don't know exactly how it happens yet, but on one mage character, i have a mystic staff and 4 mage staffs for spare mana. now comes the interesting part. the staffs gain max mana with use! i think they only gains mana if you level up while using the staff in melee, or just using the staff in melee will do the trick. but i now have a mystic staff with 106 mana, 2 mage staffs with 94 and 92 mana, another mage staff with around 60, and the last one stayed at 40. add my own mana supply of 140ish, and i have mana in access of 530 everywhere i go. while i'm using one staff, the others are recharging in my pack.
How odd. This game has so many odd quirks. For instance - if you tell a follower to wait, then cast Quench Life on them til they die, then resurrect them, it INCREASES their reactions dramatically. I have followers with 200+ reaction from this trick, though whether it will work for anyone else I don't know. Try it and tell me.