Anything a AA cell external battery pack can do, a D cell external battery pack can do longer. We used those years ago for extended running of devices like the original laptop, the AA powered Tandy Model 100 and 102, or field DXing with AA powered shortwave radios. That was pretty trick 20something years ago. You could even buy or make D cell battery packs with bean bags to support the powered device, stuck together with Velcro.

But nowadays I'd stick with Li-ion batteries. More bang for the buck, lighter weight, more versatile.

A single larger external Li-ion battery can handle two or more USB ports to simultaneously run and charge compatible devices (other than my Light & Motion headlight, which can only charge while plugged into a USB external battery). Spare Li-ion batteries with a solar charger on the rear rack or on top of the panniers.

I can recommend the Jackery USB batteries. I've used one for about six months and abused the poor thing. I rarely drop anything but for some reason I keep dropping that Jackery battery -- possibly because the darned things are shaped like a bar of soap and about as slick. Keeps on ticking. I usually keep it in a small padded handlebar bag Velcro'd across my handlebar (and retained with a backup safety strap -- which saved my bacon this weekend twice when rough roads and a crash popped the main Velcro strap loose). Run the USB cable through the bag to the device that needs charging or extended running.