Alternating current signal extraction circuit and impedance online detection method

Abstract

The invention belongs to the field of energy storage system impedance detection, and particularly discloses an alternating current signal extraction circuit and an impedance online detection method. According to the Wheatstone bridge, at least one bridge arm is adopted as an impedance unit, the rest are resistor units, and branches are asymmetric. Under a direct current condition, the impedance unit presents a fixed resistance, so that the bridge meets a balance proportion, thereby counteracting direct current bias and suppressing noise. Under the alternating current condition, the impedance value is reduced along with increasing of the frequency, bridge balance is broken, differential voltage proportional to an alternating current component is generated between output nodes due to the fact that branch structures are asymmetric, and therefore alternating current signals are accurately extracted. Compared with the prior art, the circuit effectively extracts alternating current signals while suppressing direct current noise, the signal-to-noise ratio is remarkably improved, and the problem that the alternating current signals in a direct current system are difficult to accurately extract is solved.

Description

Technical Field

[0001] This application belongs to the field of impedance detection in energy storage systems, and more specifically, relates to an AC signal extraction circuit and an online impedance detection method. Background Technology

[0002] With the rapid development of electrochemical energy storage technology, battery energy storage systems have been widely used in grid frequency regulation, renewable energy grid connection, peak shaving and valley filling, and backup power. Large-scale energy storage systems typically consist of a layered architecture of cells, modules, and high-voltage battery clusters, with terminal voltages varying by orders of magnitude from 3.2V and tens of volts to thousands of volts. During long-term operation, the internal impedance of batteries changes with cycling, aging, temperature variations, and abuse conditions. Impedance parameters have become important quantitative indicators for assessing battery state of health (SOH), consistency, lifespan, and safety risks. Therefore, achieving online impedance detection across all levels of the energy storage system is of significant engineering importance for improving battery operational safety and maintaining the stability of energy storage stations.

[0003] Currently, the mainstream method for online electrochemical impedance spectroscopy (EIS) detection is to apply a small-amplitude AC current (or voltage) excitation signal to the battery and measure its AC voltage (or current) response at the port. To ensure the system operates within a near-linear range, the amplitude of the response voltage generated by the AC injection signal should generally not exceed 1% of the battery's DC voltage, or even lower. However, in practical engineering applications, the DC bias at the ports of different levels in energy storage systems varies significantly: the cell port is typically around 3.2V, the module port can reach 24 to 80V, while the battery cluster port voltage can reach 800 to 1500V. In this context, the AC response signal, superimposed with a large DC bias, has an extremely low signal-to-noise ratio, making accurate extraction difficult. Furthermore, the voltage divider chain at the high-voltage cluster level, long-distance cables, and the bandwidth characteristics of the isolation amplifier also introduce frequency-dependent attenuation, weakening the low-frequency AC signal. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the purpose of this application is to provide an AC signal extraction circuit and an online impedance detection method, aiming to solve the problem that the extraction of AC components in existing DC systems is limited by extremely low signal-to-noise ratios and is difficult to extract accurately.

[0005] To achieve the above objectives, in a first aspect, this application provides an AC signal extraction circuit, comprising: a first bridge arm unit, a second bridge arm unit, a third bridge arm unit, and a fourth bridge arm unit; the first bridge arm unit and the second bridge arm unit are connected in series to form a first branch, the third bridge arm unit and the fourth bridge arm unit are connected in series to form a second branch, the first branch and the second branch are connected in parallel between a first input node and a second input node, and the series nodes of the first branch and the second branch are respectively defined as a first output node and a second output node, to form a first Wheatstone bridge; among the four bridge arm units, at least one bridge arm unit is an impedance unit, and the remaining bridge arm units are resistance units, and the first branch and the second branch have asymmetrical structures; wherein, the impedance value of the resistance unit is a fixed resistance value that does not change with frequency; the impedance unit... The element is configured such that: the impedance value is a fixed resistance value under DC conditions to allow the DC component to pass through, and the impedance value decreases as the signal frequency increases under AC conditions; the fixed resistance value of each bridge arm unit under DC conditions satisfies: the ratio of the fixed resistance value of the first bridge arm unit to the fixed resistance value of the second bridge arm unit is equal to the ratio of the fixed resistance value of the third bridge arm unit to the fixed resistance value of the fourth bridge arm unit; the first Wheatstone bridge is used to: when the received input signal contains both DC and AC components, make the potentials of the first output node and the second output node equal under the action of the DC component, thereby achieving DC bias cancellation; and enter an unbalanced state under the action of the AC component, thereby generating a differential voltage signal between the first output node and the second output node that is proportional to the AC component, in order to extract the AC signal.

[0006] In one embodiment, the first bridge arm unit and the fourth bridge arm unit are impedance units, or the second bridge arm unit and the third bridge arm unit are impedance units.

[0007] In one embodiment, the impedance unit includes: a first resistive element and a capacitive element; the first end of the resistive element is connected to the first end of the capacitive element and the input node respectively; the second end of the resistive element is connected to the second end of the capacitive element and the output node of the branch respectively; the first resistive element is a series, parallel or mixed combination of one or more resistors, and the capacitive element is a capacitor, or a series, parallel or mixed combination of multiple capacitors.

[0008] In one embodiment, the resistor unit includes: a second resistive element; a first end of the second resistive element is connected to an input node, and a second end of the second resistive element is connected to an output node of the branch in which it is located; the second resistive element is a resistor, or a series, parallel, or mixed combination of multiple resistors.

[0009] In one embodiment, the AC signal extraction circuit further includes: an amplitude conditioning unit, an input resistor, and a first isolation operational amplifier unit; the two input terminals of the first Wheatstone bridge are connected in parallel with the energy storage unit under test, and the two output terminals of the first Wheatstone bridge are respectively connected to the two input terminals of the amplitude conditioning unit; the two output terminals of the amplitude conditioning unit are connected in parallel with the input resistor, and then respectively connected to the two input terminals of the first isolation operational amplifier unit; the amplitude conditioning unit is configured to directly output to the first isolation operational amplifier unit when the AC signal amplitude is less than a preset value; and to limit the AC signal amplitude before outputting it to the first isolation operational amplifier unit when the AC signal amplitude is greater than or equal to the preset value; the first isolation operational amplifier unit is configured to isolate and differentially amplify the input AC component within the voltage range, and output a first AC voltage signal proportional to the AC component.

[0010] In one embodiment, the AC signal extraction circuit further includes: a first sampling unit, a second Wheatstone bridge, and a second isolation operational amplifier unit; the second Wheatstone bridge has the same structure as the first Wheatstone bridge; the first sampling unit is connected in series with the energy storage unit under test, and the two input terminals of the second isolation operational amplifier unit are connected in parallel with the first sampling unit; the two output terminals of the second isolation operational amplifier unit are respectively connected to the two input terminals of the second Wheatstone bridge; the first sampling unit is configured to convert the current signal flowing through the energy storage unit under test into a voltage signal proportional to the current signal; the second isolation operational amplifier unit is configured to isolate and amplify the voltage signal at both ends of the first sampling unit, and output the amplified voltage signal to the second Wheatstone bridge; the second Wheatstone bridge is configured to extract a second AC voltage signal proportional to the AC current component flowing through the energy storage unit under test from the voltage signal output by the second isolation operational amplifier unit.

