A brief introduction to LR41 battery thermal protection

　　Among many lithium-ion protection schemes, multi-level protection schemes have been widely adopted to obtain high safety of lithium-ion batteries. Usually multi-level protection includes active and passive protection schemes, and the basic parameters controlled are mostly voltage, current and temperature. In recent years, due to limited improvements in energy density, fast charging has rapidly become popular in order to meet customer requirements. From the perspective of design habits and certification testing, customers tend to pay more attention to voltage monitoring and protection, and ignore the over-temperature protection performance of passive protection devices that were commonly used in the past, such as PTC and MHP-TA. They use two sets of IC+Mos Active protection scheme and NTC temperature monitoring. Whether such a design is unreasonable is still debated in the industry. This article will try to briefly talk about it from the perspective of actual use.

　　As shown in Figure 1, lithium-ion batteries use aging and other mechanisms during the production process, which will expose some defective products in advance, making the products put on the market have a relatively low instantaneous failure rate. However, as the use time increases or the number of cycles increases, the internal materials of lithium-ion batteries begin to age due to factors such as chemical reactions and stress. The intuitive manifestations are capacity attenuation, volume increase and expansion, and internal resistance increase.

　　At the end of the battery life, the internal resistance of the battery may increase abnormally. At this time, maintaining high power input and output will inevitably bring about an increase in temperature. Generally speaking, for every 10°C increase in temperature, the chemical reaction rate will approximately double, and the accelerated chemical reaction will lead to accelerated aging of the battery. In other words, it’s an “autocatalytic vicious cycle.”

　　Current designs rely too much on NTC's active temperature monitoring and lack passive over-temperature protection. This design is based on the assumption that the battery temperature distribution is uniform and heat conduction is fast, but in fact both of these points are difficult to achieve. The internal temperature of the battery needs to be conducted to the NTC on the board to effectively "notify the active device to respond, and this process is accompanied by a long time and temperature difference. MHP-TA and StrapPPTC are directly connected to the tabs, and the battery The high-speed thermal conduction path is consistent with the current loop, which is the fastest way to sense temperature abnormalities inside the battery. Therefore, it is also an ideal passive temperature protection device.

　　The battery safety cycle starts with design and ends with battery end-of-life recycling. The design plan should consider the abnormal conditions of the LR41 battery throughout its life cycle and fully consider and evaluate the protection plan.

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