Cast enchantments do not last forever! Each cast with the item pays its cost in Soul Energy from the item’s Soul Energy Pool, which is calculated when the enchantment is finalized in the last step of its creation. If this would reduce the size of the pool to below zero, then the enchantment can no longer be cast and must be recharged.
Recharging an item requires that the character be trained in the Enchant skill (though no test is required), and that they have a soul gem with energy stored within it. A simple minute long ritual is required: on completion the soul energy inside the gem is transferred to the item’s pool. Any energy that would exceed the pool’s maximum size is lost and the gem is destroyed.