[0011] In one embodiment, the AC signal extraction circuit further includes: a differential amplifier unit, and an ADC and a filter unit; the first to fourth input terminals of the differential amplifier unit are respectively connected to the two output terminals of the first isolated operational amplifier unit and the two output terminals of the second Wheatstone bridge; the output terminal of the differential amplifier unit is connected to the input terminal of the ADC and filter unit; the differential amplifier unit is configured to output an analog voltage signal proportional to the AC impedance of the energy storage unit under test based on the first AC voltage signal and the second AC voltage signal; the ADC and filter unit is configured to perform analog-to-digital conversion and digital filtering on the analog voltage signal output by the differential amplifier unit to obtain a digital signal characterizing the AC impedance.

[0012] Secondly, this application provides an online impedance detection method, comprising: applying an excitation to a storage unit under test (SUT) so that its terminal voltage signal contains a DC component and an AC component of a desired frequency; acquiring the terminal voltage signal of the SUT and passing it through a first Wheatstone bridge to obtain a first differential voltage signal reflecting the AC component after DC bias cancellation; acquiring a current signal flowing through the SUT and converting it into a first voltage signal; isolating and amplifying the first voltage signal and passing the amplified signal through a second Wheatstone bridge to extract a second differential voltage signal reflecting the AC current component, wherein the structure of the second Wheatstone bridge is consistent with the structure of the first Wheatstone bridge; performing calculations based on the first and second differential voltage signals to obtain an analog voltage signal proportional to the AC impedance of the SUT at the desired frequency, and performing analog-to-digital conversion and digital filtering on the analog voltage signal to obtain a digital signal characterizing the AC impedance of the SUT.

[0013] In one embodiment, acquiring the terminal voltage signal of the energy storage unit under test and passing it through a first Wheatstone bridge to obtain a first differential voltage signal reflecting the AC component after DC bias cancellation includes: acquiring the terminal voltage signal of the energy storage unit under test and passing it through a first Wheatstone bridge to obtain an AC signal reflecting the AC component after DC bias cancellation; when the amplitude of the AC signal is less than a preset value, identifying the AC signal as the first differential voltage signal and outputting it; when the amplitude of the AC signal is greater than or equal to the preset value, limiting the AC signal, identifying the limited AC signal as the first differential voltage signal and outputting it.

[0014] Overall, the technical solutions conceived in this application have the following beneficial effects compared with the prior art: This application employs a first Wheatstone bridge structure, in which at least one bridge arm unit is an impedance unit and the rest are resistive units. The first and second branches are structurally asymmetrical, causing the circuit to exhibit different characteristics under DC and AC conditions. Under DC conditions, the impedance units exhibit fixed resistance values, working with the resistive units to ensure that the fixed resistance values ​​of each bridge arm unit under DC conditions maintain a proportional relationship. Thus, under the influence of the DC component of the input signal, the bridge remains balanced, with the potentials of the first and second output nodes being equal, achieving DC bias cancellation and eliminating interference from the DC component in AC signal extraction. Under the influence of the AC component, the impedance value of the impedance units decreases as the signal frequency increases, disrupting the bridge balance. This causes an imbalance between the first and second branches due to structural asymmetry, generating a differential voltage signal proportional to the AC component between the first and second output nodes, thereby accurately extracting the AC signal. Compared with existing technologies, this effectively improves the signal-to-noise ratio and solves the problem of inaccurate AC component extraction in DC systems. Attached Figure Description

[0015] Figure 1 This is a structural block diagram of the AC signal extraction circuit provided in the embodiments of this application; Figure 2 This is a schematic diagram of the unified port signal model at each level of the AC signal extraction circuit provided in the embodiments of this application; Figure 3 This is a circuit topology diagram of the AC signal extraction circuit provided in the embodiments of this application; Figure 4 This application provides an embodiment of the AC signal extraction circuit, which operates under DC and AC conditions. The circuit diagram shows the working principle and equivalent circuit diagram of the circuit. Figure 5 This is a schematic diagram of the simulated voltage characteristics of the AC signal extraction circuit provided in this application embodiment before and after a weak AC injection signal under a strong DC bias background; Figure 6 This is an impedance detection topology diagram of the AC signal extraction circuit provided in the embodiments of this application; Figure 7 This is a schematic flowchart of the online impedance detection method provided in the embodiments of this application.

[0016] In all the accompanying drawings, the same reference numerals are used to denote the same elements or structures, wherein: 10 is the first bridge arm unit; 20 is the second bridge arm unit; 30 is the third bridge arm unit; and 40 is the fourth bridge arm unit. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0018] In this article, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. The symbol " / " in this article indicates that the related objects are in an "or" relationship; for example, A / B means A or B.

[0019] The terms "first" and "second," etc., used in the specification and claims herein are used to distinguish different objects, not to describe a specific order of objects. For example, "first response message" and "second response message," etc., are used to distinguish different response messages, not to describe a specific order of response messages.

[0020] In the embodiments of this application, the terms "exemplary" or "for example" are used to indicate that something is an example, illustration, or description. Any embodiment or design that is described as "exemplary" or "for example" in the embodiments of this application should not be construed as being more preferred or advantageous than other embodiments or design. Specifically, the use of the terms "exemplary" or "for example" is intended to present the relevant concepts in a specific manner.

[0021] In the description of the embodiments of this application, unless otherwise stated, "multiple" means two or more, for example, multiple processing units means two or more processing units, multiple elements means two or more elements, etc.

[0022] In the current context, the signal-to-noise ratio of the AC response signal superimposed with a large DC bias is extremely low, making it difficult to extract accurately. In addition, the voltage divider chain at the high-voltage cluster level, long-distance cables, and the bandwidth characteristics of isolation amplifiers also introduce frequency-dependent attenuation, further weakening the low-frequency AC signal.

[0023] Based on this, this application proposes an embodiment of an AC signal extraction circuit. Please refer to... Figure 1 , Figure 1 This is a structural block diagram of the AC signal extraction circuit provided in the embodiments of this application.

[0024] In this embodiment, the AC signal extraction circuit includes: a first bridge arm unit 10, a second bridge arm unit 20, a third bridge arm unit 30, and a fourth bridge arm unit 40; the first bridge arm unit 10 and the second bridge arm unit are connected in series to form a first branch, the third bridge arm unit 30 and the fourth bridge arm unit 40 are connected in series to form a second branch, the first branch and the second branch are connected in parallel between the first input node and the second input node, and the series nodes of the first branch and the second branch are respectively defined as the first output node and the second output node, so as to form a first Wheatstone bridge.

[0025] Understandably, the bridge arm unit refers to the basic circuit module that constitutes the bridge, which can be specifically implemented as a passive impedance network of resistors, capacitors, inductors or combinations thereof, or an active circuit containing operational amplifiers, used to provide specific impedance or gain; the first input node and the second input node are used to receive a mixed input voltage containing DC bias and AC signal, while the first output node and the second output node provide a differential output voltage that reflects the changes in the AC signal.

