[Shadekeep]

One way of tamping down the overpowered-ness of some self-powered items is to give them a battery charge level. Self-powered items are described as drawing power from ambient mana. It makes sense that this is a low-level power field, otherwise wizards would call on it all the time instead of using their own ST. So self-powered items might need to build up a charge, similar to a battery charging from an inductive power source.

Therefore a magic item might have a 10 ST capacitance, or 20 ST, or whatever, and this charge is used when the item is active. Once it runs down, then it might recharge at about 1 ST per hour or so.

Naturally, this comes with more bookkeeping, and you'd need some kind of rule about how much it costs to give an item X ST capacitance. It also takes away some of the high fantasy element of a magic item that is perpetually "on". But it's once way to stop self-powered items from being too powerful.