2003 Other Mercedes Benz Models Alternator Problem Message.

I am getting an "Alternator Bat" message by the diagnostic computer on the dashboard. We checked with a certified mechanic and both battery (new) and alternator are working fine (output and operation is what it should be). The mechanic suggested changing the whole unit. Since the alternator itself is working fine, is it really necessary to change the whole unit, or can the faulty device that is generating this message be changed without changing the whole alternator? Thanks in advance for your help,
Macs