[0026] Specifically, the basic principle of electrochemical impedance spectroscopy is to inject a small-amplitude sinusoidal voltage or current signal near the steady-state operating point of the battery. By analyzing the amplitude and phase relationship between the excitation signal and the response signal, the frequency response function of the system under test at a specific frequency can be obtained. To ensure that the battery system is in an approximately linear operating range, the amplitude of the AC injection signal is usually selected to be within 1% of the DC operating point, thus satisfying the small-signal linearization modeling conditions.

[0027] When the above conditions are met, the battery port voltage or current signal can be expressed as a linear superposition of the DC and AC components: .

[0028] It should be noted that the voltage differences between different levels of an energy storage system are significant. The voltage of a battery cell is typically 3.2V, the module voltage is 24 to 80V, and the battery cluster voltage is 800V to 1500V. The response of the AC injection signal from a cell impedance measurement is typically 1% of the DC signal response, thus there is a significant difference between DC and AC quantities. .

[0029] Understandably, when the port signal is directly acquired via the sampling link, the ADC's range will be significantly occupied by a large DC component, making the proportion of millivolt-level AC signals extremely small and drastically reducing the acquisition accuracy. Further considering factors such as the high-voltage side amplitude conditioning circuit, cable parasitic parameters, isolation operational amplifiers, and front-end filtering networks, the overall signal path from the battery port to the sampling output can be uniformly abstracted as a linear time-invariant system, whose frequency domain relationship can be expressed as: Expanding the input signal, we have: .

[0030] in, This is the equivalent transfer function of the sampling link. In traditional sampling structures, this transfer function has the same gain characteristics for both DC and AC components in the low-frequency band, making it impossible to effectively separate the DC bias and the weak AC response at the hardware level.

[0031] The main reasons for the further attenuation of AC components at low frequencies include: high impedance in capacitive coupling, which cannot effectively couple AC components at low frequencies; reduced input impedance of the voltage divider resistor chain, which lowers the AC signal amplitude; and the high-ratio voltage division of the high-voltage battery cluster, which attenuates the AC signal proportionally. Furthermore, the bandwidth limitation of the isolation amplifier (typically >100kHz, but drift and noise limitations exist in the low-frequency range, reducing the effective signal-to-noise ratio) makes it extremely difficult to directly acquire weak AC injection signals in practical engineering environments.

[0032] To maintain consistent modeling and testing methods across the cell, module, and battery cluster levels, a unified modeling objective for multiple levels is proposed from the perspective of transfer function design. This objective is to achieve a significantly different overall transfer function characteristic across the DC and AC frequency bands through a specific hardware topology. The design objective can be expressed as: ; Where K is a constant gain that is approximately independent of frequency. This refers to the AC frequency band required for impedance testing.

[0033] It is understandable that the above two points constitute the core theoretical foundation of the hybrid Wheatstone bridge topology design proposed in this invention, and establish a unified model for the transfer function, such as... Figure 2 As shown, Figure 2 This is a schematic diagram of the unified port signal model of each level of the AC signal extraction circuit provided in the embodiments of this application.

[0034] It should be noted that this application selected the Wheatstone bridge as the basic architecture for improvement: in the four bridge arm units, at least one bridge arm unit is an impedance unit, the remaining bridge arm units are resistance units, and the first branch and the second branch have asymmetrical structures.

[0035] Understandably, among the four bridge arm units, at least one bridge arm unit is an impedance unit, and the remaining bridge arm units are resistive units. This constraint clearly defines the type and minimum number of components: "at least one" means the minimum quantity is one, but can be more than one; "remaining" refers to all other bridge arm units not classified as impedance units, which are all resistive units. Secondly, the first branch and the second branch have asymmetrical structures. This constraint is a strict supplement and specification of the first condition. Its core meaning is to define the physical connection relationship of these impedance units and resistive units in the bridge, ensuring that the first branch and the second branch do not exhibit symmetry in circuit topology.

[0036] Specifically, this means that impedance elements cannot be distributed evenly or mirror-imagely across the two branches. For example, even if both arms are impedance elements, they must be staggered, meaning one impedance element is located in the first branch and the other in the second branch, and their relative positions within their respective branches—for example, whether the upper or lower arm of a series voltage divider node—cannot be the same. This staggered connection method ensures that the sequence of bridge arm element types traversed by the first and second branches from the input node to the output node will be different, resulting in distinctly different voltage division ratio frequency characteristics in the two branches under the influence of AC signals.

[0037] Understandably, it is this inherent asymmetry determined by the topology itself that forces the bridge to operate in an unbalanced mode under AC conditions, thereby effectively converting the superimposed AC signals into differential output voltages. In other words, the arm unit types contained in the two branches are different from each other. Therefore, if all arms are impedance units, this principle is violated, or if the first and third arms are impedance units, this principle is also violated.

[0038] It should be noted that the impedance value of the resistor unit is a fixed resistance value that does not change with frequency; the impedance unit is configured such that the impedance value is a fixed resistance value under DC conditions to allow the DC component to pass through, and the impedance value under AC conditions decreases as the signal frequency increases.

[0039] It is understood that in the circuit of this application, the resistor unit and the impedance unit have clearly defined and distinct electrical characteristics. The resistor unit refers to the circuit part whose impedance value remains basically constant within the DC and target AC frequency bands. This impedance value does not change significantly with the signal frequency. It is typically implemented as a discrete resistor with a fixed resistance value or an equivalent resistance structure in an integrated circuit.

[0040] Understandably, an impedance element is a specially designed circuit network with frequency-selective characteristics, where its impedance value changes with the frequency of the excitation signal. Specifically, under DC or sufficiently low frequency conditions, the element is equivalent to a resistor with a fixed resistance value. This design aims to allow the DC component or quasi-DC bias in the circuit to pass smoothly, thereby ensuring that the bridge can establish the required static operating point under DC conditions. Under AC conditions, the impedance value of the element is configured to decrease as the signal frequency increases. This characteristic is usually achieved through an RC network formed by a resistor and a capacitor in parallel. In DC conditions, the capacitor branch is considered an open circuit, and the element impedance is its parallel resistance value. As the frequency increases, the capacitive reactance decreases, causing the overall equivalent impedance of the element to decrease.

[0041] Understandably, this differentiated design results in the bridge arm containing impedance units exhibiting a transmission path for AC signals where attenuation decreases with increasing frequency. When such impedance units are connected with purely resistive units in an asymmetrical topology to form a Wheatstone bridge, the bridge tends to maintain balance for DC signals or produce a fixed, small output that can be suppressed by subsequent circuitry, thus greatly suppressing the DC component. However, for superimposed AC signals, due to the different frequency response characteristics of the two branches, the bridge becomes unbalanced with frequency, effectively converting the AC signal into a differential voltage output. The characteristic of the impedance unit's impedance value decreasing with increasing frequency can specifically compensate for the attenuation introduced by external systems, which typically intensifies with decreasing frequency, thereby enhancing the ability to extract low-frequency AC signals.

[0042] It should be noted that the fixed resistance values ​​of each bridge arm unit under DC conditions satisfy the following: the ratio of the fixed resistance value of the first bridge arm unit 10 to the fixed resistance value of the second bridge arm unit 20 is equal to the ratio of the fixed resistance value of the third bridge arm unit 30 to the fixed resistance value of the fourth bridge arm unit 40.

