You have a LiPo that one cell needs a 'jump-start' ... or been silly to allow a deep discharge in flight that left 1 cell down and charger is throwing fits and chucking it back at you !!
How do you connect to one cell without blowing all up in your face or opening the pack ?
Simple ... get an old Futaba M series servo plug ... the ones that had thin pins that were always bending when used !! Looking at the plug ... where the wires exit - the nearest is the -ve .. the middle one is the +ve ... the furthest from cable exit is signal. Snip of the signal pin and wire. Connect remaining two wires to your favourite charger suitable plug and voila ... a plug that will plug into any two adjacent balance plug connections to access a single cell.
Use : Plug into balance lead for the cell that's down ... simple check with multi-meter will identify the two connections ... making sure + is + etc. Then connect to charger as a NiMH cell ... give it a few minutes to bring volts up to about 3.5V ... then disconnect ... reconnect LiPo to charger as normal ...
Disclaimer : No responsibility accepted for mistakes, errors, incidents or faults that others may make / experience in using above ... all at persons own risk.