I don’t see a specification for operational temperature range for the Marvels in the manual I could find on-line. I did see a 5 C minimum operational temperature spec for an earlier Phonak Audeo - that is 41 F. You might ask your Audi what the minimum operating temperature is - they should be able to find out from Phonak if they don’t know.
For my Oticon OPN S 1 aids (typical 312 zinc air battery), the manual that came with the aids (and is on-line) specifies a minimum operating temperature of +1 C (or about 34 F) with a maximum of +40 C. The storage and transport temperature range is much wider (-25 to +60 C). For the rechargeable version of the OPN S, the manual lists an operating temperature range of +41 to +104 F. No idea why the rechargeable version would have a higher minimum operating temperature. Of interest is a comment about the best charging conditions being between +41 and +95 F.
I do not know what battery is used in a Marvel but I did find a Varta document on their rechargeable Lion small cells. The operating temperature range for charge is 0 to 45 and for discharge is -20 to 60 C.
Stuart