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APPLICATION NOTE
Circuit Protection Solution for Lithium Cells
Primary lithium cells (i.e., AA and 2/3A) and rechargeable lithium cells (i.e., 18650, 17500, and prismatic) are used in many portable electronics applications, such as laptop or notebook computers.
Their popularity is based on the fact that they offer high energy density, high capacity and long cycle life, with no memory effect. Lithium cells are sensitive to faults caused by overcurrent and overtemperature conditions that may result from accidental shorting of the cell terminals and, in the case of rechargeable lithium cells, abusive charging or charger failure. For this reason, lithium cells generally require individual protection.
Their unique electrical characteristics and thin, flat form factor have coefficient) annular disc devices the circuit protection device of choice in a wide range of lithium cell applications.
Typical Protection Requirements
Lithium cells typically require a protection device with a for lithium batteries and the UL2054 standard for household rating of 15V and 40A minimum. Primary and rechargeable lithium cell protection is covered under the UL1642 Standard Technology Comparison and Design Considerations
The industry standard for protecting individual lithium cylindrical cells from overcurrent conditions is the PPTC device, in the form of an annular disc located inside the lid assembly of each cell. The PPTC device works in conjunction with other safety devices, such as separators and pressure vents. Because the design of top cap assemblies in lithium cells varies from manufacturer to manufacturer, PolySwitch annular discs devices are usually customized for each PPTC circuit protection devices are made from a conductive polymer blend of specially formulated plastics and Figure 1. PPTC annular disc devices work in conjunction with conductive particles that are sandwiched between metal foils. At normal temperature, the conductive particles form heating can take place due to an increase in ambient or cell low-resistance chains in the polymer. However, if the temperature, or can be generated by resistive heating, as in temperature rises above the device’s switching temperature, the crystallites in the polymer melt and become amorphous.
PolySwitch PPTC annular disc devices latch into a high- The increase in volume during the crystalline melting phase resistance state when a fault occurs. Once the fault is causes separation of the conductive particles and results in a removed and power is cycled, the device automatically resets non-linear increase in the resistance of the device. The and the battery is restored to normal operating conditions.
Device Selection
The dimensions of the annular disc device are typically The PPTC material selection is based on the performance determined by the cell manufacturer and are based on how characteristics required (e.g., resistance, switch temperature, the device fits inside the cap assembly. As shown in Figure 2, hold- and trip-current). The hold current, trip current, and the key measurements include: outside diameter (A), inside time-to-trip parameters should be measured after the device Most disc devices are installed into the cell through a crimping process that must be well controlled. Placing excessive pressure on the disc during crimping may result in the device failing to trip during an overcurrent event.
Additionally, precautions should be taken to prevent solvent contact with the disc, as such contact can corrode the foil or PolySwitch annular discs are custom designed according to each lithium cell manufacturer’s individual requirements.
Please contact TE for additional information about the circuit Figure 2. PPTC key measurements inside a cap assembly include (A) outside diameter, (B) inside diameter, and (C) thickness.
TE Circuit Protection
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Other logos, product and company names mentioned herein may be trademarks of their respective owners. All information, including illustrations, is believed to be accurate and reliable. However, users should independently evaluate the suitability of each product for their application. Tyco Electronics Corporation and/or its Affiliates in the TE Connectivity Ltd. family of companies (“TE”) makes no warranties as to the accuracy or completeness of the information, and disclaims any liability regarding its use.
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Source: http://www.te.com/content/dam/te/global/english/products/Circuit-Protection/knowledge-center/documents/ap-cp-solutions-for-lithium-cells.pdf