Battery life may vary depending on ambient temperature. The capacity of the battery decreases as it gets colder. At temperatures below 0°C the capacity will still be +/- 80%.
To extend the life of the batteries, keep them charged to at least 25% at all times, even when the batteries are not on or in use. Consider the summer period. Therefore, charge the batteries once a month.
Tip: make a note of this in your agenda.