Interest calculation with excluding some charges

Hi,
Everyone

My problem is, i need to calculate the Interest @ 15% pa excluding some specific cell
But i am getting -answer

In month 1
A1 is opening = 0
B1 is Charges 1 = 681
C1 is Charges 2 = 4960
D1 is interest = 0 because no outstanding on opening
E1 is Amount paid = 3000
F1 is Balance = 2641

now month 2
G1 is opening = 2641
H1 is Charges 1 = 681
I1 is Charges 2 = 4960
J1 is interest = -29 (no interest should be Charged on charges 2)
K1 is Amount paid = 0
L1 is Balance = 8253

what i need is Charges 2 should be interest free so in month 2 Interest should be calculated only on outstanding if any other than charges 2 (Rs. 4960)

In above case the interest should be ZERO

please help