I don't know if having it off the charger will necessarily prevent issues. I had an old ~2010 Macbook Pro battery pack that stopped taking a charge, but which I decided to keep around because I thought I might see if it was possible to replace the cells. Kind of forgot about it, so it sat on a desktop for maybe a year, and then I tossed it in a closet maybe a year ago. While cleaning in there recently, I found this:

https://my.mixtape.moe/kceuko.jpg https://my.mixtape.moe/zbvjdu.jpg

It was in a closed ziploc bag, which showed no evidence of high heat, but kind of disturbing in that I assumed after a year of sitting it would have fully discharged.