From the Lange website:
The life expectancy of the battery is decisively influenced by two factor the number of cycles and the natural aging process.

The battery capacity decreases with increasing number of charging and discharging processes. The life expectancy of the battery is based on the latest findings with more than 4500 SAE cycles. One SAE cycle stands for full charging of the battery and a discharge to 20% of the capacity. Partial discharge corresponds only to an equivalent portion of a full cycle. After 4500 of these SAE cycles, the capacity of the battery has decreased to 80% of the initial state. For the pilot, this means that the battery allows a minimum of 10,800,000 risers before it is replaced.

In practice, the natural aging of the batteries is more relevant. It is therefore recommended to change the batteries according to the latest findings at an average storage temperature of 20 ° C after approx. 20 years. Even then the battery capacity has decreased to 80% of the initial capacity.