[0043] It is understandable that the fixed resistance value here refers to the constant resistance value exhibited by the resistor unit under DC or extremely low frequency conditions, as well as the equivalent fixed resistance value presented by the impedance unit. This proportional relationship is set to establish a defined DC operating state for mixed input signals containing a large DC bias. Its core function is that, when this proportional relationship is satisfied, under conditions where only DC input exists, the first Wheatstone bridge composed of the first and second branches is theoretically in a DC balanced or near-balanced state. At this time, the DC differential-mode voltage generated between the first and second output nodes is theoretically zero or a very small, predictable static offset voltage. This design allows the subsequent differential amplifier circuit to efficiently suppress this DC common-mode component, thereby greatly reducing the impact of the large DC bias on the output signal and creating favorable conditions for the subsequent extraction of weak AC response signals.

[0044] It should be noted that the first Wheatstone bridge is used to make the potentials of the first output node and the second output node equal under the action of the DC component when the received input signal contains both DC and AC components, thereby achieving DC bias cancellation; and to enter an unbalanced state under the action of the AC component, thereby generating a differential voltage signal between the first output node and the second output node that is proportional to the AC component, so as to extract the AC signal.

[0045] Understandably, under the influence of the DC component, each bridge arm unit exhibits its fixed DC resistance value. By setting the resistance values ​​of each bridge arm to satisfy a specific proportional relationship, the bridge can theoretically achieve a DC balance state. In this state, the DC potential difference between the first and second output nodes is zero or a very small, predictable static offset. This means that the DC component is converted into an approximately equal common-mode voltage at the output, which can then be largely canceled or suppressed by subsequent differential amplifiers and other high common-mode rejection ratio circuits, effectively eliminating the impact of large DC bias on subsequent signal processing.

[0046] Understandably, under the influence of the AC component, since at least one bridge arm is an impedance unit whose impedance value varies with the signal frequency, and the first and second branches of the bridge have an asymmetrical design, the original DC balance condition of the bridge is broken, leading to an unbalanced state. This unbalanced state is dynamic and related to the amplitude and frequency of the AC component. As a result, a differential voltage signal proportional to the input AC component is generated between the first and second output nodes. This differential voltage signal is the effectively highlighted AC response signal extracted from the strong DC background, which can be further amplified and processed by subsequent circuits.

[0047] In summary, this first Wheatstone bridge, through its unique structural design, achieves both common-mode suppression of DC components and differential-mode extraction of AC components within the same circuit, thus solving the extraction challenges caused by extremely low signal-to-noise ratios and low-frequency signal attenuation in the prior art. The coordinated design of DC balance conditions and AC imbalance states is the core mechanism for this circuit to achieve efficient signal separation.

[0048] Furthermore, it should be noted that the impedance unit includes: a first resistive element and a capacitive element; the first end of the resistive element is connected to the first end of the capacitive element and the input node respectively; the second end of the resistive element is connected to the second end of the capacitive element and the output node of the branch respectively; the first resistive element is a combination of one or more resistors connected in series, parallel or mixed, and the capacitive element is a capacitor or a combination of multiple capacitors connected in series, parallel or mixed.

[0049] Understandably, this connection method places the first resistive element and the capacitive element in parallel. The first resistive element can physically be a single resistor or a network of multiple resistors connected in series, parallel, or a combination of both, with the aim of providing a defined total DC resistance value. Similarly, the capacitive element can be a single capacitor or a combination of multiple capacitors connected in series, parallel, or a combination of both, with the aim of providing one or a set of defined equivalent capacitance values.

[0050] Understandably, this parallel structure collectively determines the characteristics of the impedance unit: under DC or extremely low frequency conditions, the capacitive element is equivalent to an open circuit, and the impedance unit as a whole presents the fixed DC resistance value of the first resistive element, thus allowing the DC component to pass through to establish a static operating point; under AC conditions, the capacitive reactance of the capacitive element decreases as the signal frequency increases, causing the total impedance value after parallel connection to also decrease, achieving the design goal of impedance value decreasing with increasing frequency. This structure is a basic and preferred implementation of the impedance unit.

[0051] It should be noted that the resistor unit includes: a second resistive element; the first end of the second resistive element is connected to the input node, and the second end of the second resistive element is connected to the output node of the branch it belongs to; the second resistive element is a single resistor, or a combination of multiple resistors connected in series, parallel, or in a mixed configuration. Regardless of its internal configuration, its ultimate purpose is to provide a stable and frequency-invariant fixed resistance value for the resistor unit within the DC and operating frequency range. The structure of the resistor unit is simpler than that of the impedance unit. Its function is to cooperate with the impedance unit to form two asymmetrical branches and provide a defined DC voltage division ratio for the bridge circuit.

[0052] Specifically, this embodiment uses a resistor and a capacitor connected in parallel as an impedance unit, and a resistor as a resistance unit as an example. Please refer to... Figure 3 , Figure 3 This is a circuit topology diagram of the AC signal extraction circuit provided in the embodiments of this application.

[0053] exist Figure 3 In this configuration, the first bridge arm unit 10 and the fourth bridge arm unit 40 are impedance units, while the second bridge arm unit 20 and the third bridge arm unit 30 are resistance units; the same principle applies if they are interchanged. The first branch uses a high-pass filter formed by the first capacitor C1 connected in parallel with the first resistor R1, and then connected in series with the second resistor R2. The second branch uses a third resistor R3 connected in series with the fourth resistor R4, and the fourth resistor R4 is connected in parallel with the second capacitor C2. The output terminals are nodes A and B, respectively, and the voltage difference between them is the final AC output signal.

[0054] Understandably, the bridge circuit utilizes the difference in impedance characteristics of capacitors under DC and AC conditions (DC open circuit, AC short circuit) to perfectly eliminate DC bias and accurately extract weak AC ripple. Under DC conditions, C1 and C2 are considered open circuits, and the two branches become a purely resistive network with a bridge voltage division ratio of: ; .

[0055] Understandably, when , can be obtained = DC bias can be eliminated by using a differential method.

[0056] Under alternating current, capacitance behaves as follows: .

[0057] As the frequency increases, the capacitor impedance decreases rapidly, causing a change in the voltage division ratio of the bridge circuit. .

[0058] Therefore, - This eliminates the DC bias, yielding the AC response output for impedance measurement. The operating principle and equivalent circuit diagram of this topology under DC and AC conditions are shown below. Figure 4 As shown, Figure 4 This is a schematic diagram and equivalent circuit diagram of the AC signal extraction circuit provided in the embodiments of this application under DC and AC conditions.

[0059] Based on the above objectives, a complex frequency domain model of the hybrid two-branch parallel Wheatstone bridge topology is performed. The left and right sides of the bridge are respectively composed of impedance pairs (…). )and( The bridge circuit is composed of a differential output voltage and an input voltage, and the transfer function between them can be expressed as: ; in: ; ; ; .

