Cal. Rev. & Tax. Code § 6477

Any person required to make a prepayment pursuant to Section 6471 or Section 6471.5 who fails to make a prepayment before the last day of the monthly period following the quarterly period in which the prepayment became due and who files a timely return and payment for the quarterly period in which the prepayment became due shall pay a penalty of 6 percent of the amount equal to 90 percent of the tax liability, as prescribed in those sections, for each of the periods during that quarterly period for which a required prepayment was not made.