What causes NiMH batteries to fail?
Had picrel for several years, set of 4, I use them occasionally, not heavily. Have cycled through them evenly. Today it turns out 1 is dead. The other 3 still work perfectly fine, but this onesometimes measures about 1.3V, sometimes it doesn't even register at all, can't get a reading. It won't charge. It doesn't have any juice. Won't discharge. Charger says error. So pretty sure it's dead. Will dispose of at the store tomorrow. But what causes it, why would this 1 be dead but the other 3 fine?

Also have a set of 4 Eneloops (standard white). They're all still good.