Valve-regulated　lead-acid　(VRLA)　batteries　that　have　aged　on　a　float　charge　at　constant　voltage　occasionally　suffer　from　thermal　runaway.　Operating　conditions　for　a　VRLA　battery　have　been　simulated　by　changing　the　electrolyte　saturation　level　in　the　separator　and　the　ambient　temperature.　The　charge　current,　battery　temperature　and　cell　overpressure　were　measured　during　current-limited　constant-voltage　charging.　The　experiments　show　that　applied　voltage,　saturation　level　and　ambient　temperature　are　significant　variables　in　the　oxygen　cycle.　However,　the　saturation　level　of　the　electrolyte　in　the　separator　pore　volume　is　critical.　When　it　is　lower　than　80%,　thermal　runaway　occurs　readily.　Significant　corrosion　of　the　positive　grid　and　poor　conductivity　between　the　grid　and　the　active　mass　(AM)　is　also　found　in　aged　VRLA　batteries,　and　many　inactive　PbSO4　crystals　appear　on　the　negative　plates.　As　a　result,　both　positive　and　negative　plates　have　a　very　high　resistance,　which　can　accelerate　thermal　runaway.