It's noted in the package, just got mine a couple of weeks ago from a reputable eBay supplier for $10.99 shipped & was genuine after checking (2014 Version on the package), that if a battery drops too low, as in running way down, it may not be rechargeable, or won't fully charge. My 18650's are dead on the money at 4.25V when removing, though my Samsung's are fairly new & I don't let them go below 4.0V if I can help it. I can tell when they're dropping & will pull one out & test with a simple digital multimeter.
Actually I purchased the Intellicharger 12 to be a spare, to have in case my Efest LUC V2 went down, but after seeing it was right on target, versus 4.30V for the LUC V2, made it my primary charger & put the LUC V2 in it's box as a backup.
Bunnykiller raises a great point above, if doing any serious sub-ohming, batteries needs to be frequently checked & swapped. I always carry a spare set in my pocket in the plastic case that shipped with the two batteries. Swap as needed for best battery life & safety. If I had a set that wouldn't charge fully & the same happened on another charger, would purchase new batteries & properly recycle the old (drop off at a local battery store).
Any rechargeable battery has a certain number of charging cycles, this can be increased by not pulling down too much, and will certainly be decreased by pulling below 3.4V on a regular basis. I take it with a grain of salt that these can be used down to 2.4V, that's a sure fire battery killer.
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