Abstract: The present invention relates to a battery (100) comprising an electrode material (102a), an electrolyte material (104), a battery charge sensor (106, 206, 306) comprising a plasmonic sensing element (108, 208, 308) having a sensing volume (110, 210, 310) within the battery (100, 200, 401) and which upon illumination with electromagnetic radiation exhibits a localized surface plasmon resonance condition being dependent on a charge state of the battery (100, 200, 401). A system and a method for determining a charge state of a battery are further provided.