A refuelable electrochemical power source includes one or more electrochemical cells adapted to employ particulate material electrodes. The one or more cells include a cell body defining an interior cell cavity, a flow path in the cell cavity through which particulate material and fluid flow, an electroactive zone within the cell cavity, and a fluid mechanic device adjacent or within the flow path capable of filling the electroactive zone or maintaining the electroactive zone in a constantly full or maximum electroactivity condition. A method for filling or maintaining one or more cells of a refuelable electrochemical power source at a constantly full condition includes providing the above-described apparatus and constantly, periodically, or intermittently circulating particulate material and fluid through the flow path in the one or more cells, past the fluid mechanic device, so that the electroactive zone of the one or more electrochemical cells is maintained in a constantly full or maximum electroactivity condition.