Electrochromic tungsten oxide (WO3) and indium hexacyanoferrate (InHCF) thin-film electrodes, in combination with a polymer electrolyte that accommodates conduction of both K+ and H+, were assembled into a thin-film electrochromic battery (ECB). A typical InHCF-WO3 ECB (designated as IWECB) can be charged and discharged reversibly between 0.5 and 1.5 V with a theoretical voltage of around 1.24 V A hybrid K+/H+-conducting solid polymer electrolyte (SPE) was prepared through doping different amounts of KCl into poly-2-acrylamido-2-methylpropane sulfonic acid (PAMPS). The resultant SPE fulfilled the dual requirements: H+ for the WO3 and K+ for the InHCF insertion/extraction. The KCl-doping level, evaluated from the molar ratio of (KCl)/(AMPS), plays a crucial role in determining the SPE properties, including its ionic conductivity and water content, and thus strongly affects the charge-discharge characteristics of the IWECB. Furthermore, both properties of the SPE exhibited a very similar, concave-up dependence on the doping level, and a minimum existed at (KCI)/(AMPS) = 0.44. It was found that the SPE with a higher KCl-doping level could achieve a larger discharge capacity and a higher cell voltage, but would result in a poorer cycle life. Although the charge capacity of the IWECB was limited, it is enough to drive many low-watt electronic devices for several hours. Finally, the capability of the IWECB was also demonstrated by storing solar energy and by visualizing the state-of-charge (SOC), in which a highly contrasting blue-to-colorless electrochromism in response to discharging was visualized. (C) 2003 Elsevier B.V. All rights reserved.