i have been using my Turnigy 2200mah 3s lipo (25-35c) on my quad, and there is any issue with just one cell among the three: its not charging well enough, it goes to 3.5 volts only and discharges faster than the other two



is there a way to charge only than bad cell alone??
if there is any other suggestions, please let me know

May be a bad cell...or the internal resistance of that one is too low so that it discharges quickly than the other two

Any fix ?

Use a good quality balancer with custom settings not the Rs 500 type charger.  Try the balancing feature of the charger at the lowest setting.  It may take about 6 hours for the constant charging, discharging, for the charger to balance all 3 cells. 

More likely is your Lipo overheated or was abused that caused a cell to reach its limits.  Is there any puffiness?

I use a good charger and the lipo is new and I have tries balancing it twice

If you have a good charger ,than try charging at very low rate for ex; 0.4 to 0.5 amps .may work sometimes!

You can kill a lipo on the every flight if you abuse it.  Or, if the lipo had a defect, you can tell right away when you first do a static test (run the motor through the full range while holding the plane)