If first indications are correct the new Nintendo 3DS might not provide as much game-play on those long journeys as first thought. Currently the Nintendo DSi XL will give you around 13-17 hours play, while the original DS gives nearer 10-14 hours which are massive amounts of time when compared to the poultry 3-5 hours of game-play expected from the new 3DS launching next month.
When playing a 3DS title expect a battery life of about 3 hours, due to the use of the 3d generating 3 screens, but with some 3DS titles that could increase to as much as 5 hours, but not much more and as any gamer know 3-5 hours is hardly enough time for extended journeys.
Nintendo are currently working with software developers to try and improve the battery life whilst running their software titles, but first generation battery life will probably stay around 3 to 4 hours if you are playing 3DS titles.
Via Device Mag