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Lithium-ion battery thermal protection

Date: 2019-05-07
Viewed: 68
案例名称: Lithium-ion 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. 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 passive protection devices that were used before.

Over temperature protection performance of PTC and MHP-TA, they use two sets of IC+Mos active protection scheme 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, the lithium ion battery adopts a mechanism such as aging in the production process, which exposes some defective products in advance, so that the products put on the market have relatively low instantaneous failure rate. However, with the prolonged use time or the increase of the number of cycles, the internal materials begin to age due to chemical reactions and stresses, and the visual performance is that the capacity is attenuated, the volume is increased, and the internal resistance is increased.


Figure 1: '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 design is overly dependent on NTC

Active temperature monitoring without 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 board.

The NTC can effectively 'notify' the active device to react, and this process is accompanied by a long time and temperature difference. MHP-TA and Strap PPTC

Because it is directly connected to the pole, and the high-speed heat conduction path of the battery is consistent with the current loop, it is the fastest solution for sensing the abnormal temperature inside 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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