I am trying to create a spread sheet that will calculate the balance of a loan that does not require a minimum payment or even a payment for any certain amount of time, but will still add interest to the loan on monthly bases. So to put it simple I have loaned someone $2843.87 they have no time period upon when it has to be paid off or have a minimum payment per month, but I want to still add the interest of 16.9% APR a month even if no payment is made. But I want to have the ability to enter the payment manually when they do make a payment and the new balance to reflect what the balance is. This way if they make a payment of $20 one week and $50 two weeks later, then they don't make a payment for 2 months the new balance will still reflect that Interest was added when they made a payment and when they did not make a payment.

Can anyone give me a detailed formula per cell that I would need to use to make this work?