In certain battery applications, e.g. energizing tiny electrostatic oscillators useful for timepieces, the power needed is very modest (about 0.5-1.5 mu W) but the output voltage must be high (500-1500 V) and a long lifetime is desirable. These requirements can be met by an isotopic battery, based on the collection of charged particles emitted from a tritium source. The report describes the theory and present state of development of such a battery, mechanically designed to fit into the case of a wristwatch.