The fine gents over at Kotaku have listed all of the DS Lite and newly announced DSi specs in a one-on-one showdown. We see all of the expected small upgrades—the screens are a tiny bit bigger, the case ever so slightly longer and thinner. But the big surprise? Battery life bites it big time.
On low brightness, the DS Lite was rated at 15-19 hours of life. The DSi gets only 9-14 in the same conditions. That's a loss of 5-6 hours. Higher brightnesses show less battery loss for the DSi, ranging from 1-5 hours less on "high brightness" and stabilising to a 1-2 hour reduction on "highest brightness."
But a major selling point of the DS was its ridiculously great battery. And the only explanation we could maybe reason is that the DSi could be brighter than the DS Lite, meaning that each of these tiers is actually brighter than advertised in side-by-side comparison. Well, that, or Nintendo figured out how to build DSs even cheaper and let the battery suffer as a consequence.