You can connect the SB-900 or 910 to an external battery pack which is useful if you are shooting an event all day. Otherwise the SB-700 should be fine
I was hit up with an ad the other day, from
B&H Photo, or
Adorama, one of the two, selling a nifty-looking external battery pack for Nikon flash units. It was priced at $99, and said it offered .5 second recycling time, and was I think 3300 milli-ampere hours, and operated at 315 volts DC, with several hundred recharging cycles guaranteed for the battery, which supplied over 300 full-power shots. Looked like a really nice product. This one used a replaceable, $69 internal battery cluster; other types of these external packs use six or eight AA-cells, and cost less money.
Quantum Instruments has long made external flash battery packs, which provide a ton of power for a speedlight. When you have an external pack, and are shooting at wider apertures, like say f/4 or f/5.6 and using ISO 400 or so, it's really nice, since with that high an ISO level and that wide an aperture, a powerful flash does not normally need to discharge itself fully, and with a beefy battery pack, you DO get almost instant follow-up shots, often for extended sequences, at events such as say a wedding cake-cutting, or a perp walk outside the courthouse, or the high school dunk tank at the fund raiser in the gym, etc,etc..