The Fenix ARB-L18-3500U USB rechargeable lithium-ion battery has a built-in USB port and can also be charged using an external charger. It offers an extra level of protection with an anode circuit encased in a steel housing, and as the battery is fully wrapped in steel, the risk of short-circuiting due to impact or collision is drastically reduced.
Note: This item is packaged and priced as one unit - one battery.