If you own a hotspot, you probably don't just leave it sitting on your desk. Hotspots are perfectly happy running about anywhere you want to use them. But they need power. Of course, you can always plug it in to an accessory outlet in your car, but most new cars these days cut power to accessory outlets about 5 minutes after turning off the ignition.
Internal batteries would be difficult for small hotspots because they just don't have the capacity for more than a few hours of operation, and most of the time it is just an hour or two. That's okay for short term backup, but not for all-day use.
Our DMRspot and most other Raspberry Pi Zero based hotspots run very efficiently on common cell phone charging batteries. So this gives us lots of options. The problem is that these batteries are usually much larger and heavier than the hotspot itself. And, you have to attach the hotspot and battery with heavy duty rubber bands or velcro unless you want them flopping around.
We wanted a portable system that would last for a minimum of 8 hours, be fairly compact, solid, and could use and readily available batteries from a name brand manufacturer.
For this prototype, we used an Anker PowerCore+ mini 3350mAh Lipstick-Sized Portable charger battery. These are readily available on Amazon for $24. In our tests, this battery lasted for 11-1/2 hours with about 90% of that time in standby (listening) mode. These batteries also come in several colors so the battery and DMRspot can be color matched in many cases.
We designed a holder that would securely hold the DMRspot and battery, and while it is a little clunky looking, it works great. We added a 9" right angle power cable and it is all reasonably compact. The DMRspot and battery stay securely in their openings and you can move it from place to place without anything flopping around. The battery comes out easily and can be swapped with a fresh one if needed.
While not exactly pocket-sized, it suits the purpose. There are smaller batteries available and we are experimenting with other prototypes. But there are a couple of issues that we have no choice but to live with. Unfortunately, the smaller the battery, the shorter the run time will be. And, we don't like messing around with cheap batteries from dubious manufacturers.