Source: Chip Learning
Original Author: Chip Future
This article mainly discusses temperature sensors.
The operational performance and safety of lithium-ion batteries are closely related to their operating temperature range. Excessively high or low temperatures can lead to capacity degradation, increased internal resistance, and even catastrophic safety incidents such as thermal runaway. Therefore, establishing an efficient and reliable Thermal Management System (TMS) is crucial. The effective operation of this system relies on the ability to accurately and in real-time obtain temperature data from various critical points within the battery pack. In this context,NTC (Negative Temperature Coefficient) thermistors, with their high sensitivity, precision, cost-effectiveness, and ease of integration, have become indispensable temperature sensing components in new energy battery packs.
NTC thermistors are strategically deployed at multiple locations within the battery pack to monitor the temperature of the cells or modules continuously in real-time, 24/7. Once the temperature at any monitored point exceeds the preset safety threshold, theBMS will immediately initiate protective mechanisms as follows: First, actively reduce power, limiting the vehicle’s charging or discharging power to decrease battery heat generation; second, initiate strong cooling, activating or enhancing the operation intensity of the liquid cooling/air cooling systems to force heat dissipation; third, issue alarms, alerting the driver of high temperatures through the dashboard and sound; fourth, cut off the circuit, in extreme cases, controlling the relay to disconnect the high-voltage circuit to prevent further escalation. Therefore,NTC is the first and most critical line of defense against battery thermal runaway, ensuring the safety of the vehicle and its occupants.
NTC is directly adhered to the surface of the cell (especially cylindrical or prismatic cells), which is the most direct monitoring method. It is usually placed in areas considered to potentially generate the highest temperatures, such as the center of the module or areas with poor heat dissipation. Battery packs generally adopt a three-tier structure of “cell–module–battery pack.” The layout ofNTC is mainly centered around this structure, typically using surface mount technology. Each module is equipped with a fixed number ofNTC thermistors; there are cases where a module (which may contain8-12 cells) is set with3 NTC temperature sensors to monitor different positions within the module. Some automotive companies’ BMS solutions show that8 NTCs are used to monitor a battery system composed of80 cells.
NTC is a semiconductor component made by mixing and sintering oxides of metals such as manganese, cobalt, and nickel. At low temperatures, the number of freely movable electrons (carriers) inside is very small, resulting in poor conductivity and high resistance. At high temperatures, the internal electrons gain energy, become active, and many escape their bonds to become free carriers, thus improving conductivity and reducing resistance.
NTC thermistor fabrication is a precise ceramic process that begins with raw material preparation and formulation design. First, high-purity transition metal oxide powders, such as manganese oxide(MnO), nickel oxide(NiO), and cobalt oxide(Co₃O₄ etc., are selected, and precise chemical ratio calculations are performed based on the target product’s resistance value (R25) and thermal sensitivity index (B value). Then, the mixing and ball milling stage follows. The weighed raw materials are placed in a ball mill with deionized water and grinding balls for wet ball milling for several hours. This process aims to ensure thorough mixing of the various components and grind them to micron or sub-micron levels, creating favorable conditions for subsequent solid-state reactions. The slurry after ball milling is dried and then enters the pre-sintering stage. The material undergoes the first sintering at a medium temperature of800°C to1000°C, allowing solid-state reactions between the oxides to occur, initially synthesizing the main crystalline phase with a spinel structure while expelling impurities and chemically bound water, thus enhancing material stability. The pre-sintered bulk material undergoes secondary ball milling, being crushed and ground into finer, more active powder. Next is granulation, during which a binder (such asPVA) is added to the powder to form granules with good flowability for subsequent shaping. The granulated particles are then shaped under high pressure in a press into the desired forms, such as discs, beads, or plates.
The shaped green body will undergo the most critical sintering process in the entire procedure. The green body is subjected to final sintering at specific atmospheres (usually air) and at high temperatures of1100°C to1400°C. During this process, the ceramic particles form a dense microstructure through mechanisms such as diffusion and flow, grain growth occurs, and ultimately a spinel solid solution that determines theNTC semiconductor characteristics is formed. A precisely controlled sintering temperature curve is essential for achieving the desired electrical performance.
Once the ceramic body is prepared, it enters the electrode preparation and packaging stage. First, silver is coated or printed onto specific locations of the sintered ceramic piece. This is followed by silver sintering, which occurs at lower temperatures of600°C to800°C, allowing the silver electrode to form a strong, excellent conductive ohmic contact with the ceramic surface. Next is wire bonding; for lead-type devices, metal wires (such as gold) are soldered onto the silver electrodes; for surface mount devices (SMD), solderable end electrodes are made. Afterward, encapsulation and protection are performed, typically using glass glaze for sealing against high-temperature harsh environments or epoxy resin for insulation, mechanical protection, and environmental resistance. SMD types need to be mounted on ceramic substrates and covered with resin.
Finally, all finished products must undergo aging, testing, and sorting. Unstable products are screened out through electrical aging, and then the resistance value of eachNTC (R25) andB value are precisely measured in a constant temperature environment, and they are packaged according to accuracy grades (such as ±1%, ±3%, ±5%) before final shipment.
END
Reprinted content only represents the author’s views
It does not represent the position of the Semiconductor Institute of the Chinese Academy of Sciences
Editor: One Two
Responsible Editor: Catnip
Submission Email: [email protected]
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