LIke @z10user2 & @TexasBob I use a little case. Got last ones from Lloyds, they are ~ 1" x 0.5" x 0.25". whch looks like it might be a little thinner.
I keep it in a vinyl change pouch in my pants pocket. So far I’ve only mistakenly given them away once with spare change.The recipient must have been surprised. (Canada has $1 & $2 coins not bills, so having a change pouch helps).
At home, I keep the HA case on the end table by the TV, it has a little hidden slot in it to hold 2 batteries.behind the grey tool in this picture.
I replace from either whenever the aids tell me they need it, then fill that from my supply that way I’m rotating my replacement batteries with fresh stock.