Ja der Akku kann ersetzt werden, siehe Manual Seite 4: https://www.eaton.com/content/dam/eaton/products/backup-power-ups-surge-it-power-distribution/backup-power-ups/eaton-ellipse-pro-ups/Eaton%20Ellipse%20PRO%20UPS%20-%20650-800-1200-1600%20VA%20-%20Installation%20and%20user%20manual%20(multiple%20languages).pdf

Probably no user will be able to tell you this just like that ;) If I were you, I would write an email directly to the customer service so that they can clarify this.