My past four flights on the 744 to/from NRT, seat power has not worked on my laptop, even with the battery removed. I believe it is due to the system being unable to efficiently power a 90W power supply.

I just purchased an HP add-on battery. Even on high consumption settings, I get approximately 8 hours on battery, which covers most of the usable airborne time US-NRT.

ETA - did not try the power on DL276 yesterday, but did on DL275 three weeks ago, with the same results (power for about 30 seconds before kicking off, same event would occur if the power is unplugged completely; noted several other passengers in the UD with laptops and seat power in use indicators (little blue light a the bottom of the seat pedestal)).