KR101504804B1 - Apparatus and method for estimating state of secondary battery considering aging - Google Patents

Abstract

A method and an apparatus for estimating the state of a secondary battery are provided. The apparatus determines a first open circuit voltage (OCV) curve for the first electrode and a second reference OCV curve for the second electrode. The apparatus determines the shifted second OCV curve by shifting the reference second OCV curve according to the reduced capacity due to degradation of the secondary battery. The apparatus determines a cell OCV curve based on the reference first OCV curve and the shifted second OCV curve and estimates a state of charge of the secondary battery.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a secondary battery,

The present invention relates to a secondary battery, and more particularly, to a method and apparatus for estimating a state of a secondary battery in consideration of aging of the secondary battery.

As the use of portable electric appliances such as video cameras, portable phones, and portable PCs is being activated, the importance of secondary batteries, which are mainly used as driving power sources, is increasing. In particular, the lithium secondary battery has a higher energy density per unit weight and can be rapidly charged as compared with conventional lead-acid batteries and other secondary batteries such as nickel-cadmium batteries, nickel-hydrogen batteries and nickel-zinc batteries. It is actively proceeding.

Unlike conventional primary batteries, which can not be charged, secondary batteries capable of charging and discharging are under active research in the development of advanced fields such as digital cameras, cellular phones, notebook computers, and hybrid vehicles. Examples of the secondary battery include a nickel-cadmium battery, a nickel-metal hydride battery, a nickel-hydrogen battery, and a lithium secondary battery. Among them, the lithium secondary battery has a higher operating voltage than the nickel-cadmium battery or the nickel-metal hydride battery, and has excellent energy density per unit weight.

It is necessary to estimate the state of the secondary battery in order to correctly obtain the charging rate. The charge rate can also be expressed as available capacity or SOC (state of charge). Generally, the charging rate is estimated based on the open circuit voltage (OCV) of the secondary battery.

Continuous charging / discharging degrade the secondary battery irreversibly. The capacity of the secondary battery is reduced due to aging of the secondary battery, but the OCV is maintained. Therefore, if the state of the secondary battery is estimated by only the OCV without considering the aging of the secondary battery, an error may occur. .

The present invention provides an apparatus and method for estimating the state of a secondary battery in consideration of aging of the secondary battery.

In one aspect, a method of estimating a state of a secondary battery including a first electrode and a second electrode is provided. The method includes determining a reference first OCV curve as an OCV curve for the first electrode and a reference second OCV curve as an OCV curve for the second electrode, Determining a second OCV curve shifted by shifting the reference second OCV curve according to the reduced capacity, determining a second OCV curve based on the reference first OCV curve and the shifted second OCV curve Determining a cell OCV curve, and estimating a state of charge of the secondary battery based on the cell OCV curve.

The first electrode may be a cathode, and the second electrode may be an anode.

The cell OCV curve may be determined as OCV _cell = OCV _cathode -OCV _{anode , shifted} , the OCV _cathode may be the reference first OCV curve, the OCV _{anode , and the shifted} may be the shifted second OCV curve.

In another aspect, an apparatus for estimating the state of a secondary battery including a first electrode and a second electrode is provided. The apparatus includes a memory for storing a reference first OCV curve as an OCV curve for the first electrode and a reference second OCV curve as an OCV curve for the second electrode, Determining a second OCV curve shifted by shifting the reference second OCV curve according to the reduced capacity and determining a shifted second OCV curve based on the reference first OCV curve and the shifted second OCV curve, And estimating a charge state of the secondary battery based on the cell OCV curve.

In another aspect, the secondary battery device includes a secondary battery including a first electrode and a second electrode, and a controller for estimating a state of the secondary battery, wherein the controller includes an OCV (Open Circuit Voltage) A memory for storing a reference first OCV curve as a curve and a reference second OCV curve as an OCV curve for the second electrode, a memory for determining a decrease in capacitance due to the degeneration of the secondary battery, Determining a shifted second OCV curve by shifting the second OCV curve, determining a cell OCV curve based on the reference first OCV curve and the shifted second OCV curve, And estimates the state of charge of the battery.

The state of the secondary battery can be accurately estimated according to the aging of the secondary battery.

