Passing over the question of why anyone would use something so heavy as currency, it also raises a few rules questions. If most people don't know the difference between a charged battery and an empty silverdisk, you presumably need to find someone with the ability to tell them apart if you want to sell a battery at 50gp half value rather than spend it for 10gp. Are there any guidelines on where/who, or is it assumed that any town big enough to sell goods at will have someone able to identify a working battery? Or is it just a case of "this is a game of adventure, not economics- don't worry so much"?