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:
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.
This is not unexpected for LiPo batteries. The anode in LiPo battery packs degrades overtime and outgasses oxygen which is why the packs puff up. This by itself won't cause an explosion but if you puncture the pack and expose it to water or flame you might get some fireworks.
I confirm that; I did my own battery replacement on old iPhone4 that I bought cheaply on Amazon. Year later I changed the phone and never cared to sell this one. Fast forward this year to garage cleanup - same picture the battery inside expanded dramatically destroying the case. Little I know I left it outside for recycling to pick it up. Next morning all my trash is splattered everywhere and neighbor said he hear explosion. The battery got wet and exploded! I took tons of photos but it was on my previous iPhone so unable to upload any at the moment.
That's interesting, I assumed the reaction was rapid but I didn't see it happen. It's not obvious in the pics, but 2 stacked cells in the pack swelled, so I assumed it involved an electrical issue.
Related question - best way to safely dispose of old LiPo batteries? I don't want to just toss it in the regular trash (for one thing, might ignite in there.)
I.r.o. the latter question, ask the local recycling company.
Batteries are a fire hazard and a heavy pollutant if thrown in the regular trash, but most of the raw materials can be recycled if disposed of properly.
After seeing some of the old UPS batteries bulging to the point we had to cut them out of their enclosures, cracking the battery cases, etc., I've generally just classed all batteries as things to get out of the house/business as fast as possible after they're out of service.
AGM batteries are packed with fiberglass, so when oxide starts building up on the plates because of improper charging or electrolyte drying up they start pushing on the case.
Deep discharge is actually probably what caused that. In lithium ion batteries, if cell voltage drops too low it can cause chemical changes in the electrolyte which can cause an internal short.
Apple's battery fault detection is pretty good actually. Did the battery said "service battery" or "replace battery" while it was operational? If the battery status showed something similar, the battery is already damaged before storage.
I have an 2008 MacBook pro, and one of my older batteries have shown the warning. After a year or so that battery has also swollen, albeit in a milder manner than yours.
I've seen an iPad that acted completely normal despite the case separating due to oxygen expansion while under warranty. Worst part, the owner had to get rid of it since she was on vacation where a Apple store didn't exist.
The battery should have an automatic detection system for that. Some thin copper wires wound around the battery that break when the battery expands should do the trick. It could even work as a circuit-breaker for the battery itself, although sparks should be prevented.
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.