A novel fuel particle feeding mechanism that significantly improves operation of a refuelable electrochemical power source is provided. One embodiment of the mechanism provides an input of electrolyte and electrochemically active particles into one or more electrochemical cells through a feed tube. The feed tube has several embodiments that improve the flow of electrolyte and particles, and the even and complete settling of particles into individual cell cavities. Each cell cavity is completely filled, evenly and without clogging when open, and fluid communication between cell cavities is completely cut off when closed, to prevent short-circuiting of the power source during discharge or standby. Means are provided to prevent clogging of the feed tube with fuel particles, to promote complete filling of the cell cavities with fuel, and to prevent fuel particles from prematurely flowing out of the cell cavities. A method of operation is also described.