I'm trying to power my Canon SLR camera with a larger external battery pack but confused on some results from the voltage regulator. At max, the camera only pulls .5A momentarily but it's usually around 200-300mA while the camera is awake.

To power the camera off a higher capacity battery pack, I first tried four 1.5V alkaline D batteries in series (5.5V) connected to a Pololu buck/boot regulator (switching supply) that outputs 8V 2A that then goes into a battery insert in the camera. When I tried to turn the camera on, I get a notice on the screen to "change battery pack". The 6V battery input to the regulator is well within the regulator voltage input (3 V to 30 V input) and fairly close to the 8V output. After a lot of testing, I tried 8 AA batteries in series (12v) to the voltage regulator and the camera works perfectly. What would be the problem with the 5.5V pack?

Each alkaline D battery is at 1.3-1.5 so not fully charged but not dead. I thought the problem might be voltage sag below the input voltage of my regulator if they couldn't provide the needed current but the input voltage to the regulator never dips below 5V. I could current limit a power supply at 5.5V and try that but I won't have access to one for a couple days. Any guesses on why the 5.5V pack wouldn't be working?

Voltage regulator I'm using:
https://www.pololu.com/product/2572

 

I thought the problem might be voltage sag below the input voltage of my regulator if they couldn't provide the needed current but the input voltage to the regulator never dips below 5V.

And what was happening with the output voltage at that point? The camera wants ~2W, which would require half an amp at 5V. That may be more than the aging D-cells could muster.

 

First you are trying to power the camera with 8V and then with 12V with the same regulator? Maybe try 5 or 6 D-cells. You could also try 4 NEW D-cells. Since it does work with the regulator, you can likely rule that out. A higher input voltage or Newer batteries. But if 4 newer D Batteries work, I wouldn't hold your breath on the longevity of the pack.