nickel metal hydride batteries
I just posted this to a board, if you know the answer, email me! 
Do nickel metal hydride batteries have a “memory”? Do they need to be depleted entirely before recharging?
I have some great Energizer 850 mAh ones (AAA) that work w/ a 15 minute charger. I only got it a few months ago and yesterday got a new set of AAAs to go with it. The older ones (only a few months old), right out of the charger only show 2 or 3 bars out of 4 of being charged and using this tiny usb thumb drive / mp3 player which uses one AAA, I go thru 2 batteries a day. But with the new set, they come out of the charger 4 out of 4 bars of being charged and after an entire day of use they are STILL at 4 bars.
Why are my few month old ones performing so low? They’re the exact same kind. Do I need to deplete them?
There’s such a vast difference between my brand new ones and the ones that are just a few months old. Can the old ones be reconditioned? Is it usual for them to only perform well for a few months?