SAN CLEMENTE, Calif. —  Inspections are being ramped up at a California nuclear plant where a battery that powered safety systems didn't work for four years.

The Nuclear Regulatory Commission said Monday that the problem at the San Onofre plant stemmed from poor maintenance.

Regional administrator Elmo Collins says the lapse is troubling because it persisted for so long. It was inoperable from 2004 to 2008. The problem was discovered in March.

Other emergency batteries were available to supply power to plant safety systems. But the NRC said the loss of even one emergency battery reduced safety margins.

The plant is operated by Southern California Edison. The utility says it will focus on preventing a recurrence.