Abstract: There is disclosed a battery system to be used in downhole application. The battery system provides energy to operate the measurement devices associated with drilling. The system includes a plurality of cells, each comprising an electrically insulating mandrel which is shaped to fit over an inner tube, and a combination of an anode, a cathode and a solid polymer electrolyte, all disposed over the mandrel. The individual cells are mounted end to end and are interlocked together to prevent rotation of the cells relative to one another. The cells are electrically connected together and they are all mounted between an inner and an outer tube.