Batteryless energy harvesting IoT sensor nodes such as beat sensors can be deployed in millions without the need to replace batteries. They are ultra-low-power and cost-effective wireless sensor nodes without the maintenance cost and can work for 24 hours/365 days. However, they were not equipped with security mechanisms to protect user data. Data encryption and authentication can be used to secure beat sensor applications, but generating a secure cryptographic key is challenging. In this paper, we proposed an SRAM-based Physically Unclonable Function (PUF) combining a high-reliability bit selection algorithm with a lightweight error-correcting code to generate reliable secure keys for data encryption. The system employs a feature of beat sensors, in which the microcontroller is powered on to transmit the ID signals and then powered off. This fits the SRAM-based PUF requirement, which needs the SRAM to be powered off to read out its random values. The proposed system has been evaluated on STM32 Cortex M0+ microcontrollers and has been implemented to protect important data on beat sensors.