New battery. Charged for 24 hours. Light is green,

What may also help is to reset the internal charging circuit: remove the battery and hold down the Roomba's [Power] button for at least 10 seconds, then re-install the battery and charge for 16 hours.

There's a chance that the battery you bought is bad. There's a greater chance that the problem is with the charger. If you're using the dock to charge, try plugging the charger directly into the roomba just to rule that out. If that doesn't fix the problem, I would try borrowing or buying a new charger, and see if that works. If a new charger and new battery don't solve the problem, the problem must be with your roomba itself.