A liquid level closed-loop fire control system and method for an immersed energy storage battery pack

Abstract

The application discloses a liquid level closed-loop fire control system and method for an immersed energy storage battery pack, and the system comprises: an external liquid storage tank, which is the only immersed liquid storage device; a liquid supplementing device with daily and fire-fighting dual modes; a sensing module for monitoring the liquid level change rate and the temperature change rate; and a logic control module configured to: when it is monitored that the liquid level drop rate exceeds a threshold value and the event occurs prior to temperature abnormality, the early thermal runaway sign is determined and a standby state is entered; after confirming thermal runaway, full-speed liquid supplementing without liquid discharge to a target liquid level is performed, and then continuous monitoring and automatic supplementing are performed when the liquid level drops until the temperature and the liquid level change rate are stable and the liquid supplementing is stopped. The battery pack is not provided with any electric liquid discharge valve, and only a normally closed manual liquid discharge port is arranged.

Description

Technical Field

[0001] This invention belongs to the field of energy storage battery safety protection technology, specifically relating to a liquid level closed-loop fire control system and method for an immersion energy storage battery pack. Background Technology

[0002] Immersion liquid cooling technology completely submerges the battery module in an insulating coolant, utilizing the liquid's high thermal conductivity to achieve efficient heat dissipation, and has become an important technological direction for energy storage thermal management. The immersion liquid itself usually has flame-retardant properties with a high ignition point (greater than 200°C), and can be used as both a thermal management medium and a fire-fighting medium.

[0003] Regarding fire protection technology for submerged energy storage systems, several technical solutions have been proposed. One commonly used solution in the industry is to install separate submerged liquid storage tanks and fire-fighting fluid storage tanks, each equipped with a submerged liquid pump and a fire-fighting fluid pump. This solution executes a sequence of first draining the submerged liquid and then injecting the fire-fighting fluid during firefighting. Specifically, the submerged liquid in the battery pack is first drained via an electric drain valve, and then the fire-fighting fluid pump is activated to inject the fire-fighting fluid into the battery pack.

[0004] Another technical solution currently used in the industry involves installing liquid level sensors within the battery pack or prefabricated energy storage compartment, in conjunction with fire alarm detectors, to achieve fire monitoring and fire suppression activation. The liquid level sensors are primarily used for monitoring and maintaining liquid levels under normal operating conditions, while fire suppression actions are still mainly triggered by temperature or smoke detection signals.

[0005] The existing technical solutions have the following shortcomings: setting up a dual-tank and dual-pump system results in hardware redundancy and increased costs; the operation strategy of first draining the submerged liquid and then injecting fire-fighting fluid during fire fighting poses safety risks; after the fire-fighting fluid is injected, there is a lack of continuous monitoring and closed-loop feedback of the liquid level; the liquid level sensor is only used for maintaining the liquid level under normal operating conditions, and the value of the liquid level change signal in thermal runaway early warning is not fully utilized; the thermal management cycle and fire-fighting fluid replenishment are still mostly independent loops. Summary of the Invention

[0006] The technical problem this invention aims to solve is to address the shortcomings of existing technologies, such as redundant dual-tank structures, safety risks associated with emptying and refilling during firefighting, lack of liquid level closed-loop verification and continuous maintenance during firefighting, underutilization of the value of liquid level signal early warning, and the independent loops for thermal management and firefighting liquid replenishment. This invention provides a liquid level closed-loop fire control system for submerged energy storage battery packs that uses a single storage tank as a unified liquid source, a single replenishment device for unified scheduling of daily operation and maintenance and fire emergency response, prioritizes the timing of liquid level and temperature change rates for early warning, and continuously maintains the liquid level after firefighting to prevent reignition.

[0007] The core inventive concept of this invention lies in the synergistic combination of a time-series prediction mechanism that prioritizes the rate of liquid level change over the rate of temperature change, a physical architecture in which the battery pack does not have any electric drain valves, and a closed-loop maintenance mechanism that continuously monitors and automatically replenishes liquid based on both liquid level and temperature change rates after fire-fighting liquid replenishment. Through a coupling strategy of retaining the initial liquid volume without draining under limited liquid volume constraints and closed-loop on-demand replenishment, continuous and effective fire-fighting coverage of the entire thermal runaway process is achieved under the limited capacity of a single liquid tank. At the same time, the time-series priority prediction provides an early warning time window and a pre-charge trigger basis for closed-loop maintenance. The three elements form a physical-control level coupling synergy rather than a functionally independent module superposition.

[0008] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a closed-loop fire control system for the liquid level of an immersion energy storage battery pack, including an external liquid storage tank, which is set outside the battery pack and serves as the only immersion liquid storage device in the system, and also as a daily maintenance liquid replenishment source and a fire emergency liquid replenishment source; a liquid replenishment device, which connects the external liquid storage tank to the liquid inlet of the battery pack through a liquid replenishment pipeline, and has a daily maintenance liquid replenishment mode and a fire emergency liquid replenishment mode. In the daily maintenance liquid replenishment mode, it operates at a first flow rate, and in the fire emergency liquid replenishment mode, it operates at a second flow rate higher than the first flow rate at full speed; a liquid level sensing module, which is set inside the battery pack, is used to monitor the liquid level height of the immersion liquid in the battery pack in real time and calculate the liquid level change rate; a fire detection module is used to detect the temperature and temperature change rate of the cells inside the battery pack; and a logic control module is communicatively connected to the liquid replenishment device, the liquid level sensing module, and the fire detection module respectively.

