No need, the Nokia charger is smart enough not to keep charging when it is fully charged. When your N900 has a green light, that's because the battery told it that it is full. It also tells the charger, and in this state the charger is using virtually no electricity. A simple way to illustrate this is to touch the wall plug after your phone has been charging for a few minutes (yellow light), it will be warm. Now touch it after the charging has been done for a few minutes (green light), it will be cool.