The present disclosure describes aspects of in-body power harvesting using flowing fluids. In some aspects, an apparatus comprises a magnetic field generator configured to generate a magnetic field to deflect charged particles of a fluid flowing through the body in a space formed by a vessel. The charged particles include positively and negatively charged particles, which the magnetic field deflects such that the positively charged particles are deflected in a first direction within the space and such that the negatively charged particles are deflected in a second direction within the space generally opposing the first direction. The apparatus also includes electrodes configured to harvest energy based on a difference in potential between the positively charged particles and the negatively charged particles. The harvested energy is convertible by a converter coupled to the electrodes into electrical power that is usable by an electronic device.