I am much like DeafBadger and GrayEagle although I only wear one aid (the left is too far gone for it to help). I do have two different aids that I use in different purposes that take different size batteries. I keep a pack of both sizes in my purse but only change the one on a time basis without testing (my aid is too old to beep at me when the battery goes down). My change day is Sunday rather than Friday like Gray Eagle.
On a side note that came in handy for another gal one time. I was sitting at dinner with a group and the wife of a guy that is a member of the same organization that I am said "sorry I didn't get that my hearing aid battery died" or words to the effect. It turned out one of mine was the right size. She wanted to pay me for it but I had no idea what the cost of one would work out to be. Then at another event she surprised me with one in exchange!