The 7-hour charge is about right for a battery which was discharged in
normal use. The 16-hour number is for a battery shipped completely
discharged.
However, your battery seems to be doing something similar to what one of
my batteries did when it died. It would appear to become fully charged,
but wouldn't run the machine very long. Try charging the battery until the
light goes out, then take it out and measure its voltage with a low-valued
resistor across the terminals (a 50-ohm, 5-watt resistor, if you can get
one -- 470 ohms, if you can only get 1/2 watt ones.) The voltage should
start out at 13V or above and drop down to about 12V but should decrease much
more slowly after this initial drop. The Ultralight fails to boot at about
10V or so, I seem to remember.
There's a possibility that the charger is not working right, but I think
a dead battery is more likely. You can double check by getting a second
battery (handy to have anyway) and trying it out. Hopefully IME will
replace the first battery, if it proves defective.
Good luck,
Bob Armstrong
Research Laboratory of Electronics
MIT, Cambridge, MA 02139
Email: rca@rle-vlsi.mit.edu