The Lumedyne X Flash Recycle Accelerator for Nikon is an innovative way to power your flash while halving its recycling time and doubling the battery life of the flash's on-board AAs.
Due to its extremely light weight, small size and lithium battery, it's hard to call the X a battery pack. Lumedyne opts instead for calling it a "recycling accelerator" which partially explains how it functions. The X plugs into the HV input on your compatible Nikon flash, completely bypassing the electronic functions of the flash like the LCD display etc. Instead it goes straight for the "engine room" boosting your flash's recycling and filling of its capacitors. While other products that use several AA batteries operate on the same principle, the X does it in a bantam 0.6 by 1.7 by 2.8" form factor that weighs only 2.7 oz, while multiple battery solutions weigh at least 1.0 lb and require a cable that, as we all know, is a real annoyance when flipping from vertical to horizontal compositions.
The X's battery fully recharges from depletion in about 3 hours. Since it's chemically composed of lithium-ion, it doesn't have a memory, allowing "topping up" of power before it's exhausted. The battery is good for 500 cycles (charges) or about 2 years. It is replaced free of charge if the capacity has dropped 25% before that time. Charge level is displayed by pressing a button that lets you know when it's down to 1/3 charge.
The X comes with a dual USB-style international AC Charger and a dual USB-style vehicle charger both rated for up to an amp each in output. Long and short USB MICRO-B cables are also included.