In the Vita Hludowici imperatoris (Life of Emperor Louis I the Pious), written by Thegan of Trier in about 836-837, Heilwig, is mentioned as a noblewoman of Saxon descent (Eigilwi, noblissimi generis Saxonici); the wife of Welf, and the mother of Judith, the second wife of Louis, married in 819.
Probably as a widow, Heilwig was appointed as Abbess of Chelles, about 825.
Her birth date is unknown but as Judith, her eldest daughter, and possibly eldest child is thought to have been born about 800-805, a birth date for Heilwig of about 780 seems reasonable.
Although she is of Saxon descent, her parents are never named and remain Unknown.
It is possible she is related in some way to Ekbert, a Saxon Count. "A statement of Translatio S. Pusinnæ that abbess Haduwy (a granddaughter of Egbert) and Charles the Bald (grandson of Heilwig) were related in the third and fourth degree, and have suggested that this was due to a relationship between Heilwig and Egbert"
Isanbard, a count in Alemannia, who is attested from 774-806 is also proposed as her father, but there are no primary sources and it seems unlikely. Thiedrada/Theodrada, the daughter of Bernard, and grand daughter of Charles Martel is supposed to be the wife of Isanbard and mother of Heilwig, but this is impossible on chronological grounds. Thiedrada must have been born after about 772-773, which would make her a contemporary of Heilwig. Isanbard is also proposed as the father of Heilwig's husband Welf.
Some later genealogies have Wittikind (Widukind) the Saxon leader defeated by Charlemagne as her father, but again there are no primary sources for this relationship and it seems to be extremely doubtful.
She was married to Welf, perhaps in about 800 and had 4 or possibly 5 children. (Only Judith is confirmed by sources as her daughter, other sources confirm the other children as Judith's siblings and it is thought that Heilwig is their mother as well.)
Judith (c.800-805 - 19 Apr 843), Empress and Queen of the Franks
(Possibly) Hrodroh/Ruadroh, who appears in a list of names after Heilwig, Konrad (Chuonrat) and Rudolf (Ruadolf) and before Hemma,and is thought to be another son, died 1 February, year unknown.
She was appointed as Abbess of Chelles about 825, most likely through the influence of her daughter Judith, and probably after the death of Welf.
The Abbey of Chelles was renowned for its learning and "the monastery housed an important scriptorium" and the nuns were both "skilled copyists and authors of manuscripts". Some important works were completed while Heilwig was Abbess.
The Ex Transtlatione S. Baltechildis, written in 833, confirms that Heilwig was still Abbess, and had been for 8 years. (venerabilis abbatissa Hegilwich, genetrix Iudith imperatricis ... predictae Hegilwich abbatissae, quae tunc octavo anno ipsum sanctum locum)
She died on the 5 November in a year unknown, but definitely 833 or later,  and probably at the Abbey of Chelles.
Confraternitates Augienses, in Piper, P. (ed.). Necrologia Germaniae Supplement: Libri confraternitatum Sancti Galli, Augiensis, Fabariensis, Berlin, 1884, pp. 145-352. (Monumenta Germaniae Historica, digitalised)
Nelson, J.L. (1996). The Frankish World, 750-900. A&C Black. Retrieved from Google Books
Saillot, J. (1977). Les seize quariers des Reines et Impératrices Francaises (420-1920), France, Angers: SETIG.
Settipani, C. & Van Kerrebrouck, P. (1993). La préhistoire des Capétiens 481-987: Première partie - Mérovingiens, Carolingiens et Robertiens, being volume 1 of Nouvelle histoire généalogique de l'auguste Maison de France, P. Van Kerrebrouck (Ed.); Villeneuve d'Ascq, France.