To use the Fusion Core Charger in Fallout 76, you must first place it in your camp, which needs to be connected to power. You must also have two pieces of circuitry, two steel, and 20 available power.

Tip: If you intend to claim a Public Workshop to farm Fusion Cores, I recommend doing so in a Private World. Unfortunately, you can only access one by forking over cash for Fallout 1st.

The Brotherhood of Steel has a lot of Power Armors running around that have their own cores. You can pickpocket them for Fusion cores in Fallout 4. Just make sure that you don’t get caught.