There is no expiration date, but the battery still remains 60%-80% of the original capacity after 500 charge/discharge cycles. 

​These batteries experience calendar aging—gradual chemical degradation—when they're not in use. Over long periods of inactivity, they also self-discharge slowly (typically 1-3% per month), and if the charge drops too low (often below ~2.5–3.0V per cell), it can cause permanent damage, such as the battery failing to hold a charge.

They perform and last longest when used regularly (even light use), rather than when stored for prolonged periods. For the best results during extended periods of non-use:

• Store them at a partial charge: Aim for 40–60% state of charge. This minimizes stress on the battery chemistry—avoid storing them fully charged (100%) or fully discharged (near 0%), as both extremes accelerate degradation.
• Keep them cool: Ideal storage temperature is around 15–25°C (59–77°F). Avoid hot spots (like attics, cars, or near heaters) or freezing conditions, as temperature extremes speed up aging dramatically.
• Check and top up periodically: Every 2–4 months, inspect the charge level and recharge back to that 40–60% range if needed. This prevents the voltage from dropping into the danger zone and helps preserve capacity.