The PlayStation Vita is expected to last for around three to five hours of battery life while playing games. The external battery connects directly to the system without needing any installation, and should provide you with up to three times the amount of power as the stock battery. The grips you see in the render are retractable, allowing you to play with or without the dual-shock style appendages.
We can expect the peripheral in March or April, so you're going to have to wait a bit after launch if you want to play for extended periods of time without plugging the Vita into a wall socket.