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
www.te.com/japan/bu/circuitprotection (Japanese)
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including illustrations, is believed to be accurate and reliable. However, users should independently evaluate the suitability of each
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(“TE”) makes no warranties as to the accuracy or completeness of the information, and disclaims any liability regarding its use.
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indirect, or consequential damages arising from the sale, resale, use, or misuse of the product. Specifications are subject to change
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APPLICATION NOTE
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.