1 is a graph showing an example of OCV-SOC characteristics of a secondary battery having a mixed anode.
2 is a graph showing an example of discharge characteristics of a secondary battery having a mixed anode.
3 is a graph showing voltage change with time.
4 is a graph showing time characteristics of a secondary battery having a mixed anode.
Figure 5 shows the change in OCV curve with aging.
6 is a graph showing an OCV model considering aging.
7 is a graph showing cell OCV.
8 is a graph showing an example of estimating OCV in BoL.
9 is a graph showing an example of estimating OCV in EoL.
10 is a flowchart illustrating a state estimation method according to an embodiment of the present invention.
11 is a block diagram showing a secondary battery device to which an embodiment of the present invention is applied.

The secondary battery to which the present invention is applied can be used in various places such as an electric vehicle, an electric storage, a mobile phone, and the like as a chargeable electric source. An electric vehicle refers to a vehicle that includes one or more electric motors as propulsive forces. The energy used to propel the electric vehicle includes an electrical source such as a rechargeable battery and / or a fuel cell. The electric vehicle may be a hybrid electric vehicle that uses a combustion engine as another power source.

The secondary battery to which the present invention is applied includes a cathode, an anode, and a separator. Transition metals are widely used as cathode active materials for secondary batteries. The secondary battery may be a single system including one active material for each electrode.

Alternatively, the anode of the secondary battery may be a blended cathode. Positive electrode mixture may comprise a mixture comprising LiFePO _4, it expressed as Li [NiMnCo] O ₂ and abbreviations LFP represented by the acronym NMC. The NMC includes at least one transition metal selected from Ni, Mn, and Co. LFP includes an olivine-type lithium phosphate compound. LFP is also called olivine material. NMC in the mixture can have a mass ratio of 10% to 90%. For example, the following may be a mass ratio of NMC: LFP of about 7: 3, but the technical idea of the present invention is not limited thereto.

Now, the dynamic characteristics of a secondary battery having a mixed anode will be discussed first.

1 is a graph showing an example of OCV-SOC characteristics of a secondary battery having a mixed anode.

NMC: LFP is a predominant active material when SOC (State of Charge) is about 30% or more, and LFP is a predominant active material when SOC is about 30% or less because LFP is a mass ratio of 7: 3. Lithium ions are first extracted from LFP at low SOC. Because LFP has faster kinetics than NMC. Next, lithium ions are extracted from the NMC as the SOC increases.

In the graph, OCV (Open Circuit Voltage) characteristics show a rapid change between SOC of 20 ~ 40%, which is a feature that does not appear in a single system in which a conventional single active material is used. That is, when the SOC is about 30% or more, the OCV characteristics are shown according to the NMC, and when the SOC is about 30% or less, the OCV characteristics according to the LFP are shown.

2 is a graph showing an example of discharge characteristics of a secondary battery having a mixed anode.

When the voltage is below about 3.2 V, the LFP begins to affect the current. 3.2V, which LFP starts to affect, is called a transition voltage. As the LFP decreases, the discharge rate (slope of the curve in Fig. 3) decreases below the transition voltage.

More specifically, the current-voltage above transition -I = ΔV / R _NMC (ΔV is the voltage variation, the resistance value of the _NMC NMC) can be represented by a transition current -I = ΔV / R + _NMC at a voltage less than ΔV _LFP / R _LFP (? V _LFP / R _LFP is the voltage change amount and resistance value of the LFP).

3 is a graph showing voltage change with time. When the SOC is 32%, the LFP is almost empty of lithium ions and the NMC is fully lithiated.

The discharge pulse intercalates lithium into both LFC and NMC, and the dynamic voltage falls below 3.2V.

As charging begins in about 20 seconds, intra-particle relexation is performed first in each of the LMP and NMC. This causes a voltage difference between the NMC and the LFP.

Then, inter-particle relexation is performed around about 42 seconds. Lithium is transferred from the LFP to the NMC. If the discharge causes the dynamic voltage to fall below the transition voltage, the lithium first fills the LFP before the NMC is full.

4 is a graph showing time characteristics of a secondary battery having a mixed anode.

