They may be using the same batteries, which would be drained faster by the new hardware.
Guess that could be the case. Still TONS better than PSP.
• Charging Time
Nintendo DSi: Approx. 2 hours and 30 minutes
Nintendo DS Lite: Approx. 3 hours
• Battery Life
Nintendo DSi: The lowest brightness (9-14 hrs), low brightness (8-12 hours), medium brightness (6-9 hours), high brightness (4-6 hours), highest brightness(3-4 hours)
Nintendo DS Lite: The lowest brightness (15-19 hrs), low brightness (10-15 hours), high brightness (7-11 hours), highest brightness(5-8 hours)