"Mmmmmnot sure. I think I could come up with a spell that soaked up fresh background mana, but it'd have to be cast on the spot... and I think old background mana might just not be accessible that way. Probably easier to just charge a mana battery directly, so your magic thingy doesn't depend on how many big spells have been cast nearby in the last day or so. And then I still have the problem of figuring out how to design a magical object to work without a magical interface."