Protecting Rechargeable Li-Ion and Li-Polymer Batteries

Littelfuse offers designers many different protection devices to choose from in an array of form factors and device characteristics that meet the needs of their particular design.

As sleeker designs and thinner portable consumer electronics, such as smart phones, tablets, power banks, other advanced handheld electronics and emerging drone, e-bike and e-cigar market become increasingly popular, Lithium-ion and Lithium- polymer batteries, known collectively as Li-batteries, have become the “go-to” power sources of choice in these applications. As battery technology and form factors for consumer devices expand beyond traditional cylindrical cells, Li-batteries are in increasing demand due to their higher energy density, small form factors and design flexibility. These batteries, in turn, require ever-smaller circuit protection devices to help provide robust protection in thinner, lower-profile and more compact portable products.

Need for Battery Protection:
Li-batteries are particularly sensitive to faults caused by external shorts, runaway charging conditions and abusive overcharging that can result in potentially damaging overcurrent and overtemperature conditions. The overcharge, deep-discharge, or short circuit conditions that create heat can cause a Li-battery cell to bloat, rupture, or experience other issues, even fire .

Although internal cell failures are less common, an adverse event may affect any of the complex electronics on the battery pack’s PCM (Protection Circuit Module), such as the fuel gauge or charge controller. Because these components are vulnerable to these events, Li-cells using PCMs require many levels of protection against overcharge shutdown, over-discharge shutdown, overtemperature shutdown, and overvoltage and under-voltage lockout of a cell that may lead to thermal runaway and possibly failure.

Organizations such as UL, IEC and IEEE have enforced safety regulations and established test requirements for Li-ion and Li-Polymer packs to demonstrate their resilience to both short circuit and overcharge events. (For additional details refer to UL2054, “Standard for Household and Commercial Batteries”; IEEE 1725-2011 “Standard for Rechargeable Batteries in Cellular Telephones”; and IEC/EN 60950 and IEC 62133 specifications.). Moreover, certain end product applications require that the power output of a battery be limited to reduce the risk of device failures. The Limited Power Source (LPS) Test described in UL2054 is used to determine whether a cell or battery is suitable in such applications where safety issues may otherwise exist.

This application note discusses the need for protecting Libatteries against short circuit and overcharge conditions and shows how devices from Littelfuse’s can help designers achieve robust and safe battery solutions. As a pioneer of polymeric positive temperature coefficient (PPTC) resettable devices, Littelfuse has developed PolySwitch PPTC resettable devices of different form factors including strap, disc, surface-mount, weldable and reflowable products to meet different assembly requirements. These devices are supported by several material platforms for protecting battery applications. Each materials platform offers different performance characteristics and a range of thermal cut-off, or activation temperatures. In the recent years, Littelfuse also developed MHP-TA resettable compact thermal cut-off device by combining the company’s bimetal and PPTC technologies for the high capacity Li-Polymer and prismatic cell applications.