Can I safely use the batteries in my Fenix flashlight until they are fully drained?
This depends on the battery that your flashlight is compatible with. If your flashlight is compatible with disposable batteries such as alkaline or lithium, then you can drain the battery completely with no worries.
For any lithium-ion battery or rechargeable battery, we do not recommend discharging any it 100%. Draining them fully can cause them to go into a state of "deep-discharge" which can keep the battery from being recharged. Check out our article about different battery types to learn more about flashlight batteries.