You could have a bad charger or bad battery or neither.

Assume the manual is wrong.

Put the pack on the charger and wait until the lights change, give it at least 45 minutes.

If the charger is working correctly it will automatically cut the current to the battery to prevent it from being damaged.

Then put the pack in the plane and see if you can rev the motor up to full throttle.

If so bring the throttle to about 75% of full and time how long it takes before the motor stops.

Report your results.