Hi Annabel,
Like you, I have a box full of
MagicBands, including my first red one from the test period in 2013. When MagicBands were introduced,
Walt Disney World estimated that the batteries would last at least two years. I used my original band after the two-year mark as a test, and it still worked great.
Because I have several, including at least one in each color, I usually pack more than one MagicBand for each trip, just in case I am in the mood for a different color or there's a problem with one of them. I've never had a problem, but I like to be prepared. I've also started putting the year on each new one when I customize it, i.e. Wilma16. That helps me easily know which ones are the most recent.
You can also go to the MagicBands & Cards section of My Disney Experience to see your full list of active MagicBands. They are shown with most recent ones at the top and older ones at the bottom. You can match the serial number of the band with the list to help determine which are your oldest ones.
I hope that helps. Thanks for stopping by the Disney Parks Moms with your question. Enjoy your next Walt Disney World vacation!
Wilma