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.
That’s a very good safety point. I know that some CR2032 coin cell batteries now have a very unpleasant (bitter?) coating to discourage this, and I wonder if that will extend to hearing aid batteries too.
I’m not hugely concerned about this in my situation because [a] I have no kids or pets, [b] the holder lives in my wallet, and [c] the batteries friction-fit into the holder. I can tap them (moderately gently) with the end of a small screwdriver and they don’t fall out. But it’s definitely something important to be aware of!