I felt like if they refreshed on a short rest using the ability wouldn't really be a choice, you would just use one every round of combat to maximize your chance of getting even rolls that do something. I changed it to long rest, but the dice aren't expended if you roll odd. So, you keep any dice that don't work to try again later. I also noted that abilities that can be used on allies are usually more limited (like Bardic inspiration) so I modeled it off that.