when ac goes out my system goes off

PLEASE HELP ooo!!!
My Dell E4300 laptop just suddenly stop charging it gets to 35%(writes plugged in, not charging), later it dropped to about 28% then i did some windows update and then it increased gradually to 40% then stops charging. I have tried other power adapter with less volt to mine - it even worst, mine still charge but those other ones just powers my system but does charge. My guess was that the battery was bad but the irony is that even that 40% charges last normal(that is it does not drains out fast at least for about an hour). The real challange now is that i made some changes to BIOS(i change the charging to express charge) then it charged up till 50% then stopped charging only for the power AC to go out and my system goes off. My battery has a meter which indicate that the battery is half charge. Now without power ac i tried putting on my system and it's not coming up, no light nothing but with AC power it comes. I don't know what to do please help