[0060] Analysis of the expression reveals that under DC conditions (s→0), the capacitor is equivalent to an open circuit, and the bridge degenerates into a purely resistive network. When the four arms of the bridge satisfy a specific proportional relationship, the differential output is zero, achieving hardware-level cancellation of the DC bias; under AC conditions ( As the frequency increases, the impedance of the capacitor decreases significantly, the bridge balance condition is broken, and a differential voltage proportional to the input AC component is generated at the output, the amplitude and phase of which change with the frequency.

[0061] Under symmetric parameter conditions, the above transfer function can be further simplified to: .

[0062] make Then the amplitude and phase are: ; .

[0063] This symmetrical form intuitively shows that: (DC suppression); (High-frequency pass-through). 3dB cutoff angular frequency. satisfy Substituting, we get: .

[0064] The above formulas clearly show that the gain of this structure is zero at DC and approaches 1 at high frequencies. The cutoff frequency is directly determined by the product of R and C, thus providing a clear engineering basis for the frequency band design of impedance sensing.

[0065] Understandably, under AC conditions, the capacitive reactance of the first capacitor C1 and the second capacitor C2 contributes to the complementary frequency characteristics of the two branches: the voltage division ratio of the first branch increases with increasing frequency, while the voltage division ratio of the second branch decreases with increasing frequency. The difference between the two at the differential output terminal increases significantly with increasing frequency, thus effectively forming a differential amplifier with high-pass characteristics, which can effectively extract and amplify AC signals from strong DC background.

[0066] To verify the DC bias suppression function of this topology, the voltage is set to 3.2V DC + 0.03V AC, the frequency of the AC ripple signal is 10Hz, C1=C2=15uF, R1=R2=R3=R4=100kΩ. Gaussian white noise is added to the simulation, and the AC signal is filtered by a sliding window to simulate real-world conditions. Figure 5 As shown, Figure 5 This is a schematic diagram of the simulated voltage characteristics of the AC signal extraction circuit provided in this application before and after a weak AC injection signal under a strong DC bias background. The simulated waveforms show that before the signal passes through this topology, the voltage is 3.2V ± 0.03V. After passing through this topology, the differential voltage consists almost entirely of an AC signal, with an AC amplitude of approximately 30mV. This indicates that this topology has excellent suppression of DC bias, achieving accurate acquisition of weak AC injection signals under a strong DC bias background.

[0067] Furthermore, in a configuration with only one impedance unit, assume that this unit is placed in one branch. This branch achieves a frequency-sensitive response to AC signals, but the other purely resistive branch has a flat and non-frequency-selective response to AC signals. Subtracting the two, the output AC signal is the frequency-sensitive response minus the flat response, which is equivalent to a weak high-pass filter. Its ability to enhance and extract low-frequency AC signals is significantly lower than the aforementioned complementary configuration, resulting in poor performance.

[0068] Furthermore, in a three-impedance-unit configuration, two branches typically contain frequency-sensitive components, whose frequency response curves tend to be similar in shape, resulting in a weakening of the effective signal amplitude after differential subtraction. More importantly, this scheme disrupts the clear and concise correspondence between circuit parameters and frequency response. To simultaneously satisfy DC balance and obtain the desired AC characteristics, complex optimization is required in a high-dimensional parameter space, making the circuit extremely sensitive to component tolerances. This drastically increases the difficulty and cost of design and calibration, and makes it difficult to achieve stable and optimal performance.

[0069] Understandably, the configuration of the dual impedance unit enables active, coordinated differential enhancement of AC signals, while also possessing significant advantages in engineering, such as clear parameter design, ease of implementation, and optimization.

[0070] In this embodiment, a first Wheatstone bridge structure is employed, in which at least one bridge arm unit is an impedance unit and the rest are resistive units. Furthermore, the first and second branches are structurally asymmetrical, causing the circuit to exhibit different characteristics under DC and AC conditions. Under DC conditions, the impedance units exhibit a fixed resistance value, working together with the resistive units to ensure that the fixed resistance values ​​of each bridge arm unit under DC conditions satisfy a proportional relationship. Thus, under the influence of the DC component of the input signal, the bridge remains balanced, and the potentials of the first and second output nodes are equal, achieving DC bias cancellation and eliminating interference from the DC component in AC signal extraction. Under the influence of the AC component, the impedance value of the impedance units decreases as the signal frequency increases, disrupting the bridge balance. This causes an imbalance between the first and second branches due to structural asymmetry, thereby generating a differential voltage signal proportional to the AC component between the first and second output nodes, thus accurately extracting the AC signal. Compared with existing technologies, this effectively improves the signal-to-noise ratio and solves the problem of inaccurate AC component extraction in DC systems.

[0071] Furthermore, this application proposes an application embodiment of an AC signal extraction circuit to achieve impedance detection.

[0072] In this embodiment, the AC signal extraction circuit further includes: an amplitude conditioning unit, an input resistor, and a first isolation operational amplifier unit; the two input terminals of the first Wheatstone bridge are connected in parallel with the energy storage unit under test, and the two output terminals of the first Wheatstone bridge are respectively connected to the two input terminals of the amplitude conditioning unit; the two output terminals of the amplitude conditioning unit are connected in parallel with the input resistor, and then respectively connected to the two input terminals of the first isolation operational amplifier unit.

[0073] It should be noted that the amplitude conditioning unit is configured to directly output to the first isolation operational amplifier unit when the AC signal amplitude is less than the preset value; and to limit the AC signal before outputting it to the first isolation operational amplifier unit when the AC signal amplitude is greater than or equal to the preset value. The first isolation operational amplifier unit is configured to isolate and differentially amplify the input AC component within the voltage range, and output a first AC voltage signal proportional to the AC component.

[0074] It should be noted that the preset value is a voltage threshold determined by the full-scale input range of the subsequent circuit or the linear operating region of the system, and can be set by a reference voltage source and a resistor divider network.

[0075] In this embodiment, the AC signal extraction circuit further includes: a first sampling unit, a second Wheatstone bridge, and a second isolation operational amplifier unit; the second Wheatstone bridge has the same structure as the first Wheatstone bridge; the first sampling unit is connected in series with the energy storage unit under test, and the two input terminals of the second isolation operational amplifier unit are connected in parallel with the first sampling unit; the two output terminals of the second isolation operational amplifier unit are respectively connected to the two input terminals of the second Wheatstone bridge.

[0076] It should be noted that the first sampling unit is configured to convert the current signal flowing through the energy storage unit under test into a voltage signal proportional to the current signal; the second isolation operational amplifier unit is configured to isolate and amplify the voltage signal across the first sampling unit and output the amplified voltage signal to the second Wheatstone bridge; the second Wheatstone bridge is configured to extract a second AC voltage signal proportional to the AC current component flowing through the energy storage unit under test from the voltage signal output by the second isolation operational amplifier unit.

[0077] In this embodiment, the AC signal extraction circuit further includes: a differential amplifier unit, and an ADC and a filter unit; the first to fourth input terminals of the differential amplifier unit are respectively connected to the two output terminals of the first isolation operational amplifier unit and the two output terminals of the second Wheatstone bridge; the output terminal of the differential amplifier unit is connected to the input terminal of the ADC and the filter unit.

