Lithium Ion Voltage vs. Charge Status
4.2V – 100% 4.1V – 87% 4.0V – 75% 3.9V – 55% 3.8V – 30% 3.5V – 0%
Somewhere in the 3.9V or slightly below area would be ideal for storage. Just don't overdo it. I believe AW batteries generally ship about 40% charge, or you can just discharge them in a light or other device, since that is easy and safe.
Below are links to and summaries of some very informative Battery University articles.
BU-808: How to Prolong Lithium-based Batteries
Environmental conditions, not cycling alone, govern the longevity of lithium-ion batteries. The worst situation is keeping a fully charged battery at elevated temperatures. Battery packs do not die suddenly, but the runtime gradually shortens as the capacity fades.
Lower charge voltages prolong battery life and electric vehicles and satellites take advantage of this. Similar provisions could also be made for consumer devices, but these are seldom offered; planned obsolescence takes care of this.
- A laptop battery could be prolonged by lowering the charge voltage when connected to the AC grid. To make this feature user-friendly, a device should feature a “Long Life” mode that keeps the battery at 4.05V/cell and offers a SoC of about 80 percent. One hour before traveling, the user requests the “Full Capacity” mode to bring the charge to 4.20V/cell.
BU-401a: Fast and Ultra-fast Chargers
- Charging and discharging Li-ion above 1C reduces service life. Use a slower charge and discharge if possible. This rule applies to most batteries.
- All batteries perform best at room temperature and with a moderate charge and discharge. Such a sheltered lifestyle does not always reflect real-world situations where a compact pack must be charged quickly and deliver high currents. Such typical applications are drones and remote control devices for hobbyists. Expect a short cycle life when a small pack must give all it has.