The thermocell comprises: (a) a porous, conductive, catalyst-contg. oxygen electrode (13) which reacts with water to produce oxygen during charging of the cell; (b) a metal electrode (11) adapted for immersion in an aq. electrolyte and which reacts in the presence of oxygen to generate heat during discharge of the cell; (c) a separator (12) which is permeable to oxygen interposed between the oxygen electrode and the metal electrode; and (d) a housing (14) enclosing the electrodes and separator and which has an opening to permit oxygen to enter and leave the thermocell.    The oxygen electrode has a porosity of at least 20% and an oxygen overpotential of 1.4 or less, when measured against an Hg/H2SO4 reference electrode.    The housing pref. includes a removable covering, pref. a sliding door covering the opening which is pref. located adjacent to the oxygen electrode, opposite to the separator and the metal electrode. An air-pervious, hydrophobic membrane is pref. interposed between the opening and the oxygen electrode. The metal electrode pref. comprises a porous, active Pb material adapted for immersion in a sulphuric acid electrolyte, or a porous Zn material adapted for immersion in KOH electrolyte.    The catalyst is one or more metal carbides, nitrides or oxides, esp. TaN, Mo2C, NbC, TiN, WC, SiC, TiC, Mo2Si and mixts.. If the catalyst has a conductivity of less than 10 ohm-1cm-1, the oxygen electrode contains 10-75 wt. % of a hydrophobic polymer, 10-70 wt % of catalyst and a balance of electrically conductive filler (pref. carbon particles). If the catalyst has a conductivity of at least 10 ohm-1cm-1, the oxygen electrode contains 10-75 wt % hydrophobic polymer and 25-90 wt. % catalyst.    USE -   The thermocell is useful as a personal heater, e.g. in cold football stadium and esp. for warming automobile batteries to improve starting in cold climates, where battery efficiency is low at reduced temps..    ADVANTAGE -   The thermocell can be stored in a sealed plastic case or bag for portability and can be recharged in a customised charger or by a car battery.