How to do thermal protection for lithium-ion batteries

Among many lithium-ion protection schemes, multi-level protection schemes have been widely adopted to obtain the high safety of lithium-ion batteries. Generally, multi-level protection includes active and passive protection schemes, and the basic parameters of control are mostly voltage, current and temperature. In recent years, due to the limited increase in energy density, fast charging has rapidly spread 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 passive protection devices that have been used more before, such as PTC and MHP-TA over temperature protection. They use two sets of IC+Mos Active protection scheme and NTC temperature monitoring. Whether such a design is unreasonable is still controversial in the industry. This article tries to talk about one or two from the perspective of actual use.

As shown in Figure 1, the lithium-ion battery adopts aging and other mechanisms in the production process, which will expose some defective products in advance, making the products put on the market have relatively Low instantaneous failure rate. However, with the extension of the use time or the increase of the number of cycles, the internal materials of the lithium-ion battery begin to age due to factors such as chemical reactions and stress, which are intuitively manifested as capacity decay, volume increase and expansion and internal resistance increase.

At the end of battery life, the internal resistance of the battery may increase abnormally. At this time, maintaining high-power input and output will inevitably lead to an increase in temperature. Generally speaking, for every 10°C increase in temperature, the chemical reaction rate will double, and the acceleration of the chemical reaction will bring about accelerated aging of the battery. In other words, this is a vicious circle of 'autocatalysis'.

Current designs rely too much on NTC's active temperature monitoring, but lack passive over-temperature protection. This design is based on the assumption that the battery temperature is uniformly distributed and the heat conduction is fast, but in fact these two points are more difficult to achieve. The internal temperature of the battery needs to be transmitted 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. Since MHP-TA and Strap PPTC are directly connected to the tabs, and the high-speed thermal conduction path of the battery is consistent with the current loop, they are the fastest solution for sensing abnormal temperature inside the battery. Therefore, it is also an ideal passive temperature protection device.

The battery safety cycle starts from the design and ends with the recycling of the battery. The design plan should consider the abnormal conditions of the lithium-ion battery's full life cycle, and fully consider and evaluate the protection plan.

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