Researching German Mercenary

Does anyone know of a source that deals with German mercenaries who fought in the Civil War?I have an ancestor who shows up in the 1984 Veterans Census (found @ the Michigan Family History Network), and whom I know came from Hesse as a hired soldier, but can find no trace of him in the standard Civil War databases.Name: Henry Neusel, found in 1870 census in Detroit, MI.Any ideas where else I might look for his service record?Thanks in advance.
Susan Dobson