Have a little Sangean pocket radio that only had about 20 hours of use that died a couple months ago. I replaced the batteries (with more Costco AAs), but the display was still dead. It was out of warranty, so I planned to send it back for repair at a later date.

Fast forward to my first backpacking trip several weeks ago. Since I hadn't used my camping flashlights in well over a year, I decided to replace the batteries. The "new" (or unused) Costco batteries were dead. I went through several sets of batteries before I decided the large battery pack was full of dead batteries. The expiration date on those batteries was 2021 or 2022. I had a newer pack of Costco batteries I purchased prior to hurricane season this year. The worked just fine. I went on the backpacking trips without giving it a second thought.

I just happened to look at my pocket radio this morning, and wondered if I had installed dead batteries before, so I popped in batteries from the new pack and the radio came back to life.

That's the first time I've experienced that with batteries that were still within their "good by" date. Lesson learned!