Method for minimizing cell aging of a battery or battery with a device for minimizing cell aging of the battery

Abstract

Method for minimizing cell aging of a battery (1), comprising: - Recording battery parameters that describe the current state of the battery (1), - Check, using a given aging model and the battery parameters, whether the battery is in an aging state in which the cell aging of the battery (1) during a cyclic charge or discharge is less than in a rest state in which no charge or discharge of the battery takes place, - cyclic partial discharge and partial charging of the battery (1) if the battery (1) is in such an aging state.

Description

State of the art

[0001] The present invention relates to a method for minimizing cell aging of a battery or to a battery with a device for minimizing cell aging of the battery.

[0002] Electric vehicles (EVs, HEVs, PHEVs, etc.) often use electrochemical cells, such as lithium-ion or iron-metal hydride cells, as energy storage devices. When operating as energy storage devices, these cells are not in a stable equilibrium state. This means that the cells discharge with a current typically in the microampere (µA) or milliampere (mA) range. At the same time, irreversible degradation reactions occur within the cells. The cell structures undergo a change in their chemical composition, which limits their service life. These reactions are therefore referred to as cell aging. Cell aging depends on many factors, known as aging or acceleration factors. The most important of these is temperature, but the state of charge (SOC) of the cell is also crucial.These two factors largely determine calendar aging, i.e., the aging that is independent of the battery's or cell's usage. Usage-dependent aging factors (so-called cyclic aging) include, for example, the charging and discharging currents, the charge / discharge cycle, as well as temperature and other factors. Cell aging can be expressed, for example, in terms of the usable capacity C; that is, at time t0, the capacity C(t0) = C0. At later times, due to aging, C(t>t0) < C0.

[0003] Aging can be described in a model (abstractly) as a function dependent on aging factors and state: C(t)=f(AF1,AF2,…,Z1,Z2,…,C0,t)

[0004] AF1, AF2, etc. are the aging factors mentioned above. Z1, Z2, etc. describe states such as storage, loading, unloading, etc., and t is time.

[0005] An analogous description can also be given for the aging of the cell's internal resistance or other relevant properties for which the cell was designed. Disclosure of the invention

[0006] The inventive method for minimizing cell aging of a battery comprises acquiring battery parameters that describe the current state of the battery, checking, based on a predetermined aging model and the battery parameters, whether the battery is in an aging state in which the cell aging of the battery is less during cyclic charging or discharging than in a resting state in which no charging or discharging of the battery takes place, and cyclically partially discharging and partially charging the battery if the battery is in such an aging state.

[0007] The battery according to the invention, with a device for minimizing cell aging of the battery, comprises a measuring unit configured to record battery parameters describing the current state of the battery, a test unit configured to check, based on a predetermined aging model and the battery parameters, whether the battery is in an aging state in which the cell aging of the battery is less during cyclic charging or discharging than in a resting state in which no charging or discharging of the battery takes place, and a load unit configured to subject the battery to cyclic partial discharge and partial charging if the battery is in such an aging state.

[0008] The inventive method and battery are characterized by reduced cell aging. This means that such a battery, or a battery to which the method is applied, has an increased service life. This results in particular cost advantages, environmental benefits, and improved user-friendliness.

[0009] The dependent claims describe preferred embodiments of the invention.

[0010] In particular, the cyclical partial discharge and partial charging of the battery is achieved via a resonant circuit. This resonant circuit also allows for the timed synchronization of the partial discharge and partial charging, thus eliminating the need for additional clocks. Specifically, a resonant circuit that utilizes a coil and a capacitor as oscillating elements, unlike some other energy storage devices suitable for this application, is not itself subject to cell aging.

[0011] It is advantageous if the battery parameters include battery current, battery voltage, battery temperature, state of charge, and / or cell age. These parameters can be acquired using simple sensors, thus enabling cost-effective and reliable implementation.

[0012] Furthermore, it is advantageous if the cyclical partial discharge and partial charging of the battery only occurs when the battery is connected to a charger. In this case, any energy loss caused by the partial charging and discharging can be compensated for. The battery thus remains ready for use at all times.

[0013] It is equally advantageous if the energy released during battery discharge is temporarily stored for subsequent charging. This minimizes the energy required for partial charging. The battery can store energy for longer periods when no charger is connected, and energy consumption is reduced when a charger is connected.

