How much interest can be charged on past due rent?

Question Details:

We do owe some back rent from a few years ago but the landlord was very understanding when my husband lost his job. However he has been sending us notices on the past due balance, and charging interest on the balance and late fee. Also, we are here 7 years. Our lease expired after 2 years and neither party discussed renewal; so we have had no lease for the past 5 years. Is it legal to charge interest (and he has the rate fluctuating based on his loan, as high as 8.50%) on past due rent?

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