I want to store a battery powered device long term (decades) as a reference article, it will never be switched on or charged again. The problem is that it contains a small LiPo battery that will be very hard to remove.
Is there likely to be any significant risk I need to worry about? Once depleted will the battery be relatively inert?
It should be kept charged at ~3.9V/cell (~65% charge), assuming 3.7V nominal voltage. Best storage temperature is 10°C, if memory serves, or maybe it was the best for charging. Self-discharge rate is ~15%/year at that temperature (~30% at room temp), so if you charge to 4.05V (80%) every 2-3 years, it should last long enough. All values depend on the exact cell chemistry. Learned on Battery University long ago.
My phone is 7 years old and the original battery is fine.