Well, F cells are the same diameter as D cells, but are about twice the length as D cells.
I took a 6V lantern battery apart once and was somewhat surprised at what I found.
Ken W7EKB
Sent via the Samsung Galaxy S21 5G, an AT&T 5G smartphone
-------- Original message --------
Date: 1/12/23 11:00 (GMT-08:00)
Subject: Re: [Milsurplus] Yep, 'F' cells
This is just a guess as I don’t recall ever having seen a definition of an F-cell. But if they were really called that, then it is a D-cell with wire leads instead of just flat contacts.