Suppose that the current is cut off to the secondary battery after about 1 second. OCV rises sharply within 0.1 second after current interruption. Thereafter, the single system gently rises and OCV stabilizes after 5 minutes. Generally, the OCV of a single system is lower than 20mV after 1 second after current interruption. However, the mixed system increases OCV further after 5 minutes due to the above two-step relaxation. Here, the value of the OCV rising after 1 second after the current interruption is larger than 50 mV. The OCV of the mixed system is shown to stabilize after 8 minutes.

The mixing system can be defined as a rise in OCV beyond a certain range (eg, 50 mV) after a certain time (eg, 1 second) after current interruption. The value of the rising OCV may vary depending on the mass ratio of the active material or the active material to be mixed.

As shown in Figs. 1 to 4, the mixed anode exhibits a two-stage discharge / charging characteristic through two active materials. This is called two-stage relaxation.

The mixed system to which the state estimation method proposed below is applied is a secondary battery having at least one of the following characteristics.

(1) The anode contains at least two active materials.

(2) SOC-OCV characteristics exhibit two-step relaxation due to the exchange of Li ions between the cathode active materials.

(3) OCV increases above the threshold value (eg 50mV) after 1 second after current interruption. Or OCV above threshold (eg 100mV) increases after 10 seconds after current interruption.

A state estimation method according to the present invention will now be described.

Figure 5 shows the change in OCV curve with aging.

The OCV curve in the initial three cycles (e.g., three charges) is compared with the OCV curve after 621 cycles. The 'Z' on the horizontal axis corresponds to the amount of Li at the cathode and is equivalent to SOC. E.g. Z = 1 corresponds to the initial SOC 100%, and Z = 0 corresponds to the initial SOC 0.

It shows that OCV curves are changing due to aging after 621 cycles.

Therefore, in order to estimate the state of the secondary battery due to aging, it is necessary to consider the change of the OCV curve.

Hereinafter, BoL (Begin of Life) represents a state of a secondary battery having a reference initial OCV curve, and EoL (End of Life) represents a current state of a secondary battery estimated after a certain period of use.

6 is a graph showing an OCV model considering aging.

In BoL, the SOC is 100 when Z = 1.0 and the SOC is zero when Z = 0. Assuming a 20% capacity reduction in EoL, the SOC is 100 at Z = 0.8.

The graph (A) of FIG. 6 shows the curves of the anode voltage and the cathode voltage in BoL and EoL.

The graph (A) in Fig. 6 shows a curve of the anode voltage and a curve of the cathode voltage shifted by 20% _w to the left in BoL. In BoL, the curve of the anode voltage and the curve of the anode voltage in the EoL almost coincide.

7 is a graph showing cell OCV.

Cell OCV OCV _cell = OCV _cathode - OCV _anode . 7 is the same as the graph (B) of Fig. 6, and the graph (A) of Fig. 7 is a reconstruction of the cell OCV from the graph (B). In EoL we can see that the cell OCV curve is almost identical to the cell OCV curve in BoL.

From this, it can be seen that the cell OCV curve can be modeled considering the reduction of capacity due to loss of Li. That is, if the capacity decrease of Li is known, the current cell OCV curve of the secondary cell can be obtained by shifting the initial cell OCV curve.

For example, in BoL (SOC = 0, Z = 0.05) and (SOC = 100, Z = 0.9). Then, in EoL, it is degraded by about 20% with (SOC = 0, Z = 0.05) and (SOC = 100, Z = 0.7).

The capacity EoL at _EoL can be expressed as:

8 is a graph showing an example of estimating OCV in BoL.

First, in the BoL, cell OCV and anode OCV are measured to construct a cell OCV curve and a positive OCV curve. In the graph, 'Cell OCV' is a cell OCV curve constructed by using measured values, and 'Cathode OCV' is a positive OCV curve formed by using measured values.

Cathode OCV curves include OCV _{anode , BoL} = OCV _{cathode , BoL} - OCV _{cell , BoL} .

9 is a graph showing an example of estimating OCV in EoL. The anode OCV and the cathode OCV in BoL are obtained from the above-described FIG.

The negative electrode greatly influences the degradation of the secondary battery. The anode OCV curve at BoL and the anode OCV curve at EoL are substantially the same because there is almost no degradation at the anode.

