This
Nikon SB900 Flash With Mini Litetrek Battery and Cable Kit from
B&H Photo gives you one less thing to worry about when you have an extended shoot on location with flash. The Nikon SB900 is the most advanced flash in the Speedlight line, with a guide number of 131' (40 m) at ISO 100 at the 35 mm focal length. It's compatible with i-TTL, TTL, and manual exposure, with a zoom head that covers a 17-200 mm lens range, in as little as 1.2 seconds. The SB900 automatically detects if it's connected to a DX or FX format camera and adjusts accordingly.
The Mini Litetrek Battery Pack can give you up to 650 flashes at full power or 3500 flashes at 1/64 power, with recycling times from 1 to 3.5 seconds. The battery can be removed from the power module, so that exchanging a depleted battery for a fresh one is only a matter of seconds. The power module has two outlets. A Nikon-compatible cable is included.