Suunto D4f User guide
Battery indicators
Temperature or internal oxidation can affect the battery voltage. If you store your Suunto D4f for a long period or use it in cold temperatures, the low battery warning may appear even though the battery has enough capacity.
In these cases, re-enter dive mode and check the battery power. If the battery is low, the Low Battery warning comes on.
If the low battery icon appears in surface mode, or if the display looks faded, the battery may be too low. Battery replacement is recommended.
NOTE:
For safety reasons, the backlight and buzzer (sound) cannot be activated when the low battery warning is displayed.