Many people use standard USB powerbanks to power a Pi for a few hours, but these things have LiPo cells inside and thus don’t like getting cold. They can be used for a HAB flight with enough insulation, especially with something warm (like an action camera) in the payload, but if you want to make a small tracker then you really need to use Energizer Lithium cells which are rated for very cold temperatures.
There do exist some powerbanks, sold as “emergency phone chargers”, that accept 2 or 4 AA cells. I’ve tested a few, and they don’t supply a lot of current (typically 400-500mA max) but that’s enough for a Pi Zero tracker. Models with 2 AAs will run such a tracker for 6 hours or so from Lithium AAs, which is enough to track a complete flight if you’re reasonably proficient at tracking; this not an option I recommend for newbies.
To start with, buy a suitable AA emergency charger, like this one:
The case has similar height and width to a Pi Zero case, so is a good choice. Open it up to reveal the battery compartment:
On the model I bought the two halves are clipped together and are not welded or screwed, and can be separated carefully (but forcibly) with fingers; no need for tools.
The circuit board is not fixed so just lift it out. You now need to remove the USB socket (to make it easier to solder wires to the 5V output on the board) though you could leave it in place if you wish. I also removed the white LED. Here are 2 boards one before one after:
You now need to solder 2 wires, preferably red for +5V and black for 0V, to the vacated holes left by the USB socket; the one on the far left is +5V and far right is 0V:
Now fold the wires flat to the bottom of the board, keeping red on the lft and black on the right, and place the board back in the case:
I’ve run the wires through the hold that the button for on-off switch was in; if you want to retain the button then make a hole in the case for the wires; if not then you can still use the switch with a small screwdriver.
Now clip the other half of the case on, flip the powerbank over, and place next to your case Pi Zero like so:
If like me you already made the Pi Zero tracker, then it’s easier to solder to the underside of the Pi than the top side, so tin the 3 power holes (the 3 at the bottom, in the row on the right), cut the wires to length and tin those too:
And solder the wires to the holes like so (red goes to the end 2 holes; black to the next one):
As a final touch, glue or use double-sided tape to fix the battery holder to the Pi case, remembering that the battery holder has a cover for the battery compartment that wraps round to the back of the case.