The　36　or　48　V　valve-regulated　lead-acid　(VRLA)　battery　packs　have　been　widely　applied　to　the　power　sources　of　electric　bicycles　or　light　electric　scooters　in　China.　The　failure　modes　of　the　12　V/10　Ah　VRLA　batteries　have　been　studied　by　the　cycle　life　test　at　C2　discharge　rate　and　100%　depth　of　discharge　(DOD).　It　indicates　that　the　main　cause　of　the　battery　failure　in　this　cycle　duty　is　the　softening　and　shedding　of　positive　active　mass　(PAM)　rather　than　individual　water　loss,　recombination　efficiency　or　sulfation,　etc.　When　the　electrolyte　saturation　falls　to　a　certain　extent,　the　high　oxygen　recombination　current　leads　to　the　depolarization　of　the　negative　plate　and　the　shift　of　the　positive　plate　to　a　higher　potential.　The　violent　oxygen　evolution　accelerates　the　softening　of　PAM　and　the　end　of　cycle　life.　©　2008　Elsevier　B.V.　All　rights　reserved.