Hoegel was charged with 100 counts of murder.
In 15 of these cases, the court found the accusations were not beyond reasonable doubt and cleared him.
Noting the severity of his crimes, the court in the city of Oldenburg virtually ruled out Hoegel’s release from prison after 15 years, as is usual practice for a life sentence in Germany.
If the appeal is allowed, a higher court in Leipzig will examine procedural or legal errors only and not new facts.
No new evidence would be submitted.
Hoegel admitted to killing patients with lethal injections and was already serving a life sentence after being found guilty in 2015 of murdering two people.
He selected his victims in hospitals in the cities of Oldenburg and Delmenhorst in north-western Germany, injecting them with medication that led to heart failure or other complications between 2000 and 2005.