A simple, large-scale fabrication of spherical composite (BaTiO3 nanoparticle/Ca-alginate) beads was developed using the ionotropic gelation (IG) method. The energy harvesting performance of a composite bead-based nanogenerator (CBNG) was studied using various device areas (3 cm2, 11.25 cm2), different mechanical pressures (170 Pa–1.77 kPa) and bending angles, rapid hand force, capacitive loadings and electrical poling. High output (82 V, 227 μA) was generated with a simple low mechanical pressure of 1.70 kPa, which is enough to drive low power electronic devices. We demonstrate that the strip-based CBNG (S-CBNG) device can act as a self-powered wearable flexion sensor for decoding right arm finger flexion/extension movements in a periodic manner without any external battery and additional sensory circuit. It is highly desirable to classify and detect finger movements for in-patient rehabilitation, finger Braille typing and directional bending of the human body. In this work, the average peak power of the flexible S-CBNG varied from 1.23 pW to 0.4 nW for flexion/extension movements of the right arm fingers. The reliability of the sensor depends on the generated electric potential, the location of the device on the forearm, the strain applied by the fingers, and the flexibility of the S-CBNG device. The proposed work is non-invasive, robust, cost-effective and it does not require external power for wearable and smart devices.