This is from the manual for the AstroFlight #109 Lipo charger:
"If ANY cell in your pack is discharged below 2.5 volts,
permanent and irreversible chemical damage will slowly and completely
destroy the cell within 30 days. If a cell is exposed to this low state of
discharge overnight it can sometimes be saved by applying a slow
“trickle” charge until the cell voltage reaches 3.2 volts or more and
remains at this voltage after charging current is removed. By “trickle” we
mean a charge rate between 5% and 10% of cell rated mahr capacity.
For example on a 2000 mahr cells, “trickle” means 100 to 200 milliamps.
I have recovered cells as low as 1.4V by trickle charging for one to two
hours. It doesn’t always work, but I have recovered cells most of the
time. The longer the cell has been exposed to low voltage, the lower the
chances of recovery. After recovery I recommend fully charging the cell
to 4.2 volts, and then running a discharge test at a “1 Cell” rate to
measure its capacity. “1 C” means discharging a 2000 mahr cell at 2
amps. If the battery capacity has deteriorated to less then 75% of its
rating when new, it is time to discard that pack.
IF ANY CELL IS DISCHARGED TO BELOW 3.2 VOLTS BUT
ABOVE 2.5 VOLTS it is in a stable voltage region and may remain in
that condition for a long time without damage. But the internal
resistance of the cell may have become much higher than normal. If you
try to charge this cell at a normal “1 C” rate, its voltage may jump up by
as much as one volt or more. But if the charging current stopped, the
voltage will drop back the original value. This cell is not ACCEPTING
the charge. Stop at once. If you continue to charge at a “1 C” rate,
the cell will puff up and be ruined and may even start BURNING. Always
monitor the voltage behavior of any lithium battery pack for the first
minute to be certain that the charge is progressing normally. In multi cell
packs that have no taps this is about the only way of detecting a problem.
If your cell has a separate connector with battery taps, use a battery
balancer like our Astro Blinky #106. It will detect a low voltage cell even
if the total pack voltage looks OK. If you do not have a balancer or a
balance connector on your pack you can still detect the problem by
observing the abnormal voltage rise when attempting to charge at the “1
C” rate. Good condition packs will show a very small voltage rise of less
than 0.1 volts per cell. A three cell pack in good condition will only
show a 0.1 to 0.3 volts rise. If you observe a large jump in voltage, stop.
Something is wrong with your pack."
Hope that helps.