A photonic crystal electrical property indicator for a battery comprising: an electrically conductive working electrode and an electrically conductive counter electrode; an electrically-responsive photonic crystal material in contact with at least one of said first and second electrically conductive electrodes; an ionically conductive electrolyte provided between the working and counter electrodes; and electrical conductors for connecting the indicator to terminals of the battery; wherein the photonic crystal material has a reflectance spectrum that is responsive to a change in at least one electrical property of the battery, thereby providing an optically detectable indication of the at least one electrical property of the battery.