Electrical – Li-ion Battery Shelf Life Calculation (Including Temperature)

Yes, Lithium-ion bateries degrades over time, even if unused. It is due to a chemical/physical reaction that starts right after manufacturing of the cells.

I can't answer on the state of your batteries. Because it highly depends on the storage temperature and also the charging level of the cells during the storage.

I don't remember the optimal number, and if you know it, don't hesitate to modify my answer, but AFAIK it was around 60%. Thus if you want to reduce the aging effect, you should store your cells at 60% charge. I don't remember the optimal value for the temp.

Li-Poly batteries have a useful voltage range of 3.0v to 4.2v --under 3.0v they are effectively discharged, and 4.2v they are fully charged. Both the protection circuit in the battery itself and the special L-Poly charger chips limit the high-end voltage (since going above this value can cause the battery to vent and catch fire).

Contrary to popular belief, it is not best to charge the chip to the maximum voltage of 4.2v before putting it in storage; rather a voltage of around 50% of full charge (3.6 to 3.7v) is preferred. This is borne out in the table in Andreas' answer. 3.7v is also the typical voltage the battery is charged to when it leaves the factory, and is considered the nominal working voltage for the battery as well.

In my own work, the company I am contracting at has had over a thousand cells in storage at room temperature for 18 months to over two years without any issues, with only a slight voltage drop. I realize that is anecdotal evidence.

The important thing is before using any Li-Poly cells that have been in storage is to measure their voltage before being hooked up to a charger. If they have dipped below 3v (or show no voltage at all, because the battery's internal discharge protection circuit has kicked in), I would safely get rid of them -- they are damaged goods. (Some would argue that has long as they haven't tripped the battery's low limit, which might be 2.5v, they can still be rescued.) And of course if a battery shows any signs of expansion (starting to look like a little pillow), out it goes.