Building the 
Steel Generator
Your initial bunker spawn has limited power. To charge batteries and run electrical devices, you'll need to construct a steel generator at your base.
Gather the required materials—primarily steel and industrial components found throughout bunkers. Place the generator in a central location within your base, ideally near where you store equipment and batteries.
Charging Your Batteries
Once the generator is running, place empty batteries into it. The generator will recharge them over time. You can queue multiple batteries to charge sequentially, so bring all your dead batteries back to base after exploration runs.
Keep at least one fully charged 
battery in your off-hand at all times. When your 
flashlight dims during exploration, swap in the fresh battery by sneaking and right-clicking to remove the old one, then right-clicking again with the new battery in your off-hand to install it.
As you progress, consider building multiple generators or expanding your power setup to handle more simultaneous charges, especially if you're exploring frequently.