Nuclear Regulatory Commission regional administrator Elmo Collins says in a statement Monday that inadequate maintenance procedures were to blame and the lapse is troubling because the condition persisted for so long.
Other emergency batteries were available to supply power to plant safety systems, but the NRC says the loss of even one emergency battery reduced the plant's safety margins. It was inoperable from 2004 to 2008.
Southern California Edison operates the plant. Spokesman Gil Alexander says there's no immediate comment on the NRC announcement.