What this phone needs is 3000 mAh. When I was using my N810 tethered to an E71, I could get a normal day's use out of the two of them. Both have a 1500 mAh battery. Do the math...

The N900 is great tech, but it chews down its battery like there's no tomorrow. And well it might, 'cause if you start in the morning, there'll be no afternoon :-)