a battery problem with s duos2

I have a strange problem with my phone gt-s7582 cm12.1 first of all, I have battery drain sometimes from 15% to 0% and sometimes from 30% to 0% and not only in cm12.1 but also on the stock rom but this not the problem actually this OK for me. the strange problem is that I can't charge my phone to 100% at first I could only charge to 94% then after 2 days I could charge to 75% only and now I can't charge more than 60%. I have bought 2 batteries and the problem even with the stock rom . something else that when I change the battery I get the same battery %. so if now the phone says that the battery is 40% and I changed the battery it will give me 40%. please anyone tell me how can I solve these problems.