All the stories I ever heard had to do with lithium ion, which is fussier (extreme intolerance of overcharging) and involves more reactive chemicals. In that past year, I have been hearing a lot fewer horror stories.
NiCd and NiMH is easily overcharged by slow charging. NiCd usually tolerates slow-charge overcharge well, with a slight voltage depression that can generally be fixed by full discharge followed by full charge without overcharging. NiMH appears to me less tolerant of overcharging, and prefers fast chargers because faster charging enables better detection of full charge. Smart chargers for NiCd and NiMH tend to have charge times 4 hours or less.
- Don Klipstein ( snipped-for-privacy@misty.com)