[0009] The logic control module is configured to: under normal operating conditions, control the fluid replenishment device to maintain the fluid level in the battery pack at the working level using the routine maintenance fluid replenishment mode; acquire the fluid level change rate and temperature change rate in real time; when the fluid level change rate shows a decrease in fluid level and the rate of decrease exceeds the preset fluid level safety threshold, and this occurs before the cell temperature exceeds the preset temperature safety threshold, it is determined to be an early sign of thermal runaway, and the fluid replenishment device is switched to fire standby mode in advance, making the fluid replenishment device ready to immediately perform full-speed fluid replenishment; when thermal runaway is confirmed, control the fluid replenishment... The device operates at full speed in fire emergency replenishment mode, injecting immersion fluid into the battery pack until the liquid level reaches the preset fire target level. During fire replenishment, the battery pack does not undergo any draining operation. After fire replenishment is completed, the liquid level change rate and temperature change rate are continuously monitored. When the liquid level drops from the fire target level to the preset replenishment trigger level, replenishment is automatically triggered. When the cell temperature drops below the safe temperature threshold, and both the temperature change rate and the liquid level change rate remain stable within their respective preset stable ranges for at least a preset duration, thermal runaway is considered completely suppressed, and replenishment is stopped. The battery pack is not equipped with any electric drain valve or automatic drain device; only a manual drain port is provided at the bottom of the battery pack. The manual drain port is a normally closed manual ball valve, which is closed under normal operating conditions and is only manually opened and closed by the operator during maintenance.

[0010] In this invention, the liquid level change rate (dL / dt) refers to the change in the height of the immersion liquid level per unit time, measured in mm / min; the temperature change rate (dT / dt) refers to the change in the cell temperature per unit time, measured in °C / min. The fire standby state refers to the liquid replenishment drive system entering a standby state, enabling the currently configured liquid replenishment drive device to respond to full-speed liquid replenishment commands at any time without experiencing a start-up delay. Specifically, for the variable frequency pump embodiment, this state is when the variable frequency controller has completed the pre-charging of the DC bus capacitor and the motor is energized and ready; for the high-pressure storage tank embodiment, this state is when the tank pressure is maintained at the working pressure and the outlet valve control circuit is energized and ready; for the gravity valve embodiment, this state is when the valve controller has completed self-testing and is waiting for a full-open command. A hard-wired input port refers to an electrical interface directly connected via physical wires, distinct from interfaces that transmit signals via communication buses or wireless methods; its signal transmission is unaffected by communication protocols or software faults.

[0011] Compared with existing technologies, this invention has the following advantages: First, the system structure is simplified, achieving unified scheduling through a single storage tank and a single replenishment device, reducing the number of hardware components required for storage tanks and pumping systems. Second, by eliminating any electrically operated drain valves in the battery pack's physical architecture, the safety risk of high-temperature battery cells being exposed to air due to accidental opening of drain valves is eliminated at the source. Third, by using the liquid level change rate as an independent and prioritized criterion for thermal runaway early warning, and establishing a time-priority judgment logic where the liquid level change rate criterion precedes the temperature criterion, the warning time is advanced by tens of seconds to several minutes. Fourth, after fire-fighting replenishment is completed, continuous monitoring and automatic replenishment are performed using both liquid level and temperature change rate criteria, forming a closed-loop liquid level maintenance to prevent reignition. Fifth, the multi-signal cross-verification logic using four heterogeneous criteria—temperature, gas, liquid level, and electrical—reduces the risk of fire-fighting malfunctions. Attached Figure Description

[0012] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0013] Figure 1 This is a schematic diagram of the overall system.

[0014] Figure 2 This is a cross-sectional view of the internal structure of the battery pack.

[0015] Figure 3 This is a topology diagram of the control signals.

[0016] Figure 4 This is a flowchart of a four-stage progressive control logic.

[0017] Figure 5 A schematic diagram of the "three-out-of-two" anti-malfunction logic for multi-signal cross-verification.

[0018] In the diagram: 1 External liquid storage tank, 2 Battery pack, 21 Liquid inlet, 22 Manual drain outlet, 3 Liquid replenishment device, 4 Liquid replenishment pipeline, 41 Electric liquid inlet valve, 42 Check valve, 5 Mechanical liquid level switch, 6 Composite gas detector, 7 Battery module. Detailed Implementation Example 1

[0019] This embodiment uses a 261kWh industrial and commercial energy storage integrated cabinet as a specific application scenario to describe the invention in detail.

[0020] This energy storage DC cabinet is an outdoor cabinet structure with dimensions of 1020mm wide, 1400mm deep, and 2470mm high. The entire cabinet weighs approximately 2700kg and has an IP54 protection rating. The DC-side parameters are as follows: battery type is lithium iron phosphate (LFP) 314Ah cells; battery system configuration is 1P260S (5 battery packs in series); rated voltage is 832V; operating voltage range is 728~936V; nominal energy is 261.248kWh; and rated charge / discharge rate is 0.5P.

[0021] Battery pack 2 adopts an immersed liquid-cooled PACK structure. The cells are LFP 314Ah square aluminum-cased cells, with dimensions of 174.3mm (length), 71.5mm (width), and 206.9mm (height), a nominal voltage of 3.2V, and a nominal energy of 1004.8Wh. Each battery pack 2 consists of 52 cells connected in series in a 1P52S configuration. Battery pack 2 has a nominal voltage of 166.4V, an operating voltage range of 145.6–187.2V, a rated capacity of 314Ah, and a nominal energy of 52.2496kWh. Battery pack 2 measures 840mm (length), 1222mm (width), and 300mm (height) and weighs approximately 400kg. Five battery packs 2 are housed within the entire energy storage DC cabinet, arranged in a vertical, layered manner, with one battery pack 2 on each layer, and the layers are separated and secured by load-bearing supports.

[0022] An external liquid storage tank 1 is located on top of the energy storage DC cabinet and is fixedly installed on one side of the cabinet. The external liquid storage tank 1 is a stainless steel rectangular box structure with an effective volume of 80L. The installation height of the external liquid storage tank 1 is approximately 0.8–1.0m higher than the top of the battery pack 2 inside the cabinet. The external liquid storage tank 1 stores the same type of immersion liquid, which has a flash point greater than 200℃. It serves as both a daily maintenance replenishment source and a fire emergency replenishment source; that is, it is the only immersion liquid storage device in this embodiment. The top of the external liquid storage tank 1 has a vent and an internal air filter to maintain pressure balance between the tank and the outside environment. The external liquid storage tank 1 contains a liquid level sensor, a hydrostatic level transmitter with a range of 0–1.5m, outputting a 4–20mA current signal. This sensor monitors the remaining liquid level in the external liquid storage tank 1 and issues an alarm signal when the liquid level falls below a preset value, prompting maintenance personnel to replenish the liquid.

[0023] The replenishment pipeline 4 is a 25mm inner diameter plastic pipe, which connects sequentially along the liquid flow direction to the bottom outlet of the external storage tank 1, the inlet electric valve 41, the replenishment device 3, the check valve 42, and the inlet 21 of each battery pack 2. The inlet 21 of each battery pack 2 is connected to the main pipeline of the replenishment pipeline 4 via parallel branches, each with an independent branch electric valve. Both the inlet electric valve 41 and the branch electric valves are normally closed electric ball valves with a nominal diameter of DN25 and an operating voltage of DC24V. The replenishment device 3 is a variable frequency adjustable magnetic drive pump equipped with a frequency converter. Its speed is controlled by a 4-20mA analog signal output from the logic control module, allowing switching between routine maintenance replenishment mode and fire emergency replenishment mode. In routine operation and maintenance fluid replenishment mode, the fluid replenishment device 3 operates at a low speed of 20% to 30% of its rated speed, corresponding to a flow rate of 2 to 5 L / min; in fire emergency fluid replenishment mode, the fluid replenishment device 3 operates at 100% of its rated speed, corresponding to a flow rate of 50 to 100 L / min. The one-way valve 42 is a spring-loaded check valve with a nominal diameter of DN25, installed in a direction that allows liquid to flow from the fluid replenishment device 3 to the battery pack 2 while preventing backflow.

[0024] Each battery pack 2 has a pre-installed manual drain port 22 at its bottom. The manual drain port 22 is a normally closed manual ball valve with a DN20 diameter and a 316L stainless steel body. It is operated by a handle. Under normal operating conditions, the handle is in the closed position, and the ball valve is fully closed. To drain, the operator can open the ball valve using a wrench or by directly rotating the handle 90°. After draining, the handle is rotated 90° in the opposite direction to return it to the closed position. The manual drain port 22 is only for manual draining operations during non-emergency situations such as equipment inspection, maintenance, or system decommissioning. The entire system does not have any form of electric drain valve or automatic drain device. Physically, there is no possibility of draining during firefighting operations, thus eliminating the safety risks caused by false sensor alarms, control program errors, or actuator malfunctions leading to accidental opening of the drain valve from a physical architecture perspective. The absence of an electric drain valve limits the physical structural characteristics of the product upon completion of manufacturing; the absence of a drain operation during fire-fighting replenishment limits the control behavior characteristics of the product under fire-fighting operation conditions. The two limit the same invention from different dimensions, and their protection scopes are complementary rather than overlapping.

[0025] The liquid level sensing module is located inside each battery pack 2, with each battery pack 2 containing an independent liquid level sensing unit. The liquid level sensing module employs a three-level redundancy architecture, including a main liquid level sensor, redundant liquid level sensors, and a mechanical liquid level switch 5. Both the main liquid level sensor and the redundant liquid level sensor are magnetostrictive liquid level sensors, with a measurement range of 0–400 mm and an output current signal of 4–20 mA. The main liquid level sensor is installed at a first position on the inner wall of the battery pack 2, and the redundant liquid level sensor is installed at a second position on the inner wall of the battery pack 2. The second position and the first position are diagonally arranged within the battery pack 2, forming spatial redundancy. The output signal of the redundant liquid level sensor is independently transmitted to the logic control module for cross-verification with the signal from the main liquid level sensor. When the difference between the two liquid level readings exceeds a preset tolerance, the logic control module determines a sensor fault and issues an alarm. The mechanical liquid level switch 5 is installed at four key liquid level threshold points: the low limit liquid level, the working liquid level, the fire-fighting target liquid level, and the ultra-high limit liquid level. The switching signal of the mechanical liquid level switch 5 is directly connected to the hard-wired input port of the logic control module, that is, the digital input channel directly connected by physical wires, providing hardware-level independent verification that is not affected by communication protocols and software faults.

[0026] The fire detection module includes a distributed temperature sensor array and a composite gas detector 6. The distributed temperature sensor array consists of multiple K-type thermocouples, with at least 16 measuring points arranged within each battery pack 2, located on the surface and terminals of each cell, with a temperature measurement range of -40℃ to +300℃. The composite gas detector 6 is located in the center of the upper space inside each battery pack 2, simultaneously detecting CO, H2, and volatile organic compounds.

[0027] The logic control module includes a linkage control host, installed in the control compartment of the energy storage DC cabinet. The linkage control host connects to the main liquid level sensor, redundant liquid level sensors, and the liquid level sensor in the storage tank via 4-20mA analog signal lines to receive liquid level signals; it connects to the mechanical liquid level switch 5 via a digital input channel to receive switch signals; it connects to the distributed temperature sensor array via a thermocouple input module to receive temperature signals; it connects to the composite gas detector 6 via an analog signal line or RS485 digital communication line to receive gas concentration signals; it connects to the frequency converter controller via a 4-20mA analog output channel to control the rotation speed of the replenishment device 3; it connects to the inlet electric valve 41, each branch electric valve, and the controllable pressure relief valve via digital output channels to send control commands; and it communicates bidirectionally with the battery management system (BMS) via a CAN communication bus to obtain health status data such as cell voltage and internal resistance. The linkage control host has pre-set daily maintenance replenishment control logic, fire emergency replenishment control logic, and a four-stage progressive linkage control program.

[0028] This embodiment defines four key liquid level thresholds, from lowest to highest: low limit liquid level, working liquid level, fire-fighting target liquid level, and ultra-high limit liquid level. The working liquid level is located at the cell tabs, i.e., the top of the cell. Under normal operating conditions, the immersion liquid level is flush with the cell tabs, ensuring the tabs' insulation protection and heat dissipation requirements. Taking a 314Ah cell as an example, the cell height is 207.0±0.8mm, and the working liquid level corresponds to a liquid level height of approximately 207mm (based on the bottom of the enclosure). The low limit liquid level is located 50mm below the top of the cell, i.e., 50mm below the cell tabs. When the liquid level drops to this value, the cell tabs are completely exposed above the liquid surface, posing a short-circuit risk and a potential for localized overheating. The system determines that the cell is in a dangerous exposed state and immediately triggers emergency liquid replenishment. Taking a 314Ah cell as an example, the low limit liquid level corresponds to a liquid level height of approximately 157mm. The target liquid level for fire suppression is located at a preset height above the top of the battery cell. This preset height is determined based on the distance between the top cover of the battery pack 2 and the top of the battery cell. A balance must be struck between utilizing the limited upper space to form a liquid protective layer and retaining a safety margin to prevent liquid from touching the top cover. Taking a typical structure where the top of the battery cell is about 30mm from the inner top cover of the battery pack 2 as an example, the target liquid level for fire suppression is located 20-25mm above the top of the battery cell, corresponding to a liquid level height of about 227-232mm, retaining a safety margin of 5-10mm. The upper limit liquid level is located above the top of the battery cell but does not touch the top cover of the battery pack 2. This is the highest permissible liquid level inside the battery pack 2. Exceeding this value may cause the liquid level to touch the top cover, resulting in liquid overflow or excessive pressure on the tank. The system will trigger overflow protection and stop replenishing liquid. Taking a structure where the top of the battery cell is about 30mm from the inner top cover as an example, the upper limit liquid level is located 27-28mm above the top of the battery cell, corresponding to a liquid level height of about 234-235mm, which is the safety limit position 2-3mm below the inner top cover.

[0029] The working process of this embodiment is executed according to a four-stage progressive control logic.

[0030] Phase 1, Normal Thermal Management State. During the normal charging and discharging operation of battery pack 2, the logic control module continuously receives liquid level signals from the main liquid level sensor and redundant liquid level sensors, calculates the effective liquid level (taking the lower of the two values ​​to ensure a safety margin), and maintains the liquid level of each battery pack 2 within ±10mm of the working liquid level using the routine maintenance replenishment mode. When the liquid level of a battery pack 2 is lower than the working liquid level by -10mm, the logic control module opens the corresponding inlet electric valve 41 and branch electric valve of that battery pack 2, sending a low-frequency drive signal to the replenishment device 3. The replenishment device 3 starts at a low speed of 20% to 30% of the rated speed, pumping the immersion liquid in the external storage tank 1 into the battery pack 2 through the replenishment pipeline 4 at a small flow rate. The replenishment device 3 typically operates for 30 seconds to 5 minutes each time, using an intermittent working mode with extremely low energy consumption. When the liquid level returns to the working liquid level, the logic control module stops the replenishment device 3 and closes all electric valves. The fire detection module continuously monitors temperature and gas data and is in standby mode. The logic control module simultaneously receives signals from the liquid level sensor of the storage tank. When the remaining liquid level in the external storage tank 1 is lower than the preset value, it sends a replenishment alarm signal to the station-level monitoring system.

[0031] Phase Two, Thermal Runaway Warning State. The logic control module continuously calculates the liquid level change rate dL / dt while monitoring temperature data. When the liquid level change rate shows a drop in liquid level and the rate of drop exceeds a preset liquid level safety threshold (for example, this threshold can be selected within the range of 2mm / min to 10mm / min, preferably 5mm / min, and the specific value is determined by the battery system thermal runaway calibration experiment), it indicates that the immersion liquid has begun to vaporize due to local overheating; or when the temperature of any cell exceeds the warning threshold of 60°C or the temperature rise rate exceeds the preset threshold, the logic control module determines that it has entered the warning state. In the warning state, the logic control module shuts down the circulation pump to stop liquid circulation and prevent heat from being carried to other areas; sends a preparatory command to the liquid replenishment device 3 to switch the liquid replenishment device 3 from the normal mode to the fire standby state (for the variable frequency pump in this embodiment, this state is the standby state where the variable frequency controller has completed the pre-charging of the DC bus capacitor and the motor is in the excitation ready state); and sends a warning signal to the station-level monitoring system. The core innovation lies in the timing-priority judgment logic: when the rate of liquid level drop exceeds the liquid level safety threshold, it is triggered first; and only after several seconds to tens of seconds does the temperature rise above 60°C. The logic control module determines this as an early sign of thermal runaway and initiates an emergency response in advance. The physical signal of liquid level drop due to high-temperature vaporization responds faster than the temperature signal. This timing-priority logic can advance the warning time by tens of seconds to several minutes and provide highly specific triggering conditions for fire standby—it only enters standby mode when the liquid level change precedes the temperature change in time, avoiding frequent triggering of standby due to liquid level sensor noise. Furthermore, the logic control module introduces an associated time window mechanism when judging early signs of thermal runaway: only when the rate of liquid level drop exceeds the threshold and the abnormal temperature rise of the battery cell occurs within the same associated time window (as an example, the window length can be selected from 30 seconds to 10 minutes, calibrated by the thermal characteristics of the battery system), and the liquid level signal appears before the temperature signal, is it confirmed to enter the thermal runaway warning state, avoiding miscorrelation between non-thermal runaway events such as slow leakage and delayed temperature changes.

[0032] Phase Three, Firefighting Action Status. The logic control module performs a comprehensive judgment based on multi-signal cross-validation logic. Thermal runaway confirmation requires at least two of the following four conditions to be met simultaneously: Temperature condition—more than half of the points in the distributed temperature sensor array have temperatures exceeding the firefighting action threshold of 80°C; Gas condition—the composite gas detector 6 detects thermal runaway characteristic gas and the concentration continues to rise; Liquid level condition—the liquid level sensing module detects that the liquid level has dropped below the lower limit, or the rate of liquid level drop continues to increase and is statistically positively correlated with the rate of temperature change of at least one cell in the distributed temperature sensor array over a continuous time series (as an example, statistical positive correlation can be defined as a Pearson correlation coefficient of 0.7 or higher between the increase in the rate of liquid level drop and the increase in the rate of temperature rise within a continuous sampling period); Electrical condition—BMS feedback corresponds to an abnormal drop in cell voltage or a sharp increase in internal resistance. By introducing a time-series correlation criterion between the rate of liquid level drop and the rate of cell temperature rise, it is possible to effectively distinguish between two scenarios: liquid level drop caused by leakage but temperature rise not synchronously, and liquid level drop accompanied by rapid temperature rise caused by thermal runaway vaporization, thus avoiding cyclical confirmation based solely on the same liquid level change rate signal.

[0033] Upon confirmation of thermal runaway, the logic control module executes the fire suppression action. Since the system lacks an electric drain valve and the manual drain port 22 is normally closed, there is no possibility of automatic draining during the entire fire suppression process, completely eliminating the risk of evacuation operations from a physical architecture perspective. The fire suppression action is completed within 10-30 seconds, executed in three actions in chronological order. Action 1 (0-2 seconds): The logic control module sends a closing command to the electric inlet valve 41, which temporarily closes. Simultaneously, it confirms that the manual drain port 22 is normally closed. At this time, the replenishment device 3 remains in fire suppression standby mode, and the liquid level in the battery pack 2 is maintained near the level at which the fire suppression action was triggered. Action 2 (2-10 seconds): The logic control module sends a full-speed drive signal to the replenishment device 3, which switches to the fire emergency replenishment mode and operates at full speed. Simultaneously, it sends opening commands to the inlet electric valve 41 and the corresponding branch electric valve of the battery pack 2, rapidly injecting immersion liquid from the external storage tank 1 into the battery pack 2. The liquid level in the battery pack 2 rises rapidly and linearly from its current level. The temperature continues to rise to its peak and then begins to decrease due to the heat absorption of the immersion liquid. The gas concentration continues to rise and then stabilizes due to the opening of the controllable pressure relief valve. Action 3 (10-30 seconds): The logic control module continuously receives real-time feedback signals from the liquid level sensor. When the liquid level reaches the fire target level, it sends a stop command to the replenishment device 3 and a close command to the inlet electric valve 41 and the branch electric valve. The fire replenishment injection is complete, and the process proceeds to Stage 4.

[0034] Phase Four: Submersion Maintenance and Reignition Monitoring. After the fire-fighting fluid replenishment is completed (the fluid level reaches the target fire-fighting level), the system enters the submersion maintenance and reignition monitoring phase. During this phase, the logic control module simultaneously monitors the following three key parameters: fluid level change rate dL / dt—during the continuous heat release process of thermal runaway, the submerged fluid is continuously vaporized and discharged from the pressure relief port, and the fluid level will show a downward trend again, with dL / dt showing a negative value; cell temperature T and its change rate dT / dt—when the internal thermal reaction of the cell has not yet ended, the temperature will remain at a high level or slowly rise; and the difference between the current fluid level and the target fire-fighting fluid level ΔL.

[0035] The logic control module performs continuous liquid replenishment control according to the following logic. When the liquid level drops from the fire-fighting target liquid level to a preset offset below the fire-fighting target liquid level (taking 10mm as an example), and the cell temperature is still higher than the preset replenishment temperature threshold (such as 60℃) or the temperature change rate is still positive (indicating that the internal thermal reaction of the cell has not yet ended and the immersion liquid is still in a state of continuous vaporization), the system automatically triggers replenishment injection—the replenishment device 3 operates at full speed in the fire emergency replenishment mode to replenish the immersion liquid into the battery pack 2 until the liquid level is restored to the fire-fighting target liquid level. This process can be repeated multiple times, forming a cycle of liquid level drop, automatic replenishment, liquid level restoration, drop again, and replenishment again, ensuring that the cell is always in a state of complete immersion during the continuous heat release of thermal runaway. As a preferred control method, the system can also be set with a feedforward triggering mechanism based on the liquid level change rate, that is, when the liquid level drop rate exceeds the liquid level safety threshold again, replenishment injection will begin before the liquid level drops to a fixed trigger point, so as to reduce response delay. As an alternative, once the target fire-fighting liquid level is reached, the logic control module can switch the replenishment device 3 to low-speed compensation mode. Through proportional-integral control, the replenishment flow rate follows the vaporization loss rate in real time, maintaining the liquid level constant near the target fire-fighting liquid level. Once the temperature rise has completely subsided, it switches to liquid level stabilization monitoring and ultimately shuts down. The aforementioned feedforward triggering and low-speed compensation modes can be used independently or in combination to address thermal runaway conditions of varying severity.

[0036] The logic control module continuously determines the conditions for stopping liquid replenishment. Stopping liquid replenishment requires the simultaneous fulfillment of two conditions: Condition 1 (thermal reaction termination criterion) – the cell temperature T drops below the safe temperature threshold of 50℃, and the temperature change rate dT / dt stabilizes within the range of -0.5℃ / min to 0.5℃ / min, with the stabilization time meeting a preset duration (this preset duration is determined based on the cell's heat capacity and the vaporization characteristics of the immersion liquid within the battery pack), indicating that the internal thermal reaction of the cell has essentially terminated; Condition 2 (liquid level stability criterion) – the liquid level change rate dL / dt stabilizes within the range of -1mm / min to 1mm / min, with the stabilization time meeting the aforementioned preset duration, indicating that the vaporization rate of the immersion liquid has essentially stopped. The temperature criterion confirms from the solid-phase heat conduction level that the internal heat source of the cell has been extinguished, and the liquid level criterion confirms from the liquid phase mass conservation level that the vaporization of the immersion liquid has stopped. Only when both conditions are met simultaneously can the hidden risks of a slow decrease in liquid level due to residual heat even after the heat source has been extinguished, or a temperature gradient still existing inside the cell even though the liquid level has stabilized, be eliminated. To avoid frequent resets of the stability timer due to measurement noise or minute instantaneous heat release within the battery cell, the logic control module uses a sliding window filter to filter the temperature change rate and liquid level change rate. If a disturbance exceeding the stability range is shorter than the preset transient tolerance time (for example, 5 seconds), it is considered a glitch and filtered out without interrupting the cumulative timer, ensuring the robustness of the thermal runaway suppression termination judgment. Only when both of the above conditions are met simultaneously does the logic control module determine that thermal runaway has been completely suppressed, issue a liquid replenishment stop command, close the liquid replenishment device 3 and the liquid inlet electric valve 41, and confirm that the maintenance process can begin. If either condition is not met, the system maintains the immersion state and continues monitoring. The immersion time is no less than 30 minutes. After confirming that the temperature has dropped below 50°C and there is no risk of reignition, the operator removes the battery pack 2 and manually drains the liquid through the manual drain port 22 before proceeding with the maintenance process.

[0037] Supplementary manual draining safety procedures: After confirming complete suppression of thermal runaway and that all cell temperatures have dropped below 50°C and remained below this level for 30 minutes, disconnect all electrical connections to the battery pack. Before draining, introduce nitrogen gas into the battery pack through the inert gas purging port, replacing at least five times the pack's internal volume. Operators must wear high-temperature resistant, chemical-splash-proof protective equipment and use explosion-proof tools. Slowly open the manual drain port; the discharged high-temperature liquid will flow through a detachable pipe into a safety collection container with a cooling jacket. The generated gas will be separated and discharged by a flame arrester, eliminating the risk of static electricity and backfire. Example 2

[0038] This embodiment demonstrates another lower-level implementation of the three-level redundancy architecture of the liquid level sensing module.

[0039] Unlike Example 1, the main liquid level sensor and redundant liquid level sensor use float-type fireproof and explosion-proof liquid level sensors instead of magnetostrictive liquid level sensors. Float-type liquid level sensors utilize the mechanical displacement of a float rising and falling with the liquid surface to drive a potentiometer or magnetic element to output a continuous liquid level signal, offering advantages such as simple structure and low cost.

[0040] The mechanical level switch 5 uses a reed switch. A reed switch contains a pair of normally open contacts encapsulated within a sealed glass tube. When a permanent magnet installed inside the float moves up and down with the liquid level and approaches the reed switch, the contacts close, outputting a switching signal. The reed switch eliminates the need for a mechanical linkage penetrating the sealed structure; instead, it drives the contacts through magnetic coupling, avoiding dynamic sealing issues and providing better chemical compatibility and reliability in long-term immersion environments.

[0041] The diagonal arrangement of the redundant level sensor and the main level sensor can be flexibly adjusted to suit different battery pack shapes. For a cuboid battery pack, the main level sensor is installed at the left front corner of the inner wall of the battery pack, and the redundant level sensor is installed at the right rear corner of the inner wall of the battery pack, with the two forming the maximum spatial distance along the diagonal of the cuboid. For a cylindrical battery pack, the main level sensor and the redundant level sensor are installed 180° opposite each other circumferentially on the inner wall of the cylinder, achieving radial spatial redundancy.

[0042] Another implementation of the hard-wired input port is an independent hardware voting circuit. The main control chip of the logic control module runs cross-validation and voting logic software programs, and simultaneously sets up an independent hardware voting circuit. The signal from the mechanical level switch 5 is also connected to the input of the hardware voting circuit. The hardware voting circuit operates independently of the main control chip's software. In the most extreme case of main control chip software failure or communication interruption, the protection logic can still be directly triggered through the hardware voting circuit, further improving the system's safety and integrity level. Example 3

[0043] This embodiment demonstrates an alternative driving method for the replenishment device 3—a high-pressure storage tank solution.

[0044] Unlike Example 1, the external liquid storage tank 1 is a high-pressure tank with a carbon fiber wound aluminum alloy inner liner composite material structure, designed for a working pressure of 1.5–2.0 MPa. The high-pressure tank has an inflation port on top, allowing pre-pressurization via an external high-pressure nitrogen cylinder or air compressor. The liquid replenishment device 3 is replaced by a high-pressure gas-driven device, which includes a pneumatic diaphragm pump located between the high-pressure tank and the battery pack 2, or directly utilizes the gas pressure inside the high-pressure tank to drive the immersion liquid to be rapidly injected into the battery pack 2 through the liquid replenishment pipeline 4. The configuration of the inlet electric valve 41 and the one-way valve 42 is the same as in Example 1.

[0045] In the fire emergency replenishment mode, the logic control module controls the opening of the inlet electric valve 41. The high-pressure gas in the high-pressure storage tank rapidly pressurizes the submerged liquid into the battery pack 2 through the replenishment pipeline 4, eliminating the need for a high-power replenishment pump. The fire standby state corresponds to the storage tank pressure being maintained at the working pressure and the outlet valve control circuit being energized and ready. The advantage of the high-pressure storage tank solution is that fire replenishment does not rely on an external power source—even if the energy storage DC cabinet is completely de-energized, the pressure energy stored in the high-pressure storage tank can still drive a complete fire replenishment operation, achieving intrinsically safe energy supply for fire protection. Example 4

[0046] This embodiment demonstrates another alternative driving method for the fluid replenishment device 3—a passive gravity fluid replenishment scheme.

[0047] Unlike Embodiment 1, the replenishment device 3 is replaced by a gravity self-regulating valve. The gravity self-regulating valve is installed on the replenishment pipeline 4, located between the bottom outlet of the external liquid storage tank 1 and the inlet 21 of the battery pack 2. The installation height of the external liquid storage tank 1 is higher than the preset distance of the highest liquid level of the battery pack 2. This preset distance is determined according to the required gravity-driven head, typically 1.5–3.0 m, to ensure sufficient gravity-driven flow rate in the fire emergency replenishment mode.

[0048] Under normal operating conditions, the gravity-controlled self-regulating valve remains closed or slightly open to maintain the small flow rate required for routine maintenance fluid replenishment. In fire emergency replenishment mode, upon confirmation of thermal runaway by the logic control module, a full-open command is sent to the gravity-controlled self-regulating valve. The valve fully opens, and the immersion fluid in the external storage tank 1 is injected into the battery pack 2 via the replenishment pipeline 4 by gravity. The gravity-controlled self-regulating valve operates by a proportional electromagnet whose valve core position is controlled by an electrical signal output from the logic control module. It can continuously adjust between two modes: small-flow routine replenishment and large-flow fire emergency replenishment. The fire standby state corresponds to the valve controller completing its self-test and awaiting a full-open command.

[0049] The advantage of the passive gravity-based liquid replenishment scheme is that it requires no external energy source—the replenishment power comes from the gravitational potential energy generated by the liquid level difference between the external storage tank 1 and the battery pack 2. In the most extreme case of a complete power outage of the energy storage DC cabinet, if the gravity self-regulating valve adopts a normally open design (automatically opening upon power failure), the system can automatically complete fire-fighting liquid replenishment by gravity without any electrical control, achieving intrinsically safe fire-fighting functionality. However, the fire-fighting liquid replenishment flow rate of the gravity-based scheme is limited by the liquid level difference and is usually lower than that of the active pumping scheme, making it suitable for applications with relatively relaxed requirements for fire response time. Example 5

[0050] This embodiment demonstrates a parameter calibration method for the preset replenishment temperature threshold and the preset duration for stopping replenishment after firefighting.

[0051] The preset replenishment temperature threshold can be set differently depending on the stage of thermal runaway. During the initial continuous heat release period after fire-fighting fluid replenishment, the internal thermal reaction of the battery cell is most intense, and the vaporization rate of the immersion fluid is highest. At this time, the preset replenishment temperature threshold can be set to a higher value of 120℃. Replenishment injection is only triggered when the battery cell temperature is still at an extremely high level, avoiding excessive depletion of the external reservoir 1 fluid volume due to frequent replenishment during the active phase of thermal runaway. During the cooling monitoring period after the continuous heat release period, the intensity of the internal thermal reaction of the battery cell has significantly decreased, but localized high-temperature points may still exist. At this time, the preset replenishment temperature threshold can be lowered to 60℃ to maintain high monitoring sensitivity for residual heat, ensuring timely replenishment of the immersion fluid lost due to residual vaporization. The specific boundary between the two stages can be automatically determined based on the changing trend of the battery cell temperature—when the battery cell temperature drops from its peak and dT / dt turns negative and remains so for a period of time (e.g., 5 minutes), it is considered that the battery has entered the cooling monitoring period from the continuous heat release period.

[0052] The preset time for stopping liquid replenishment is determined based on the thermal capacity of the battery cells and the vaporization characteristics of the immersion fluid within the battery pack, and can be calibrated through thermal runaway suppression experiments. Taking a 314Ah lithium iron phosphate cell as an example, a single-cell thermal runaway triggering experiment (such as overcharge triggering or heating triggering) was conducted, and time-series data of cell temperature and liquid level were recorded after emergency liquid replenishment. Experimental results show that from the time the cell temperature drops below 50℃ and dT / dt enters the stable range of -0.5℃ / min to 0.5℃ / min, the liquid level change rate dL / dt caused by immersion fluid vaporization typically requires a time lag of 3 to 4 minutes to enter the stable range of -1mm / min to 1mm / min (due to the temperature gradient inside the cell causing a difference between the surface temperature and the internal temperature). To ensure a safety margin, a preset time of 5 minutes is set—that is, after both the temperature and liquid level change rate criteria are met, an additional 5 minutes are waited, and liquid replenishment is stopped only after confirming that neither criterion repeats itself. For cells of different capacities, the preset duration should be adjusted accordingly due to the different thermal capacities and thermal runaway durations of the cells. The calibration should be completed during the type testing phase of battery pack 2 and determined based on the specific parameters of the battery type, immersion fluid type, and system configuration.

[0053] As a variation of this embodiment, in another implementation, the preset duration for stopping fluid replenishment is based on the thermal runaway suppression characteristic time constant of battery pack 2. Determined. Characteristic time constant. The following calibration experiments were conducted: During the type test of battery pack 2, after triggering thermal runaway of a single cell and completing fire-fighting liquid replenishment, time-series data of the cell temperature T(t) was recorded from the moment the fire-fighting target liquid level replenishment was completed, as a function of time. Exponential decay fitting was performed on the data during the temperature decrease phase. ,in The ambient reference temperature is ΔT, where ΔT is the initial temperature rise. This is the characteristic time constant. The preset duration is... to --when The cell temperature has dropped by about 95% from its peak value. It had already decreased by approximately 99.3%.

[0054] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of patent protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of patent protection of the present invention.

Claims

1. A closed-loop fire control system for the liquid level of an immersion energy storage battery pack, characterized in that, include: An external liquid storage tank (1) is located outside the battery pack (2). The external liquid storage tank (1) is the only immersion liquid storage device of the system and also serves as a daily maintenance liquid replenishment source and a fire emergency liquid replenishment source. The replenishment device (3) is connected to the external liquid storage tank (1) and the liquid inlet (21) of the battery pack (2) through the replenishment pipeline (4). The replenishment device (3) has a daily operation and maintenance replenishment mode and a fire emergency replenishment mode. In the daily operation and maintenance replenishment mode, it operates at a first flow rate, and in the fire emergency replenishment mode, it operates at a second flow rate higher than the first flow rate at full speed. A liquid level sensing module is installed inside the battery pack (2) to monitor the liquid level height of the immersion liquid inside the battery pack (2) in real time and calculate the liquid level change rate. A fire detection module is used to detect the temperature and temperature change rate of the cells inside the battery pack (2); The logic control module is communicatively connected to the liquid replenishment device (3), the liquid level sensing module, and the fire detection module, respectively. The logic control module is configured as follows: Under normal operating conditions, the liquid replenishment device (3) is controlled to maintain the liquid level in the battery pack (2) at the working level in the daily operation and maintenance liquid replenishment mode; The liquid level change rate and the temperature change rate are acquired in real time. When the liquid level change rate is characterized by a drop in liquid level and a drop rate exceeding a preset liquid level safety threshold, and the occurrence of this situation occurs before the time when the cell temperature exceeds the preset temperature safety threshold, it is determined to be an early sign of thermal runaway. The liquid replenishment device (3) is switched to the fire standby state in advance, so that the liquid replenishment device (3) is in a ready state that can immediately perform full-speed liquid replenishment. When thermal runaway is confirmed, the liquid replenishment device (3) is controlled to run at full speed in the fire emergency liquid replenishment mode to inject immersion liquid into the battery pack (2) until the liquid level reaches the preset fire target liquid level. During the fire liquid replenishment process, the battery pack (2) does not perform the liquid draining operation. After the fire-fighting liquid replenishment is completed, the liquid level change rate and the temperature change rate are continuously monitored. When the liquid level drops from the fire-fighting target liquid level to the preset replenishment trigger liquid level, the replenishment injection is automatically triggered. When the cell temperature drops below the safe temperature threshold, and the temperature change rate and the liquid level change rate are both stable within their respective preset stable ranges and remain stable for no less than a preset duration, it is determined that thermal runaway is completely suppressed and the liquid replenishment is stopped. The battery pack (2) is not equipped with any electric drain valve or automatic drain device. The bottom of the battery pack (2) is only reserved with a manual drain port (22). The manual drain port (22) is a normally closed manual ball valve, which is closed under normal working conditions and is only opened and closed manually by the operator during maintenance.

2. The liquid level closed-loop fire control system for the submersible energy storage battery pack according to claim 1, characterized in that, The liquid level sensing module adopts a three-level redundancy architecture, including: The main liquid level sensor is installed at the first position on the inner wall of the battery pack (2) and is used to output a continuous liquid level signal to the logic control module; A redundant liquid level sensor is installed at a second position on the inner wall of the battery pack (2). The second position and the first position are diagonally arranged inside the battery pack (2). The output signal of the redundant liquid level sensor is independently transmitted to the logic control module for cross-verification with the signal of the main liquid level sensor. Mechanical level switches (5) are set at key liquid level threshold points in the battery pack (2). The switching signal of the mechanical level switches (5) is directly connected to the hard-wired input port of the logic control module. The key liquid level threshold points include low limit liquid level, working liquid level, fire target liquid level and ultra-high limit liquid level.

3. The liquid level closed-loop fire control system for the submersible energy storage battery pack according to claim 1, characterized in that, The fire detection module includes: A distributed temperature sensor array, consisting of multiple thermocouples, is arranged on the surface and at the terminals of each cell in the battery pack (2); A composite gas detector (6) is installed in the space above the inside of the battery pack (2) for simultaneously detecting CO, H2 and volatile organic compounds.

4. The liquid level closed-loop fire control system for the submersible energy storage battery pack according to claim 3, characterized in that, The logic control module is further configured such that the determination of thermal runaway confirmation adopts multi-signal cross-validation logic, which requires at least two of the following four conditions to be met simultaneously: Temperature conditions: The temperature at more than half of the points in the distributed temperature sensor array exceeds the fire alarm threshold; Gas conditions: The composite gas detector (6) detects a gas with thermal runaway characteristics and the concentration continues to rise; Liquid level conditions: The liquid level sensing module detects that the liquid level has dropped below the lower limit liquid level, or the rate of liquid level drop continues to increase and is statistically positively correlated with the rate of temperature change of at least one cell in the distributed temperature sensor array in a continuous time series; Electrical conditions: The battery management system reports an abnormal drop in the corresponding cell voltage or a sharp increase in internal resistance.

5. The liquid level closed-loop fire control system for the submersible energy storage battery pack according to claim 1, characterized in that, The logic control module is further configured as follows: After the fire-fighting liquid replenishment is completed, when the liquid level drops from the fire-fighting target liquid level to a preset offset below the fire-fighting target liquid level, if the cell temperature is still higher than the preset replenishment temperature threshold or the temperature change rate is still positive, the replenishment injection will be automatically triggered to restore the liquid level to the fire-fighting target liquid level. The safe temperature threshold is 50℃, the preset stable range of the temperature change rate is -0.5℃ / min to 0.5℃ / min, the preset stable range of the liquid level change rate is -1mm / min to 1mm / min, and the preset duration is determined based on the heat capacity of the battery cell in the battery pack (2) and the vaporization characteristics of the immersion liquid.

6. The liquid level closed-loop fire control system for the submersible energy storage battery pack according to claim 1, characterized in that, The external liquid storage tank (1) is a high-pressure storage tank. The high-pressure storage tank is driven by high-pressure gas to quickly inject the immersion liquid into the battery pack (2) through the replenishment pipeline (4). The replenishment device (3) is replaced by a high-pressure gas driven device.

7. The liquid level closed-loop fire control system for the submersible energy storage battery pack according to claim 1, characterized in that, The replenishment device (3) is replaced by a gravity self-regulating valve. The installation height of the external liquid storage tank (1) is higher than the preset distance of the highest liquid level of the battery pack (2). The gravity self-regulating valve automatically opens when the liquid level in the battery pack (2) is lower than the fire target liquid level, and uses gravity to inject the immersion liquid in the external liquid storage tank (1) into the battery pack (2).

8. The liquid level closed-loop fire control system for the submersible energy storage battery pack according to claim 1, characterized in that, The preset liquid level safety threshold is 2 mm / min to 10 mm / min.

9. The liquid level closed-loop fire control system for the submersible energy storage battery pack according to claim 1, characterized in that, The logic control module is also configured to: when determining early signs of thermal runaway, confirm that the liquid level change rate is manifested as a drop in liquid level and the rate of drop exceeds a preset liquid level safety threshold and the cell temperature exceeds a preset temperature safety threshold within a preset associated time window, which is 30 seconds to 10 minutes.

10. A closed-loop fire control method for liquid level of an immersion energy storage battery pack, applied to the system described in any one of claims 1 to 9, characterized in that, Includes the following steps: The external liquid storage tank (1) is used as the only immersion liquid storage device. The liquid level in the battery pack (2) is maintained at the working liquid level through the liquid replenishment device (3) in the daily operation and maintenance liquid replenishment mode. The liquid level height of the immersion liquid in the battery pack (2) is monitored in real time and the liquid level change rate is calculated. At the same time, the temperature of the battery cell and the temperature change rate are monitored. When the liquid level change rate is manifested as a drop in liquid level and the rate of drop exceeds the preset liquid level safety threshold, and the occurrence of this situation occurs before the time when the cell temperature exceeds the preset temperature safety threshold, it is determined to be an early sign of thermal runaway, and the liquid replenishment device (3) is switched to fire standby status in advance. When thermal runaway is confirmed, the liquid replenishment device (3) is controlled to run at full speed in the fire emergency liquid replenishment mode to inject immersion liquid into the battery pack (2) until the liquid level reaches the preset fire target liquid level. During the fire liquid replenishment process, the battery pack (2) does not perform the liquid draining operation. After the fire-fighting liquid replenishment is completed, the liquid level change rate and the temperature change rate are continuously monitored. When the liquid level drops from the fire-fighting target liquid level to the preset replenishment trigger liquid level, the replenishment injection is automatically triggered; otherwise, the monitoring continues. When the cell temperature drops below the safe temperature threshold, and both the temperature change rate and the liquid level change rate remain stable within their respective preset stable ranges for at least a preset duration, thermal runaway is determined to be completely suppressed and liquid replenishment is stopped; otherwise, monitoring continues.

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