Numeria uses "Silverdisks" as currency.
These are expended batteries that can't be further recharged (there's a 20% chance of a battery failing when attempting to recharge them). This is (maybe) problematic for a couple of reasons. My players certainly hate them.
First, Numeria Land of Fallen Stars, uses "silverdisk" for both charged and uncharged batteries (using "dead silverdisk" for the latter). Whereas Iron Gods uses "silverdisk" exclusively for the dead ones and "battery" for the charged ones. This is isn't explained anywhere at all, and it wasn't until I encountered "battery" about halfway through Fires of Creation that I realized the discrepancy.
Numeria, Land of Fallen Stars also says a dead battery is worth only 1 gp, whereas the Technology Guide says it's worth 10 gp.
Secondly, a silverdisk weighs one pound. If it's worth 10 gp (as per the Technology Guide) it is heavier than 10 gp by 4/5 of a pound. It is half as heavy as 100 sp. The 1 gp Numeria, Land of Stars version is even worse, of course. There's one treasure in Lords of Rust with 244 silverdisks and another with 390! There's no game mechanical reason, (and no suggestion that Numeria is uniquely gold poor) that PCs can't convert them freely into gold and platinum. I am fairly sure that the authors of Numeria, Land of Fallen Stars didn't have the final draft of the Technology Guide yet when they proposed them as currency. I wonder if the authors of Iron Gods thought about the weight at all either.
I can see this making some sense as currency, certainly it's better than 100 sp, but in this volume it seems fairly ridiculous. I think the intent was to give some regional flavor but the end result is that PCs exchange them as soon as possible. I made a joke about this being a thing the Technic League encourages so they can get rid of broken batteries...