[0014] In particular, the battery is only partially discharged and partially charged cyclically when its state of charge is above a certain threshold. Complete discharge of the battery, or discharge below a level required by any other components connected to the battery, is avoided.

[0015] It is advantageous if the load unit of a battery according to the invention comprises a resonant circuit. A resonant circuit can simultaneously enable the timed synchronization of the partial discharge and partial charging of the battery, thus eliminating the need for additional clock generators. In particular, a resonant circuit that utilizes a coil and a capacitor as oscillating elements, unlike some other energy storage devices suitable for this application, is itself not subject to cell aging.

[0016] Furthermore, it is advantageous if the battery, equipped with a device for minimizing cell aging, is a vehicle battery, particularly a traction battery. Especially in this environment, high performance demands are placed on batteries, while costs are subject to considerable pressure. Both requirements are met by a battery according to the invention.

[0017] Specifically, the battery is a lithium-ion cell or an iron-metal hydride cell. These cells exhibit particularly favorable properties regarding cell aging for this process. Brief description of the drawings

[0018] Exemplary embodiments of the invention are described in detail below with reference to the accompanying drawing. The drawing shows: Fig. 1 a flowchart of the method according to the invention in an exemplary embodiment, and Fig. 2 a schematic representation of a battery according to the invention in an exemplary embodiment. Embodiments of the invention

[0019] The cell aging, or overall aging, of a battery results from calendar aging and cyclic aging. Calendar aging and cyclic aging are superimposable in most batteries. Cell aging can be described (abstractly) as a function f(AF1, AF2, ..., Z1, Z2, ..., C0, t) that depends on the aging factor and the battery's state. Here, AF1, AF2, etc., are the aforementioned aging factors. Z1, Z2, etc., describe states such as storage, charging, discharging, etc., and t is time. There are also electrochemical structures in batteries where the time derivative of a function describing the battery's cell aging has a minimum at a specific operating state. Thus, the rate of cell aging is minimal in this operating state.

[0020] It has been observed in many batteries that cyclic aging is reduced the smaller the cycle (charge and discharge interval). If, at the same time, calendar aging is inhibited by cycling (i.e., by charging and discharging the battery), then simple storage, as is typical, is less advantageous than cycling with small cycles. Therefore, by inducing small cycles, also known as mini-cycles, the aging rate of a corresponding electrochemical system can be reduced, or the battery's lifespan can be increased.

[0021] Fig. Figure 1 shows a flowchart of a method according to the invention for minimizing cell aging of a battery 1 in an exemplary embodiment. The method is preferably carried out cyclically.

[0022] In the first process step S1, battery parameters describing the current state of battery 1 are recorded. These parameters include, for example, battery current, battery voltage, battery temperature, state of charge, and / or cell age. Typically, the battery parameters recorded are based on an aging model that describes the cell aging of battery 1. These parameters can be acquired using suitable sensors. Many batteries already have such sensors, which can be used accordingly.

[0023] In a second process step S2, a test is performed using a predefined aging model and the battery parameters to determine whether the battery is in an aging state in which the cell aging of battery 1 during cyclic charging and discharging is lower than in a resting state where the battery is not being charged or discharged. Such an aging model can be determined either from theoretical considerations or through a series of experiments. For example, several identical batteries could be operated under different battery parameters and conditions in order to measure battery aging after a defined period (e.g., by determining capacity or internal resistance) and to approximate an aging function from this data.

[0024] In the embodiment described here, the aging model is defined by an aging function that describes the maximum battery capacity over time as a function of the recorded battery parameters and at least one battery state. The battery state is either a first state, in which the battery is cyclically charged or discharged, or a second state, in which the battery is not cyclically charged or discharged. The derivative of this aging function is calculated. This derivative can be interpreted as the rate of aging, i.e., the decrease in capacity over time. A first result is calculated based on the current time, the recorded battery parameters, and assuming the first state. A second result is calculated based on the current time, the recorded battery parameters, and assuming the second state.The first result is compared with the second result.

