The　effect　of　fumed　silica　(F-SiO2)　density　on　the　performance　of　valve-regulated　lead-acid　(VRLA)　batteries,　including　the　conductivity　of　H+　ions　and　the　diffusion　of　HSO4-　ions　in　the　gel　electrolyte,　the　redox　capacity　of　lead,　and　the　evolution　of　hydrogen　and　oxygen,　have　been　studied　over　a　wide　range　of　F-SiO2　densities.　The　results　show　that　the　influence　of　F-SiO2　density　is　not　monotonous　and　always　yields　a　maximum　(or　inflection)　effect　at　c.　5　to　6　wt%　silica,　before　and　after　which　the　effect　of　F-SiO2　differ.　At　lower　F-SiO2　densities,　in　general,　gel　formation　favours　an　improvement　of　the　electrochemical　behaviour.　However,　it　may　produce　an　adverse　effect　on　the　electrochemical　properties　if　F-SiO2　density　further　increases.　Battery　tests　at　a　range　between　1　to　3　wt%　silica　support　this　observation,　in　which　the　discharge　capacity,　the　utilisation　of　active　materials,　and　the　charge　efficiency　are　all　enhanced.　(C)　The　Electrochemical　Society　of　Japan,　All　rights　reserved.