If the right light is always flashing amber (even when you have charged the PC for 3-4 hours) it probably indicates that the battery is failing.
Copied straight from the manual:
the left light:
Turns on when you turn on the computer or pulses when the computer is in a power management mode
the right light:
If the computer is connected to an electrical outlet, the light operates as follows:
" Solid green: The battery is charging.
- Off: The battery is adequately charged, the computer is turned off, or no battery is installed.
" Rapidly flashing green: An error has occurred regarding the battery.
If the computer is running on a battery, the light operates as follows:
" Off: The battery is discharging normally with use.
" Flashing amber: The battery charge is low