Less is known about Æþelberht than that of his joint ruler, Beonna, of whom little is known. The 12th century monk Symeon of Durham wrote (500 years later) that Ælfwald divided between three successors, e.g., Beonna, Æþelberht, and Hun. Even this account is suspect due to a possible transcription error.
As of 2013, there has been only one coin found bearing the name of Æþelberht I. It wasn’t until 1992 that a coin was found at Burrow Hill, Butley, Suffolk bearing the name Æþelberht, among others minted by Beonna. Until that time, the king’s name was believed to be Alberht.