[0078] It should be noted that the differential amplifier unit is configured to output an analog voltage signal proportional to the AC impedance of the energy storage unit under test based on the first AC voltage signal and the second AC voltage signal; the ADC and filter unit are configured to perform analog-to-digital conversion and digital filtering on the analog voltage signal output by the differential amplifier unit to obtain a digital signal characterizing the AC impedance.

[0079] Specifically, please refer to Figure 6 , Figure 6 This is an impedance detection topology diagram of the AC signal extraction circuit provided in the embodiments of this application.

[0080] It should be noted that the AC ripple voltage response signal detection circuit mainly consists of a first Wheatstone bridge topology, an amplitude conditioning circuit, and an input resistor R. SNS It consists of an isolation operational amplifier chip, a differential amplifier circuit, an ADC, and a programmable digital filter.

[0081] It should be noted that the battery port voltage signal is first directly input to the first Wheatstone bridge topology. In this bridge, the physical mechanism that capacitors exhibit significantly different impedance characteristics under DC and AC conditions is utilized to keep the bridge in balance under DC conditions, with the theoretical output node potential difference being zero. However, under AC conditions, the bridge becomes unbalanced, thus forming a differential voltage signal at the output that strictly corresponds to the injected AC component. Through this structure, without proportional scaling of the DC bias and AC response, the large DC bias at the battery port is canceled out at the bridge level by hardware, retaining only the AC ripple component at the output, achieving high signal-to-noise ratio extraction of the AC response. This processing method avoids high DC bias entering the input of the isolation operational amplifier, reducing the impact of device common-mode range, DC drift, and signal amplitude scaling on the accuracy of the AC signal from the source.

[0082] Specifically, the AMC1311B isolation operational amplifier chip is selected to isolate the voltage signal. It has a wide input dynamic range, high common-mode rejection ratio and low gain error characteristics, and can maintain the measurement accuracy of weak AC signals under different voltage levels.

[0083] Understandably, a tiered AC amplitude conditioning circuit is used to address the voltage differences between different levels of the battery system. At the cell level, since the AC response amplitude is typically tens of millivolts, after the DC bias is canceled by an improved Wheatstone bridge, the output AC signal can be directly input to the isolation operational amplifier without saturation. At the module level, the AC response amplitude is typically hundreds of millivolts. After bridge processing, it remains within the linear input range of the isolation operational amplifier. Under the condition of satisfying amplitude constraints, an additional high-voltage scaling network can be omitted to reduce the attenuation of low-frequency AC signals.

[0084] Understandably, at the battery cluster level, due to the significant increase in AC response amplitude, this invention sets up a scaling network at the bridge output to limit the AC signal amplitude within the input range allowed by the isolation operational amplifier before performing isolation amplification.

[0085] Understandably, the weak AC voltage ripple is fed into a differential operational amplifier after passing through an isolation operational amplifier chip. This amplifies the amplitude of the weak AC signal to a range suitable for ADC sampling and further suppresses the common-mode component of the signal. The differential output is then fed into the ADC after passing through an anti-aliasing filter. A high-precision ADC is selected to perform small-signal measurement. Finally, a programmable digital filter is connected for digital filtering, synchronous detection, and impedance calculation, achieving accurate extraction of the voltage ripple response.

[0086] Specifically, the AC current excitation signal detection path is used to acquire the current excitation signal at the battery port under AC injection conditions. Its overall structure is consistent with the voltage response detection path in terms of signal processing flow, but the front-end signal acquisition method differs. The current excitation signal detection circuit mainly consists of a shunt or shunt resistor R. SHUNT It consists of an isolation operational amplifier chip, a second Wheatstone bridge topology (which is consistent with the first Wheatstone bridge topology), a differential amplifier circuit, an ADC, and a programmable digital filter.

[0087] In current sensing schemes, unlike voltage sensing with scaling, this scheme uses a sampling unit, such as a shunt or a precision shunt resistor R. SHUNT The AC current ripple at the battery port is converted into a voltage signal. Since the shunt itself has the ability to scale a large current into a small voltage, the circuit is connected to an isolated operational amplifier chip via the shunt. The subsequent circuit uses a hybrid Wheatstone bridge topology to cancel the DC bias, thereby achieving the acquisition of the current excitation signal.

[0088] In this path, the AC current at the battery port first passes through a shunt or a precision shunt resistor R. SHUNT The signal is converted into a linear voltage signal. This voltage signal is then input to an isolation operational amplifier chip for electrical isolation and first-stage amplification. The AMC1302 isolation operational amplifier chip is preferred, as it is designed specifically for current sensing applications and has good linearity and common-mode rejection performance.

[0089] Understandably, the equivalent voltage signal of the current ripple from the isolated output is input to the second Wheatstone bridge. Similar to the voltage sensing link, the bridge is in a balanced state under DC conditions, but becomes unbalanced at the bridge nodes under AC conditions. The DC bias is canceled out by hardware, allowing for lossless extraction of the weak AC signal. The subsequent process is consistent with the voltage path.

[0090] In the design process of the above-mentioned multi-level signal conditioning scheme, this application fully considers the constraint relationship between the small-signal linearization principle in electrochemical impedance detection and the selection of circuit parameters. To ensure the accuracy of impedance measurement results, the amplitude of the AC injection signal is usually selected to be within 1% of the battery's DC operating point, so that the battery system and sampling link both operate in the linear range.

[0091] The hybrid Wheatstone bridge used in this application is a fully linear passive network structure. Under the premise of satisfying the DC balance condition, it exhibits linear differential response characteristics to AC small signals. The AC differential voltage output by the bridge is proportional to the input AC component and will not cause compression, clipping, or nonlinear distortion of the AC signal. Therefore, it can maintain the authenticity of impedance information under the 1% small signal condition.

[0092] In another embodiment, the AC current excitation signal can also be detected separately. That is, in addition to the first Wheatstone bridge, the AC signal extraction circuit also includes: a second sampling unit and a third isolation operational amplifier unit; the second sampling unit is connected in series with the energy storage unit under test, and the two input terminals of the third isolation operational amplifier unit are connected in parallel with the first sampling unit; the two output terminals of the third isolation operational amplifier unit are respectively connected to the two input terminals of the first Wheatstone bridge.

[0093] It should be noted that the second sampling unit is configured to convert the current signal flowing through the energy storage unit under test into a voltage signal proportional to the current signal; the third isolation operational amplifier unit is configured to isolate and amplify the voltage signal across the second sampling unit and output the amplified voltage signal to the first Wheatstone bridge.

[0094] It is understandable that the second sampling unit, the third isolation operational amplifier unit, and the first Wheatstone bridge are essentially the same as the first sampling unit, the second Wheatstone bridge, and the second isolation operational amplifier unit in the above embodiments, and will not be elaborated here.

[0095] In conjunction with the above embodiments, this application also proposes an embodiment of an online impedance detection method. Please refer to... Figure 7 , Figure 7 This is a schematic flowchart of the online impedance detection method provided in the embodiments of this application.

[0096] In this embodiment, the online impedance detection method includes steps S10 to S60.

[0097] Step S10: Apply an excitation to the energy storage unit under test so that its terminal voltage signal contains a DC component and an AC component of the desired frequency.

