Hermetic seal for a fluid fill hole and related method for use in an implantable electrochemical cell

An electrochemical cell suitable for use in an implantable medical device includes a case and a cover attached to the case. The cover is formed of a first material and has an outer surface and an inner surface defining a thickness thereof. The cover also has a fluid fill hole formed therein. The fluid fill hole has an outer diameter at the outer surface and a smaller inner diameter at the inner surface. A hermetic seal is formed within the fluid fill hole of a second material that is softer than the first material and that is deformed to assume the approximate shape of the fluid fill hole.