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Lithium-ion battery thermal protection circuit design

Keywords: PTC, NTC, lithium battery
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. The basic parameters of control are mostly voltage, current and temperature. In recent years, due to the limited increase in energy density, in order to meet the requirements of customers, rapid charging has rapidly spread. From the perspective of design habits and certification testing, customers tend to pay more attention to voltage monitoring and protection, while ignoring the over-temperature protection features of passive protection devices such as PTC and MHP-TA. They use two IC+Mos active protection schemes and NTC temperature monitoring. Whether there is irrationality in such a design, there is still debate in the industry. This article tries to talk a little bit about the actual use.

As shown in Fig. 1, some defective products will be exposed in advance by aging mechanism in the production process of lithium-ion batteries, which makes the products put into market have relatively low instantaneous failure rate. However, with the prolongation of service time or the increase of cycle times, due to chemical reactions and stress, the internal materials of lithium-ion batteries begin to aging. The intuitive performance is capacity attenuation, volume expansion and internal resistance increase.
Lithium-ion battery "bathtub" curve
Figure 1: Lithium-ion battery "bathtub" curve

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

Current designs are overly dependent 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 the heat conduction is fast, but in fact these two 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 react, a process that is accompanied by longer time and temperature differences. MHP-TA and Strap PPTC are the fastest solutions for sensing internal temperature anomalies in the battery due to their direct connection to the tabs and the high-speed thermal conduction path of the battery. It is therefore also an ideal passive temperature protection device.
The battery safety cycle begins with design and ends with battery scrap recycling. The design plan should consider the abnormal situation of the full life cycle of the lithium ion battery, fully consider and evaluate the protection scheme.
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