Most portable mobile devices employ rechargeable lithium-ion batteries. This lithium-ion battery usually suffers from the possibility of explosion due to heat generation from surrounding atmosphere or internal deficiency during charging or at overuse. To solve these problems, most rechargeable batteries have a built-in protection circuit module (PCM). The resistance of a properly processed $VO_2$ critical temperature sensor (CTS) is changed dramatically at a critical temperature of around $68^{circ}C$, which can replace some bi-metal, NTC, or PTC sensors embedded in PCM. Such $VO_2$ CTS consumes a very small current at the level of natural discharge. Experimental results showed that this CTS could be applied to a PCM as the PCM could protect the battery while keeping its power consumption at minimum.