Battery　accidents　due　to　thermal　runaway　have　been　frequently　reported.　The　operating　principles　that　take　place　in　lithium-ion　cells,　valve-regulated　lead-acid　(VRLA)　batteries,　nickel-cadmium　(Ni-Cd),　and　nickel-metal　hydride　(Ni-MH)　cells　during　overcharge,　and　the　factors　that　lead　to　thermal　runaway　are　reviewed.　The　VRLA　batteries　make　use　of　the　same　oxygen　cycle　principle　as　Ni-Cd　and　Ni-MH　cells.　Thermal　runaway　of　all　three　types　of　cells　is　attributed　to　the　self-accelerating　processes　of　temperature　rise　and　unrestricted　overcharge　current　increase　at　unsuitable　constant　voltage　(CV).　The　safety　issue　for　lithium-ion　cells　is　of　greater　concern.　During　overcharge,　the　lithiated　carbon　anode　and　delithiated　cathode　materials　are　very　reactive.　The　decomposition　of　the　cathode　materials　and　solid　electrolyte　interface　(SEI)　film　and　the　reactions　of　the　electrolyte　with　the　negative　and　positive　electrodes　are　all　exothermic　reactions.　Any　of　the　exothermic　reactions　above,　or　external/internal　short　circuits,　may　trigger　self-heating　of　the　cell　and　lead　to　combustion　and　even　explosion.　Therefore　it　is　vital　that　the　lithium-ion　cells　be　managed　individually　in　the　cell　string　during　charging.　©　2009　Elsevier　B.V.　All　rights　reserved.