How do you recharge the venom edge?

Solution 1:

No. Once you consume all the charges of a charged item, the magical energy is gone for good. You might still want to hold onto the base item if it's useful. For example,

the Flame Blade in the Halls of Fire on Level 6

is a perfectly good Longsword once its charges are depleted.

I've found a couple of items in Grimrock that are permanently "charged" with some kind of magical/elemental energy, but they're usually very challenging to get, and they can't release a spell on command. For instance,

the Dismantler sword in the Vault occasionally deals shock damage

when you wield it. I haven't found any items that are both permanently charged and can release energy on command like the limited-charge items do.