging throughput value of the electrical energy storage unit (41) and on a state-of-charge difference, which is obtained from the states of charge at two different time instants; obtain a correction factor (32) for determining a capacity value using the first mathematical model on the basis of the determined first capacity value and the determined second capacity value; determine a third capacity value of the electrical energy storage unit (41) using the first mathematical model based at least on the obtained correction factor (32) and on the operating time of the electrical energy storage unit (41), on a third charging throughput value of the electrical energy storage unit (41), or both.

8. An electrical energy storage system (40) comprising an electrical energy storage unit (41) and a device (42) according to claim 7.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Advantageous embodiments of the invention are described in more detail in the following description and are shown in the figures,

(2) in which:

(3) FIG. 1 is a flow diagram of the disclosed method according to an embodiment;

(4) FIG. 2 is a schematic diagram of a modelled open-circuit voltage characteristic curve containing two first regions and three second regions;

(5) FIG. 3 is a schematic diagram of a variation of the capacity of an electrical energy storage unit according to the first mathematical model including correction factor; and

(6) FIG. 4 is a schematic diagram of the disclosed electrical energy storage system according to an embodiment.

DETAILED DESCRIPTION

(7) In all the figures, the same reference signs denote identical device components or identical method steps.

(8) FIG. 1 shows a flow diagram of the disclosed method for determining the capacity of an electrical energy storage unit according to an embodiment.

(9) In a first step S11, a first capacity value of the electrical energy storage unit is determined using a first mathematical model based at least on an operating time and/or on a charging throughput of the electrical energy storage unit.

(10) The first mathematical model may have the following form:

(11) $C\left(t\right)=\frac{C2-C1}{t2-t1}t+C2$

(12) It is assumed here that the capacity C(t) of the electrical energy storage unit depends linearly on the operating time t. The capacity thus decreases linearly with increasing operating time. C2 and C1 are here capacity values, and t2 and t1 the corresponding operating times at which these capacity values were acquired, where t2 denotes a later time instant than t1. This model determination can be carried out in laboratory trials, for instance. Thus the first mathematical model can be used to determine a first capacity value of the electrical energy storage unit.

(13) In a second step S12, a second capacity value of the electrical energy storage unit is determined using a second mathematical model based at least on a second charging throughput value of the electrical energy storage unit and on a state-of-charge difference. Said state-of-charge difference is obtained here from the states of charge at two different time instants.

(14) The second mathematical model may have the following form, for example:

(15) $Q_{\mathrm{cell}}=\frac{\frac{1}{36}.Math.{\int }_{t3}^{t4}\mathrm{Idt}}{{\mathrm{SOC}\left(\mathrm{OCVz}\right)}_{t4}-{\mathrm{SOC}\left(\mathrm{OCVz}\right)}_{t3}}$

(16) where Qcell is the capacity value determined by the second mathematical model, and the time instants t3 and t4 are the two different time instants. The electrical current is integrated over time between the two time instants t3 and t4 in order to determine the net amount of charge that has flowed. Dividing by the state-of-charge difference between the two time instants t3 and t4 then yields the second capacity value.

(17) Then in a third step S13, a correction factor for determining a capacity value using the first mathematical model is obtained on the basis of the determined first capacity value and the determined second capacity value. This is also illustrated in FIG. 3.

(18) In a fourth step S14, a third capacity value of the electrical energy storage unit is determined using the first mathematical model, this being done on the basis of the correction factor obtained for the first mathematical model and on the operating time and/or on a third charging throughput value of the electrical energy storage unit.

(19) The third capacity value determined in this manner can be used, for example, in an electronic control unit for better prediction of lifetime and/or range, and can improve the prediction of the power available, allowing the electrical energy storage unit to be operated in an improved way.

(20) FIG. 2 shows a schematic diagram of a modelled open-circuit voltage characteristic curve 21 containing two first regions 41, 42, the open-circuit voltage values of which are used to determine the states of charge, and three second regions, the open-circuit voltage values of which are not used to determine the states of charge. The values of the open-circuit voltage are plotted on the vertical axis OCV, and the state-of-charge values are plotted on the horizontal axis SOC, resulting in the open-circuit voltage characteristic curve 21.

(21) FIG. 3 shows a schematic diagram of a capacity variation 31 of an electrical energy storage unit according to the first mathematical model including correction factors 32, which correction factors are obtained at two different time instants t1 and t2. The values of the capacity of the electrical energy storage unit that are obtained from the first mathematical model are plotted on the vertical axis CAP, and the operating time of the electrical energy storage unit is plotted along the horizontal axis t, resulting in the capacity variation 31.

(22) Including the correction factor, the model cited above by way of example has the following form:

(23) $C\left(t\right)=\frac{C2+\Delta Ckorr-C1}{t2-t1}t+C2$

(24) where ΔCkorr denotes the relevant correction factor. For FIG. 3, this means that ΔCkorr is a positive value, because the first mathematical model has overestimated the loss in capacity over time. The model can be improved by the disclosed method, resulting in more accurate capacity values.

(25) FIG. 4 shows a schematic diagram of the disclosed electrical energy storage system 40 according to an embodiment. The electrical energy storage system 40 comprises an electrical energy storage unit 41 and a device 42 for determining the capacity of the electrical energy storage unit. Said device 42 comprises a means 44 that is configured to perform the steps of the disclosed method. The device 44 in turn controls a power-electronics component 43 that, for instance, regulates the supply of energy to an electric motor.