Why does my LED lights get brighter after I unplug then replug them back into battery pack?

[AC Question]

I am testing the brightness of LED strips and have them wired into battery packs. After 2 days of being lit, I noticed some of them were dimmed. I unplugged them from their battery packs. The next day, I replugged them into their respective battery packs and they were all bright again. Can someone please explain this?