Recharging items with spell charges is simple.1. Find a merchant that will buy the item and sell it to them.
2. Immediately buy the item back from the merchant.
It usually costs a little gild but when you buy it back it is fully charged. This applies to any usuable item with multiple cjarges.
Thanks Henry two more thanks to add about multiple charges: Some things appear to have charges (a small number in the corner) but somehow aren't usable. Those aren't recharged (why should they )
And careful with the flamethrower: if it (or even worse, a mana stone) reach 0 charges, it disappears. Everything else can be used up completely, and will still exist, and be rechargable, but those items won't (Iirc). Oh, and in my humble opinion the flamethrower is crap