A spare or replacement battery for the Petzl Swift RL headlamp.
Petzl says:
Rechargeable battery for SWIFT RL headlamp.
The Lithium-Ion rechargeable battery ensures high performance for the SWIFT RL headlamp, even at low temperatures. It charges via micro USB port and has a luminous charging indicator.
Petzl's ACCU Swift RL is a replacement battery for the Petzl Swift RL. Alternatively, it's great to have a spare battery for emergencies or longer trips where you will burn through a battery due to after dark activities.