A dummy AA-battery allows you to 'short out' one position in a battery pack so that you can get a different voltage. For example, if you have a four-cell pack and want to power a circuit with 4.5 volts rather than 6 volts, you can replace one of the cells with a dummy battery.
Another use: you could connect a power-supply into a battery pack with one of these. Just ensure you get your polarities right, and you probably need a few dummies for the other position(s) in your pack. Don't use an external power-supply AND a real battery together. That's a bad idea.
I wasn't the first one to think of this, and I found a couple of other designs on Thingiverse for it as well, but they were all missing some nuance that I wanted in my design. And given that it's just a cylinder, it's a pretty quick thing to throw together.
I made my first pass a bit better, and it works so well that I decided to share it here.
You simply string a wire through it, and loop through the holes like sewing a button and you get a good contact in most battery holders.
I looped through the positive (bump) end 3-times with 22-gauge wire and a 'Z' pattern on the negative (flat) end. Works for me.
Need a AAA battery? You can probably scale it if you wish.
Should print well with .2mm resolution as well, if that's what your kit does.
Made an arched interior so it will print without supports just fine.
I used a raft with 4mm extra size to ensure I got good adhesion. With a tall item I wanted to be sure It didn't flop over half way through and make a mess. Go without if you are confident in your bed-adhesion.
Wiring it up.
You can use any old wire laying around. I just stripped the ends, but really insulation on the inside bits doesn't really add any value. Don't go with too small of a gauge - remember you can get a fair bit of current out of a AA battery and you don't want it to go 'open' on you.
Oh here's a thought you could use super-fine wire and it could be a fuse?
A pair of tweezers or fine needle-nose pliers are useful for helping you with the threading.