NOVELTY - A battery half cell comprises: greater than or equal to 1 carbon fiber as negative electrode, where carbon fiber comprises several layers with carbon atoms having graphite structure, and having ability of intercalating metal ions; and carbon fiber is at least partially coated with greater than or equal to 1 electrically insulating polymer layer acting as ionically conducting electrolyte, has been applied with electro-driven polymerization reaction, is permeable for metal ions, has stiffness of greater than or equal to 0.5 MPa, has electrical ionic conductivity of greater than or equal to 10-10 S/m and electrical resistivity of greater than or equal to 1010 Omega m.    USE - As battery half cell, useful for forming battery (all claimed) useful for vehicles such as electric vehicles, cell phones, and airplanes.    ADVANTAGE - The electrically insulating polymer layer of carbon fibers acting as ionically conducting electrolyte, is applied with electro-driven polymerization. Using electro-driven polymerization provides battery where polymer layer may be electrically insulating despite its thinness. The polymerization is initiated on all bare carbon surfaces. Thus uncoated spots are avoided so that coating entirely or essentially free of defects is achieved, and provides perfectly coated surface entirely free or essentially free of parts without coating. When polymerization has progressed so that polymer layer is electrically insulating polymerization reaction automatically stops. As soon as there is uncoated spot polymerization reaction will start on uncoated bare carbon fiber. Uncoated spots may otherwise cause undesired short circuit. As a result, battery or cell is provided where short-circuits of cell can be avoided despite use of very thin layer of polymer. The possibility to use thin polymer layer reduces at same time distance over which metal ions must be transported thus retains sufficient ion conductivity/mobility. Further, process itself is self inhibitory which provides robust and reliable battery eliminating need of costly testing during manufacture. At the same time, relatively small dimensions of battery cell enable scalability where large number of carbon fibers, are arranged in parallel. The process gives covalent bonds to coated carbon fibers and insulating polymer layer. This has the advantages of giving very high adhesion of polymer layer. The half cell battery by using carbon fibers has additional advantage of possibility to use same design independently of desired capacity of battery. If higher real energy density is required, battery can simply be made thicker while other properties remain unchanged. Thus still makes use of all active material and full depth of electrodes, whereas batteries with higher cell voltages can in principle be constructed by connecting several cells together in series. Thus battery is scalable. The storage capacity is increased by adding another carbon fiber. Polymer layer, coated on carbon fiber, having stiffness of greater than or equal to 0.5 MPa takes up mechanical load unlike liquid electrolytes and thus makes them more robust than other comparable electrolytes. The carbon fibers have inherent strength which makes them suitable for use in structural batteries which are able to take up mechanical load, and can replace traditional heavy, bulky and structurally parasitic batteries in electric vehicles and provide essential weight saving potential. Further weight and volume of parasitic cell components such as separator and current collectors can be kept at minimum, and higher specific and gravimetric energy densities are achievable, superior even to conventional cell designs based on same active materials. The ease of manufacturing not including any costly or time-consuming manufacturing steps provides advantages compared to most other microbattery designs. The short distance and large cross-section between negative and positive electrode materials makes it possible to utilize electrolytes with poor conductivity without creating large ohmic losses; whereas traditional laminar structure results in electrode distances of at least several tens of micrometer, and corresponding large ohmic potential drop. The design of battery also allows for more or less free forming of battery, as long as electrically insulating polymer layer acting as electrolyte can be applied.    DETAILED DESCRIPTION - A battery half cell comprises: greater than or equal to 1 carbon fiber as negative electrode, where carbon fiber comprises several layers with carbon atoms having graphite structure, and having ability of intercalating metal ions; and carbon fiber is at least partially coated with greater than or equal to 1 electrically insulating polymer layer acting as ionically conducting electrolyte, has been applied with electro-driven polymerization reaction, is permeable for metal ions, has stiffness of greater than or equal to 0.5 MPa, has electrical ionic conductivity of greater than or equal to 10-10 S/m and electrical resistivity of greater than or equal to 1010 Omega m. The electrically insulating polymer layer has been applied with electrodriven polymerization reaction. The carbon fiber has length of greater than or equal to 1 cm, stiffness of 100-000 GPa and strength of 1-10 GPa in longitudinal direction of carbon fiber. The insulating polymer layer has thickness of 10-800 nm (preferably 100-600 nm). The metal ions are Li+ ions. INDEPENDENT CLAIMS are included for the following:    (1) battery comprising the battery half cell, and further comprising material as positive electrode applied outside and in contact with electrically insulating polymer layer, with positive electrode material further comprising greater than or equal to 1 type of metal ion, and battery further comprising conductor adapted to collect electrical current from positive electrode material; and    (2) method for manufacturing battery half cell involving: providing carbon fiber as negative electrode carbon fiber comprising several layers with carbon atoms having graphite structure, and having ability of intercalating with metal ions; coating carbon fiber at least partially with greater than or equal to 1 electrically insulating polymer layer acting as electrolyte, where insulating polymer layer is applied with electro-driven polymerization reaction, is permeable for metal ions, has stiffness of greater than or equal to 0.5 MPa, has electrical ionic conductivity of greater than or equal to 10-10 S/m and electrical resistivity of greater than or equal to 1010 Omega m.    DESCRIPTION OF DRAWING(S) - The figure shows set-up comprising coated carbon fiber (CF) roving in direct contact with lithium metal foil for battery half cell.