Assume that a capacity degradation of about 6.2% occurs. Since 93.8% of Li ions are inserted into the cathode, Z = 1 in EoL corresponds to Z = 0.938 in BoL. _Obtain the OCV _{anode and shifted} by shifting the negative OCV curve by the reduced capacity.

From the anode OCV curve and the shifted cathode OCV curve, the cell OCV curve at _EoL can be obtained as follows: OCV _{cell , EoL} = OCV _{cathode , BoL} - OCV _{anode , shifted} .

The cell OCV curve thus obtained is shown as 'Cell OCV (EoL, model)' in FIG. 9, which is almost the same as the experimentally obtained result (indicated as 'Cell OCV (EoL, experiment)').

10 is a flowchart illustrating a state estimation method according to an embodiment of the present invention.

The reference second OCV curve, which is the OCV curve for the second electrode of the secondary battery, is determined based on the reference first OCV curve as the OCV curve for the first electrode of the secondary battery at step S410. Let the first electrode be an anode and the second electrode be a cathode. The reference first OCV curve is the positive OCV curve at BoL and the reference second OCV curve is the negative OCV curve at BoL.

The decrease in capacity due to degradation of the secondary battery is determined (S420).

The shifted second OCV curve is determined by shifting the reference second OCV curve according to the reduced capacity (S430). The cathode OCV curve at BoL can be shifted by a reduced amount to determine the shifted OCV curve.

The cell OCV curve is determined based on the reference first OCV curve and the shifted second OCV curve (S440). The cell OCV curves at EoL from the anode OCV curve and the shifted cathode OCV curve at BoL can be obtained as follows: OCV _{cell , EoL} = OCV _{cathode , BoL} - OCV _{anode , shifted} .

The charging state of the secondary battery is estimated based on the cell OCV curve (S450). Once the cell OCV curve of the degenerated secondary cell is determined, the measured cell OCV can be applied to the cell OCV curve to determine the corresponding SOC.

11 is a block diagram showing a secondary battery device to which an embodiment of the present invention is applied.

The secondary battery 110 supplies electric power to the load 180. [ The secondary battery 110 includes an anode, a cathode, and a separator. The anode of the secondary battery 110 may be a blended cathode.

The load 180 may be an electric motor attached to the electric vehicle. An electric vehicle refers to a vehicle that includes one or more electric motors as propulsive forces. The energy used to propel the electric vehicle includes an electrical source such as a rechargeable battery and / or a fuel cell. The electric vehicle may be a hybrid electric vehicle that uses a combustion engine as another power source.

The temperature sensor 130 measures the temperature of the secondary battery 110. The current sensor 140 measures the current of the secondary battery 110. The voltage sensor 150 measures the voltage of the secondary battery 110. The measured temperature, current, and voltage are provided to the controller 120.

The controller 120 estimates the state of the secondary battery 110. The above-described state estimation method of the secondary battery can be implemented by the controller 120. [

The controller 120 includes a memory 121 and an estimator 125. [ The memory 121 stores a reference OCV curve. The estimator 125 determines a cell OCV curve and estimates the state (e.g., SOC) of the secondary battery 110. [

The estimator 125 may include a processor, an application-specific integrated circuit (ASIC), other chipset, a logic circuit, and / or a data processing device. When the embodiment is implemented in software, the above-described techniques may be implemented with modules (processes, functions, and so on) that perform the functions described above. The module is stored in the memory 121 and can be executed by the processor. The memory 121 may be internal or external to the processor and may be coupled to the processor by various well known means.

Controller 120 may be part of a battery management system (BMS). Or when the power supply system is mounted in an electric vehicle, the controller 120 may be part of an electronic control unit (ECU) of the electric vehicle.

In the above-described exemplary system, the methods are described on the basis of a flowchart as a series of steps or blocks, but the present invention is not limited to the order of the steps, and some steps may occur in different orders or simultaneously . It will also be understood by those skilled in the art that the steps shown in the flowchart are not exclusive and that other steps may be included or that one or more steps in the flowchart may be deleted without affecting the scope of the invention.

