Disclaimer: don't try this at home. Safety first, fire extenguisher nearby, stop/drop/roll, etc.
I had a K3 that sat long enough the battery was unchargeable through normal means. As an experiment I took the battery out and used a hobby grade multi-chemistry charger on it.
I monitored the charging and there was no swelling or warming. The battery has behaved normally in the ~year since the reanimation. It may be an outlier (and few will have reason to buy
a hobby-grade charger but that was my experience.
For the curious, chargers like this are used to charge batteries for serious model airplanes, cars, etc. Not toy R/C.