[0025] If the first result value is less than or equal to the second result value, the battery is in an aging state in which the cell aging of battery 1 during cyclic charging or discharging is greater than or equal to that in a resting state in which the battery is not being charged or discharged. In this case, the procedure branches back to the first process step S1.

[0026] If the first result value is greater than the second result value, the battery is in an aging state in which the cell aging of battery 1 during cyclic charging or discharging is less than in a resting state in which the battery is not being charged or discharged. The procedure then branches into a third process step S3.

[0027] In the third process step, S3, battery 1 undergoes a cyclic partial discharge and partial charge. This brings the battery to its initial state, thereby reducing cell aging compared to its resting state. Such cyclic partial discharge and partial charge of battery 1 can be achieved via a resonant circuit. In this process, the battery excites the resonant circuit by supplying energy, thus slightly discharging battery 1. When the resonant circuit reaches a defined oscillation state, energy is supplied back to the battery, recharging it. Alternatively, other energy storage devices can be used. For example, a charge exchange between two batteries could occur within a battery pack.

[0028] In the embodiment of the method described here, the process branches back to the first process step S1 after a predefined time. The number of partial charging or partial discharging cycles performed in the third process step depends essentially on the duration of these cycles. If the partial charging or partial discharging is carried out via a resonant circuit, the duration of a cycle can typically be determined by dimensioning a capacitance or an inductance in the resonant circuit.

[0029] The procedure can further include an initial step in which it is monitored whether battery 1 is connected to a charger. The procedure only continues if a charger is connected. In practice, losses occur during cyclic partial discharge and partial charging of battery 1. Thus, the battery's state of charge can decrease due to this cyclic partial discharge and partial charging. To prevent this, it is therefore advantageous to compensate for these losses by charging the battery via the charger. For example, energy could be supplied to the resonant circuit via the charger.

[0030] Alternatively or additionally, the process can include a fourth step that monitors whether the battery's state of charge (SOC) exceeds a threshold. Cyclic partial discharge and partial charging of the battery only occurs if this condition is met. Such a threshold could, for example, be 90% of the maximum SOC. Since losses occur in practice during cyclic partial discharge and partial charging of battery 1, and the battery's state of charge can decrease as a result of this process, this prevents the battery from entering a state where a potentially required charge is no longer available. Furthermore, deep discharge of battery 1 is also prevented.

[0031] Fig. Figure 2 shows a schematic representation of a battery 1 according to the invention in an exemplary embodiment. In this embodiment, the battery 1 comprises a single battery cell 6 with a positive and a negative contact. In other embodiments, however, the battery 1 can also comprise several battery cells connected in parallel or in series.

[0032] A device 5 for minimizing cell aging of battery 1 is connected to the two contacts of battery cell 6. The cell aging of battery 1 refers in particular to the cell aging of battery cell 6.

[0033] The device 5 comprises a measuring unit 2, which is configured to acquire battery parameters describing the current state of the battery cell 6. For this purpose, the measuring unit 2 includes a voltage sensor, a current sensor, and a temperature sensor. Furthermore, the measuring unit 2 may include a timer for recording a time. Logic within the measuring unit 2 allows the state of charge of the battery 1 to be determined from the voltage detected by the voltage sensor and the current detected by the current sensor. The measuring unit 2 is thus suitable for implementing the first process step S1 described in the exemplary embodiment of the method according to the invention. The acquired battery parameters, i.e., the voltage, current, temperature, and time, are transmitted to a test unit 3.

[0034] The device comprises the test unit 3, which is configured to check, based on a predefined aging model and the battery parameters, whether the battery 1 is in an aging state in which the cell aging of the battery 1 during cyclic charging or discharging is less than in a resting state in which no charging or discharging of the battery 1 takes place. For this purpose, the second process step S2 described in the exemplary embodiment of the method according to the invention is carried out by a computing unit included in the test unit. The aging function could be stored in a digital memory as a mathematical function. If the test unit 3 determines that the cell aging of the battery 1 during cyclic charging or discharging is less than in a resting state, the test unit outputs an activation signal.Test unit 3 is thus a unit that measures the condition and aging factors of the cell and, based on a pre-parameterized aging model, determines whether, under the external conditions (the given battery parameters), cyclic partial discharging and partial charging of the battery or purely calendar-based aging results in overall lower cell aging. If cyclic partial discharging and partial charging of battery 1 results in overall lower cell aging, a load unit 4 is connected to battery 1, which ensures this cyclic partial discharging and partial charging of battery 1.

