I’ve personally killed an m18 battery by having it ‘discharge’ outside of a proper tool. (Rigged up to some temp lighting.)

Once drained, the charger would no longer re-charge, and if I remember correctly, after opening the pack up, cells were at very low voltage.

Same here with a Dewalt 20V. I left it in an adapter to a Ryobi tool. I really should have known better, too.