The　battery　temperature,　H2SO4　distribution,　Pb2+　ion　concentration　and　composition　of　the　plates　during　the　plate　soaking　of　the　12　V　12　Ah　valve-regulated　lead-acid　(VRLA)　battery　are　studied.　A　simulated　cell　composed　by　two　pure　Pb　plates　and　the　absorptive　glass　mat　(AGM)　separator　is　used　to　investigate　the　growth　of　the　lead　dendrite　in　the　separator,　which　is　verified　by　analyzing　the　faulty　batteries　after　the　formation　and　the　failure　batteries　after　the　usage.　It　is　found　that　the　H2SO4　is　exhausted　very　quickly　after　filling　and　the　separator　near　the　plates　can　become　neutral　during　soaking.　Low　acidity,　high　temperature　and　small　PbSO4　particle　size　will　increase　the　Pb2+　ion　concentration.　Higher　Pb2+　ion　concentration　makes　more　PbSO4　tend　to　deposit　on　the　coarse　glass　fibers,　develop　along　them　and　even　enwrap　the　entire　fiber.　And　the　fine　PbSO4　crystals　are　continually　transformed　into　large　PbSO4　crystals　via　dissolution-deposition.　In　the　subsequent　charge,　these　PbSO4　crystals　will　be　reduced　to　the　club-shaped　lead　dendrites　and　may　lead　to　short　circuit　of　the　battery.　©　2015　Elsevier　B.V.　All　rights　reserved.