Cal. Bus. & Prof. Code § 10242.5

(a) A charge imposed for late payment of an installment due on a loan secured by a mortgage or deed of trust on real property shall not exceed an amount equal to 10 percent of the installment due, except that a minimum charge of five dollars ($5) may be imposed when the late charge permitted by this section would otherwise be less than that minimum charge.
The charge permitted by this section may be assessed only as a percentage of the increment of any installment due that is attributable to principal and interest.
(b) No charge may be imposed more than once for the same late payment of an installment. No late charge may be imposed on any installment which is paid or tendered in full within 10 days after its scheduled due date, even though an earlier maturing installment or a late charge on an earlier installment may not have been paid in full. For purposes of this subdivision, a payment or tender of payment made within 10 days of a scheduled installment due date shall be deemed to have been made or tendered for payment of that installment.
(c) A late-payment charge may be imposed pursuant to this subdivision for the payment of any balloon payment more than 10 days after the date due. The charge shall not exceed an amount equal to the maximum late charge that could have been assessed with respect to the largest single monthly installment previously due, other than the balloon payment, multiplied by the sum of one plus the number of months occurring since the late-payment charge began to accrue. For purposes of this subdivision, “month” means the period between a particular day of a calendar month and the same day of the next calendar month.