Ultrathin galvanic cells, which can comply with a variety of form factors and electronic system packages, are of technological importance, as they show promise for flexible electronic systems. Here we describe a high energy density flexible galvanic cell, which is non-toxic and environmentally friendly. It operates with a zinc anode and hydrated ruthenium(IV) oxide cathode, where RuO2·nH2O nanoparticles are utilized in amounts that are not cost-prohibitive. As the battery utilizes aqueous electrolytes, it is safe in operation, which enables its use in a number of settings and surroundings. It can be optimized for volume manufacture at low cost. Given that they function at much lower cell voltage than Li-ion batteries do, Zn–RuO2·nH2O cells can be recharged remotely, at a conveniently low voltage, by harvesting, for example, radio-frequency (RF) energy or microwaves. As an additional asset, Zn–RuO2·nH2O electrodes enable battery-supercapacitor hybrid power sources. At present, this cell demonstrates a specific charge capacity of 84.4 mAh per cm2 of projected electrode area, which is, so far, the largest value reported for thin film cells. Also, its cycle life of up to 400 charge–discharge cycles is very promising for use as a secondary battery.