The item can hold a number of charges equal to its Tier. You can spend 1 charge of Mana Flux as a Free Action on your turn to add +1 charge to the item; you can do this as often as you wish, so long as you have Mana Flux to spend on it.
As a Free Action on your turn, you can expend any number of charges from the item to gain that many charges of Mana Flux. You lose any charges that exceed your Mana Capacity, as normal.
Charges held by the item last indefinitely so long as you are Attuned to it.