[0098] Understandably, this step is the excitation application stage. An AC excitation signal containing a specific frequency component is applied to the energy storage unit under test using an excitation source. This AC excitation signal is superimposed on a DC operating point. The DC component simulates the actual operating bias voltage of the energy storage unit, while the AC component is a small AC signal with a single frequency or a specific spectrum. Its frequency is the desired frequency, which is the target frequency for subsequent impedance analysis. This step ensures that the terminal voltage signal of the energy storage unit contains both a large DC bias and a weak AC response component at the same frequency as the excitation.

[0099] Step S20: Acquire the terminal voltage signal of the energy storage unit under test, and pass it through the first Wheatstone bridge to obtain the first differential voltage signal reflecting the AC component after canceling the DC bias.

[0100] Step S20 includes: acquiring the terminal voltage signal of the energy storage unit under test and passing it through a first Wheatstone bridge to obtain an AC signal reflecting the AC component after the DC bias is canceled; when the AC signal amplitude is less than a preset value, identifying the AC signal as a first differential voltage signal and outputting it; when the AC signal amplitude is greater than or equal to the preset value, limiting the AC signal, identifying the limited AC signal as a first differential voltage signal and outputting it.

[0101] Understandably, this step involves front-end processing of the voltage signal at the energy storage unit terminals. This signal is input to the first Wheatstone bridge. This bridge performs high-precision cancellation of the DC component based on its pre-defined arm resistance ratios. Simultaneously, utilizing its structural asymmetry and the frequency characteristics of its impedance units, it extracts the AC signal reflecting the desired frequency AC component from the mixed signal.

[0102] To further improve the system's reliability and dynamic range, this step includes a post-processing judgment mechanism: the extracted AC signal enters an amplitude detection and comparison circuit. The preset value is a voltage threshold determined by the full-scale input range of subsequent circuits or the system's linear operating region, which can be set via a reference voltage source and a resistor divider network. If the amplitude of the AC signal is less than this preset value, it is considered to be in the linear amplification region and is directly output as the first differential voltage signal to the next stage. If its amplitude is greater than or equal to the preset value, it means the signal may be too strong, posing a risk of saturation or damage to subsequent circuits. In this case, the signal first passes through a limiting circuit to limit the signal peak within a safe range, and then the limited signal is recognized as the first differential voltage signal and output. This mechanism ensures that even when the AC response signal amplitude unexpectedly increases, the system can still safely output a clamped voltage reflecting the presence of the AC signal.

[0103] In addition, this can also be achieved through optional structural configurations. When detecting the impedance of a cell / module, if the output amplitude of the hybrid bridge (<1%×80V=±0.8V) is within the allowable input range of the subsequent analog circuit, signal conditioning can be performed directly. However, when detecting the impedance of a battery cluster, the maximum AC output amplitude of the hybrid bridge is <1%×1500V=±15V, which is far beyond the range that the subsequent analog circuit can withstand. Therefore, a scaling network (e.g., 1 / 10) should be added to reduce the voltage to within ±1.5V before it enters the subsequent signal conditioning circuit.

[0104] Understandably, the preset value is not necessarily a single value; it can be multiple values ​​or ranges to achieve more accurate and safer signal processing.

[0105] Step S30: Acquire the current signal flowing through the energy storage unit under test and convert it into a first voltage signal.

[0106] Understandably, a precision sampling resistor of known resistance is connected in series in the charging and discharging circuit of the energy storage unit under test. The current signal flowing through the energy storage unit generates a proportional voltage drop across this sampling resistor; this voltage drop is the first voltage signal. By measuring the voltage across this sampling resistor, accurate current information can be indirectly obtained. This current signal also contains both DC and AC components.

[0107] Step S40: The first voltage signal is isolated and amplified, and the amplified signal is passed through a second Wheatstone bridge to extract a second differential voltage signal that reflects the alternating current component. The structure of the second Wheatstone bridge is the same as that of the first Wheatstone bridge.

[0108] Understandably, firstly, since the measurement system may have a high common-mode voltage with the power ground, the first voltage signal needs to be isolated and amplified. This can be achieved through an isolation amplifier to ensure system safety and suppress common-mode interference. Subsequently, the isolated and amplified signal is input to the second Wheatstone bridge. The structure of the second Wheatstone bridge is identical to that of the first Wheatstone bridge; the types of its bridge arm components, their connection relationships, and parameter design principles are the same as the first bridge. Its function is similar to that of the first Wheatstone bridge: suppressing the DC component in the current signal and extracting the AC current component corresponding to a desired frequency in step S10, outputting a second differential voltage signal. Using bridges with identical structures ensures that the voltage and current signal channels have highly consistent amplitude-frequency and phase-frequency characteristics, laying the foundation for subsequent accurate impedance calculation.

[0109] Step S50: Based on the first differential voltage signal and the second differential voltage signal, an analog voltage signal proportional to the AC impedance of the energy storage unit under test at the desired frequency is obtained. The analog voltage signal is then subjected to analog-to-digital conversion and digital filtering to obtain a digital signal characterizing the AC impedance of the energy storage unit under test.

[0110] Understandably, the obtained analog voltage signal is fed into an analog-to-digital converter (ADC) for sampling and quantization, converting it into a digital sequence. Subsequently, this digital sequence undergoes digital filtering, such as using a digital lock-in amplification algorithm or a narrowband digital bandpass filter, to further suppress quantization noise, power frequency interference, and noise from other frequency bands. This allows for the precise extraction of a digital signal that uniquely characterizes the AC impedance at the desired frequency. This digital signal can be directly used for the state assessment of the energy storage unit, such as calculating internal resistance and electrochemical impedance spectroscopy.

[0111] Compared with the prior art, the beneficial effect of the impedance online detection method provided in this application directly stems from the use of the first and second Wheatstone bridges in its core signal processing steps. Since these two bridges adopt the same improved architecture as the aforementioned circuit embodiment, they inherit all the core performance advantages of the circuit, fundamentally improving the initial signal-to-noise ratio of signal extraction; ensuring the consistency of voltage and current signal channel processing, and guaranteeing the accuracy of impedance calculation; and providing stability and feasibility for system-level applications.

[0112] It should be understood that expressions such as “comprising” and “may include” used in this application indicate the existence of the disclosed functions, operations, or constituent elements, and do not limit one or more additional functions, operations, and constituent elements. In this application, terms such as “comprising” and / or “having” are to be interpreted as indicating a particular characteristic, number, operation, constituent element, component, or combination thereof, but not to exclude the existence or possibility of adding one or more other characteristics, numbers, operations, constituent elements, components, or combinations thereof.

[0113] Furthermore, in this application, the expression "and / or" includes any and all combinations of the associated listed words. For example, the expression "A and / or B" may include A, may include B, or may include both A and B.

