title_body: In what situations do puffy batteries pose a risk of ignition?
upvoted_answer: The risk of a puffy battery exploding during storage is still comparatively low, you shouldn't underestimate it however.
  During charging the risk of explosion is at its peak. This is also the reason why you charge your batteries slower than you discharge them (i.e. to lower the risk of puffing and explosions, as well as to keep wear and tear low).
  During use, the risk is quite high as well, though I'd say it's lower than while charging.
  Always, always get rid of them as quickly as you can. Exploding LiPos are not a joke. Besides, a puffy LiPo has a far lower total capacity, rendering it basically useless for flying anyway (though don't be fooled - the explosion is still more than dangerous).