Not sure if yours is beyond help. But this worked for me on 2 Phantom batteries. Yeah I screwed up and let them sit too long.
I use an old Heathkit Variable Power Supply. And a Volt meter. Variable Power Supplies are available at Amazon.
I charge the battery using a voltage just a bit higher that the battery reads at the moment. The Heatkit also limits the current. I keep it at minimum. Dont know what that was because no meter in the power supply and I did not bother to measure it.
After a while I check the voltage. If it is equal to the charge voltage I crank it higher. Little bit at a time. .25 Volts or so/
I keep going until it reaches something close to the minimum spec for the battery. Put it on its regular charger and VOILA!
Both Phantom batteries give me the same flight as before.
Thanks for the advice,Actually I got to know the defective cell and tried to charge it through a Energy charger set on NIcd/Nimh at .1 A.It just managed to reach 1V and that too after charging and discharging itself a number times.It seems that the cell is unable to hold the charge to and gets limited at about 1V.