Mortgage interest charged when you make "EARLY" large pricipal payment?

I will try to use realistic numbers related to above question. I had first mortgage on my primary residence with balance of over $120,000 in early 2009. The mortgage was a first with 15 year and 5.25% interest. Regular monthly payment was $2,210. I started to make additional $10,000 'principle' payment... show more I will try to use realistic numbers related to above question. I had first mortgage on my primary residence with balance of over $120,000 in early 2009. The mortgage was a first with 15 year and 5.25% interest.
Regular monthly payment was $2,210.
I started to make additional $10,000 'principle' payment from 1/1/2009.
My rationale was to get credit for this amount from the day bank put it on their books. Sometimes it was 20 or 15 or 10 days earliar than monthly due date.
The question is why bank would charge interest based on last months balance? They did not explain why, but just kept replying that it is the bank policy. If you make large payments you would not get credit for lower principle to calcualte monthly interest.
This went on for 2 -3 months with me hoping it would be resolved in my favor. Than, somewhere in March-April I had principle balance of $75,000. And again I paid that amount 20 days ealiar and asked for reduced interest. AGAIN, I was charged for balance of over $77,000 for whole month.
Can someone explain if I can fight this further when the Chase Home Finance is not explaining anything than saying it is their policy.

Now, I have taken out an line of credit from Bank of America for $75,000 with current rate of 2.99% per year. I keep paying additionaql premium every month on diffrent dates amounting to almoswt $10,000 towards the principle. I DO GET FULL CREDIT BY EACH DAY OF REDUCED INTEREST ON THIS LINE OF CREDIT.

Why I get credit from B of A while Chase does not explain???

What agency can I contact for California residence loan from Chase???
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