Ask HN: What's the chance of a battery in an old forgotten device catching fire?
I occurred to me that I've got 12 year old smartphones in boxes in the attic. Should I be worried?
What happens to lithium batteries over time?
What happens to lithium batteries over time?
Depends on the type of lithium battery. Primary lithium batteries (not rechargeable) can hold a charge for years/decade and high rate ones can be a long term hazard.
However lithium secondary batteries (rechargeable) in your old phones discharge faster. Few percent a month. Likely they're dead with no stored energy to create a fire hazard.