Claims (19)

A method for estimating a state of a secondary battery including a first electrode and a second electrode,
Determining a reference first OCV curve that is an OCV curve for the first electrode and a reference second OCV curve that is an OCV curve for the second electrode;
Determining a decrease in capacity due to degradation of the secondary battery based on a ratio between a maximum amount and a minimum amount of the active material contained in the initial secondary battery and a difference between a maximum amount and a minimum amount of the active material included in the aged secondary battery;
Shifting the reference second OCV curve according to the reduced capacity to determine a shifted second OCV curve;
Determining a cell OCV curve based on the reference first OCV curve and the shifted second OCV curve; And
And estimating a state of charge of the secondary battery based on the cell OCV curve. The method according to claim 1,
Wherein the first electrode is a cathode and the second electrode is an anode. 3. The method of claim 2,
Wherein the cell OCV curve is determined as OCV _cell = OCV _cathode -OCV _{anode , shifted} , wherein the OCV _cathode is the reference first OCV curve, the OCV _{anode , and the shifted} is the shifted second OCV curve. Way. 3. The method of claim 2,
Wherein the anode comprises a first active material and a second active material. 5. The method of claim 4,
Wherein the first active material comprises at least one transition metal selected from Ni, Mn and Co. 6. The method of claim 5,
The first active material is a state estimation method, characterized in that, it expressed as Li [NiMnCo] O _2. 5. The method of claim 4,
Wherein the second active material comprises an olivine material. 8. The method of claim 7,
State estimation method, characterized in that the second active material comprises LiFePO _4. An apparatus for estimating a state of a secondary battery including a first electrode and a second electrode,
A memory for storing a reference first OCV curve which is an OCV curve for the first electrode and a reference second OCV curve which is an OCV curve for the second electrode;
Determining a decrease in capacity due to degradation of the secondary battery based on a ratio between a maximum amount and a minimum amount of the active material contained in the initial secondary battery and a difference between a maximum amount and a minimum amount of the active material included in the aged secondary battery,
Determining a shifted second OCV curve by shifting the reference second OCV curve according to the reduced capacity,
Determining a cell OCV curve based on the reference first OCV curve and the shifted second OCV curve,
And an estimator for estimating a state of charge of the secondary battery based on the cell OCV curve. 10. The method of claim 9,
Wherein the first electrode is a cathode and the second electrode is an anode. 11. The method of claim 10,
Wherein the cell OCV curve is determined as OCV _cell = OCV _cathode -OCV _{anode , shifted} , wherein the OCV _cathode is the reference first OCV curve, the OCV _{anode , and the shifted} is the shifted second OCV curve. Device. 12. The method of claim 11,
Wherein the anode comprises a first active material and a second active material. 13. The method of claim 12,
Wherein the first active material comprises at least one transition metal selected from Ni, Mn and Co. 14. The method of claim 13,
State estimator, characterized in that the first active material is represented by Li [NiMnCo] O _2. 13. The method of claim 12,
Wherein the second active material comprises an olivine material. 16. The method of claim 15,
Wherein the state estimator, characterized in that the second active material comprises LiFePO _4. A secondary battery including a first electrode and a second electrode;
And a controller for estimating a state of the secondary battery, wherein the controller
A memory for storing a reference first OCV curve which is an OCV curve for the first electrode and a reference second OCV curve which is an OCV curve for the second electrode;
Determining a decrease in capacity due to degradation of the secondary battery based on a ratio between a maximum amount and a minimum amount of the active material contained in the initial secondary battery and a difference between a maximum amount and a minimum amount of the active material included in the aged secondary battery,
Determining a shifted second OCV curve by shifting the reference second OCV curve according to the reduced capacity,
Determining a cell OCV curve based on the reference first OCV curve and the shifted second OCV curve,
And an estimator for estimating a state of charge of the secondary battery based on the cell OCV curve. 18. The method of claim 17,
Wherein the first electrode is a cathode and the second electrode is an anode. 19. The method of claim 18,
Wherein the cell OCV curve is determined as OCV _cell = OCV _cathode - OCV _{anode , shifted} , wherein the OCV _cathode is the reference first OCV curve, the OCV _{anode and the shifted} is the shifted second OCV curve. Device.

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