As with any rechargeable battery, one aspect of the life depends on how well it is maintained...avoiding extreme temperatures, etc. The Fenix warranty for all Fenix rechargeable batteries is 1 year. That's not saying the battery pack will last ONLY one year. We have seen many lasting longer than that. Charge cycles come into play also.
In the industry, this is pretty standard:
The overall lifespan of a battery is expressed in charge cycles. One cycle is a complete discharge followed by a complete charge. It stands to reason that the life expectancy in years varies depending on the use to which the batteries are put. In general, rechargeable batteries can stand from 500 to 800 charge cycles.
Here's a good read. A bit technical but interesting.
READ HERE
Let us know if you have other questions.
Approximately how long can the internal battery pack last until it start failing or not keeping the charge anymore?