These are cases for batteries of types AA, AAA and ½ AA.
Originally I designed the version for the ½ AA battery, as those batteries are used by my DANA RS pump. In order to always have a spare battery with me, I created this key ring battery case. After switching to a DANA i pump, I also needed a AAA battery case, so I modified my original design. To somewhat wrap up the bunch, I also added a full AA battery case as well.
I'd recommend a 20mm (AA) or 16mm (AAA) keyring to attach it to your bunch of keys.
You may print or modify them and also give them away for free, but you are not allowed to sell them without prior permission.
My thanks go to wikipedia for the battery dimensions and to aubenc for his Knurled Surface Library v2.