[0114] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. An AC signal extraction circuit, characterized in that, include: The bridge consists of a first bridge arm unit, a second bridge arm unit, a third bridge arm unit, and a fourth bridge arm unit; the first bridge arm unit and the second bridge arm unit are connected in series to form a first branch, the third bridge arm unit and the fourth bridge arm unit are connected in series to form a second branch, the first branch and the second branch are connected in parallel between a first input node and a second input node, and the series nodes of the first branch and the second branch are respectively defined as a first output node and a second output node to form a first Wheatstone bridge; In the four bridge arm units, at least one bridge arm unit is an impedance unit, and the remaining bridge arm units are resistance units, and the first branch and the second branch have asymmetrical structures. Wherein, the impedance value of the resistor unit is a fixed resistance value that does not change with frequency; the impedance unit is configured such that: the impedance value is a fixed resistance value under DC conditions to allow the DC component to pass through, and the impedance value under AC conditions decreases as the signal frequency increases; the fixed resistance value of each bridge arm unit under DC conditions satisfies: the ratio of the fixed resistance value of the first bridge arm unit to the fixed resistance value of the second bridge arm unit is equal to the ratio of the fixed resistance value of the third bridge arm unit to the fixed resistance value of the fourth bridge arm unit; The first Wheatstone bridge is used to: when the received input signal contains both DC and AC components, make the potentials of the first output node and the second output node equal under the action of the DC component, thereby achieving DC bias cancellation; and enter an unbalanced state under the action of the AC component, thereby generating a differential voltage signal between the first output node and the second output node that is proportional to the AC component, so as to extract the AC signal.

2. The AC signal extraction circuit as described in claim 1, characterized in that, The first bridge arm unit and the fourth bridge arm unit are impedance units, or the second bridge arm unit and the third bridge arm unit are impedance units.

3. The AC signal extraction circuit as described in claim 1 or 2, characterized in that, The impedance unit includes: a first resistive element and a capacitive element; The first end of the resistive element is connected to the first end of the capacitive element and the input node; the second end of the resistive element is connected to the second end of the capacitive element and the output node of the branch. The first resistive element is a combination of one or more resistors connected in series, parallel, or in a mixed configuration, and the capacitive element is a capacitor, or a combination of multiple capacitors connected in series, parallel, or in a mixed configuration.

4. The AC signal extraction circuit as described in claim 1, characterized in that, The resistor unit includes: a second resistive element; The first end of the second resistive element is connected to the input node, and the second end of the second resistive element is connected to the output node of the branch in which it is located. The second resistive element is a resistor, or a combination of multiple resistors connected in series, parallel, or in a mixed configuration.

5. The AC signal extraction circuit as described in claim 1, characterized in that, The AC signal extraction circuit further includes: an amplitude conditioning unit, an input resistor, and a first isolation operational amplifier unit; The two input terminals of the first Wheatstone bridge are connected in parallel with the energy storage unit under test, and the two output terminals of the first Wheatstone bridge are respectively connected to the two input terminals of the amplitude conditioning unit; the two output terminals of the amplitude conditioning unit are connected in parallel with the input resistor, and then respectively connected to the two input terminals of the first isolation operational amplifier unit; The amplitude conditioning unit is configured to directly output to the first isolation operational amplifier unit when the amplitude of the AC signal is less than a preset value; and to limit the amplitude of the AC signal before outputting it to the first isolation operational amplifier unit when the amplitude of the AC signal is greater than or equal to the preset value. The first isolation operational amplifier unit is configured to isolate and differentially amplify the input AC component within a voltage range, and output a first AC voltage signal proportional to the AC component.

6. The AC signal extraction circuit as described in claim 5, characterized in that, The AC signal extraction circuit further includes: a first sampling unit, a second Wheatstone bridge, and a second isolation operational amplifier unit; The second Wheatstone bridge has the same structure as the first Wheatstone bridge; The first sampling unit is connected in series with the energy storage unit under test, and the two input terminals of the second isolation operational amplifier unit are connected in parallel with the first sampling unit; the two output terminals of the second isolation operational amplifier unit are respectively connected to the two input terminals of the second Wheatstone bridge. The first sampling unit is configured to convert the current signal flowing through the energy storage unit under test into a voltage signal proportional to the current signal; the second isolation operational amplifier unit is configured to isolate and amplify the voltage signal across the first sampling unit and output the amplified voltage signal to the second Wheatstone bridge. The second Wheatstone bridge is configured to extract a second AC voltage signal from the voltage signal output by the second isolation operational amplifier unit, which is proportional to the AC current component flowing through the energy storage unit under test.

7. The AC signal extraction circuit as described in claim 6, characterized in that, The AC signal extraction circuit further includes: a differential amplifier unit, and an ADC and filter unit; The first to fourth input terminals of the differential amplifier unit are respectively connected to the two output terminals of the first isolation operational amplifier unit and the two output terminals of the second Wheatstone bridge; the output terminal of the differential amplifier unit is connected to the input terminal of the ADC and filter unit. The differential amplifier unit is configured to calculate and output an analog voltage signal proportional to the AC impedance of the energy storage unit under test based on the first AC voltage signal and the second AC voltage signal. The ADC and filtering unit are configured to perform analog-to-digital conversion and digital filtering on the analog voltage signal output by the differential amplifier unit to obtain a digital signal characterizing the AC impedance.

8. The AC signal extraction circuit as described in claim 1, characterized in that, The AC signal extraction circuit further includes: a second sampling unit and a third isolation operational amplifier unit; The second sampling unit is connected in series with the energy storage unit under test, and the two input terminals of the third isolation operational amplifier unit are connected in parallel with the first sampling unit; the two output terminals of the third isolation operational amplifier unit are respectively connected to the two input terminals of the first Wheatstone bridge. The second sampling unit is configured to convert the current signal flowing through the energy storage unit under test into a voltage signal proportional to the current signal; The third isolation operational amplifier unit is configured to isolate and amplify the voltage signals at both ends of the second sampling unit, and output the amplified voltage signal to the first Wheatstone bridge.

9. An online impedance detection method, characterized in that, include: An excitation is applied to the energy storage unit under test so that its terminal voltage signal contains a DC component and an AC component of the desired frequency. The terminal voltage signal of the energy storage unit under test is acquired and passed through a first Wheatstone bridge to obtain a first differential voltage signal that reflects the AC component after the DC bias is canceled. The current signal flowing through the energy storage unit under test is collected and converted into a first voltage signal; The first voltage signal is isolated and amplified, and the amplified signal is passed through a second Wheatstone bridge to extract a second differential voltage signal that reflects the AC current component, wherein the structure of the second Wheatstone bridge is the same as that of the first Wheatstone bridge. Based on the first differential voltage signal and the second differential voltage signal, an analog voltage signal proportional to the AC impedance of the energy storage unit under test at the desired frequency is obtained. The analog voltage signal is then subjected to analog-to-digital conversion and digital filtering to obtain a digital signal characterizing the AC impedance of the energy storage unit under test.

10. The online impedance detection method as described in claim 9, characterized in that, The terminal voltage signal of the energy storage unit under test is acquired, and a first differential voltage signal reflecting the AC component is obtained by passing it through a first Wheatstone bridge after canceling the DC bias, including: The terminal voltage signal of the energy storage unit under test is acquired and passed through a first Wheatstone bridge to obtain an AC signal that reflects the AC component after the DC bias is canceled. When the amplitude of the AC signal is less than a preset value, the AC signal is identified as the first differential voltage signal and output; When the amplitude of the AC signal is greater than or equal to the preset value, the AC signal is limited, and the limited AC signal is identified as the first differential voltage signal and output.

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