I'm looking for a power pack for my camera system.  It takes 4 AAs that don't last long and can add up fast.  The power adapter that goes with it converts from 12v.  So, I know I need a power adapter with 12v out as a feature.  The camera's always worked on a 12v circuit fused at 7.5 amps, so I know the output doesn't need to exceed that.  I'm still trying to lock down how many amps the camera draws when recording.

The best I can get from the manual is 4-5 hours with (4) AA alkaline batteries, up to 10 hours with (4) AA lithium batteries.

I'm not having any luck narrowing down what would do the job.  I was thinking it'd be nice if it could act as a pass-through power source so there's constant power regardless, but if the power pack will run the camera for 15 hours or more non-stop on a single charge, it'd not be a big deal to charge it normally at the end of the day.  I have no idea of what capacity I should look for.  

So what did you come up with?

Look around for battery substitutions; over the years I've seen a few things that are shaped just like a pair of AA or AAA batteries, and are just there to fill the space and pass through power. You might need to notch the battery cover and dab it with silicon to keep the wire in place and keep weather out, but these exist. I used to use them back when I was dabbling with circuitry design, back in the day.

Then maybe a few of these: https://www.amazon.com/EasyAcc-24000mAh-Waterproof-Portable-Charger/dp/B01B73I5ZU