by Matthew Galla Materials and Process Development
Circuit protection for rechargeable lithium-based battery cells and packs is a critical design consideration. Overcurrent and overtemperature conditions that can result from accidental shorting or abusive/runaway charging can raise temperatures high enough to damage components and cause substantial equipment damage.
Early circuit-protection designs relied on fragile one-shot fuses to provide total current interruption for overcurrent protection. However, because the majority of fault conditions that a battery pack encounters are relatively infrequent or intermittent events, a resettable protection device is generally preferable.
Today, Li-ion packs typically include an active overvoltage and overcurrent detecting safety circuit (IC and MOSFETs) as the primary pack protection, and a polymeric positive temperature coefficient (PPTC) device in series as a second level of protection. Although the semiconductor circuitry is considered reliable, there are conditions under which failure of the primary protection may occur, such as excessive electrostatic discharge, high temperature, or oscillation during a short circuit condition. In these cases, the PPTC device provides both short-circuit and overcharge protection.
For emerging Li-ion cell technologies, such as Li-P and Li-Mn, the increased level of safety inherent in the cell design has led some OEM pack designers to eliminate the IC safety circuit and rely on a PPTC device as the primary short-circuit and overcharge protection (Fig. 1.) An optional secondary passive element, in the form of a current fuse or thermal fuse, is sometimes included if protection is needed beyond the rated voltage of the PPTC device.
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