I wear hearing aids in both ears, which take small and easy-to-lose size 312 batteries. These are button-like batteries about 8mm (0.3in) in diameter, and 3.5mm (0.14in) thick.
One of the problems with every pair of hearing aids I’ve ever owned is that they give you about a 30-minute warning of the battery dying, and then it will just turn off. As you can probably imagine, this is not very helpful The boxes the batteries come in are bulky, and not particularly portable. Individual batteries easily fall out of pockets/wallets, get buried under other things, or lose the protective sticker that keeps them inert. (The batteries are air-activated, and start slowly losing charge if the sticker is removed.)
As a visual aid, here’s a AAA battery beside two hearing aid batteries (with and without the sticker), and the box they come in:
To make my life easier, I decided to make something more portable. I carefully measured, designed, and cut a small holder out of semi-frosted 3mm acrylic, capable of keeping 2-3 weeks’ worth of batteries close at hand.
It’s thin enough to fit into my wallet or the small pocket in a pair of jeans, and large enough to not easily get lost. The batteries themselves fit in snugly, making use of the slight tapering on the battery and the slight tapering from the laser to stop them just sliding through.
And finally, to give you a sense of scale, here’s a AAA battery beside the finished holder: