I. Introduction
In the current booming development of new energy, lithium-ion batteries have received widespread attention due to their high energy density and wide range of applications. However, the problem of thermal runaway caused by battery overheating has become a major challenge restricting its development. According to statistics from the National Fire Protection Association (NFPA) in the United States, 38% of fire accidents caused by battery overheating worldwide in 2022 were due to the failure of temperature protection devices. Therefore, in the design of battery systems, reasonable selection of thermostats is of key significance for ensuring the safe operation of batteries.
II. Core Technical Parameter System of Thermostats
(I) Temperature Threshold and Accuracy
1. Threshold Setting
Different chemical systems of batteries have different temperature tolerances, so the temperature threshold of the thermostat needs to match accordingly. For example, the upper limit of lithium iron phosphate batteries is usually set at 65℃±2℃, and the lower limit is -20℃±3℃; while the upper limit of ternary lithium batteries is 55℃±1℃, and the lower limit is -30℃±5℃. This precise threshold setting can ensure that when the battery is in a dangerous temperature range, the thermostat responds in time to avoid thermal runaway.
2. Accuracy Level
Different application scenarios also have different accuracy requirements for thermostats. For example, electric vehicle PACKs require an allowable error of ≤±0.5℃ and commonly use thin-film platinum resistance (RTD); household energy storage systems allow an error of ≤±1.5℃ and mostly use NTC thermistors; industrial drones allow an error of ≤±2℃, and bimetallic mechanical switches are more suitable. Taking Tesla's 4680 battery pack as an example, it uses a PT1000 platinum resistance to achieve temperature monitoring accuracy of 0.1℃ at the module level, which fully demonstrates the importance of high-precision temperature monitoring for battery safety.
(II) Response Time (τ)
The response time of the thermostat is determined by the thermal conduction path design and is mainly divided into three types. The surface mount type has a response time of 8 - 15 seconds, suitable for BMS board-level monitoring; the probe insertion type has a response time of 3 - 8 seconds, applicable for cell-level monitoring; the capillary type has a response time of 1 - 3 seconds, mainly used for emergency shutdown scenarios. Laboratory data shows that when the internal temperature rise rate of a cell exceeds 10℃/s, shortening the response time by 1 second can reduce the probability of thermal runaway by 27%, indicating the importance of a fast response in suppressing thermal runaway.
(III) Contact Capacity and Lifespan
1. Electrical Characteristics
For resistive loads, the recommended contact capacity is rated current ×1.2 times, and silver alloy contacts are used; for inductive loads, the required contact capacity is rated current ×2 times, and magnetic blowout + RC snubber circuit is needed at the same time. This design can meet the current carrying and arc extinguishing requirements under different load types, ensuring the safety of the circuit in case of overload or short circuit.
2. Mechanical Life
Low-end mechanical switches generally have a mechanical life of 5,000 - 10,000 times, while high-end magnetic maintain relays can reach more than 1 million times. A longer mechanical life means that the thermostat is more reliable in the long-term operation of the battery system, reducing the trouble and cost of frequent replacement.
III. Four Major Selection Decision Factors
(I) Environmental Adaptability
1. Temperature Shock
When selecting a thermostat, it is necessary to ensure that its working temperature range is 20℃ wider than the battery's extreme temperature. For example, if the battery's extreme temperature is 60℃, then the thermostat's working temperature range should at least reach -40℃ to 80℃, so as to adapt to various complex environmental temperature conditions and ensure normal operation under different working conditions.
2. Vibration Level
For automotive environments, the thermostat needs to meet the IEC 60068 - 2 - 6 standard (10 - 2000Hz/15g). This is because vehicles will be subjected to various frequencies and amplitudes of vibration during driving. Only the thermostat that meets this standard can work stably in such a harsh environment without malfunction or damage due to vibration.
3. Protection Level
Outdoor energy storage systems usually require a protection level of IP67 or above (dustproof and waterproof) to prevent the intrusion of external dust and moisture, which may affect the performance and lifespan of the thermostat; underwater equipment needs to adopt pressure-balanced packaging (withstand 10bar water pressure) to ensure reliable operation in the high-pressure underwater environment.
(II) Failure Safety Mode
1. Normally Open (NO)
The normally open thermostat closes the circuit when the temperature exceeds the standard, suitable for series connection in the power supply main circuit. When the battery temperature is normal, the circuit is in an open state. Once the temperature exceeds the standard, the thermostat closes and cuts off the power supply circuit, thereby protecting the battery and related equipment.
2. Normally Closed (NC)
The normally closed thermostat opens the circuit when the temperature exceeds the standard, mostly used in safety interlock systems. In this mode, the circuit is normally conductive. When the temperature abnormally rises, the switch opens to block the current from continuing to flow, playing a safety protection role.
3. Bistable Type
The bistable thermostat requires manual reset to prevent automatic restoration of power supply and triggering a secondary accident. This type of switch does not automatically return to its initial state after the temperature returns to normal but needs manual intervention for reset operations, enhancing the safety and reliability of the system.
(III) Intelligentization Requirements
1. Basic Function Level
At the basic protection level, a general mechanical switch can meet the simple overtemperature protection needs. For example, in the battery protection of some small electronic devices, the mechanical switch can cut off the power supply when the temperature is too high, preventing further damage to the device.
2. Status Monitoring Level
NTC thermistor + comparator circuit can realize temperature monitoring functions. By comparing the temperature signal collected by the thermistor with the set threshold, when the temperature exceeds the threshold, a corresponding control signal is output to achieve real-time monitoring and preliminary warning of the battery temperature.
3. Predictive Maintenance Level
Digital temperature sensors (such as DSC) combined with AI algorithms can predict the risk of thermal runaway in advance. By analyzing a large amount of temperature data and learning from it, AI algorithms can predict the trend of battery temperature changes and take preventive measures in advance to improve the safety and reliability of the battery system.
4. Cloud Management Level
Intelligent thermostats with unique UIDs can achieve remote monitoring and management. With the help of Internet of Things technology, the temperature data of batteries can be uploaded to cloud servers. Engineers can view battery status in real-time through mobile apps or computer software, centrally manage and monitor multiple battery systems, and timely discover and handle potential problems.
(IV) Total Cost of Ownership (TCO)
1. Procurement Cost
When selecting, one should not only focus on the price of the thermostat itself but also consider its overall value in the battery system. For example, although some high-end intelligent thermostats have higher procurement costs, their high precision, high reliability, and long lifespan can reduce maintenance costs and fault risks in the entire life cycle of the battery system, which may be more economical in the long run.
2. Operation and Maintenance Cost
Choosing a thermostat with self-diagnosis function can reduce the workload and cost of later maintenance. The self-diagnosis function can monitor the working status of the thermostat in real-time. When a fault occurs, it can promptly issue an alarm and provide fault information, facilitating maintenance personnel to quickly locate and solve problems, reducing downtime and maintenance costs.
3. Empirical Case
After an energy storage station adopted digital thermostats instead of traditional analog switches, its operation and maintenance costs were reduced by 62%. This is because digital thermostats have higher precision and reliability, reducing battery damage and safety accidents caused by the failure of temperature protection devices. At the same time, they also reduced the frequency and cost of manual inspection and maintenance.
IV. Typical Scene Selection Strategies
(I) Electric Vehicle Power Batteries
Electric vehicle power batteries work in a complex and variable environment, so they have extremely high requirements for thermostats. Adopting multi-layer composite temperature monitoring, including cell surface + tab + cell bottom gradient detection, can comprehensively master the temperature distribution of batteries. Digital thermostats with self-check functions can ensure stable operation in complex electromagnetic environments without interference. For example, during high-speed driving or charging of vehicles, digital thermostats can accurately monitor battery temperature, timely detect and handle abnormal situations, and ensure driving safety.
