A thin film battery comprises a substrate with a front side and a back side. A first battery cell is provided on the front side of the substrate, the first battery cell including an electrolyte between a pair of electrodes. A second battery cell is provided on the back side of the substrate, the second battery cell also including an electrolyte between a pair of electrodes. The battery is capable of providing an energy density of more than 700 wh/l and a specific energy of more than 250 wh/kg. A method of annealing a deposited thin film is also described.