With any portable device, battery life is very important. The original DS claimed up to 10 hours, and according to a recent article at GameSpot, the DS Lite engineers note that the battery for the DS Lite is not only smaller, but also has 20% more capacity (an excellent read for anyone interested on the design ideas behind the Lite).
I’ve been getting 8-10 hours pretty easily, unless I start using the WiFi. Then battery life, as with most portable wireless units, drops off. The battery indicator does change to red at the 30% mark, so it is much like the Windows Mobile battery indicator, or your car’s fuel tank. You still have a little more juice to give.
I do hope that Nintendo releases some sort of car or USB adapter – just to make getting power a little easier when those heavy Wi-Fi sessions come around.