At approximately 13:50 PDT, a Beech A200 suffered a collapsed main landing gear on landing roll at Boeing Field. There were no injuries. The flight returned to its departure airport after experiencing a total electrical failure shortly after departure. The pilot said both DC generator caution lights illuminated on the aircrafts annunciator panel. He declared an emergency and turned back toward Boeing Field. He lowered the gear manually, but the gear collapsed on rollout. The pilot could not recall placing the engine ignition/start switches OFF at the completion of the engine start sequence, as required. The aircrafts battery failed a capacity check in the as-found condition, but subsequently accepted a charge and, once charged, passed a capacity check. After charging, the battery was reinstalled in the aircraft and an examination of the aircrafts electrical system showed no electrical power distribution problems.