I have seen this before: one of the tradesmen at work gave me two Makita 18V batteries that the charger had spat the dummy at. Both had lower voltages than the charger was happy about.
I put both in turn on to charge using my benchtop supply, for an 18V Li battery it needs to go to 4.2V per cell or 21V for the pack, and set the current limit to 1.5A, kept a careful eye on them while they charged.
Both packs took charge and recovered, they got to 21V and the PSU dropped the current back to zero, just like a charger would.
I gave the packs back to Jeremy, and he tells me that they are fine now and do recharge OK.
Pros: packs took charge, recovered
Cons: lower voltages