(II) Household Energy Storage Batteries
Household energy storage batteries are usually installed in indoor or outdoor environments, so it is necessary to balance cost and reliability. The dual metal sheet + electronic redundancy design can effectively prevent false triggering. The built-in 1-hour temperature change rate filtering algorithm can cope with the situation that the household electricity load fluctuates greatly. For example, during the night when electricity consumption is low and batteries are charged, there may be certain temperature fluctuations. This filtering algorithm can avoid false triggering of the thermostat due to short-term temperature changes and ensure normal charging and use of batteries.
(III) Industrial Drone Batteries
Industrial drones face harsh conditions such as high temperature and high vibration during flight, so they have special requirements for the impact resistance and lightweightness of thermostats. The MEMS temperature sensor and protection IC integrated module can quickly respond to temperature changes and ensure that the battery temperature is always within a safe range during flight. For example, when performing high-intensity tasks during drone flight, motor heat generation may cause the battery temperature to rise. This integrated module can quickly sense and take corresponding measures to prevent battery thermal runaway accidents.
V. Industry Warnings and Cutting-Edge Technologies
(I) Common Misunderstandings
1. Threshold Setting Error
A company set the protection threshold of lithium iron phosphate batteries at 70℃, causing the cells to work at high temperatures for a long time and eventually leading to thermal runaway. This reminds us that when selecting, we must set reasonable temperature thresholds according to the characteristics of the battery chemical system. Otherwise, it may bring serious safety hazards.
2. Ignoring Vibration Environment
Part of automotive energy storage systems do not fully consider vibration factors and choose ordinary mechanical switches. During vehicle driving, these switches may malfunction or be damaged due to vibration. This emphasizes that in automotive and other vibrating environments, it is necessary to choose thermostats that meet corresponding vibration level standards.
3. Blindly Pursuing Intelligence
Some manufacturers rely too much on AI algorithms while ignoring hardware infrastructure construction, resulting in inaccurate temperature monitoring in actual operation. This indicates that while pursuing intelligence, we cannot ignore the stability and reliability of hardware. The two should complement each other.
(II) Cutting-edge Technologies
1. AI Prediction and Early Warning
AI prediction and early warning technology based on big data analysis and machine learning algorithms can anticipate the risk of battery thermal runaway in advance. By analyzing a large amount of historical temperature data, battery performance parameters, and environmental factors, a prediction model is established to evaluate the safety status of batteries in real-time, providing enough reaction time for operation and maintenance personnel to take preventive measures.
2.Application of New Materials
The application research of new materials such as graphene in thermostats is emerging. These materials have excellent thermal conductivity, strength, and stability, which are expected to greatly improve the performance of thermostats. For example, graphene-coated heat sinks can more efficiently conduct heat and improve the heat dissipation efficiency of thermostats, thereby enhancing their ability to control battery temperature.
3. Wireless Communication Technology
The introduction of wireless communication technologies such as Bluetooth and LoRa makes data transmission between thermostats and cloud platforms more convenient and efficient. Through wireless connections, remote monitoring, parameter adjustment, and fault diagnosis can be realized, further improving the intelligence level and operation and maintenance efficiency of battery management systems.
VI. Conclusion
Thermostats play a vital role in the thermal management of power lithium batteries. Their selection is a complex process involving multiple factors, requiring comprehensive consideration of technical parameters, decision factors, application scenarios, and industry development trends. With the continuous progress of technology, thermostats will develop towards intelligence, high precision, high reliability, and miniaturization in the future to meet the growing safety and performance needs of power lithium batteries. In practical applications, appropriate thermostats should be selected according to specific situations, and attention should be continuously paid to industry dynamics and technological development. The selection strategy should be optimized in time to ensure the safe and stable operation of power lithium battery systems.
