Really GOOD to know...!!!!
I had few package of new ones from my audiologist when I got Max aids. I noticed when I got home that I still have some left overs to used up before I'd use the new ones. When I first got Max, there were new batteries, it lasts me about a week and half. Not bad . (remember I didn't wear them 100% of the times, was still getting used to them)
Then it was time to change it, few days ago, I used the old package (expiry date is Feb. 2013)..it didn't last long. and I can hear the difference, its not as sharp or loud as the newer batteries were.
So more the reasons to used up the old ones so you dont waste them but always check for the dates before you buy them and make sure its last longer than a year. (the new ones are expired in 2014!)