Yeah, when you have atleast 1 charge, you have a +1 attack roll and deal 1 additional lightning damage; after 5 charges it does 1-8 lightning damage. The item description explains how you get charges.
2 things in Act 3 lets you get from 20 to 24:
House of Grief by sacrificing Thayan Forbidden Knowledge to the Mirror of Loss and a hat (Birthright) you buy at Sorcerous Sundries.
Mind you, it's not very necessary to min/max to beat the game. Never bothered watering enemies nor did I know you could cast Twinned Haste let alone cast Twinned Chain Lightning until after I finished.
