Several members are evidently using two (2) D cells in parallel to power the filament chain of receivers such as the GRC-9, BC-611, and GRC-109. I've mentioned a  "Melt Down" problem with this procedure numerous times on my BC-611 and BA-48 page.
        Over the years I and several other members  have experience multiple instances of D cell battery packs consisting to two Alkaline Manganese Dioxide cells wired in parallel  creating very high temperatures. We call this phenomenon "Melt Down."  In BC-611's the case gets so hot you could not pick up the radio if you touched the area where the battery pack was inserted. Fortunately  each time the "Melt Down" occurred the radio was either in the field or on the bench and not being stored or on the shelf.
         In each case the manufacture was Duracell or Energizer.  It is mentioned  briefing in their safety sheets that the "Batteries may explode or leak, and cause burn injury, if recharged. . ."  I hate to mention brand names but these manufactures make an excellent battery and thus their cells have enough energy to cause "Melt Down" which occurs when  one battery is trying to charge the other battery.
        In most cases two D cells are overkill in powering the filament chain of our military reeivers when one battery would  suffice. I recommend that if you insist on using two cells in parallel that you insert the batteries only for the exact time that you need the receiver to be powered and that you monitor for over heating. Remove the batteries when not in use.  Do not store the battery pack on the shelf  in a parallel configuration. It has been reported that batteries that were exposed to high temperatures in the sunlight prior to use were prone to "Melt Down."

Safety sheets for these batteries should be  downloaded and read.

K4CHE