I have a 2s 7.4v 1000mah battery with this motor,  http://www.rcbazaar.com/product.aspx?productid=4089 . I put 6x4 prop on it. It used to work fine. But last week i saw saw that the battery was charged full at 8.2v and when i handlaunched the plane at 80% throttle the low voltage buzzer started beeping and the motor stopped spinning and the plane crashed but when i reduced the throttle to zero it stopped beeping. What caused the sudden drop in voltage?.I charged the battery with this charger http://www.rcbazaar.com/product.aspx?productid=992again and all lights turned green on it but when i connected the buzzer i saw that one cell had 4.18 and the other was at 4.0.
i connected the motor again and gave only 15%throttle and i saw that only the 1st cell was losing charge and the other was stuck at4.18. Please help. What can i do to make it run normal again and balance charge it with the charger i have. Please help.

Can you test the cell which is 'losing charge' for a while and the post the results here?(For eg.,Voltage drop)
The problem can be either battery, esc or with the low voltage alarm.

The cell is facing fierce voltage sag. Try to change cell .

Is it a Wolfpack LiPo? I had one cell go bad on a 3S 2200 mAh pack. It would sag very low (below 3V) when any load was applied. I removed the cell to make a 2S pack I use with my FPV goggles. However, dismantling/ modifying LiPos can be very dangerous and I would not advise it unless you are very confident of your skills.

The cell losing voltage gone bad. It is not balancing problem.Try replacing battery with  good battery before it do more damage to your plane.

Yeah its very dangerous task to desolder and resolder the new lipo cell

it is a Wolfpack lipo. So there is no way to save my lipo?  and should buy a new one? please suggest a good company lipo that will not give me such problem.

The wolfpack specs are bloated. Especially the c rating .
IMHO
For normal plane flying any good lipo is enough.
But personally i try to stay away from nano techs and wolfpacks.
Go for bonka, tattu or dinogy for high perfomance.