[0035] The device also includes the load unit 4, which is configured to subject the battery 1 to cyclic partial discharge and partial charging if the battery 1 is in an aging state where cyclic partial discharge and partial charging of the battery 1 results in an overall lower cell aging. The load unit is connected to the test unit 3 in such a way that the latter can be activated by the activation signal. If activation by the activation signal occurs, an electromagnetic resonant circuit comprised of the load unit is connected to the positive and negative contacts of the battery cell 6 in such a way that the resonant circuit is set into oscillation by energy emitted by the battery cell 6.During a return oscillation, the resonant circuit is connected via a charging circuit to the positive contact and the negative contact of battery cell 6 in such a way that battery cell 6 is charged via the resonant circuit.

[0036] In its simplest form, such a resonant circuit consists of a capacitor, an inductor, and a resistor of the lowest possible value. The resonant circuit can be connected to battery cell 6 via a switch. Battery cell 6 discharges into the resonant circuit. Subsequently, battery cell 6 is recharged via the resonant circuit. The oscillation is damped. To prevent the battery from discharging, the cyclic partial discharge and partial charging of battery 1 could only occur when connected to an electric charger. In this case, for example, a charging current could be detected by measuring unit 2, and activation of load unit 4 could be prevented if there is no charging current or if the charging current is too low. Similarly, activation of load unit 4 could be prevented if the state of charge of battery 1 falls below a residual charge threshold (e.g., 90% of the state of charge).By appropriately selecting the capacity and inductance, a cycle amplitude and cycle frequency can be determined that are adapted to the specific aging characteristics of battery cell 6. The cycle amplitude is described by the amount of energy that battery cell 6 releases or receives during cyclic partial discharge and partial charging. The cycle frequency describes the duration of one cycle of cyclic partial discharge and partial charging.

[0037] In addition to the above written revelation, explicit reference is made to the revelation of the Fig. 1 to 2 referred.

Claims

[1] Method for minimizing cell aging of a battery (1), comprising: - Recording battery parameters that describe the current state of the battery (1), - Check, using a given aging model and the battery parameters, whether the battery is in an aging state in which the cell aging of the battery (1) during a cyclic charge or discharge is less than in a rest state in which no charge or discharge of the battery takes place, - cyclic partial discharge and partial charging of the battery (1) if the battery (1) is in such an aging state. [2] Method according to claim 1, characterized by , that the cyclical partial discharge and partial charging of the battery takes place via a resonant circuit. [3] Method according to any one of the preceding claims, characterized bythat the battery parameters include battery current, battery voltage, battery temperature, state of charge and / or cell age. [4] Method according to any one of the preceding claims, characterized by that the cyclical partial discharge and partial charging of the battery only occurs when the battery is connected to a charger. [5] Method according to any one of the preceding claims, characterized by , that the energy released during discharge of the battery is temporarily stored for subsequent charging of the battery. [6] Method according to any one of the preceding claims, characterized by , that the cyclic partial discharge and partial charging of the battery only occurs when the battery's state of charge is above a threshold. [7] Battery (1) with a device for minimizing cell aging of the battery (1), comprising: - a measuring unit (2) which is set up to record battery parameters that describe the current state of the battery, - a test unit (2) which is set up to check, based on a predefined aging model and the battery parameters, whether the condition of the battery (1) is an aging state in which the cell aging of the battery (1) during a cyclic charge or discharge is less than in a rest state in which no charge or discharge of the battery (1) takes place, and - a load unit (3) configured to subject the battery (1) to a cyclic partial discharge and partial charge if the battery (1) is in such an aging state. [8] Battery (1) with a device for minimizing cell aging of the battery (1) according to claim 7, characterized by that the load unit comprises a resonant circuit. [9] Battery (1) with a device for minimizing cell aging of the battery (1) according to claim 7, characterized by , that the battery (1) is a vehicle battery, in particular a traction battery. [10] Battery (1) with a device for minimizing cell aging of the battery (1) according to claim 7, characterized by that the battery is a lithium-ion cell or an iron-metal hydride cell.

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