Lithium batteries have one of the lowest self-discharge rates, and self-discharging does not mean that they ever reach the level of deep discharge that turns them into short-circuited battery bombs (or actively-damaging levels of discharge in general). The voltage is important, and if you've got nothing that can actively lower it by drawing power from them it will simply stabilize at a certain safe level. They are being stored at a certain charge level because it puts them into the chemically most stable state - but other charge levels only accelerate the natural degeneration, it does not actively damage them (in contrast to actively charging or discharging them). I've once got a few unused Thinkpad batteries that were stored without special care as replacement parts, they were being discarded due to switching to a newer series with different batteries. They were still in prime condition after over 5 years, with up to 98% of their original capacity.